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LES Approaches to Combustion
W P Jones
Department of Mechanical Engineering
Imperial College London
Exhibition Road
London SW7 2AZ
SIG on Combustion Science,
Technology and Applications,
Imperial College London
28th September 2017
LES Approaches to combustionLES Approaches to combustion
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Turbulent Flows: DNS
Many reactions of type
exp( )
Pressure 1 bar, 0.01 mm
Pressure 40 bar, 0.001 mm
n A BA
B
r
r
A B C
Y YEr aT
RT W
Inert Flows9
4
3
4
3
Re
Re
Re
Resolve the smallest scales
Number of mesh points, N
Time step
CPU time
xyx
t
Combusting Flows
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Modelling Approaches
Reynold/Favre Averaged Approaches (RANS)
• Quasi-steady and quasi-homogeneous assumptions
• 40-50 years research
Large Eddy Simulation (LES)
• Assumptions: scale separation, i.e. high turbulence Reynolds numbers
• Resolve large scale energy containing motions responsible for
transport.
• Model fine scale dissipative motions / replace physical viscosity with
sub-grid scale (sgs) viscosity.
• sgs viscosity provides mechanism for dissipation.
• Mean profiles, Reynolds stresses, turbulent kinetic energy etc.
insensitive to sgs viscosity.
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Large Eddy Simulation
Introduce a spatial filter:
where
Generalised moments:
The filter width is given by:
3
3
, , ,
,, , ,
,
t t G d
tt t G d
t
x x x x x
xx x x x x
x
( , ) 0x xG 3, 1 ;G dx x x
2 2, , i j i ju u u u etc.
1 3
x y z
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Filtered Equations
Continuity𝜕 𝜌
𝜕𝑡+𝜕 𝜌 𝑢𝑖𝜕𝑥𝑖
= 0
Momentum
𝜕 𝜌 𝑢𝑖𝜕𝑡
+𝜕 𝜌 𝑢𝑖 𝑢𝑗
𝜕𝑥𝑗= −
𝜕 𝑝
𝜕𝑥𝑖+
𝜕
𝜕𝑥𝑗µ
𝜕 𝑢𝑖𝜕𝑥𝑗
+𝜕 𝑢𝑗
𝜕𝑥𝑖−2
3
𝜕 𝑢𝑘𝜕𝑥𝑘
𝛿𝑖𝑗 −𝜕𝜏𝑖𝑗
𝑠𝑔𝑠
𝜕𝑥𝑗+ 𝜌𝑔𝑖
Scalars
𝜕 𝜌 𝜙𝛼𝜕𝑡
+𝜕 𝜌 𝑢𝑗 𝜙𝛼
𝜕𝑥𝑗=
𝜕
𝜕𝑥𝑗 𝜌𝐷𝜕 𝜙𝛼𝜕𝑥𝑗
+ 𝜌 𝜔𝛼 𝝓,𝑇 −𝜕𝐽𝛼,𝑗
𝑠𝑔𝑠
𝜕𝑥𝑗
Closures are required for the sub-grid stress 𝜏𝑖𝑗𝑠𝑔𝑠
, sub-grid flux 𝐽𝛼,𝑗𝑠𝑔𝑠
and
the filtered rate of formation 𝜌 𝜔𝛼 𝝓, 𝑇 terms.
with CS determined dynamically i.e. CS= CS(x,t)
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LES: Justification
Large scale energy containing motions responsible for turbulent transport.
Energy transfer from large to small scales.
Energy Dissipation via viscosity at the smallest scales.
Energy dