role of pvc in swirl combustion systems
TRANSCRIPT
A review of oscillation mechanisms and the role of the precessing
vortex core (PVC) in swirl combustion systems
Nicholas Syred *
School of Engineering, Cardiff University, Queens Buildings, The Parade, Cardiff, Wales CF24 0YF, UK
Received 3 September 2004; accepted 13 October 2005
Available online 19 January 2006
Abstract
This paper reviews the occurrence of the precessing vortex core (PVC) and other instabilities, which occur in, swirl combustion
systems whilst identifying mechanisms, which allows coupling between the acoustics, combustion and swirling flow dynamics to
occur.
Initially, the occurrence of the PVC in free and confined isothermal flows is reviewed by describing its occurrence in terms of a
Strouhal number and geometric swirl number. Phase locked particle image velocimetry and laser doppler anemometry is then used
to describe the three-dimensional flow fields, which are generated when swirling flow is discharged into an open environment. This
shows the presence of a rotating and precessing off centred vortex and associated central recirculation zone (CRZ), extending up to
one burner exit diameter. The presence of axial radial eddies close to the burner mouth, in and around the CRZ, is clearly shown.
Typically one large dominant PV is found, although many harmonics can be present of lower amplitude. The occurrence of these
phenomena is very much a function of swirl number and burner geometry.
Under combustion conditions the behaviour is more complex, the PVC occurrence and amplitude are also strong functions
of mode of fuel entry, equivalence ratio and level of confinement. Axial fuel entry, except at exceptionally weak mixture ratios,
often suppresses the vortex core precession. A strong double PVC structure is also found under certain circumstances.
Premixed or partially premixed combustion can produce large PVC, similar in structure to that found isothermally: this is
attributed to the radial location of the flame front at the swirl burner exit. Provided the flame is prevented from flashing back to the
inlets values of Strouhal number for the PVC were excited by w2 compared to the isothermal condition at equivalence ratios
around 0.7. Confinement caused this parameter to drop by a factor of three for very weak combustion.
Separate work on unconfined swirling flames shows that even when the vortex core precession is suppressed the resulting
swirling flames are unstable and tend to wobble in response to minor perturbations in the flow, most importantly close to the burner
exit.
Another form of instability is shown to be associated with jet precession, often starting at very low or zero swirl numbers. Jet
precession is normally associated with special shapes of nozzles, large expansions or bluff bodies and is a different phenomenon to
the PVC. Strouhal numbers are shown to be at least an order of magnitude less than those generated by the PVC generated after
vortex breakdown.
Oscillations and instabilities in swirl combustion systems are illustrated and analysed by consideration of several cases of stable
oscillations produced in swirl burner/furnace systems and two where the PVC is suppressed by combustion. The first cases is a low
frequency 24 Hz oscillation produced in a 2 MW system whereby the PVC frequency is excited to nearly six times that for the
isothermal case due to interaction with system acoustics. Phase locked velocity and temperature measurements show that the flame
is initiated close to the burner exit, surrounding the CRZ, but is located inside a ring of higher velocity flow. Downstream the flame
has expanded radially past the high velocity region, but does not properly occupy the whole furnace. This allows the flame and
Progress in Energy and Combustion Science 32 (2006) 93–161
www.elsevier.com/locate/pecs
0360-1285/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.pecs.2005.10.002
* Tel.: C44 29 20874318; fax: C44 29 20874939.
E-mail address: [email protected]
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–16194
swirling flow to wobble, exciting instability. The next family of oscillations reviewed occur in a 100 kW swirl burner/furnace
systems whereby oscillations in the w40 Hz range are excited with flow fields akin to those found in pulsating combustors where
the flow is periodically stopped in the limit cycle of oscillation. The phase locked velocity and temperature measurements show a
number of mechanisms that can excite oscillation including substantial variations in shape and size of the CRZ during the limit
cycle of oscillation, and wobble of the whole flame and flow as shown by negative tangential velocities close to the centre line.
Analysis is then made of a high frequency w240 Hz oscillation in the same 100 kW swirl burner/furnace system, this oscillation
being caused by minor geometry changes. The flame was shown to not fully occupy the furnace, allowing irregular wobble and
precession of the flow and flame to develop, being especially noticeable close to the outer wall. The addition of an exit quarl to the
swirl burner is shown to substantially reduce the amplitude of oscillation by eliminating the external recirculation zone (ERZ),
reducing flow/flame wobble and variations in the size and shape of the CRZ. The quarl used was designed to largely occupy the
space normally taken up by the ERZ.
Two gas turbine combustor units firing into chambers are then considered, strong PVCs are developed under isothermal
conditions, these are suppressed with premixing in the equivalence number range 0.5–0.75. PVC suppression is attributed to the
equivalence ratios used, the burner configuration, location of the flame front and associated combustion aerodynamics. Other work
on an industrial premixed gas turbine swirl burner and can showed the formation of strong helical coherent structures for
equivalence ratios greater than 0.75. LES studies showed the PVC contributed to instability by triggering the formation of radial
axial eddies, generating alternating patterns of rich and lean combustion sufficient to reinforce combustion oscillations via the
Rayleigh criteria.
Finally, it was concluded that coupling between the acoustics and flame/flow dynamics occurs through a number of mechanisms
including wobble/precession of the flow and flame coupled with variations in the size and shape of the CRZ arising from changes in
swirl number throughout the limit cycle. Remedial measures are proposed.
q 2005 Elsevier Ltd. All rights reserved.
Keywords: Swirl combustors; Precessing vortex core (PVC); Reverse flow zones; Oscillation mechanisms
Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
2. Vortex core and jet precession . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
2.1. General characteristics of the PVC under isothermal conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
2.2. Effect of confinement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
2.3. Precessing jets and jet burners . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
3. Combustion and the PVC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
4. Vortex breakdown, modelling of the PVC and related phenomena, comparison with experiment . . . . . . . . . . . . . 126
5. Oscillations in swirl burner furnace systems, related systems and associated driving mechanisms . . . . . . . . . . . . 135
5.1. Driven PVC oscillations in the 2 MW swirl burner/furnace system, 100% axial fuel entry . . . . . . . . . . . . 135
5.2. Helmholtz and other resonances and vortex wobble /precession in a 100 kW swirl burner/furnace system,
partial premixing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140
5.3. Characterisation of high frequency oscillations in a 100 kW swirl burner furnace system, partial
premixing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143
5.4. Combustion oscillations in a swirl burner combustion chamber systems and suppression of the PVC . . . . 148
5.4.1. Instabilities generated in industrial premixed gas turbine combustor systems . . . . . . . . . . . . . . . 149
6. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
6.1. Interaction between the above effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
7. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
1. Introduction
The use of swirl-stabilised combustion is wide-
spread, including power station burners, gas turbine
combustors, internal combustion engines, refinery and
process burners [1]. The mechanisms and benefits of
swirl stabilised combustion are well documented and
depend in most instances on the formation of a central
toroidal recirculation zone which recirculates heat and
active chemical species to the root of the flame, allows
flame stabilisation and flame establishment to occur
in regions of relative low velocity where flow and the
Nomenclature
A constant in modified Strouhal number for PJ
burner
B constant in modified Strouhal number for PJ
burner
CRZ central recirculation zone formed by swir-
ling flow
d inlet orifice diameter of precessing jet
burner
D upstream PJ inlet orifice diameter, mm
De exhaust diameter of swirl burner, m
Dfe diameter of central exhaust of furnace
Do diameter of furnace or confinement vessel,
m
ERZ external recirculation zone
f frequency
Iis intensity of oscillation, w/cm2, derived from
pressure measurements for the isothermal
PVC
I intensity of oscillation relative to Iis, the
value for the isothermal PVC
Isothermal used to describe operation of a swirl
combustor without combustion where the
unit is fed air at ambient temperature
k kinetic energy of turbulence
LES large eddy simulation modelling
LDA laser Doppler anemometry
Lcontract length of contraction nozzle on end of
furnace
Lf flame length
Lfurn length of parallel section of furnace in swirl
burner furnace
Linlet length of inlet duct to swirl burner furnace
LPC lean premixed combustion
mair mass flowrate of air
NGV nozzle guide vanes in gas turbines
~p pressure
PIV particle image velocimetry
PJ precessing jet as characterised at University
of Adelaide
PVC precessing vortex core
Q volumetric flowrate, m3/s
QH thermal input, kW
r radius, m
re exit radius, m
ro radius of furnace or confinement, m
rs radius of bluff body in exit of Sydney swirl
burner, m
r* r/ro
rms root mean square of a signal
RANS reynolds averaged Navier–Stokes
modelling
Re reynolds number
Ri modified richardson number, (1/r)((r/(r)
(W2/r))/(U/(r)2
S swirl number, unless stated otherwise,
always derived from device geometry,
defined as ratio of axial flux of angular
momentum to axial flux of axial momentum,
non-dimentionalised by the exhaust radius
Scr critical swirl number when direction of
precession changes
Sg swirl number for the Sydney swirl burner—
ratio of integrated bulk tangential to
primary bulk axial air velocities measured
via LDA just above burner exit annulus.
The geometric swirl number S is 90% of this
value
SSN Strouhal number for the Sydney swirl
burner, 2frs/Ws
Strouhal number the common definition, fDe/ub, is
used throughout. ub is derived from the
isothermal burner flowrate and is based on
the burner exhaust area. Where the original
data used the definition fDe3/Q, this has
been converted
PJ Strouhal fd/ub—one definition of Strouhal num-
ber for the PJ nozzle number
u axial velocity, m/s
ub average bulk burner exit axial velocity, Q
outlet area available for flow (isothermal
conditions assumed), m/s
ub bulk flow velocity through the PJ inlet
orifice, m/s.
VBD vortex breakdown
w tangential velocity at a specific radius r, m/s
Ws bulk or average tangential velocity as
measured by LDA in exhaust annulus of
Sydney swirl burner, m/s
x axial distance
x 0 axial distance from exit of PJ nozzle
F equivalence ratio
g directional intermittency, % of negative
samples in ‘bin’ used to collect velocity
samples from LDA
3 turbulence dissipation rate
r gas density, m3/kg
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161 95
Fig. 1.1. Schematic diagram of processing leading to CRZ formation [1]: (1) tangential velocity profile creates a centrifugal pressure gradient and
sub-atmospheric pressure near the central axis; (2) axial decay of tangential velocity causes decay of radial distribution of centrifugal pressure
gradient in axial direction; (3) thus, an axial pressure gradient is set up in the central region towards the swirl burner, causing reverse flow.
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–16196
turbulent flame velocity can be matched, aided by the
recirculation of heat and active chemical species [1,2].
These processes are illustrated in Fig. 1.1 and arise as
follows:
– Swirling flow generates a natural radial pressure
gradient due to the term w2/r.
– Expansion through a nozzle causes axial decay of
tangential velocity and hence radial pressure
gradient.
– This in turn causes a negative axial pressure gradient
to be set up in the vicinity of the axis, which in turn
induces reverse flow and the formation of a CRZ.
– Where the tangential velocity distribution is of
Rankine form [1] (i.e. free/forced vortex combi-
nation), the central vortex core can become unstable,
giving rise to the PVC phenomena.
– The formation of the CRZ is thus dependent on
the decay of swirl velocity as swirling flow expands.
A typical toroidal recirculation zone formed at the
exhaust of a swirl burner is shown in Fig. 1.2 for a swirl
number of 1.57 and shows the large bubble of time
mean recirculated flow that is formed with here 12% of
the flow being recirculated [3].
With confinement this process is modified, the rate
of decay of swirl velocity is considerably reduced,
hence the size and strength of the CRZ formed [1,2].
This is illustrated by results from a swirl burner
furnace system for the combustion of low calorific
values gases from carbon black plants [4]. The
combustion system is illustrated in Fig. 1.3 and
consists of a variable swirl number burner with
separate flow controls for axial and tangential
premixed air and fuel. This is fired into a refractory
lined chamber, the confinement ratio for the swirl
burner, Do/De is 2, whilst the Lfurn/Do ratio for the
furnace is 2.5. Isothermal velocity results are shown in
Figs. 1.4–1.6. The tangential velocity distribution,
Fig. 1.4, close to the burner exit at x/DoZ0.11 shows a
peak velocity of w17 m/s at r*Z0.55; by x/DoZ0.33
this peak velocity has been maintained whilst moving
radially inwards to r*Z0.35. These tangential velocity
profiles are then conserved until the end of the furnace.
This initial change in tangential velocity profiles
induces complex axial velocity profiles and reverse
flow zone patterns, Fig. 1.5, and also a PVC close to
the burner exhaust. Throughout the furnace a region
of forward axial exists on the axis, extending to
r*w0.3–0.4. An annular reverse flow zone, centred at
Fig. 1.2. Stream function distribution at swirl burner exhaust showing typical recirculation zone set up in the exhaust of a swirl burner, isothermal
conditions, SZ1.57. PVC is located on boundary of reverse flow zone [3].
Fig. 1.3. Schematic diagram of refractory lined swirl burner furnace system for combustion of low Calorific value gases from carbon black plants
[4]: (1) inlet for tangential premixed gas and air; (2) inlet for axial premixed gas and air. Swirl number variation achieved by varying proportions of
above. Do/DeZ2.:LfurnZ2.5: air flow rateZ2.85 kg/s: SZ1.36.
Fig. 1.4. Distribution of tangential velocity in system of Fig. 1.3 [4].
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161 97
r*Z0.5, develops between x/DoZ0.11 and 0.69,
virtually disappearing by x/DoZ1, although there is
evidence of a weak intermittent zone to the end of
the furnace. Associated velocity vectors are shown
in Fig. 1.6 and show the development of the annular
CRZ.
The conservation of swirl velocity and hence,
angular momentum along the furnace length causes
the PVC formed near the burner exit to be of higher
frequency, but lower amplitude, than that formed by a
free, unconfined, expansion. Moreover, this conserva-
tion of swirl velocity also means that there is
potential for the formation of further PVCs in the
furnace exit downstream. This is discussed later in
Section 2.
Despite the advantages of swirl stabilised combus-
tion there is a well known propensity for instability to
Fig. 1.5. Distribution of axial velocity in system of Fig. 1.3 [4].
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–16198
develop and again there is an extensive literature in this
area [5–19].
Recent focus has been on lean premixed combustion
(LPC) as used with many modern gas turbine systems
to ensure low NOx emissions. Premixed flames are
by nature more susceptible to static and dynamic
Fig. 1.6. Velocity vectors in system of Fig. 1.3 illustrating flow
patterns [4].
instabilities due to the lack of inherent damping
mechanisms. The resulting absence of diffusive mixing
times leaves flames sensitive to acoustic excitation
from sound waves with flame response dependent upon
the amplitude, frequency and nature of acoustic wave
impingement. If conditions are favourable, periodic
fluctuations in the heat release will match the natural
resonant frequency of one or more of the geometrical
components of the combustor, or related natural fluid
mechanic mechanisms, resulting in self-excited
thermo-acoustic instabilities. The mechanism respon-
sible for the maintenance of limit-cycle heat-driven
oscillations was originally proposed by Rayleigh [20]
and refers to the relationship between the pressure wave
and rate of heat release. This paper discusses natural
fluid dynamic and related instabilities, occurring in
swirl combustors and related systems, which can excite
or increase periodic heat release. A major focus here is
the influence of vortex core precession and precessing
vortex cores (PVC).
The actual mechanism of the coupling effect
between the flow/flame dynamics appears to arise
from flow instability feeding into unsteady heat
release/combustion processes, which then feed instabil-
ity via coupling with acoustic modes of oscillation
and amplification via the Rayleigh criterion. Associated
work has shown that in high pressure process plant
containing large ductwork runs and cyclone separators
low frequency high amplitude pressure oscillations can
arise from coupling between natural modes of acoustic
oscillation and the vortex core precession (PVC)
generated in the cyclone separator, Yazabadi et al.
[21,22]. Similarly, Kurosaka [23] has shown that the
cooling effect produced by the Ranque–Hilsch tube
relies on the presence of strong PVCs, with up to six
strong harmonics being readily detectable, typical
fundamental frequencies were 2–7 kHz, being a near
linear function of inlet velocity. Suppression of the
PVC could be achieved by fitting 12 quarter wave
damping tubes radially around the circumference of the
tube and tuning their frequency to that of the PVC so
that they worked in anti phase.
It is often difficult to analyse the role of the PVC, its
influence on instability and indeed its presence in
combustion systems. The occurrence of the PVC is a
function of swirl number (S) [1,2], the presence of a
CRZ (normally SO0.6–0.7 for vortex breakdown, the
PVC and a CRZ to occur [1,2]), as well as the mode of
fuel entry, combustor configuration and equivalence
ratio. It has been shown that axial fuel entry normally
suppresses the PVC amplitude substantially, whilst
premixed fuel and air can restore its presence and
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161 99
indeed can considerably excite it [2]. This is of course
extremely important with premixed and partially
premixed combustors. Here again, the effect of
confinement ratio on the swirling flow is important as
discussed in Section 5.
This paper, thus commences with a review of
relevant work and then uses recent and new data to
analyse the role of the PVC and the associated CRZ,
relative to other factors which influence instability in
swirl combustion systems.
2. Vortex core and jet precession
The concepts to be discussed in this paper are
initially best illustrated by reference to the swirl
burner shown in Fig. 2.1. This is of simple
configuration with two circular inlets firing into a
circular chamber, which leads via a sudden contrac-
tion to the exhaust, normally 50% of the diameter of
the main chamber. The area of the tangential inlets
can be varied by removable inserts to give swirl
numbers in the range 0.75 upwards. Fuel can be
introduced by several methods, including axially
along the centre line and premixed with the air by
introduction via a premixing system just before the
tangential inlets. This device normally produces a
central recirculation zone (CRZ) in the exhaust over
its operational range giving excellent flame holding
capabilities.
This burner has been extensively used to describe
and illustrate the phenomena of the precessing vortex
core (PVC) [18,24–31]. Fig. 2.2(a) and (b) shows PIV
images obtained from this device under isothermal
conditions, operating with SZ2.6 [29]. Here, a laser
sheet fired horizontally close to the exhaust of the
burner in the tangential radial plane has illuminated
fine oil particles and with a double pulse laser unit
Fig. 2.1. Schematic diagram of generic swirl burner—swirl number adjusta
backwards into swirl chamber to prevent flashback. Nominal thermal input
enabled velocity vectors to be derived in the
tangential radial direction. The central axis is marked,
together with the diameter of the burner exhaust.
Fig. 2.2(a) shows the main vortex is displaced from
the central axis and is precessing about the central
axis of symmetry, here with a frequency of 140 Hz.
In this figure, the PIV image is superimposed on top
of the phase averaged tangential radial velocity
contours obtained from LDA at the same section.
The PVC can be seen to generate a central region of
negative tangential velocity due to the co-ordinate
system used for velocities in this plane. In this type of
unit the PVC phenomena persists for about 1–1.5 exit
diameters in free air. Fig. 2.2(b) shows another PIV
image showing evidence of the presence of a second
PVC [30]. A schematic representation of the flow
patterns associated with the PVC is shown in
Fig. 2.3(a), with a typical periodic signal obtained
from a pressure transducer inserted in the burner
exhaust flow shown in Fig. 2.3(b) [2]. Normally, the
PVC frequency increases quasi-linearly with flowrate.
A visualisation of PVC obtained under combus-
tion conditions is shown in Fig. 2.4(a) [30]. Here,
10% of the fuel is injected axially into the burner
where it is entrained into a low-pressure region in
the PVC centre. The remainder of the natural gas
fuel is premixed with the air upstream of the
tangential inlets and produces a non-luminous blue
flame. This consumes most of the available oxygen
and hence the fuel in the PVC burns fuel rich on the
PVC boundary as it is starved of oxygen. The
structure of the PVC extends to about 1.5 diameters
downstream of the burner exit before breaking up.
There is evidence from many sources that the PVC
is helical in nature [29–33], wrapping itself around
the reverse flow zone boundary, as shown in
Fig. 2.4(b) [33].
ble from 0.75 upwards via use of tangential inserts. Exhaust extends
100 kW [27–29].
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161100
2.1. General characteristics of the PVC under
isothermal conditions
For isothermal conditions, the frequency of the PVC
can be readily characterised by a Strouhal number and
Fig. 2.2. (a) Isothermal PIV instantaneous velocity vector plot just above ex
instantaneous velocity vector plot just above exit of swirl burner, Fig. 2.1,
the swirl number, S [1,2,24,25,32,33]. The Strouhal
number is a weak function of Re and asymptotic values
have been used for Fig. 2.5 (data has been gathered
from many sources, it should be noted that the Strouhal
number used on this and subsequent figures, fDe/ub, is
it of swirl burner, Fig. 2.1, showing one PVC [30]; (b) isothermal PIV
showing two PVCs [30].
Fig. 2.3. Processes associated with the PVC [30]: (a) schematic diagram of the flow patterns; (b) pressure fluctuation against time trace obtained
from pressure transducer located at lip of swirl burner, Fig. 2.1.
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161 101
different by the factor {P/4} to that used in the source
data [1,2,21,22,26,32–39], fDe3/Q, to ensure common-
ality with other literature reviewed). The relationship is
clearly a function of burner/swirl flow system configur-
ation. There are a number of effects here; the data from
large power station boilers was obtained from large-
scale systems and indicates a scale effect in that high
values of Strouhal number are obtained for low swirl
numbers. Strouhal numbers of more than two are
produced for Swirl numbers of one. Swirl burners and
cyclone combustors with non divergent exhausts with
no centre bodies produce data which fit onto the same
curve, giving values of Strouhal number of w0.86 for a
swirl number of 1. The well known Ijmuiden movable
block swirl burner [1,2,39], gives a Strouhal number of
w0.37 for the same swirl number of 1, indicating the
unit is not that effective in generating swirling flow.
This type of unit also uses a large central fuel injector,
and produces CRZ down to low swirl conditions,
resulting in the occurrence of a PVC type structure at
low swirl levels. These results were obtained with zero
or very low fuel jet velocities and thus do not arise from
precession of the central fuel jet: this has been
confirmed by separate PIV studies [39]. Cyclone dust
separators similar values of Strouhal number as for the
Ijmuiden movable block swirl generator, indicating that
the vortex finder (facing backwards into the cyclone
chamber to prevent boundary layer egress of particles
into the exhaust) is having deleterious effects on swirl
generation and hence, the Strouhal number.
Fig. 2.4. (a) Visualisation of single PVC with separate axial fuel
injection into a premixed flame, swirl burner as Fig. 2.1 [30]; (b)
visualisation of helical nature of the PVC from Chanaud [33].
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161102
Interesting results have been reported from the group
at the University of Sydney where a swirl burner is
formed by forcing swirling flow through the thin
annulus formed when a large bluff body is inserted into
the exhaust of a swirl flow system, Fig. 2.6 [40–45].
Fuel is introduced via a small central jet, which can be
of high velocity. This configuration produces extremely
complex flows, with the occurrence of multiple
recirculation zones due to the interaction of the swirling
flow, bluff body and the high velocity central fuel jet.
As discussed in [1] this can result in several flame
types; work at the international flame research
foundation (IFRF), the Netherlands, show that there
are at least two main types, the so called Type I flame
where a high velocity fuel jet fires through the CRZ,
followed by a wide zone of instability as the jet velocity
was reduced, finally resulting in a stable, more
common, Type II flames. The reported occurrence of
precession in this type of burner, Fig. 2.6, is very
dependent on the swirl number, central jet velocity and
bulk flow velocities [40,41]. Precession is reported
down to swirl numbers of 0.28 with high central jet
velocities relative to the bulk inlet velocity, and is
associated with precession of the central fuel jet as
opposed to a PVC interacting with the outer boundary
of the CRZ. As discussed later this form of precession is
characterised by values of Strouhal number an order of
magnitude less than shown in Fig. 2.5.
The persistence of preccessional frequencies to low
swirl numbers in the power station burners, Fig. 2.5,
arises from the use of large central fuel injectors, bluff
body stabilisers, and is differentiated from jet preces-
sion by the values of Strouhal number, which indicate
PVC form. All the other data from a range of different
swirl burners and cyclone combustors lack fuel
injectors and large bluff bodies in the exhaust flow
[21,22,34–38]. These units only produce PVC signals
beyond the normally accepted level of swirl for the
formation of a CRZ and vortex breakdown for SO0.5.
The Strouhal number data for these units thus collapses
to one separate curve, Fig. 2.5, as does that for cyclone
separators.
The structure of the PVC has been quantified under
isothermal conditions using the swirl burner of Fig. 2.1,
and LDA techniques whereby the PVC pressure signal
is used to phase lock and overlap velocity data to
produce the rotating velocity field associated with the
PVC, Fig. 2.7(a)–(c) (x/DeZ0.007) and Fig. 2.8(a)–(c)
(x/DeZ0.78), SZ1.5 [25]. Three diagrams are shown
for the rotating tangential (a), axial (b) and radial
velocities (c). Each diagram shows the average rotating
velocity field over the full 3608 of the burner. Close to
the burner exit, Fig. 2.7(a), the rotating tangential
velocity shows considerable variation in the q direction,
with a small but significant area of negative tangential
velocity near to and around the axis of symmetry, due to
the effect of the PVC, this is also shown on Fig. 2.2(a).
This is an important effect due to the presence of the
PVC and arises from the convention used to designate
measured tangential velocities, reference to Fig. 2.2(a)
is useful here. High levels of tangential velocity are
confined to a banana shaped sector of 1208 close to the
outer wall. There is an area of low tangential velocity
diametrically opposite to the high velocity region of
w8 m/s, reflecting the inlet velocity. The angular
position of maximum rotating axial velocity,
Fig. 2.7(b), closely matches that of the rotating
tangential velocity, indicating that much of the flow
leaves the burner in a thin banana shaped segment
inclined upwards at an angle of 458. The reverse
flow zone has a relatively high axial velocity value of
K7 m/s and is displaced substantially from the central
axis, extending from r/reZ0 to 0.7 and over a phase
angle of 1008. The rotating radial velocity levels,
Fig. 2.5. Variation of Strouhal number with Swirl number, asymtotic high reynolds number values, data for four distinct groups of devices [1,2,21,
22,25,26,31,34,35,37–39].
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161 103
Fig. 2.7(c), are somewhat lower than the other two
components. Of particular note is the kidney
shaped region of negative radial velocity (corre-
sponding to inwards flow), extending from the central
axis to r/rew0.77. Correlation of the tangential, axial
and radial velocities close to the burner exhaust,
Fig. 2.7, shows there is a large PVC present which
leads the reverse flow zone by about 90–1008 in phase.
These appear to be linked but distinct structures as most
of the volume of the PVC is in a region of forward flow.
By one exit diameter downstream the flow has nearly
returned to axisymmetry, although the PVC and off
centred reverse flow zone could still be detected at x/
DeZ0.78, Fig. 2.8(a) and (b). The region of negative
tangential velocity has shrunk considerably, although
its existence was still well defined. The maximum value
of tangential velocity had also decayed from 24
(Fig. 2.7(a)) to 9.5 m/s. Rotating axial velocity levels,
Fig. 2.8(b), were also more uniform, although the
banana shaped area of flow is still evident for both axial
and tangential velocities, although moved by about
2308 in the flow direction. Clearly, as indicated by
Fig. 2.4 the PVC and associated structures are helical in
nature, having been twisted by w2708 between the
sections shown in Figs. 2.7 and 2.8 (x/DeZ0.07 and
0.78); see also Fig. 2.4(b).
Phase locking of the PVC and associated phenomena
clearly looses some information and this is illustrated
by the two instantaneous PIV images shown in Fig. 2.2,
SZ1.7 [30]. Fig. 2.2(b) shows a state where two PVCs
can be distinguished, Fig. 2.2(a) shows a state where a
single PVC exists. The single PVC dominates this flow,
intermittently jumping to a two PVC state. This PIV
data has been subsequently analysed to give phase
locked axial radial velocity vectors, Fig. 2.9(a) and (b),
at two different cross-sections separated by 908.
Especially in Fig. 2.9(b), the presence of axial radial
eddies can be seen in and on the boundary of the CRZ
whilst, Fig. 2.9(a) shows that a phase angle change of
908 causes these eddies to diminish significantly. Other
work [2] using water models has shown the existence of
axial radial eddies produced by a swirl burner, SZ1.86.
Here, the eddies appear to be periodically shed from the
end of the CRZ and propagate downstream through the
expanding flow, accompanied by large scale motions or
flapping of the CRZ and shear layer. Other workers
have reported similar phenomena with swirling flames
including Roux et al. [45], Masri et al. [42], Syred et al.
Fig. 2.6. Swirl burner developed by sydney university group [41–44].
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161104
[46], but without the presence of the large PVC.
Dorrestein [47] found large radial axial eddies on the
edge of a swirling flame and attributed large amplitude
system oscillations to acoustic coupling with these
eddies.
2.2. Effect of confinement
The effect of confinement has an extremely
important effect upon the PVC and its related
instabilities. As discussed by Syred and Dahmen [48],
Syred and Beer [2], Gupta et al. [1], confinement can
dramatically alter the size and shape of the CRZ and
ERZ formed as the swirl burner flow expands into a
furnace or combustion vessel. It can also induce weak
regions of forward axial flow on the central axis inside
the CRZ [2,46]. Confinement ratio, Do/De, is the
dominant factor, the smaller this ratio the larger is the
effect. Other important factors include the level of
swirl, equivalence ratio and whether or not a quarl or
sudden expansion is used on the burner exit. As
discussed in Section 1 the CRZ formed by an
unconfined swirl burner arises because of the sudden
expansion and associated entrainment effects on the
edge of the swirling flow [1]. This causes decay in swirl
velocity profile, which in turn generates strong radial
and axial pressure gradients creating the CRZ.
Inevitably, any form of significant confinement will
affect this process and alter the size and shape of the
CRZ, whilst also normally causing an ERZ to form as
the flow sticks to the external wall. As the PVC is
closely associated with the boundary of the CRZ
confinement has considerable effects as discussed by
Fick [30].
Available results are summarised in Fig. 2.10(a) and
(b) for isothermal flow in the small 100 kW burner of
Fig. 2.1 (unconfined flow) and for confined flow in the
swirl burner/furnace system of Fig. 2.10(c). Fig. 2.10(a)
shows that for a confinement ratio of 2 (see Fig. 2.10(c))
Strouhal number is scarcely affected by this relatively
high level of confinement until a swirl number of about
1.3, when sudden difference occur, more than doubling
the unconfined value for SO1.5. This continues up to
the maximum swirl number characterised of 4.78.
There is still sufficient swirl left in the flow for another
vortex breakdown to occur, Fig. 2.10(a), in and just past
the exhaust of the furnace, Fig. 2.10(c), creating
another, separate PVC (this can be readily observed).
Chao et al. [49] reported a similar phenomena finding
two natural frequencies in different regions of an
undisturbed swirling flow field; a transition region was
also found where both instabilities co-existed, as found
by Fick [30]. Thus, in the exhaust of the furnace shown
in Fig. 2.10(c) a PVC was found whose value of
Strouhal number was considerably less than that found
in the main furnace just after the burner exit. There was
an effect of Reynolds number as especially at low swirl
numbers, SZ0.5, the effect did not appear until
high flow rates and furnace exit average axial velocities
of 12 m/s, SZ0.5 (ReZ48,000). This decreased to
4.5 m/s, for SZ1.5 (ReZ18,000). Fig. 2.10(b) shows
the relationships between the various frequencies at
high Re. For SO1.7 the effect of confinement is to
increase the frequency of the PVC formed in the swirl
burner exhaust by w2.1. The PVC then formed in the
furnace exhaust has a frequency w30% of that formed
just downstream of the burner exhaust. The occurrence
of this secondary PVC is unfortunate, as it can easily
Fig. 2.7. (a) Phase locked tangential velocity contours above swirl burner exit, x/DeZ0.07, burner as Fig. 2.1 [25]; (b) phase locked axial velocity
contours above swirl burner exit, x/DeZ0.07, burner as Fig. 2.1 [25]; (c) phase locked radial velocity contours above swirl burner exit, x/DeZ0.07,
burner as Fig. 2.1 [25].
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161 105
Fig. 2.7 (continued)
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161106
become another mechanism for driving instability.
Other work on cyclone dust separators has shown that
the PVC can travel around bends [21,22]. Thus, where
instability is a problem the use of inline exhausts for the
burner and furnace/combustion chamber as shown in
Fig. 2.10(c) is probably undesirable.
Unfortunately, detailed results are only available for
a confinement ratio of 2, Do/DeZ2 [30]. Clearly, larger
confinement ratios would produce values of Strouhal
number somewhat in between those for the unconfined
and confined cases shown in Fig. 2.10(a) and (b).
Anacleto et al. [50] studied swirling flow with and
without combustion in a LPP combustor model as
shown in Fig. 2.11 using a number of techniques and a
variable angle swirl generator. Swirl vane variation
between 0 and 608 could achieved, giving a swirl
number range of up to 1.6, whilst a wide range of Re
could be covered. Flow passes through the vaned
swirler, with an outer diameter of 120 mm, and then is
converged to a 50 mm diameter, 110 mm long pre-
mixing section, before passing through a 40 mm
contraction to the final combustion chamber of
110 mm diameter, Fig. 2.11. The PVC was character-
ised under isothermal conditions just past the 40 mm
contraction, both with and without the final combustion
chamber. The Strouhal number is shown as a function
of swirl number and Re, Fig. 2.12(a) and the pressure
difference, central flow axis to the wall of the 50 mm
diameter chamber, Fig. 2.12(b) at a position just past
the tip of the fuel injector, Fig. 2.11. In region I for S!0.5 no PVC is detectable, vortex breakdown occurs in
region II with the formation of a PVC. Strouhal
numbers then decrease from the initial value with
increasing swirl until values of Sw0.9. Subsequent
increases in swirl number produces the expected
increase in Strouhal number as indicated for other
systems in Fig. 2.5. The effect of the final combustion
chamber on the PVC is small, Fig. 2.12(a), with the
largest deviation occurring for SZ0.88. Thus, the
processes determining the formation of the PVC in this
system are governed by those occurring in the first
50 mm diameter premixing chamber, Fig. 2.11. The
pressure difference curves, Fig. 2.12(b) shows the
changes in flow structure occurring with vortex
Fig. 2.8. (a) Phase locked tangential velocity contours above swirl burner exit, x/DeZ0.78, burner as Fig. 2.1 [25]; (b) phase locked axial velocity
contours above swirl burner exit, x/DeZ0.78, burner as Fig. 2.1 [25]; (c) phase locked radial velocity contours above swirl burner exit, x/DeZ0.78,
burner as Fig. 2.1 [25].
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161 107
Fig. 2.8 (continued)
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161108
breakdown between states I and II and II and III. The
Strouhal number data for SO0.9 and high Re compares
well to the data of Fig. 2.5 and shows isothermal
Strouhal numbers just a little higher in value than those
produced by many other swirl burners and cyclone
combustors.
2.3. Precessing jets and jet burners
Studies have carried out at the University of
Adelaide on Precessing Jets (PJ) and their application
to burners in cement kilns and similar installations
[51–60]. Significant advantages in terms of reduced
NOx emissions have been shown on gas fired cement
kilns and promise is shown when firing pulverised coal
[52–54]. The associated fundamental work has
included studies of oscillating two-dimensional jets of
varying aspect ratios, triangular jets and most relevant
to this work oscillating or fluidic jets [51,55]. The
relevance to the PVC and swirl instabilities is that there
are many similarities in the mechanisms from which
the PJ originates and the characteristics of the jets so
generated. Fig. 2.13(a) and (b) schematically illustrates
the processes occurring with the PJ [51], with the entire
jet precessing about the axis of the system. Fig. 2.13(a)
shows a schematic of the processes occurring, whilst
Fig. 2.13(b) shows a water model visualisation,
obtained via a thin light sheet illuminating the central
axial radial plane. The unit consists of a cylindrical
chamber with a small axisymetric sharp edged inlet
orifice at one end and an exit lip at the other. Flow
enters the sharp edged orifice and expands into the
chamber where it attaches asymmetrically to the wall,
with substantial internal flow recirculation, Fig. 2.13(a).
The asymmetry causes the reattaching flow to precess
about the axis of the device, producing a precessing exit
flow. The lip and large transverse pressure gradients
near the outlet together steer the exit flow through a
large angle, towards the axis and across the face of the
nozzle outlet [51,56]. As a result the PJ entrains large
quantities of external fluid, some 5–6.8 that of an
equivalent free turbulent jet. Later versions of the PJ
nozzle have a centre body located just before the exit,
Fig. 2.14, to improve the regularity of the precession.
Fig. 2.15 shows phase locked LDA measurement of
axial velocity past the PJ nozzle exit at a PJ frequency
Fig. 2.9. (a) Phase locked PIV image in axial radial plane at exit of swirl burner, Fig. 2.1, plane 155–3358 [30]; (b) phase locked PIV image in axial
radial plane at exit of swirl burner, Fig. 2.1, plane 65–2458 [30].
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161 109
Fig. 2.10. (a) Variation of Strouhal number with swirl number,
isothermal conditions, data for unconfined and confined flow [30]; (b)
frequency ratio for the PVC between confined and unconfined states
[30]; (c) schematic diagram of swirl burner furnace system, swirl
burner as Fig. 2.1 [30]. Inserts are used in the tangential inlets to alter
swirl number. DeZ75 mm. Burner exit protrudes backwards into
swirl chamber to prevent flashback.
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161110
of 7.5 Hz. Flow leaving the PJ nozzle, Fig. 2.15, at
x 0/dZ0.67, assumes a banana or kidney shape region
of high velocity, moving downstream the flow has
returned to near symmetry by x 0/dZ1.93. These results
are similar to those obtained with the PVC, Figs. 2.7(b)
and 2.8(b). However, in operation the frequency and
motion of the PJ is more irregular than the PVC with
considerable signal jitter. This lead to several studies
produced by a mechanically rotating PJ nozzle where
the flow field was much more regular [59,60]. Wong
[57] describes a new phase locking technique to analyse
the complex motion from conventional non-rotating PJ
nozzles using two separate hot wire probes to produce
triggering signals for LDA or PIV systems. Earlier
phase locked techniques were unable to detect the
direction of rotation of the PJ. In this way, different and
consistent and parts of the cycle can be phase averaged
to obtain detailed velocity maps. Fig. 2.16 shows the
general flow characteristics of the PJ analysed by this
technique derived from instantaneous PIV, phase,
precession direction resolved phase averaged PIV and
surface flow visualisations. This shows a number of
smaller vortices and structures which the earlier phase
locked LDA technique had missed, for instance on the
exit nozzle, centre body and in the flow around the
precessing jet.
Wong [57] also discusses in detail various methods
of non-dimensionalising the frequency data from
various designs of PJ to produce Strouhal number
data. He proposes a modified Strouhal number for PJ
systems based on the inlet orifice diameter, d,
precessional frequency, bulk flow velocity through the
inlet orifice and two constants, A and B, which are
representative of system length scales. The derived
Strouhal number range from 0.008 to 0.06. This
contrasts with previously derived Strouhal number
values of w0.001–0.005 [51] using precessional
frequency, inlet PJ orifice diameter and corresponding
bulk flow velocity (fd/ub). These values are much lower
those obtained for the PVC, ranging from 0.2 to 2 or
more, although there are differences in definition.
In combustion situations, the driving fluid is usually
high-pressure gaseous fuel, typically natural gas [52],
although pulverised coal versions have been developed
[54]. The PJ creates a rapid decay in mean velocity
close to the nozzle and generates large-scale flame
structures with reduced shear relative to a simple free
turbulent jet [56]. As a result, the natural gas initially
burns in an oxygen deficient region and produces a
flame of excellent stability and high emissivity, unusual
for natural gas flames. This enhances radiant heat
transfer and can reduce NOx emissions by between 20
and 60% [52,53]. The high entrainment rates ensure
that downstream as the large-scale structures breakup,
good mixing occurs with good final fuel burnout.
Extensive experimental work shows that combustion
Fig. 2.11. Vaned swirler, prevapourisation and combustion chamber of Acacleto et al. [50].
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161 111
has little effect on the PJ structure and characteristics
as the initial processes which generate the PJ occur
upstream of the combustion process. There is parallel
here with the work of Ancacleto et al. [50], where the
PVC was generated upstream of the combustion
process and thus was not affected.
Fig. 2.12. Effect of Swirl Number on Strouhal Number and Pressure
Drop Parameter [50].
3. Combustion and the PVC
Combustion processes make the behaviour and
occurrence of the PVC more complex. The form of
the PVC and associated flows can be similar to that
found in isothermal flows [1,2,26,30,37,38,50]. The use
of axial or tangential fuel entry alone [1,2,26,37,38] can
suppress the amplitude of the PVC by an order of
magnitude or more and its frequency/occurrence
becomes a complex function of flow rate, equivalence
ration and mode of fuel entry. The PVC occurs more
readily with premixed combustion [1,2,30]. The
occurrence of the PVC is a very strong function of
the position where the flame is radially located and this
is highly dependent on the mode of fuel entry. This is
illustrated in Fig. 3.1(a)–(c) which summarises three
main flame types that can be found with a large 1 MW
rated unconfined swirl burner fired on natural gas, SZ1.86. The burner is schematically shown in Fig. 3.1(d)
and has no flashback protection to guard the eight
inlets. The first flame [2], Fig. 3.1(a), shows a result
with premixed air and natural gas where the flame is
actually located in the air/fuel inlets and the PVC is
considerably excited in frequency and amplitude. This
is an extreme result from a large unconfined flame
where the premixed flame has flashed back to the eight
slit tangential inlets through which the air/natural gas is
fired. The flame is thus mainly contained inside the
burner and is extremely noisy. A strong PVC signal was
readily seen and the results for a range of f are shown
for Strouhal numbers as a function of Re, Fig. 3.2(a).
Flame extinction occurred beyond fw0.68. The high-
est excitation of the PVC frequency occurred for
fw0.68 producing a value of Strouhal number
increased by a factor of 4 on the isothermal result.
This effect steadily decreases for reducing equivalence
ratios. Simple calculations indicate that this Strouhal
number increase can be described if allowance is made
for the acceleration of the gases due to combustion in
Fig. 2.13. (a) Schematic of processes occurring in the PJ burner [51]; (b) water model visualisation via an axial radial slit light of the processes
occurring in the PJ burner [51].
Fig. 2.14. Improved version of the PJ burner with centre body [56].
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161112
the tangential inlets as the value for bulk velocity, ub, in
the Strouhal number is based on the isothermal
flowrate. This premixed condition is unusual and
normally undesirable as considerable overheating and
distortion of the inlets can occur. Reference to Figs. 2.1
and 2.10(c) shows that an extension to the exhaust
nozzle is normally fitted to prevent flashback. This
produces very different flames, which are now
primarily stabilised downstream in the exhaust nozzle.
Fig. 3.1(b) shows the type of flame produced by axial
fuel injection in the same large burner; here the main
part of the flame is located downstream of the burner
exit, but parts of the flame surrounding the CRZ extend
Fig. 2.15. Phase locked LDA axial velocity contours in exhaust of PJ burner [56].
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161 113
back into the burner mouth and indeed right down to the
burner back plate, as a thin tulip shaped column.
Similar flames are produced with axial fuel injection by
the burners of Figs. 2.1 and 2.10(c). The PVC
amplitude is nearly always suppressed by at least an
order of magnitude compared to the isothermal state
(until very small equivalence ratios [2,61]).
Fig. 3.1(c) shows another flame produced by
tangential fuel entry with the flame located at an
intermediate radial position between that shown for the
flames of Fig. 3.1(a) and (b). Again, the PVC amplitude
is suppressed by at least an order of magnitude.
Fig. 3.2(b) shows the effect of axial and tangential
fuel entry upon Strouhal number for the suppressed
PVC as a function of Reynolds number and equivalence
ratio for a 1/5 scale model of the burner of Fig. 3.1(d).
There are differences between axial and tangential fuel
entry, but at high Reynolds numbers the values of
Strouhal number are approaching 80–90% of the
isothermal state. At exceptionally low equivalence
ratios (f!0.02) a large PVC reappears with axial fuel
injection [2,51] and this is the configuration upon which
the stability analysis discussed later was carried
out. The radial location of the flame front close to the
burner exhaust is important as this can give rise to
unfavourable/favourable gradients of rwr and density
conducive to PVC formation/suppression.
Claypole [26,37,38] used a natural gas fired swirl
burner of similar configuration to that of Fig. 2.1, but
with four inlets. Fig. 3.3(a) shows the effect of
combustion upon PVC rms pressure amplitude for
centreline axial fuel injection via spectral analysis of
signals obtained from a pitot tube located at the burner
exhaust lip. The dramatic reduction in amplitude by up
to a factor of 15 can be observed. Premixed fuel and
air was shown to only slightly affect the PVC under the
stated conditions, Fig. 3.3(b). Fig. 3.4 shows the
occurrence of the PVC for a range of Swirl numbers
and flow rates (Re). PVCs only occur beyond a flow rate
of w600 l/min (ReZ40,000) and this is where vortex
breakdown occurs as there is no CRZ formed at lower
flowrates. For 0.8OSO1.8 two PVCs are observed
of approximately equal intensity. For higher Swirl
numbers a single PVC reappears, but with multiple
Fig. 2.16. General flow characteristics of the PJ, derived from instantaneous PIV, phase and precession direction resolved phase averaged PIV and
surface flow visualisations [57].
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161114
harmonics, whilst for S!0.8 only one PVC could be
detected. A visualisation of a two PVC state in the swirl
burner of Fig. 2.1 with axial fuel injection is shown in
Fig. 3.5. A stainless steel mirror enabled simultaneous
images to be obtained in the axial/radial and tangential/
radial directions. Inside the burner the two PVCs rotate
in mesh and then spiral outwards in a helical manner as
they leave the burner exit. The PVCs persist for about
1.5 exhaust diameters downstream of the exhaust, being
of similar length to the single PVC visualised in
Fig. 2.4.
For fZ0.89, Fig. 3.6, the Strouhal number ranged
from w0.8 (SZ0.63) to 0.32 (SZ1.26), 0.3 (SZ1.53).
The value for SZ0.9 shows a sudden jump as a double
PVC mode is established with values of Strouhal
number dropping from 0.86 to 0.2 as the mode switched
from single to double PVC. This behaviour is quite
different from the isothermal state [26,37,38] where
there is a steady increase of Strouhal number with swirl
number, Fig. 2.5. The variation of Strouhal number for
the second harmonic of the PVC shows the same trends,
Fig. 3.7. A radial fuel injector reduced the coherence of
the PVC somewhat.
Available data on PVC frequencies in combustion
systems has been assembled in Figs. 3.8 and 3.9, this data
has been derived from references [1,2,26,30,31,37,38].
Fig. 3.8 shows the variation of Strouhal number for the
PVC with 100% axial fuel entry with swirl number
varying from 0.73 to 3.43 as a function of equivalence
ratio, all the flames being unconfined. An equivalence
ratio of 0 conveniently corresponds to the isothermal
state. For the lowest swirl number of 0.732, there is a trend
of increasing PVC frequency with equivalence ratio,
changing as the swirl number increases due to the
occurrence and formation of double PVC structures with
changes in equivalence ratio.
Fig. 3.1. Effect of different modes of fuel injection, SZ1.8 [2]; (a)
premixed flame with excited PVC, premixed natural gas and air, fZ0.52; (b) effect of central axial fuel injection, fZ0.952; (c) effect of
tangential fuel entry, fZ0.952; (d) schematic diagram of burner.
Fig. 3.1 (continued)
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161 115
For partially premixed conditions, Fig. 3.9 (100 kW
burner of Fig. 2.1, 10–50% of the fuel injected axially,
SZ0.76), a different situation pertains and although
higher harmonics of the PVC were present, the first
harmonic always dominates. This is why with
unconfined flames, SZ1.76, there is a steady increase
in PVC frequency with equivalence ratio, the value
doubling from the isothermal Strouhal number of 0.86–
1.64 at fZ0.71, but then dropping down again to
values between 0.56 and 0.96 as the burner is operated
up to fZ2. The level of axial fuel injection varied from
0 to 50%, depending on the equivalence ratio. For
unconfined flames the technique is limited by the blow
off limits of the combustor.
Fig. 3.9 also shows results from a large 2 MW swirl
burner furnace system, Fig. 3.10(a) and (b) (0.7!S!1.6, De/DoZ0.5, swirl burner four times geometric
scale up unit of Fig. 2.10(c)). Here, because the furnace
Fig. 3.2. The effect of Reynolds number and equivalence ratio f upon Strouhal number [2]: (a) premixed natural gas and air and isothermal state-
large combustor, SZ1.86; (b) 1/5 scale model combustor axial and tangential fuel entry, SZ1.86.
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161116
is refractory lined to investigate the combustion of low
calorific value gases, much wider blow off limits can be
investigated with different modes of fuel entry. Seventy
to one hundred % of the natural gas fuel was injected
axially, the rest was premixed in the inlets with the air.
The value of Strouhal number drops dramatically from
that of the isothermal state, 1.2 to w0.5 for f between
0.1 and 0.3, and then rises steadily back to the
isothermal value for fw1, then steadily increases
again with increasing f. There is only a small effect of
Fig. 3.3. Effect of combustion upon the PVC with (a) 100% axial fuel
injection; and (b) premixed, SZ1.98, DeZ75 mm, equivalence ratio
0.89 [26,37,38].
Fig. 3.4. Occurrence of the PVC for a range of swirl numbers as a
function of Swirl number and flow rate (Re). Hatched area shows
region of single PVC, square hatched area shows region of double
PVC [26,37,38].
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161 117
Swirl number and mode of fuel injection, upon Strouhal
number for fw1. As the PVC frequency varied quasi-
linearly with flowrate changes it was evident that the
PVC was not driven by system acoustics.
The isothermal value of the Strouhal number for the
2 MW system is 1.2, Fig. 3.9. Reference to Fig. 2.10(b)
shows a frequency ratio confined to free (Do/DeZ2) of
1.5 and thus, the equivalent Strouhal number for the
2 MW unit firing into free air is w0.8. This is a
reasonable match to the isothermal result for the
unconfined SZ1.76, 100 kW unit, Strouhal number
0.88, when differences in swirl number are allowed for.
For partially premixed conditions only one dominant
PVC was normally found, although there was always
evidence of other harmonics.
The work of Anacleto et al. [50] provides an
interesting contrast here. In their LPP system,
Fig. 2.11, a vaned type swirl surrounds a central hub
containing a fuel injector/atomiser. The swirling flow is
converged to a small diameter, but long chamber,
where centrally injected liquid fuel is pre-vapourised.
Combustion occurs downstream of this section in a
larger diameter chamber. The mixing and flow
characteristics in the combustion chamber are shown
to be strongly influenced by the formation of a large
PVC in the first pre-mixing chamber. However, as the
PVC has had significant opportunity to develop in the
first chamber, there is no suppression, and the Strouhal
number with combustion is very similar to the
isothermal state.
The stability of rotating flow may be analysed via
the work of Rayleigh on flow stratification [62] and
consideration of stratification parameters such as
modified Richardson numbers, Ri, as proposed by
Beer et al. [63]. The stability criterion proposed by
Rayleigh was that a system is:
– stable if rwr increases with r (solid body rotation)
– neutrally stable rwr is constant with r (free vortex)
– unstable if rwr decreases with r
Syred et al. [61] characterised the flow containing a
single PVC with combustion via axial fuel injection in
the burner of Fig. 3.1(d) at very low equivalence ratios,
fZ0.02, using phase locked fluctuating temperature
measurements and flow analysis. The rotating tempera-
ture fields obtained are shown in Fig. 3.11(a) and (b).
Here, the natural gas was completely entrained into the
Fig. 3.5. Co-incident pairs (a and b), (c and d), (e and f) of double PVC images from natural gas fired swirl burner, SZ1.77, via high-speed video. Images obtained via inclined stainless steel mirror.
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Fig. 3.6. Variation of Strouhal number for first harmonic PVC with
flow rate and swirl number equivalence ratio, 0.98, DeZ75 mm
(1 m3/minZreynolds number 67,500) [37,38].Fig. 3.8. Variation of unconfined strouhal number with equivalence
ratio, 100% axial fuel injection, 100 kW unit, Fig. 2.1 [1,2,26,30,31,
37,38].
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161 119
PVC and burnt on the boundary, the precessional
motion developing in the last half diameter before the
exhaust. By the exhaust the PVC was about 39% of the
exhaust radius and trailed a wake of burning hot gas,
achieving maximum temperatures of 1250 8C on the
edge of the PVC. Phase locked distributions of angular
momentum (rwr), Fig. 3.12, showed negative gradients
in and around the PVC, followed by a neutral region
just past the PVC, then an outer annulus with positive
gradients at larger radii showing stable flow. Further
analysis of this flow was carried out using the modified
Richardson number, Ri, which is the ratio of the
centrifugal forces in a field with density gradients to
the shear forces. Stabilising effects occur for values of
Fig. 3.7. Variation of Strouhal number for the second Harmonic of the
PVC with flow rate and swirl number [26,37,38].
RiO0. As the radial density gradient distributions
showed large negative values in and around the PVC,
Fig. 3.13(a) and (b), Ri becomes negative and thus
confirms the unstable nature of the flow region around
the PVC. Re-examination of the flames produced by
axial fuel injection, Fig. 3.1(b) shows that at the burner
exit the flame is burning in and around the central
vortex core region at quite a small diameter, typically
w0.2 De. There is little opportunity for negative
gradients of rwr and temperature (hence, density
gradient) to develop and thus precession of the vortex
core is minimised. The flame can only expand radially
when velocities have decayed due to the downstream
expansion of the flow, allowing matching to occur
between the flow and flame speed. This also causes a
downstream displacement of the CRZ; conditions in
and around the CRZ are then not favourable to
significant PVC formation.
Recent work from several sources [42–46,64] has
shown some light on stability of swirling flames when the
PVC is suppressed. Roux et al. [45] modelled the flows
within an atmospheric complex swirl combustion system
using compressible large eddy simulation (LES), acoustic
analysis and experiments in both isothermal and reacting
flows with methane as fuel. A vaned type swirler fired into
a square combustion chamber was used whilst the fuel,
methane, was premixed with the air. Reasonable
agreement between predictions and experimental
measurements was found. Under combustion conditions
(fZ0.75, mairZ12 g/s, QHZ27 kW) a PVC found under
isothermal conditions was suppressed. Here the combus-
tion aerodynamics are strongly influenced by an acoustic
Fig. 3.9. Variation of Strouhal number with equivalence ratio with partial premixing, 2 MW and 100 kW units [1,2,26,30,31,37,38].
Fig. 3.10. Photograph and schematic diagram of swirl burner/furnace
system, four times scale up of 100 kW system, operated with 25%,
tangential inserts to give Swirl No. 1.155 [30]: (a) photograph; (b)
schematic.
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161120
588 Hz 3/4 wave mode. A snapshot of an LES prediction
of an instantaneous temperature isosurface is shown in
Fig. 3.14(a) with a compact but irregular flame located
close to the burner exhaust. Mean temperature profiles
show a regular flame form [45], and this is constructed
from the average of thousands of snapshots as shown in
Fig. 3.14(a). This irregular instantaneous flame form is
clearly susceptible to distortion and coupling with
acoustic and other natural modes of oscillation of the
system. Selle et al. [64] carried out a LES simulation of a
swirl stabilised Siemens industrial gas turbine burner
firing into a square combustion chamber under atmos-
pheric conditions firing natural gas with preheated air at
673 K. The burner was constructed from two sections, a
central axial swirler is used to inject some air, whilst the
majority of the air is injected by a so-called diagonal
swirler. Fuel is normally injected in the diagonal swirler
through holes located on both sides of the swirl vanes.
Measurements were taken of mean and rms velocities for
hot and isothermal cases, in addition thermocouples were
used to obtain temperature fields under combustion
conditions. Under isothermal conditions PVC was
predicted and measured, but completely suppressed by
combustion and a 1000 K temperature iso-surface from
the LES work is illustrated in Fig. 3.14(b). This clearly
shows again the turbulent nature of the flame/flow
interaction where pockets of fresh gas are periodically
shed from the main flame zone and burn downstream.
A central core of hot gas is stabilised along the burner axis
by the CRZ, this core is attached to the face of the axial
swirler. The pressure field structure with combustion
corresponds with and induces an acoustic mode of the
chamber not analysed.
Syred et al. [46] have shown that swirling flames
with a suppressed PVC are susceptible to irregular
disturbances and hence, coupling with acoustic or other
modes of oscillation and indeed re-establishment of
a large PVC structure in certain circumstances [30].
Fig. 3.10 (continued)
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161 121
The burner of Fig. 2.1 is operated with axial fuel entry
alone at fZ0.456, whilst the PVC amplitude is
substantially suppressed so little residual trace can be
found. A salt solution is injected with the fuel so that
the high temperature regions of the flame are well
visualised by the fluorescence of the sodium chloride.
This volatilises early in the high temperature regions
inside the burner, enabling high frequency images to be
obtained at 1 kHz, with an exposure of w0.1 ms (no
laser is used, this is just natural flame luminescence).
Successive images are subtracted from each other to
give a measure of the change of intensity especially
towards the edge of the flame, Fig. 3.15. These
subtracted images are then analysed to give a mean,
rims and local intensity value. Both a side and top,
tangential/radial, views are obtained via the use of
stainless steel mirror. This plot thus gives information
on the fluctuation of flame intensity for frequencies up
to 1 kHz for a 1 s time frame (the storage limit of the
camera was 1000 frames, based on LDA experience
probably 10,000 images are needed to obtain better
statistics). In particular, it shows that the edge of the
flame is highly intermittent with instantaneous fluctu-
ations up to six times the mean, this occurring within
about 1–1.5 burner exit diameters. Circumferentially,
there is also a very large non-uniformity as shown by
the top view, and tangential radial mixing appears to
dominate on the edge and top of the flame. The data can
be further analysed, Fig. 3.16. Here, four successive
flame images, again each separated by 1 ms have been
analysed in terms of the flame shape, the 5 and 95%
areas of maximum intensity have been identified.
Analysis of the behaviour of volatilised sodium in
flames indicates that the flame boundary corresponds to
the outer contour and a temperature around 650 8C. As
to be expected the downstream flame shows substantial
variation in shape, but most interestingly the flame just
leaving the burner exit shows considerable variation in
its diametric location and it appears that the flame is
physically wobbling or precessing with no regular
frequency that could be detected, there are also
indications of this in Fig. 3.15. The top view of the
flame shows that it is non-circular in shape and
considerably distorted. there have been similar reports
of this phenomena by other workers [40–42,45].
Examination of the cine film and still images so
derived shows that the flame is sensitive to small
disturbances and is easily disturbed by flow or acoustic
perturbations, especially downstream of the burner exit.
Clearly, the presence of a quarl (or conical burner
outlet) which guides the expansion of the flow can also
serve to damp significant eddy movements on the
outside of the flame as it expands past the burner exit,
(discussed in more detail later). However, it does little
to suppress the irregular circumferential movements
shown in the top views of Figs. 3.15 and 3.16, or the
irregular end section of the flame. The 95% intensity
contour also suggests that the boundary of the highest
temperature regions of the flame and central reaction
zone is also varying considerably, Fig. 3.16, probably
also corresponding to an irregular fluctuation in the size
and shape of the CRZ and associated shear layer.
Fig. 3.17 shows flame boundaries derived from an
analysis of three separate successive side views. Here, it
is clear that between images 783 and 784 there has been
Fig. 3.11. Phase locked rotating temperatures (8C) obtained from compensated thermocouples in swirl burner of Fig. 3.1(d), fZ0.02 [61]: (a)
x/DeZK0.52; (b) x/DeZ0.
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161122
Fig. 3.12. Phase locked radial distribution of angular momentum
flux [61].
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161 123
massive movement in 1 ms, corresponding to mean
flame front velocities of up to 40 m/s in places, more
commonly 20 m/s. The radial movement of the whole
flame close to the burner exit is evident between images
783 and 784, with a radial displacement in 1 ms of
5.2 mm, or 10% of the flame diameter at this point.
As discussed earlier one reason for the suppression of
the large PVC is the very small column of hot
recirculating flow in the exhaust of the burner, surrounded
by a thin annular flame, this being contained within a high
velocity annulus of swirling flow where the flame will not
stabilise. Flame expansion occurs downstream of the
burner exhaust and this flame then surrounds the CRZ
located well downstream of the burner exhaust (i.e. flame
of Fig. 3.1(b)) in a region not conducive to PVC
formation.
The work of Roux et al. [50] is noteworthy in that it
shows a swirl flow combustor with premixed fuel and
air where the PVC is suppressed. The reasons for this
appear to lie with the configuration of the system,
location of the flame front and the development of the
swirl flow system and CRZ. In particular, swirling flow
enters an annulus surrounding a centre body and then is
forced into a contraction before entering the combus-
tion chamber. The processes occurring are illustrated
by Figs. 3.18(a), (b) and 3.19(a), (b) (fZ0.75, mairZ12 g/s, QHZ27 kW), which show velocity and tem-
perature profiles just inside the combustion chamber.
The important features are as follows.
For isothermal flow the tangential velocity close to the
entrance to the combustion chamber follows a Rankine
distribution [1,2], Fig. 3.18(a), with a steady rise from the
central axis to a peak in the forward shear layer (forced
vortex) followed by a decay towards the walls (free
vortex). This type of distribution continues downstream
with steady decay of velocity levels, Fig. 3.18(a), with
transference of angular momentum to the external flow
and smoothing out of the profile by xZ25 mm.
In contrast, the tangential velocities with combus-
tion show, Fig. 3.18(b), that at the entrance to the
combustion chamber (xZ1.5 mm) there is little
tangential velocity in the central region of flow.
Significant transference of angular momentum to this
region does not occur until xZ35 mm, Fig. 3.15(b). All
the tangential velocity is concentrated in an annular
flow region on entry to the combustion chamber.
Essentially, as there is little angular momentum in the
central region of the flow there is no real vortex core
(normally this region has a forced vortex distribution
[1,2]) and nothing to precess.
The axial velocity profiles under combustion
conditions, Fig. 3.19(a), show that the initial annular
jet flow rapidly diverges and gives rises to a large
toroidal recirculation zone and is of high velocity
w25 m/s. The corresponding temperature profiles,
Fig. 3.19(b), shows recirculation of very hot combus-
tion products back to the root of the flame at xZ1.5 mm. These hot recirculated gases are extremely
viscous and appear to substantially reduce the
transference of angular momentum into the central
region, thus producing conditions not favourable to
PVC formation. The presence of a centre body restricts
the upstream location of the CRZ and due to the high
velocity levels the flame cannot flash back and allow a
PVC to develop as reported in [1,2]. The configuration
of the centre body is important here, it consists of a
tapering cone leading from the axial swirler and
terminating at small diameter at the entrance to the
combustion chamber. There is thus some restriction of
flow on the central axis but not enough to induce a
substantive bluff body flow and allow a PVC to form as
reported in [39].
Although the PVC has been suppressed with
premixed combustion an acoustic 3/4 wave for the
whole device is amplified at 588 Hz and interacts with
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161124
the instantaneous the flame structure. Obviously under
appropriate conditions considerable excitation can
occur.
Selle et al. [64] also showed suppression of the PVC
with combustion in an Industrial gas turbine swirl
stabilised combustor. There are considerable simi-
larities to the results of Roux [45] in terms of the
velocity and temperature profiles produced close to the
burner exhaust when firing into the combustion
chamber. Especially, noticeable is the experimental
LDA results showing that there is negligible tangential
velocity in the central region of flow at x/DeZ0.35, this
only develops for values of x/DeO0.6 (LES results
differ). Again the temperature profiles show a CRZ
completely filled with hot gas at the main combustion
chamber temperature (w1650 K), surrounded by an
initially cold, just starting to burn, annular jet of fuel
and air, thus creating similar conditions to Roux et al.
[45]. Thus, there is a situation where there is initially no
swirl velocity in the central region, hence, no vortex
Fig. 3.13. Phase locked distribution of radial density Gradient: (a) cross-se
various phase angles.
core to precess. Similarly, use of the Rayleigh criteria
for stratified flows [62] and consideration of the
modified Richardson number, Ri, shows that
– as angular momentum flux, rwr, is very low in the
central region of flow close to the burner axis,
positive gradients exist due to the strongly swirling
annular jet entering the combustion chamber, thus
promoting stability;
– in terms of the modified Richardson number, Ri,
density gradients and centrifugal force gradients are
positive from the central region outwards to the
annular swirling jet, again promoting stability;
– this analysis applies equally to the work of Roux
et al. [45]. Unfortunately, neither Roux et al. [45]
nor Selle et al. [64] define a swirl number for their
configurations.
Reddy et al. [66] used PIV with a 508 vaned swirler
(swirl number S estimated at about 1) firing
ction at burner exhaust, x/DeZ0; (b) radial distribution at x/DeZ0,
Fig. 3.13 (continued)
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161 125
isothermally into a square combustion chamber and
found both a central CRZ, and also, not unexpectedly, a
series of corner eddies. The swirl vanes are fitted to a
large flat centre body. The PVC was clearly visualised
well downstream of the swirler at x/DeZ2.5. Unfortu-
nately, the effect of combustion was not investigated.
Paschereit and Gutmark [65] described and analysed
the effectiveness of passive combustion control
methods applied to a low-emission swirl stabilised
industrial combustor. Several axisymetric and helical
unstable modes were identified for fully premixed and
diffusion type combustion. The combustion structures
associated with the different unstable modes were
visualised using phase locked images of OH chemilu-
minescence and analysed using cross-correlation
between OH detecting fibre optics. Four different
thermo-acoustic instability modes were forced to
occur by adjusting the acoustic boundary conditions
for different operating conditions. Each of the four
modes was due to different acoustic and or flow modes.
Three of the modes reported were of helical form, both
with premixed combustion and diffusion flames. The
Strouhal numbers ranged from (all Strouhal numbers
corrected to that used in this text) 0.59 (axisymetric
structure, premixed), to 1.19 (helical structure, pre-
mixed, 2.05 (helical structure, diffusion) to 7.97 (helical
structure, premixed). The helical structures appear to of
PVC form. One form of instability that contributed to
the pressure oscillations was movement of the CRZ and
initiation of vortex breakdown. Three passive control
methods were discussed and reviewed in the paper:
† Miniature vortex generators installed around the
circumference of the burner exit to induce instability in
the Kelvin–Helmholtz vortices formed at that point.
These instabilities disrupted the roll-up of the vortices,
thus reducing the source of regular oscillating heat
release, and disrupting amplification via the Rayleigh
criteria [20]. This technique reduced high frequency
oscillations and at the same time suppressed low
frequency instabilities. Some nozzle designs yielded
Fig. 3.14. (a) Snapshot of an LES prediction of 1250 K instantaneous
temperature, iso-surface [45]. Note compact but irregular flame
located close to the burner exhaust; (b) Snapshot of an LES prediction
of 1000 K instantaneous temperature iso-surface produced by
industrial gas turbine combustor [64].
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161126
over 10 dB suppression of high and low frequency
instabilities.
† An elliptically shaped burner, which essentially
has two volute or scroll inlets, induces axis-switching
dynamics in the large-scale swirling vortices formed in
the combustor. These are characterised by several
azimuthal unstable modes that reduce the coherence of
the vortices. Such geometry prevented coupling with
acoustic modes and resulted in suppression of instabil-
ities by over 25 dB for a wide range of flame
temperatures and power levels. In addition, NOx and
CO were reduced due to enhanced mixing and
increased turbulence.
† Extended pilot fuel lance protruding into the
plenum of the burner was used to stabilise the point of
vortex breakdown (VBD). Tests in high and low
pressure combustion conditions showed the VBD was
highly sensitive to combustor pressure fluctuations,
thus leading to another mechanism for thermoacoustic
excitation. A longer lance prevents this interaction and
was implemented in gas turbines in the field.
Krebbs et al. [67] undertook a detailed acoustic
analysis of a swirl stabilised gas turbine combustor and
described design and modelling methodologies aimed
at evolving configurations which minimise acoustic
response and excitation, hence interaction with any
nascent PVC.
In contrast, Ancacleto et al. [50] used a system with
a vaned swirler firing through a convergence, followed
by a first stage premixing chamber and then the
combustion chamber, Fig. 2.11. The first stage-mixing
chamber allowed the PVC to develop and this then
continued through into the combustion chamber so
combustion had little effect on the PVC.
A summary of flame types with and without the PVC
is summarised in Table 1. Six different flame types are
identified; the state of the PVC is indicated and whether
or not it is suppressed or not. What is clear from the
above is that even when the large PVC is absent the
resulting swirl stabilised flame is very sensitive to small
disturbances and can follow an irregular precessional
motion, which translates to large irregular motion of the
flame brush.
4. Vortex breakdown, modelling of the PVC and
related phenomena, comparison with experiment
The occurrence of the PVC is normally linked to
the phenomena of vortex breakdown and the
occurrence of CRZ. There is considerable evidence
from analytical and experimental studies that
precessional motion can exist at low swirl numbers
when CRZs are not present, although there do appear
to be significant differences to the PVC occurring
after vortex breakdown; this is discussed later in this
section. Sarpkaya [68] provided the first very
detailed experimental study of the vortex breakdown
phenomena and showed that the form, type and
occurrence were very much a function of Swirl
number and Re, Fig. 4.1. In his rig, vortex core
precession only started after the formation of the
initial breakdown bubble. Two main types of vortex
breakdown were identified, being a function of Swirl
number and Re. An extensive review of vortex
breakdown has been made by Lucca-Negro and
O’Doherty [69]. The paper reviews experimental,
numerical and analytical studies, as well as descrip-
tions, types and forms of the phenomenon. Although
a clearer picture of the flow structures produced has
emerged, a complete description of the phenomena
has not emerged. As the vortex breakdown phenom-
ena is normally regarded to be a pre-cursor to PVC
Table 1
Summary of flow and flame characteristics and the precessing vortex core
Flame/flow type Combustion intensity
other effects
PVC intensity and frequency (f) Strouhal no. fDelub
Correlation
Pressure drop Remarks
Isothermal Confinement doubles
PVC frequency for SO1.5
Iis!5 w/cm2 for SZ1.8 Good at high Re with/
without furnace.
Strong function of
Swirl no.
Audible low frequency
noise. Large PVC
(a) Premixed fuel and
air. Combustion
extends back
High comb. iIntensity;
Lfw1–5 De
Iw20Iis fpvcw3fisothermal Fair By factorw3 of
isothermal
Wide blow off limits,
violent flame
oscillations large
PCVs present
(b) Diffusion flames
axial fuel entry
Medium combustion
intensity: Lfw3–5 De
Iw0.01–0.1Iis fwfisothermal Poor at low Reynolds
no.
w90% of isothermal Exceptionally wide
blow off limits: PVC
suppressed
(c) Diffusion flames
axial fuel entry very
low f
Very weak combustion
flame burns on PVC
boundary
Iw0.8Iis fw0.85fisothermal Good at high Reynolds
numbers
w85% of isothermal Flame burns on PVC
boundary. Large PVC
present
(d) Tangential fuel
entry, diffusion flame
Medium combustion;
Lfw2–3 De
Iw0.01–0.1Iis fwfisothermal Poor at low Reynolds
numbers
Up to twice isothermal Narrow blow off limits:
flame quiet. PVC
suppressed
(e) Partially premixed
flashback prevented
back to inlets by
extension of exit see
Fig. 2.10
Medium combustion
intensity: Lfw1–2 De
Iw0.5–0.8Iisothermal f function of mixture
ratio and confinement,
Fig. 3.9
Fair, function
equivalence ratio
Similar to isothermal Axial fuel entry for
10–50% of fuel, rest
premixed. Large PVC
present
(f) Premixed confined
LPP configuration
Fwup to 0.75
High comb. intensity PVC suppressed Not applicable Not applicable PVC suppressed due to
system configuration
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Fig. 3.15. Intensity variations from a free natural gas, swirling flame, burner of Fig. 2.1, non-dimentionalised by maximum intensity value.
Simultaneous horizontal and vertical views. Intensity variations obtained by subtracting successive images obtained from camera with 1 ms interval
(exposurew0.1 ms) [46].
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161128
formation there is clearly a fruitful area for further
research here.
Experimental studies have indicated that for many
different systems (cyclone combustors, dust separators,
hydrocyclones, burners, fluidic vortex devices, vortex
whistles, Ranque–Hilsch tube, turbine runners [1,33,
71]) and different fluids the PVC is a phenomena which
only occurs when vortex breakdown has occurred
beyond critical levels of Swirl number, Re, and for
certain configurations [1,2,25,32,33,37,38,70,71].
Other work [39,40,41,44] has shown that preces-
sional motion may persist to very low values of swirl
number if a swirling jet is fired into a large expansion or
a centre body of significant size is present in the burner
exhaust. For zero swirl the flow past a centre body will
naturally induce a CRZ due to flow separation. As the
level of swirl is increased there is some form of vortex
breakdown occurring/change in recirculation zone/
CRZ structure leading to PVC formation [39],
providing any central fuel jet is of limited velocity as
otherwise the mechanism can differ with precession of
the central fuel jet.
Hallet and Gunther [78] while studying the flow within
a dump combustor, with expansion ratios Do/De ranging
from 1.25 to 3.0, visually observed jet precession within
the combustor chamber. They further concluded that
precession in a dump combustor was not beneficial for
mixing and did not pursue the matter further.
Dellenback et al. [79] conducted a series of
experiments with upstream swirl in a long pipe flow
to further observe the precession phenomenon. They
used an expansion ratio of Do/De of 1.94 and varied
the upstream swirl number from 0.05!S!0.4 for
ReZ30,000 and 100,000. Jet precession direction was
found to be related to Swirl number. At low Swirl
numbers the precession of the jet is opposite to that of
the upstream swirl. When the swirl increases past a
critical swirl number (Sw0.15), the flow precesses in
the same direction as the upstream swirl. However, the
air bubble visualisation technique was not able to
resolve jet precession direction for values for S!0.05.
The results were extrapolated to conclude that at a swirl
number of zero, no precession occurs. The other region
where precession direction was difficult to resolve
was at the critical swirl number, Scr. The authors
interpolated the data before and after the crossover
point and reasoned that no precession exists at the
critical swirl number (Scrw0.15).
There have been many attempts to model the PVC
phenomena using a number of tools ranging from
Fig. 3.16. Top four images show instantaneous successive images from sodium, seeded flame, contours show 5 and 95% intensity levels (images
move sequentially from left to right, top to bottom). Note irregularity both in the axial radial and tangential radial planes. Bottom left figure shows
normalised mean rms intensity from 1000 images [46].
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161 129
analytical models to CFD and LES. One of the earliest
studies was that of Sozou and Swithenbank [72]. They used
an inviscid model of a vortex core embedded in an axial
flow and a perturbation technique by presuming small,
wave-like disturbances of variables about the asymmetric
flow. The intention was to model high frequency travelling
tangential waves, but the numerical solution converged to a
slow wave or PVC solution. Reasonable agreement with
the data of Chanaud [33] and Vonnegut [71] was found.
Avramenko et al. [73] extended this work by
considering an axisymetric swirling flow with radial
velocities that were an order of magnitude lower than
axial or tangential velocities. Cylindrical polar co-
ordinates were used with the assumption that unper-
turbed velocities and turbulent viscosity are functions
of radius only. The analysis eventually reduces to a
second order differential equation for the perturbed
tangential velocity amplitude. With the assumptions of
a linear form for the unperturbed tangential velocity
and considering only angular perturbations, analytical
solutions are then derived for the perturbed velocity
amplitudes in terms of Bessel functions and an
analytical solution for the Strouhal number in terms
of an effective Reynolds number. The model predicts
Fig. 3.17. Flame boundaries derived from an analysis of three separate successive axial/radial views, burner of Fig. 2.1, exit diameter 80 mm [46].
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161130
that the PVC frequency is only a weak function of
viscosity, but linearly dependent upon mean inlet
velocity, in agreement with experiments. The predic-
tion of the PVC frequency was improved by using
values of turbulent viscosity that varied linearly with
radius, adopting complex forms of the solutions,
invoking the variational principle. This produced a
general expression for the Strouhal number which
included the effects of axial perturbations and an
effective turbulent viscosity function [74]. Reasonable
agreement between experimental velocity measure-
ments and the theory was shown, Fig. 4.2.
Bowen et al. [74] extended this work further by
utilising variational techniques based on the principle
that solutions will tend to a state in which energy is
minimised given certain conserved quantities. They
used a stream function vorticity approach for two-
dimensional inviscid incompressible flow over a disc.
They then expanded these equations by Bessel–Fourier
functions, whilst using several variational theorems
which allow critical points of kinetic energy under the
constraints of conserved quadratic entropy and angular
momentum to be derived. Families of relative equilibria
solutions were produced, the first solution representing
the axisymetric case, the second term in higher order
solutions representing sets of vortices rotating about
each other. Prediction of the rotating flowfields
produced by the swirl burner of Fig. 2.1 were
qualitatively in agreement with the experimental
phase locked isothermal data produced near to the
burner exhaust, Fig. 2.7, with the dominant features of
the flow present. The model predicts the existence of a
second peak of tangential velocity opposite to the main
tangential peak, indicative of a second vortex, again
confirmed by experiment, Fig. 2.2(b). The central
region of negative tangential velocity is also well
predicted.
The first attempt to use CFD to characterise and
describe the PVC was by Sato [75,76]. Fluent with a
three-dimensional axisymetrical grid was used to model
the swirl burner furnace combination of Fig. 2.10(c).
Although a non-time dependent analysis was used he
showed for the isothermal state that the flow would
easily perturb and stick to a sidewall producing
structures similar to those experimentally recorded
and shown in Figs. 2.7 and 2.8. Bowen et al. [74] and
Lucca-Negro [77] extended the work of Sato using
Fluent and the RNG and RSM turbulence models
operating in a time dependent mode. Good qualitative
agreement between the CFD predictions and the
measured PVC characteristics, Figs. 2.7 and 2.8 were
found, although there was a tendency for the CFD
predictions to revert to axisymetry over time.
Guo et al. [80] used the CFX code and a VLES kK3
turbulence model approach for time dependent
analysis of turbulent swirl flow passing into a sudden
expansion, Do/DeZ5, ReZ105. The flow was unstable
over the whole swirl number range from 0 to 0.48, with
a large PVC type structure normally being present. The
analysis shows that with zero swirl the limit cycle is a
mixture of precession and flapping oscillation: the
flapping motion is significant up to SZ0.5. Increase of
Fig. 3.18. (a) Isothermal tangential velocity profiles in the combustion chamber. O LDA:LES [45]; (b) tangential velocity profiles in the combustion
chamber-combustion conditions. O LDA:LES [45].
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161 131
Fig. 3.19. (a) Axial velocity profiles in the combustion chamber-combustion conditions. O LDA:LES [45]; (b) temperature profiles in the
combustion chamber. O thermocouples:LES [45].
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161132
Fig. 4.1. Occurrence of the vortex breakdown phenomena [68].
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161 133
swirl number beyond 0.08 makes the precessional
motion dominant with more regular limit cycles. At the
same time the precessional frequency drops until it
reaches a critical swirl number, Scrw0.23, and then
increases again with Swirl number, but in the reversed
direction. Dellenback et al. [79] found similar results
experimentally, but with Scr at 0.15. Two instantaneous
visualisations of the precessional flow for swirl
numbers of 0.13 and 0.25 are shown in Figs. 4.3(a),
(b) and 4.4. The change in pressional direction should
be noted. Beyond values of SZ0.48 vortex breakdown
occurs with the formation of a CRZ and ERZ. The
Strouhal number, Fig. 4.5, varies linearly with swirl
number as found elsewhere, Fig. 2.5, although there is a
Fig. 4.2. Comparison of predicted and measured axial velocities at
exit of swirl burner [73].
sharp change at Scr, 0.23. The definition of Strouhal
number used here is as follows:
2ffiffiffiffip
p
� �Do
De
� �2 f De
ub
� �
Compared to conventional definitions of Strouhal
number the inclusion of the terms ð2=ffiffiffiffip
pÞðDo=DeÞ2
increases the value by more than 28, and thus the values
in Fig. 4.5 must be divided by this value to compare
with Fig. 2.5, i.e. giving values ranging from 0 to
0.0075. The modelled processes are thus similar to
those occurring with precessing jets [51–57], where
values of (a comparable) Strouhal number between
0.001 and 0.005 were found. This is hardly surprising
considering the extent of the jet expansion (Do/DeZ5)
and the low level of swirl. Thus, this form of precession
is quite different to that associated with the CRZ and
which normally occurs for higher values of S, where
values of Strouhal number are from 0.2 upwards.
Guo et al. [41] used RANS kK3 time dependent
calculations and extensive measurements to character-
ise swirl flow instabilities in the Swirl burner developed
at the University of Sydney, Fig. 2.6 [41–44], primarily
for non reacting flows. Data from the earlier work of
Al-Abdeli et al. [44] was used for comparison. For
isothermal conditions they showed that for ujZ66 m/s
and ubZ16.3 m/s increases in Swirl number, Sg,
eventually lead to the detection of distinct frequency
peaks indicative of precession. This was initiated at
SgZ0.34 with a 20 Hz irregular oscillation, leading to
stable strong precession at SgZ0.4, again at 20 Hz.
Further increases of Sg to 0.57 produced a further peak
at 28 Hz, followed at SgZ0.68 and 0.91 peaks at w28
and 26 Hz, respectively. Increase of Sg to 1.59 showed
no distinct frequencies, but considerable noise. A lower
jet velocity of 50 m/s for SgZ0.4 gave a frequency
peak of 17 Hz, whilst a jet velocity of 90 m/s for SgZ0.57 gave a 35 Hz frequency. As discussed earlier there
are obviously interactions between the swirl, bluff body
and central jet which are difficult to separate. High
velocity central jets are well known to cause substantial
changes in flow patterns both for bluff body [81] and
swirl flows [1,2] and further work is needed to separate
effects.
RANS prediction of the Strouhal number variation
with Sg are shown in Fig. 4.6(a) and (b). Fig. 4.6(a)
(ubZ16.3 m/s) shows good agreement with measured
and predicted vales of Strouhal number: Fig. 4.6(b)
(ubZ29.7 m/s) shows poorer agreement for a higher
bulk fluid velocity. Here again the Strouhal number
(SSN) is defined unconventionally, see the
Fig. 4.3. Instantaneous visualisation of swirl flow showing its spiral nature, SZ0.13 [80]: (a) isosurface of axial velocity; (b) image showing
corresponding vortex core location.
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161134
nomenclature. The values of Strouhal number may be
converted to the conventional definition (fDe/ub) by the
parameter 1.2Sg. If this is carried out the values of
Strouhal number reduce to a range of w0.04 (SgZ2.1)
to 0.1 (SgZ1) maximum. For high swirl numbers, SgO0.8 these values of the SSN and conventional Strouhal
number are very low w0.1–0.04, indicating again that
this is different to conventional swirl burner systems as
described by Fig. 2.5. This is confirmed by Fig. 4.7
which shows snapshots the isothermal flow evolution
and structure change with variation in Sg. Regions of
reverse flow are displayed by the contour line u!0. For
low values of Sg, 0.35, the central jet deflects little and
the flow is virtually symmetric. A downstream
recirculation bubble appears at about x/DeZ1.67
from the burner face at SgZ0.6. This bubble appears
to restrict the central jet movement as the precessional
frequency is reduced by subsequent growth of bubble
size. A recirculation ring exists in a stagnant region
behind the burner face. The extent of the downstream
bubble increases with Sg, eventually merging with the
upstream recirculation zone. Subsequent increases of
Fig. 4.4. Instantaneous visualisation of swirl flow, SZ0.25, note
change of direction of precession from Fig. 4.3 [80].
Sg cause the bubble to intersect the central jet so the jet
precesses within the confined space created by the
recirculating bubble. As has been commented earlier
these very complex structures differ very significantly
from the conventional CRZs discussed in [1,2]. Limited
reacting flow studies were undertaken where it was
found that increasing heat of reaction of the fuel
suppressed precession. The Strouhal number results
from the Sydney Swirl Burner show that the precession
generated is very similar to that of the precessing jets of
references [51–58].
Wegner et al. [82] used time dependent RANS, LES
and experiments to characterise isothermal swirl flow
instability in an Ijmuiden type of movable block swirl
generator [1,2], that has been extensively studied in
the EU funded TECFLAM programme. The device is
shown in Fig. 4.8, together with the computation grid
used. As can be seen the computational grid extended
back into the device and to the sets of inlets used to vary
the swirl level. The RANS method employing a full
Reynolds stress model was able to capture the PVC
phenomena both qualitatively and quantitatively in
Fig. 4.5. Variation of Strouhal number with Swirl number, without
CRZ presence [80].
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161 135
parts. Good accuracy was achieved for predictions of
PVC frequency, however the energy contained in the
coherent motion of the PVC was significantly under-
predicted by the unsteady RANS. Measured and
predicted values of Strouhal number for SZ0.75 are
in agreement with the results of [39], which are already
plotted in Fig. 2.5.
As has been discussed earlier, Roux et al. [45]
carried out an LES study of the atmospheric flow from a
gas turbine combustor into a large furnace chamber,
Figs. 3.11(b) and 3.15(a). For the isothermal state a
major PVC 540 Hz oscillation was found in the main
chamber as well as a strong second acoustic mode at
360 Hz. The PVC frequency did not coincide with any
major acoustic modes of the system and was the
dominant mode of oscillation. A visualisation of the
540 Hz PVC at the exit of the swirl burner in
the combustion chamber is shown in Fig. 4.9, and the
helical nature is clear. The Strouhal number is
estimated at around 0.7–0.8 from available data and
Fig. 4.6. Variation of Strouhal number with swirl number, Sydney
swirl burner [41] for bulk fluid velocities ubZ16.3 and 29.7 m/s.
this is agreement with the data for the swirl burner of
Figs. 2.1 and 2.5 for a geometric swirl number of about
1. Fig. 4.10 shows the measured and modelled pressure
fluctuations for two positions in the system. The PVC
can be seen to be only influential in the combustion
chamber, whilst an acoustic resonance dominates in the
inlet plenum. Combustion results and the suppression
of the PVC are discussed in the next section.
Selle et al. [64] studied a Siemens industrial gas
turbine burner firing into a combustion chamber using
LES and detailed experimental results. The burner is of
complex geometry with both a central axial and an outer
diagonal swirler. An instantaneous visualisation of the
isothermal predicted PVC is shown in Fig. 4.11 for this
configuration in the form of a snapshot of a pressure
isosurface, the rotational frequency is 275 Hz. The flow
inside of the spiral structure is recirculating in a CRZ,
with the entire structure, PVC and CRZ rotating about the
central axis causing large pressure perturbations. The
sense of the rotation of the whole spiral, as a structure, is
that of the surrounding swirling flow, but the sense of the
winding of the spiral is opposite to that of the swirl. Not
enough information is provided to calculate Strouhal and
Swirl numbers. Again suppression of the PVC with
combustion is discussed in the next section.
5. Oscillations in swirl burner furnace systems,
related systems and associated driving mechanisms
In order to explain in part the driving mechanisms
for instability in swirl stabilised combustion systems,
it is useful to characterise the complex mechanisms
occurring under oscillation conditions and flow
conditions where the PVC is suppressed. There are
few articles in the literature which quantify the
processes occurring under regular, stable, oscillatory
conditions whilst analysing the underlying processes,
apart from Fick [30], Froud, [19,85], Dawson, [83,86],
Syred et al. [18,61,84], Rodriquez-Martinez [28,29],
Roux et al. [45], Ancacleto et al. [50], Schildmacher et
al. [87–90]. In each of these references, analysis has
been made of the flow and other structures in swirl
combustion systems oscillating under representative
but very different conditions using a variety of
techniques including phase locked LDA, LDA, phase
locked temperatures via fine wire thermocouples and
LES. A variety of different oscillations have been
investigated with a range of different driving mechan-
isms, these are discussed below. This is complemented
by two studies [45,64] where the PVC is suppressed by
combustion.
Fig. 4.7. Sydney swirl burner, RANS visualisation of instantaneous flow-field showing evolution with Sg for ubZ16.3 m/s. Contour lines show
reverse flow zones where axial velocity is zero [41].
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161136
5.1. Driven PVC oscillations in the 2 MW swirl
burner/furnace system, 100% axial fuel entry
This occurred with the large 2 MW swirl burner/
furnace system shown in Fig. 3.10(a) and (b). This
involved a 24 Hz oscillation [30,84], identified as of
large, high amplitude, PVC form. It occurred with
100% axial fuel injection (no premixing), an equival-
ence ratio of 0.092. This gave a value of Strouhal
number of 4.8, some four times that expected with the
isothermal state or 5–8 times that with partial
premixing, Fig. 3.9. Indeed under these conditions a
suppressed PVC would have been expected. This
oscillation only occurred at low values of equivalence
ratio as indicated, but over a significant range of
flowrates, and there are similarities to the combustion
state reported in [61] and previously described in
Section 3. Acoustic analysis of the swirl burner/furnace
Fig. 4.8. Ijmuiden movable block swirl generator and computational
grid [82]: (a) schematic of device; (b) computational grid.
Fig. 4.9. Isosurface of low pressure to visualise the isothermal PVC
formed at the exit of the swirl burner [45].
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161 137
system showed that there were a number of different
acoustic modes corresponding to this resonance,
including basic Helmholtz and inlet travelling waves.
It thus appears that the large PVC is being driven
by coupling with acoustic resonances of this system.
The oscillation was analysed by measurement of phase
averaged temperature (via compensated fine wire
thermocouples), axial and tangential velocities, as
shown in Figs. 5.1–5.3. Fig. 5.1 shows the rotating
axial and tangential velocities x/DeZ0.5 below the
burner exit in the furnace. In comparison with the flow
field found with the PVC freely exhausting in open air,
(Figs. 2.7 and 2.8) there is less circumferential non-
uniformity, especially with the axial flow, Fig. 5.1(a).
The tangential flow shows an elongated, elliptical
shaped higher velocity region extending over phase
angles of 200–2508, whilst due to the confinement the
flow has not spread radially as much as the unconfined
system, Fig. 5.1(b). There is evidence of a small CRZ in
the centre of the flow, Fig. 5.1(a), and this matches the
characteristics of the open flame found with axial fuel
entry in the vicinity of the burner exhaust, Fig. 3.1(b).
Here, almost all the combustion takes place past the
burner exhaust, there being a thin narrow tulip shaped
CRZ which extends back into the burner and the back
plate. The phase averaged tangential velocity,
Fig. 5.1(b) shows a small central region of negative
tangential velocity, characteristic of the PVC. The
rotating temperature field, Fig. 5.1(c), shows that
combustion is occurring in a region surrounding the
small CRZ of diameter about 0.56De, just within the
annular high velocity regions shown in Fig. 5.1(a) and
(b). The irregular nature of the outer periphery of the
flame appears to be due to shear effects from this high
velocity region, with the flame moving into lower
velocity regions opposite to the high velocity PVC
region. Combustion occurs in regions with velocities up
to about 7 m/s. Thus here, at x/DeZ0.5 a fairly stable
combustion region is surrounded by the rotating PVC
and associated flows. Correlation of Fig. 5.1(c) and (b)
shows there is a small trailing arm of hot 1200 8C
combustion gases which have expanded into a low
tangential velocity region of flow for phase angles 120–
1808, thus increasing the diameter of the flame in this
region by 40% or more: there are similarities to
Fig. 3.11(b). This periodic variation in heat release
can be one of the feedback mechanisms for the
Rayleigh criteria. There is also clear evidence of a
large external recirculation zone near to the outer walls
as shown by the negative axial velocities here.
Further downstream at x/DeZ1.5, Fig. 5.2a–c, phase
averaged axial and tangential velocities have become
much more uniform circumferentially; the CRZ has
Fig. 4.10. Pressure fluctuation spectra for isothermal flow at two locations [45]. Solid line experiment, dashed line LES.
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161138
expanded somewhat in size, Fig. 5.2(a), but is still off
centred and of kidney shape. There again is evidence of
an external recirculation zone close to the outer walls as
shown by reversed axial velocities, Fig. 5.2(a).
Residual vortex core precession is also present,
Fig. 4.11. Pressure iso-surface visualisation of the isothermal pv
Fig. 5.2(b), in the centre of the flow as shown by
negative tangential velocities. The phase averaged
temperature contours, Fig. 5.2(c), show that here the
flame has expanded radially beyond the regions of
highest velocity, being about 1.3De in overall diameter.
c generated by an industrial gas turbine swirl burner [64].
Fig. 5.1. Phase averaged characterisation of oscillating flow in the
2 MW swirl burner furnace at x/DeZ0.5 below burner, 100% axial
fuel injection, fZ24 Hz [30]: (a) axial velocity; (b) tangential
velocity; (c) temperature 8C.
Fig. 5.2. Phase averaged characterisation of oscillating flow in the
2 MW swirl burner furnace at x/DeZ1.5 below burner, 100% axial
fuel injection, fZ24 Hz [30]: (a) axial velocity; (b) tangential
velocity; (c) temperature 8C.
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161 139
The axial velocity contours in a single axial radial plane
at a phase angle of 1058, Fig. 5.3, are typical of those
found. The CRZ is not symmetrical, being of annular
form and slightly titled to one side; only one view is
presented as the differences between successive phase
angles is small. The driving mechanism for the
Rayleigh criteria appears to arise from small variations
in the diameter of the flame, Figs. 5.1(c) and 5.2(c);
examination of the original data indicates that
throughout the cycle at x/DeZ1.5, the flame diameter
(as characterised by the 1035 8C contour) contracts by
up to 15% or more, especially between phase angles of
190 and 2608 on the exterior boundary and 270–908
internally. This variation in heat release appears to be
sufficient to provide the requisite driving mechanism
Fig. 5.3. Axial velocity contours at a phase angle of 1058 for the
2 MW burner, fZ24 Hz, 100% axial fuel injection [30].
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161140
for large amplitude oscillations. Examination of the
overall flow field suggests that as the flame is not
impinging on the wall in the system it is free to wobble
radially in response to external perturbations (such as
arise from acoustic resonances), like the flames shown
in Fig. 3.14(a) and (b), and observed by other workers
[42,45]. This has resulted in a large PVC type of
resonance for a situation where PVC suppression would
normally occur. Clearly, stabilisation methods for such
flames require methodologies to reduce the wobble at
the base and to better stabilise the flame downstream by
avoiding the weak, doubtless intermittent, flow regimes
between the flame and the outer walls.
5.2. Helmholtz and other resonances and vortex wobble
/precession in a 100 kW swirl burner/furnace system,
partial premixing
The next type of resonance generated in a swirl
burner/furnace system is quite different, as discussed by
Froud [31], Froud et al. [19,85]. Here, the swirl burner/
furnace system of Fig. 2.10(c) was operated in a wide
range of different modes with the deliberate aim of
stimulating regular oscillations so the system could be
appropriately described and driving mechanisms ident-
ified [31]. One such case for a swirl number of 1.5 is
shown in Figs. 5.4 and 5.5 [31,85], where a 900 mm long
extension has been added to the exhaust of the furnace
(same diameter as the swirl burner exhaust). This caused
a high intensity regular oscillation in the system as
characterised by Figs. 5.4 and 5.5 where first and second
harmonic frequencies and amplitudes as a function of
equivalence ratio are shown. For a constant fuel flow rate
(110 l/min), the air flow rate is varied such that a range of
equivalence ratios from 0.4 to 1.2 is covered. Other
variables include the isothermal PVC frequency and the
Helmholtz frequency of the system calculated by
assuming that the gases in the furnace act as the
capacitance and the flow oscillates in the extended
furnace exhaust pipe as the neck. An average combus-
tion temperature is assumed to give the result shown in
Fig. 5.4. Other acoustic resonances were investigated,
but did not fit the data. Many interesting features are
shown:
– A high-amplitude, low-frequency resonance occurs
for equivalence ratios 0.57–0.83, the frequency of
which is close to that of the predicted Helmholtz
oscillation.
– A second resonance of much reduced amplitude
occurs over the equivalence ratio range 0.57–0.83
with frequencies in the range 130/140 Hz, some three
times higher than the first harmonic. The frequency
varies quazi linearly with equivalence ratios between
0.57 and 0.83, hence with flow rate as the natural gas
flow rate is held constant at 110 l/min.
A PVC structure could also be seen to be forming in
the exhaust of the furnace as discussed by Fick [30],
see Fig. 2.10(a) and (b) plus associated text.
Similar results were achieved with axial fuel entry
alone, different fuel flow rates and variable furnace
exhaust extension pieces. Longer extension pipes gave
sharper resonant peaks and much higher amplitudes of
oscillation.
Fig. 5.4. Oscillation frequencies as a function of equivalence ratio for swirl burner furnace, Fig. 2.10(c), 900 mm exhaust extension, 50% axial fuel
injection, partially premixed [31,85].
Fig. 5.5. Corresponding first and second harmonic oscillation
amplitudes as a function of equivalence ration (arbitrary units) [31,85].
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161 141
For all cases combustion was not complete until the
gases had entered the final extension pipe section, as
shown by surface temperatures.
As the resonance was essentially driven by a
Helmholtz oscillation the frequency was constant over
a wide range of equivalence ratios and flow rates and
hence, it is difficult to immediately associate the PVC
or related flow instabilities with this resonance.
Phase averaged tangential, axial velocities and
temperatures (again using compensated thermocouples)
over the oscillation cycle, just past the swirl burner exit
and in the furnace, were used to characterise the
mechanisms of oscillation in the system for an
equivalence ratio of 0.671 and an oscillation frequency
of 41 Hz, Fig. 5.6 (phase angle 08), 5.7 (phase angle
908), 5.8 (phase angle 1808), 5.9 (phase angle 2708)
[83].
Examination of the axial velocity levels through the
system, Figs. 5.6–5.9, shows the device is acting like a
pulsating combustors as the axial flow is virtually
stopped between phase angles of 90 and 1808. It is only
at a phase angle of 08 that a conventional type of swirl
burner flow exists with a continuous flow stream
leaving the burner exit, hitting the furnace wall at about
x/Dew0.5De and then staying attached as it moves
through the furnace. A large CRZ exists in the centre of
the flow, with some evidence that it extends down to the
end of the furnace. Highest levels of tangential velocity
are not reached until x/Dew1 downstream of the swirl
burner exit for all phase angle shown, 0, 90, 180, 2708,
Figs. 5.6–5.9 The fluctuating temperature measure-
ments, Fig. 5.6, show that at a phase angle of 08,
combustion is confined to a central rod shaped region in
and around the CRZ and this appears to act as a pilot
flame through the oscillation. At a phase angle of 908,
Fig. 5.7, the rising pressure of the oscillation has
virtually stopped flow entering the furnace from the
swirl burner, leaving a weak, annular CRZ close to the
swirl burner exit and a weak ERZ. The flame has
weakened in the central region, but a flame front can be
seen to be propagating backwards down the wall of the
combustor, Fig. 5.7 in the low velocity region. Flow
continues to swirl with high tangential velocities in the
main section of the furnace, Fig. 5.7. Both Figs. 5.6 and
5.7 show very significant levels of negative tangential
velocity in and around the centre line along the whole
length of the furnace (phase angles 0 and 908). This
must be associated with vortex core precession or some
form of vortex wobble. Fig. 5.8 (phase angle of 1808)
shows the flame front has now moved completely down
the outer wall and become joined to the central region
of combustion.
The corresponding axial velocity contours, Fig. 5.8,
show that apart from small regions towards the far end
of the furnace, axial velocities are low, creating
conditions favourable for flame stabilisation in the
main section of the furnace. Quite high levels of
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161142
tangential velocity still persist in the downstream
section of the furnace, Fig. 5.8. Finally, Fig. 5.9
shows results for a phase angle of 2708. The axial
velocity contours show that flow is reissuing from the
swirl burner exhaust as the pressure in the furnace
reduces, this can also be seen for the tangential
velocities. The outer annular flame front has retreated
up the furnace, just leaving the central rod shaped pilot
flame. Again Fig. 5.9 shows significant levels of
negative tangential velocity in and around the centre
line of the system, indicative vortex precession or
wobble. Both Figs. 5.8 and 5.9 show areas of negative
tangential velocity in the region of the external
recirculation zone, close to the burner exit for r/roO0.5, again indicative of unstable swirling flow. The
negative tangential velocities in and around the centre
line of the system thus indicate that as in the previous
case, with the 2 MW swirl burner/furnace system, the
flow is wobbling radially as it leaves the swirl burner
exit (where there is little or no combustion, only some
recirculation of hot gases via the CRZ back into the
burner exit). Wether this wobble is irregular or regular
in nature is difficult to resolve as some data smearing
does occur with phase locked LDA and temperature
measurements and there may be undetected generation
Fig. 5.6. Phase averaged (phase angle 08) temperat
of axial radial eddies from any PVCs that are present.
Simple calculations based on the maximum tangential
velocity and its radius as it leaves the swirl burner exit
(phase angles 270 and 08) indicate that a double PVC
may be present with a frequency of about 80 Hz for part
of the cycle. However, flame wobble probably produces
circumferential variation of heat release, triggering the
formation of axial radial eddies, generating alternating
patterns of rich and lean combustion sufficient to
reinforce combustion oscillations via the Rayleigh
criteria, especially as the flame propagates back along
the furnace wall towards the swirl burner exhaust at a
phase angle of 1808, Fig. 5.8, as with the 2 MW system
previously described.
More recently, Rodriquez-Martinez [27,29], Daw-
son et al. [86] have extended the work on the 100 kW
swirl burner/furnace system, Fig. 2.10(c), producing a
number of stability maps similar to that shown in Figs.
5.4 and 5.5. Of relevance here is the detailed phase
averaged velocity characterisation of a low frequency
(41 Hz, SZ2.18, equivalence ratio 0.9) system
oscillation, this time excited by travelling waves in
the inlet pipe. This lead to instantaneous flow reversal
in the pipes over part of the limit cycle oscillation.
The configuration of the furnace was changed slightly
ures, 8C, axial and tangential velocities [85].
Fig. 5.7. Phase averaged (phase angle 908) temperatures, 8C, axial and tangential velocities [85].
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161 143
from that of Froud [31,85], with a reduced furnace exit
diameter, Dfe/DeZ0.7, and small changes to the
length of the furnace taper and length. Inside the swirl
burner furnace system there were some differences in
the flow patterns generated, but there still clearly
existed a fundamental pattern of the flow into the
furnace from the swirl burner being periodically shut
off over the limit cycle of oscillation. As angular
momentum and swirl velocities were largely con-
served in the swirl burner and furnace system over the
limit cycle oscillation, the considerable variation in
axial velocity caused large variation in swirl number
and hence, size and shape of the CRZ. No negative
tangential velocities were found with the measured
tangential velocities inside the system, although
directional intermittency measurements clearly
showed that the centre line of the vortex in the
furnace was wobbling off centre more than 30% of the
time. Similar effects were found close to the outer
wall, but the internal measurements were not as
detailed as those of Froud [31,85]. Detailed phase
locked axial and tangential velocities were taken just
above the top of the furnace exit. Fig. 5.10(a)–(e)
shows the phase averaged pressure trace used for
triggering purposes and illustrates there are inputs
from several harmonics in the system; similarly the
geometry and geometrical ratios used are illustrated.
The phase averaged axial velocity, Fig. 5.10(a) shows
that although flow is issuing from the furnace exit for
all phase angles, the velocity and hence, flow rate
doubles over most of the section for phase angles 240–
758. The associated axial directional intermittency
plot, Fig. 5.10(c), shows that there is some irregular
wobble for all phase angles, the most intense effect
being between 0 and 908. The tangential velocities,
Fig. 5.10(b), show a very different pattern with the
most intense swirling flow being confined to phase
angles between 300 and 908. The corresponding
tangential directional intermittency plot, Fig. 5.10(d)
shows that this flow is very unstable over the whole
limit cycle of oscillation, both close to the outer wall
and in the central region of flow. Instantaneous flow
reversal is occurring up to 40% of the time for phase
angles 250–458 close to the outer wall and again this
infers a high level of vortex wobble and/or precession,
probably originating from excitation of the swirling
flow leaving the swirl burner and entering the furnace
as discussed in the data from Figs. 5.6 to 5.9.
Fig. 5.8. Phase averaged (phase angle 1808) temperatures, 8C, axial and tangential velocities [85].
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161144
5.3. Characterisation of high frequency oscillations in
a 100 kW swirl burner furnace system, partial
premixing
Rodriquez-Martinez [27,29] and Dawson [28,83,86]
have extended the work with the generic 100 kW swirl
burner/furnace system of Fig. 2.10(c) with 60% of the
fuel being premixed with the air, the rest being
introduced axially to investigate not only low frequency
oscillations but those in the range w240/260 Hz, again
produced by subtle changes in furnace geometry,
Fig. 5.11. The two geometries are designed to contrast
the effects of a sudden expansion and a quarl. Fig. 5.12
contrasts pressure and frequency spectra data from the
two different configurations, a and b refer to geometry
2a and c and d to geometry 2b with a quarl inserted. The
quarl substantially reduces the amplitude of the
oscillation. The spectral analysis for both cases,
Fig. 5.12(b) and (d) shows the predominant peak of
the Helmholtz resonance at w240 Hz, although there is
also a low frequency peak present for both cases at
around 40 Hz, corresponding to the bulk mode low
frequency oscillation previously described. Fig. 5.13
shows the corresponding rms pressure and frequency as
a function of equivalence ratio. The high frequency
oscillation persists for an equivalence ratio range
w0.55–0.85, dependent on the case, reverting to the
low frequency bulk mode oscillation beyond these
limits, w40/50 Hz, although at much lower amplitude
levels. The effect of the quarl is seen to substantially
reduce the amplitude of oscillation, case 2b compared
to the case without it, case 2a, over virtually the whole
of the equivalence ratio range where this high
frequency oscillation is found. The quarl has little
effect on the amplitude of the low frequency
oscillations. Simultaneous measurement of light emis-
sion and pressure from the system enables a Rayleigh
index to be constructed, which showed, as to be
expected that maximum excitation occurred in the flow
region immediately downstream of the swirl burner exit
in the furnace [27,29,85]. The high frequency oscil-
lations are attributed to near in phase coupling of a
natural Helmholtz resonance of the swirl burner and
furnace with the combustion process and swirl
dynamics. The exhaust of the swirl burner acts as the
neck of the resonator, and periodic heat release occurs
via the mechanisms discussed above [27,83], including
wobbling and precessional motion of the swirling flow
Fig. 5.9. Phase averaged (phase angle 2708) temperatures, 8C, axial and tangential velocities [85].
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161 145
as it leaves the burner exit, as well as shed partially
burning radial axial eddies.
Detailed phase locked velocity measurements for
both this high frequency oscillation case are shown
in Figs. 5.14 and 5.15, case 2a, and 5.16, case 2b
[27,29,86]. The phase locked velocity levels for case
2a, Fig. 5.14 show considerable differences from those
found with the low frequency oscillation, Figs. 5.6–5.9,
in that the flow through the swirl burner is not
periodically arrested, but slows and accelerates in
tune with the near sinusoidal pressure wave shown on
Fig. 5.14(e). Fig. 5.14 compares and contrasts axial and
tangential velocities as well as their directional
intermittencies for six phase angles. Most variation
occurs with the axial velocity which shows the CRZ
expanding and contracting with the sinusoidal pressure
wave, becoming detached and quite weak at a phase
angle of 2408, whilst extending well down into the swirl
burner and the burner back plate at a phase angle of 08.
This arises because the variations in axial velocity and
hence flow rate into the furnace cause a variation in
swirl number (as in previous cases) estimated from 0.8
at phase angle 2408, rising to nearly four at phase angle
08. Changes in size and shape of recirculation zones are
well known to produce substantial pressure pertur-
bations and this also probably adds to the mechanisms
contributing to instability. An external recirculation
zone is also evident as the swirling jet fires into the
furnace. Axial directional intermittency levels show
that much of the swirling jet entering the furnace is
quite stable, but with layers of significant intermittency
on the sides as it interacts with the CRZ and ERZ. The
phase locked tangential velocities have very small
regions of negative tangential velocity on the centre
axis (indicative of vortex wobble), but significant
regions close to the outer wall, whose size and location
vary considerably over the pressure cycle. The
tangential directional intermittencies are possibly the
most revealing showing very considerable intermit-
tency approaching 80% close to the wall for some phase
angles.
Again, as with the 2 MW swirl burner system and
the 24 Hz PVC type oscillation, the flow and hence,
flame is wobbling and precessing in the furnace,
possibly several PVCs are spiralling in the system
over part of the limit cycle oscillation at a much
higher frequency. Reference to Fig. 5.12 shows that
there is some modulation on the pressure signal for
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161146
case 2a, with a superimposed frequency probably
originating from the low frequency 40/50 Hz peak
evident on the power spectrum. This effect is also
illustrated by Fig. 5.15(a) and (b), which show the
phase averaged axial and tangential velocities and
associated directional intermittencies, Fig. 5.15(c) and
(d) just above the furnace exit for case 2a. The results
are quite different to the low frequency oscillation
results above the furnace exit, Fig. 5.10. The
tangential velocity field is virtually uniform above
Fig. 5.10. Phase locked velocity levels just above furnace exit [28,29,83] at X
velocities, axial directional intermittency, tangential directional intermitten
(a) Phase averaged axial velocities; (b) phase averaged tangential veloc
intermittency; (e) pressure trace; (f) schematic diagram of swirl burner furn
the furnace exhaust, whilst there is still some
variation in the axial velocity through the pressure
cycle for phase angles 90–2858. However, the
directional intermittency levels for both axial and
tangential velocities are both very high, although anti-
phased. Again this is indicative of wobble in the main
flow leaving the burner which is amplified by the
combustion process and Helmholtz resonance.
The corresponding data from case 2b with the quarl,
is shown in Fig. 5.16, although with a restricted set of
/De Z 0.52. Phase averaged axial velocities, phase averaged tangential
cy, pressure trace, schematic diagram of swirl burner furnace
ities; (c) axial directional intermittency; (d) tangential directional
ace.
Fig. 5.10 (continued)
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161 147
data owing to the quarl interfering with laser access.
The presence of the quarl has made the axial velocity
contours much more uniform over the oscillation cycle
with a more uniform and consistent CRZ, albeit of
reduced size. Apart from a phase angle of 08, the ERZ
has disappeared giving much more stable flows in this
region and much lower values of directional inter-
mittency. Again the tangential velocity profiles are
quite uniform over the oscillation cycle, whilst the
levels of directional intermittency are substantially
reduced, both at the wall and towards the central axis.
Above the furnace exit the flows were much more
stable with substantially reduced levels of directional
intermittency [27,29]. Doubtless, better shaping of the
quarl section could have improved these results and
reduced the pressure amplitude even further.
The Rayleigh Criteria for stratified flows [62] is useful
here. Although temperature measurements are not
available examination of tangential velocity contours
and associated directional intermittencies, Fig. 5.14(c)
and d, Fig. 5.15(b) and (d), configuration 2a, shows
significant levels of negative tangential velocity in the
region of the ERZ and near the swirl burner exit over at
least 60–70% the oscillation cycle. Thus, at the burner exit
in the furnace, moving radially outwards from the
entering annular, highly swirling shear flow, gradients
of angular momentum, rwr, must be negative, thus
confirming the unstable nature of this region. For
configuration 2b and the corresponding Fig. 5.16(c) and
(d) there are no regions of negative tangential velocity and
thus certainly stability in the outer region of flow close to
the walls is much improved.
Even for dilute combustion systems operating beyond
an equivalence ratio of 0.6, thus putting them beyond the
range of the high frequency oscillation with this
configuration, the role of the quarl in stabilising wobbling
or irregular precessing swirling flow is evident, as well as
the improvement of the gradient of rwr. Clearly, the flow
and flame stabilisation methods proposed herein cannot
eliminate the acoustic response of the system.
The mechanism of instability and coupling thus
appears to be irregularities in the flame boundaries
and/or reaction surfaces/areas, primarily associated
with wobble or precession of the main vortex, possibly
distortions of the CRZ, axial/radial eddy shedding from
the shear layer, triggered by PVCs. Associated with this
CRZ distortion is the production of a PVC whose radius
of precession is governed by the motion and distortion
of the CRZ, and the actual instantaneous level of swirl
at a given point in the oscillation cycle.
Other work on Industrial gas turbines using CFD has
shown precessing vortices leaving the combustor can
exhaust, passing through and attaching to the turbine
guide vanes, causing overheating problems [87],
Fig. 5.11. Swirl burner/furnace configurations to produce 240/260 Hz oscillations [27,28,86]. Differences between configuration 2a and 2b involve
the removable insert which forms a conical exit or quarl at the exit of the swirl burner as it enters the furnace.
.
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161148
confirming the results shown in Fig. 2.10; this is
discussed in more detail later.
Here the reader is referred to the work of references
[65,67], where methods for detailed acoustic analysis of
gas turbine combustors and systems are described,
together with amelioration techniques, such as small
vortex generators, elliptical burners and enlarged,
better located lances.
5.4. Combustion oscillations in a swirl burner
combustion chamber systems and suppression
of the PVC
As discussed earlier in Sections 3 and 4, Roux et al.
[45] have made a very detailed study of the flow
characteristics of a vaned type swirl burner firing into a
square combustion chamber using both modelling, LES,
acoustic analysis and experimental measurements, pri-
marily LDA. Section 4 described the isothermal
characterisation of the system and the appearance of
a strong PVC signal, both measured and predicted at
540 Hz, located close to the region where the swirling
flow fires into the combustion chamber. A weaker 340 Hz
acoustic mode exists everywhere in the system. Measured
and calculated velocities and temperatures have been
presented earlier in Fig. 3.15(a), (b), 3.16(a) and (b),
whilst an instantaneous LES 1250 K isosurface was
shown in Fig. 3.11 for combustion conditions, fZ0.75.
For this mode of combustion the PVC is suppressed
as discussed earlier in Section 3 whilst two self excited
acoustic modes appear experimentally around 300 and
570 Hz. They correspond to the first two modes of the
combustor, 1/4 and 3/4 wave, respectively, with the 3/4
wave being the most amplified from 360 Hz (iso-
thermal) to 570 Hz (combustion). Both the LES and
Helmholtz acoustic solver gave good correlation with
the experimental data, differences from experiment
being attributed to errors in the acoustic boundary
conditions. Fig. 5.17 shows the field of rms pressure
taken from the LES predictions along the chamber axis
together with the modal structure predicted by the
Helmholtz solver for the 3/4 wave mode. Even though
the LES signal contains the signature of all modes, its
shape matches the structure of the 3/4 wave predicted
by the Helmholtz solver. Unlike the rms pressure
profile for the isothermal flow, the match between the
Helmholtz solver and the LES is good everywhere,
Fig. 5.12. Time series traces of pressure signals at burner wall and associated power Spectral densities for configuration 2a (a and b) and 2b (c and
d), Fig. 5.11 [27,29,86].
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161 149
even in the combustion chamber, confirming that the
whole flow is locked on the 3/4 wave mode.
Apparently, changes in flow structure due to combus-
tion, especially the distribution of swirl flow (see
Section 3), have altered the characteristic of any
nascent PVC such that its frequency is well displaced
from that of the Helmholtz resonance and amplification
is unable to occur. This is in contrast to the oscillations
reported in the 2 MW swirl burner furnace system of
Fig. 3.10(a) and (b), Section 5.1. Similar results for the
suppression of the PVC with combustion have been
reported by Selle et al. [64] for an Industrial LPP gas
turbine combustor. The conditions causing suppression
of the PVC appear to be very similar to those reported
above [45]. These results on the suppression of the PVC
need to be treated with caution as the swirl combustors
were not operating with a conventional combustor can,
where there is a high level of confinement. For
conditions of high swirl even when PVC does not
develop near to the swirler, the intense Rankine vortex
so formed can give rise to PVC in the exhaust of the
combustor can [87].
The next section discusses the effect of equivalence
ratio on suppression of oscillation, including the PVC,
and important effects are highlighted. Finally, the
effects of vortex core precession in the exhaust of a
combustor can are described.
Fig. 5.13. Stability maps for high frequency oscillations, systems 2a
and 2b, as a function of equivalence ratio [27,29,86]: (a) pressure rms;
(b) frequency Hz.
5.4.1. Instabilities generated in industrial premixed gas
turbine combustor systems
Schildmacher et al. [88,89] have described a series
of experiments undertaken on an industrial gas turbine
combustor to investigate various instability modes, the
test rig burner and combustor liner are illustrated in
Fig. 5.18(a) and (b). Initial investigation of the
isothermal flow indicated a vortex shedding phenomena
whose frequency was linearly proportional to flowrate
[88]. The accompanying large eddy simulation studies
[90] showed that there was PVC which triggered vortex
Fig. 5.14. Phase averaged contour plots at six phase angles for configuration 2a [27,29,86]: (a) axial velocities; (b) axial directional intermittency;
(c) tangential velocities; (d) tangential directional intermittency; (e) phase averaged pressure trace: directional intermittency is the phase averaged
% of negative samples, contour cut off at 3%.
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161150
shedding in the shear flow region at the burner mouth.
This is the same phenomena as reported in Dorresten
[47] and the radial axial eddy phenomena discussed in
Section 2.1. In addition, investigations of fuel concen-
tration showed that alternating patterns of rich and lean
fuel concentration is generated by this vortex shedding,
though time averaged fuel concentration were axisy-
metrical and much more homogeneous [88,89,91].
Under combustion conditions with premixing of the air
and fuel, pressure fluctuations were found to strongly
increase with equivalence ratio, Fig. 5.19, starting at
fZ0.66. The pressure amplitude at peak frequency was
twice the turbulent combustion noise level at nominal
operating conditions without oscillations, fZ0.5.
Fig. 5.15. (a) Phase averaged axial (left) and (b) tangential (right) velocity contour plots just above the furnace nozzle exit (x/DeZ4.0) for
configuration 2a, sudden expansion. Velocities normalized by the mean inlet flow velocity uiZ4.3 m/s [27,29,86]; (c) contour plots of the directional
intermittencies of the axial (left) and (d) tangential (right) velocities just above the furnace nozzle exit (x/DeZ4.0) for configuration 2a [27,29,86].
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161 151
The amplitude of the oscillations grew steadily with f,
in contrast to a sudden excitation which can often
happen in other systems. For fO0.77 the pressure
amplitude was more than 50 times higher than the
turbulent noise at nominal operation. Phase locked
velocity measurements were used to analyse the
variation of local swirl number over the oscillation
cycle for the highest amplitude oscillations, Fig. 5.20,
fZ0.83. This shows the strongest fluctuation is
between 0.1!S!0.8 and is located in the reaction
zone (x/DeZ0.63, 0.56). The periodicity of the signal
indicated the presence of coherent structures, probably
PVC induced or derived. Fig. 5.20 also indicated that
for a short time around 1508 phase angle, swirl
stabilisation of the flame is interrupted, which may
cause strong strain rates in the reaction zone and local
flame quenching. Only minor fluctuations of swirl were
recorded outside of the recirculation zone. This work
compliments that discussed in Sections 5.2 and 5.3
where considerable variation of swirl number (derived
from integrating the measured phase locked velocities
across the flow field) through the oscillation cycle was
shown [27–29,81,84].
Even when barely audible oscillations were gener-
ated at fZ0.71 the amplitude was still five times higher
than the turbulent combustion noise: phase locked
velocity measurements showed velocity fluctuations
were of identical frequency to that of the pressure field.
No frequency harmonics were present, the phase
averaged velocity profiles being sinusoidal in form,
whilst the swirl level only varied between 0.35!S!0.5. No definite frequency peak could be found in the
transition region when oscillation started, fZ0.66. The
work concluded that there was a very strong impact of
the heat release on the generation of coherent
structures. For combustion the oscillation frequency
Fig. 5.16. Phase averaged contour plots at six phase angles for configuration 2b with quarl inserted, Fig. 5.11 [27,29,86]: (a) axial velocities; (b)
axial directional intermittency; (c) tangential velocities; (d) tangential directional intermittency: directional intermittency is the phase averaged %
of negative samples, contour cut-off at 3%.
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161152
only changes slightly with normalised flow and the
Strouhal number decreases, leading to the conclusion
that resonant frequencies are linked to the acoustic
eigen frequencies of the system and are not too
dependent on the burner air flowrate.
This work provides an interesting contrast to that
of Roux et al. [45] and Selle et al. [64]. Roux et al.
operated at an equivalence ratio of 0.75, Selle et al.
Fig. 5.17. Field of rms pressure predicted via LES along the chamber axis to
the 3/4 wave mode [45].
at 0.5. Although configurations differ, reference to
Fig. 5.19 shows that the work of Roux et al. at fZ0.5 is
beyond the range of equivalence ratios where excitation
of high amplitude PVC type oscillations can be
expected, whilst that of Selle et al. is only just in the
range where excitation is initiated. Here also the effect
of swirler expansion appears to be important, as the
swirler was fired into a square furnace for these two
gether with the modal structure predicted by the Helmholtz solver for
Fig. 5.18. Industrial gas turbine combustor and combustion chamber
[89]: (a) schematic of test Rig; (b) schematic of swirl burner.
Fig. 5.20. Phase locked analysis of swirl number over oscillation cycle
[89].
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161 153
cases. In contrast, Schildmacher [8,89] used the
complete swirler and combustor can system, giving
higher levels of confinement and a smoother transition
from the swirler to the can.
A very interesting CFD study of a Siemens high
swirl dry low emissions gas turbine combustor [87]
Fig. 5.19. Effect of equivalence ration on pressure amplitude and
frequency [89].
has been produced, arising from development tests on
a 13.4 MW Cyclone engine. Problems arose from
observed interactions between the exhaust flow from
the combustor can and the first row nozzle guide vanes
of the turbine. A schematic diagram of the combustor
arrangement is shown in Fig. 5.21, the system is
designed for dual fuel operation. Temperature indicat-
ing paint was used to verify operating temperatures in
the first row nozzle guide vanes and high temperatures
were found on the six guide vanes having their leading
edge closest to the axis of the six combustors used on
the development engine. Fig. 5.22 shows an example
of one of these vanes (termed a central NGV) and for
comparison an example of a non-central NGV. The
figure shows high temperatures on the suction side of
the central vane and at the hub platform immediately
downstream of this. A degree of flow visualisation is
shown by the leading edge film cooling whose tracks
can be clearly seen on the temperature indicating
paint.
A three-dimensional time dependent CFD analysis of
the system was carried out using the computational
domain shown in Fig. 5.23. A special version of the
turbulent Reynolds Stress model was necessary to
reproduce measured behaviour [87]. Analysis of the
upstream section was first carried out and used to derive
inlet boundary conditions for the full CFD analysis
which covered the combustor can and nozzle guide
vanes. Fig. 5.24 shows a vector plot of the combustor
front end extracted at an arbitary time step. The transient
nature of the flow is evident with a large radial axial eddy
and the formation of PVC (not shown). Fig. 5.25 shows
Fig. 5.21. Schematic diagram of siemens high swirl dry low emissions gas turbine combustor [87].
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161154
the temperature at a plane through the centre of the
combustor and an iso-surface of relatively high vorticity,
and indicates the central core of the Rankine vortex
formed. Moving away from the central core the vorticity
drops, given that the outer potential vortex is irrota-
tional. The vortex core so generated in the centre of the
can passes through the NGVs, Fig. 5.26, where the
vortex core is again visualised by a surface of relatively
high vorticity. The vortex core is directed towards the
leading edge of the central NGV; however it is also
attracted by the low pressure on the suction side of this
vane. A second rotation is thus set up near the hub over
the pressure surface, due to a large variation in incidence
angle with span induced by the vortex core. The vortex
core passes the leading edge of the vanes at about 40%
span and migrates towards the hub through the NGV
passage. This migration appears to be due to the core
being attracted by the locus of lowest static pressure and
an interaction between the vortex cores and secondary
flows set up within the NGV. These results are consistent
with the experimental ones from the development
engine.
6. Discussion
Most swirl combustion systems are designed with a
Swirl number SO0.5 to generate a CRZ for flame
stabilization purposes. When a PVC appears it is linked
and possibly coupled with the CRZ. Typically, it is of
helical form and is wrapped around a distorted
asymmetrical CRZ. This flow combination also excites
secondary flows especially radial axial eddies, and
recent LES work indicates that these eddies, shed from
the edge of an inlet shear flow can propagate down-
stream and help to initiate thermoacoustic instability.
Strouhal numbers are usually in the range 0.2–1.5
unless distorted by acoustic coupling.
There are other forms of precession, associated with
jets as shown by the work at the University of Adelaide.
This shows that such jet precession can occur with zero
to quite high Swirl numbers and a variety of different
configurations up to and beyond vortex breakdown.
Usually this is forced precession of a central initially
non-rotating jet. Below a critical swirl number, Scr,
between 0.15 and 0.23 the precession is a mixture of
flapping motion and precession, beyond this it is
dominated by precession with a change in rotational
sense. Strouhal numbers are one to two orders of
magnitude lower than those generated with conven-
tional swirl combustors with a large PVC and CRZ.
Under isothermal conditions the frequency of the
PVC can be characterised for a range of different swirl
flow systems by a Strouhal and Swirl number. There is
evidence that a central fuel injector or bluff body of
significant size can allow the formation of the PVC to
much lower levels of swirl than previously thought
especially when the central fuel jet is of low velocity.
The effect of high levels of confinement (Do/DeZ2)
upon the isothermal PVC is to increase the value
of Strouhal number by more than 2 for SO1.5. The
occurrence of a further vortex breakdown and
associated PVC of different frequency was noted in
the exhaust of the furnace in a swirl burner/furnace
system, and has also been noted by others in diverse
systems. Offset or other arrangements of furnace
exhaust may be beneficial here in eliminating this
source of the PVC.
Phase locked LDA and PIV data showed that the
PVC in isothermal swirling flow is characterised by the
formation of regions of negative tangential velocity in
the near the central axis coupled with elliptical/banana
shaped regions of high axial and tangential flow
close to the burner wall just above the burner exit.
Fig. 5.22. Temperature indicating paint results from first stage NGVs. View of leading edge of central and non Central NGVs [87].
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161 155
The associated CRZ is also distorted, displaced radially,
and precesses about the central axis. The flow normally
returns to near axi symmetry by x/Dew1–1.5. PIV
studies showed the formation of axial radial eddies in
and around the CRZ near the swirl burner exit, whilst
water models and other experimental work showed
the shedding of axial radial eddies further downstream
both from the outside of the jet flow and from the end
of the recirculation zone. LES work and experiments
have shown the presence of the PVC in many simulated
gas turbine combustion chambers, especially under
isothermal conditions, being of helical form.
Fig. 5.23. Geometry of the computational domain [87].
Fig. 5.24. Time dependent CFD velocity snapshot at a diametrical plane at the head of the combustor. Velocity vectors are coloured with the
velocity component in the direction of the combustor axis. Note the transient radial axial eddy [87].
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161156
Qualitative agreement between the LES predictions and
measurements is steadily improving.
Under combustion conditions the behaviour of the
PVC becomes much more complex. Except at
Fig. 5.25. Temperature contours at a diametrical plane through the
exceptionally weak equivalence ratios, 100% axial
fuel injection suppresses the PVC amplitude by more
than an order of magnitude, although its residual
presence can still be detected in many systems. One
combustor and an iso-surface of relatively high vorticity [87].
Fig. 5.26. View of the vortex core approaching the leading edge of the central NGV. The vortex core is visualised by an iso-surface of relatively high
vorticity [87].
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161 157
reason for this suppression of the PVC appears to be the
radial location of the flame front and the pushing of the
main region of flame stabilisation and CRZ formation
well downstream. The flow appears to be stabilised in
the burner exhaust by an annular region comprising a
rotating flame surrounding a small column of hot
recirculating flow which extends to the back-plate of
the burner. This is surrounded by another annular
region of high axial and tangential velocity where the
flame cannot stabilise. Analysis using the Rayleigh
criteria for the stability of stratified flows and a
modified Richardson number has shown this is a very
stable condition. The PVC can be excited when the
flame front can move into the outer region of high
velocity flow.
Values of Strouhal number are very much a function
of Swirl number, less so of equivalence ratio, also being
complicated by the occurrence of double PVC for
certain swirl number ranges for 100% axial fuel
injection. Partial premixing can change this pattern
with the excitation of the PVC frequency by up to a
factor of 2 for equivalence ratios w0.7.
The effect of confinement and partial premixing
for weak equivalence ratios, 0.1–0.3, shows the value
of Strouhal number being reduced by up to a factor
of 3 compared to the isothermal state. Although
100% axial fuel injection generally suppresses the
amplitude of the PVC, the swirling flames so
produced are still unstable and susceptible to small
perturbations in the flow especially in the burner
exit. The flames were essentially shown to wobble
with large changes in flame shape between
successive 1 ms separated cine images. Similar
findings arise from consideration of snapshot flame
temperature images from LES studies.
Analysis of the mechanism of oscillation of swirl
burner/furnace systems has been carried out in the
context of the Rayleigh criteria and describing how,
with a number of different excitation conditions, the
system flow and flame characteristics can serve to add
heat in phase with naturally occurring acoustically
generated pressure nodes.
The first case describes how, in a large 2 MW swirl
burner/furnace system with 100% axial fuel injection, a
high amplitude PVC oscillation is generated by
resonance with the systems natural frequencies. High
amplitude PVC would not be normally found in this
condition, or be of very low amplitude. Here, the flame
initially stabilises in a low velocity region around the
forming CRZ and inside an annular region of high axial
and tangential flow velocities. The flame propagates
outwards into low velocity regions, giving a circumfer-
ential variation in heat release. This effect propagates
downstream such that the flame engulfs the PVC
region, but is still irregular circumferentially as the
flame propagates into any available low velocity region
of flow. The flame never touches the furnace walls and
is surrounded by a weak area of low velocity flow
which often reverses direction. This flame is thus
unconstrained, can readily wobble, shed axial/radial
reacting eddies, contributing to instability and the
oscillation.
The second case describes low frequency oscil-
lations in a 100 kW swirl burner/furnace system,
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161158
excited by a Helmholtz resonance. Here the oscillation
is akin to that found in pulsating combustors as the flow
is periodically arrested over part of the limit cycle of
operation. A central cylindrical shaped pilot flame
exists over all the oscillation cycles, just downstream of
the burner exit, with an annular flame front moving
axially back into the furnace as the flow is arrested.
Phase locked measurements show negative levels of
tangential velocity on and around the central axis
through the limit cycle, indicative of vortex core
precession, albeit of an irregular nature.
Complimentary work on a similar system with a
similar frequency of oscillation, but excited by
travelling waves in the inlet pipes showed some vortex
wobble in the furnace, but essentially a precessing flow
in the furnace exhaust with significant levels of
especially tangential directional intermittency, adding
to evidence that the whole flame is wobbling, thus
deforming the flame externally near to the furnace walls
and internally in and around the CRZ, the primary
flame stabilisation region. This thus again provides the
variation in heat release rate necessary for the Rayleigh
criteria and excitation of oscillations.
The third case uses the same swirl burner furnace
system with some minor changes to the furnace
geometry and this time generates a high frequency
oscillationw240 Hz, via a Helmholtz excited reson-
ance. Two cases are compared, one with a quarl or
conical section inserted at the swirl burner exit, one
without. Without the quarl the flame does not properly
fill the furnace and has considerable wobble. The quarl
produces a flame which substantially fills the furnace
section and thus gives a substantial reduction in
oscillation amplitude over a wide range of equivalence
ratios. Outside of this range the amplitude of oscillation
falls considerably, the frequency dropping back to that
of the low frequency case (w40 Hz), the quarl having
little effect. Again the effect of flow/flame wobble/irre-
gular precession is brought out via the phase locked
measurements, especially the directional intermitten-
cies. The quarl is shown to especially reduce
intermittency, negative tangential velocities and vari-
ation in CRZ size and shape over the limit cycle of
oscillation. Here, the Rayleigh criteria for the stability
of stratified flows is useful in explaining the instability
of the ERZ formed without the quarl.
A substantive body of work has now been generated
on industrial gas turbines using a variety of techniques,
both experimental and numerical, RANS, LES, phase
locked velocity measurements, PIV and advanced
acoustic analysis. In all these systems the presence of
the PVC is reported under isothermal conditions, but
with combustion suppression often occurs for equival-
ence ratios ranging from 0.5 to 0.75. This is a function
of the system configuration, the type of swirl flow
generated and the absence of swirl and angular
momentum in the central region of flow close to the
burner exit. Effectively there is no vortex core to
precess. However, as the equivalence ratio moves into
the range greater than 0.75, the flame front moves
further into the annular shear flow entering the
combustion chamber with the result that severe
oscillations can develop, dependent on system geome-
try and flame front location often with the presence of
helical coherent structures of PVC form.
Thus, the coupling between swirl combustion and
acoustic oscillations (apart from the case of PVC
excitation) appears to arise from regular variations in
heat release rate arising from the following:
† Swirl flow and hence, flame wobble or irregular
precession, causing circumferential and hence, axial
variations in flame shape, combustion aerodynamics,
CRZ and hence, the initial region of flame formation
and stabilization. The PVC is influential here via flow
coupling triggering the formation of axial radial eddies
from the edge of the shear flow and the CRZ, generating
alternating patterns of rich and lean combustion
sufficient to reinforce combustion oscillations via the
Rayleigh criteria.
† This is reinforced as the limit cycle of oscillation
causes natural variations in the swirl number, primarily
due to variation in axial flow rate into or through the
system, there being less variation in the swirl flow
velocity over the limit cycle. This in term cause natural
variation in the size and shape of the CRZ, in accord
with the Swirl number variation. Again this affects the
initial region of flame stabilisation/formation as the
CRZ moves axially in and out of the burner exit and this
again can reinforce oscillation.
6.1. Interaction between the above effects
Remedial effects which can be used on combustors
include:
– the use of higher swirl levels should produce more
regular and stronger CRZs that are less susceptible
to deformation by pressure fluctuations. This will
generate stronger PVCs, but providing these are well
controlled and regular should not cause problems,
providing there is a fundamental mismatch to major
acoustic modes of oscillation;
– control of the wobble of the central flow and flame
appears to be important in reducing the regular and
N. Syred / Progress in Energy and Combustion Science 32 (2006) 93–161 159
irregular precession of the flow and flame, a quarl or
carefully shaped exhaust section can be useful here
to remove the ERZ and ensure the flame properly
fills the furnace;
– an off centred furnace exhaust may well be
beneficial in eliminating the formation of other
PVCs, whilst also altering the fundamental acoustic
modes of oscillation of the combustor;
– the use of minature vortex generators to distort the
generation of PVC and axial radial eddies;
– the use of elliptical burners which again distort the
generation of the PVC, axial radial eddies and other
coherent structures;
– The use of substantive pilot lances to stabilise the
point of vortex breakdown and location of the CRZ.
– Investigation of the acoustic response of the system
and derivation of techniques to give acoustic
mismatch to other resonant frequencies.
Finally it must be noted that vortex cores that form in
the exhaust of a combustor may easily deleteriously
interact with other downstream components
7. Conclusions
This paper has reviewed recent work on instability
and oscillations in swirl burner and combustion
systems, using a range of existing and new data on
open and confined swirl combustors, and related them
to the occurrence of instability in such systems. Based
on this, an analysis of the underlying mechanisms by
which naturally occurring acoustic and other reson-
ances can be reinforced is given. A number of remedial
methods are discussed.
For the future, there is a need for many more
fundamental experimental investigations of these
types of flow both to elucidate the coupling methods
between the PVC and excitation of combustion
oscillations as well as the exact mechanisms by
which suppression of the PVC occurs. Examination
of the occurrence and role of the PVC in the exhaust
of combustor cans is also needed. Complimentary
LES and related work is needed for validation and
extrapolation purposes.
Acknowledgements
Professor N. Syred gratefully acknowledges the Royal
Academy of Engineering award of a Global Research
Award, also the facilities provided by the School of
Mechanical Engineering, Adelaide University during his
sabbatical leave. The financial support of the European
Union via several programmes is acknowledged for much
of the more recent work carried out at Cardiff University.
The assistance of Dr Andy Crayford with the diagrams is
gratefully acknowledged.
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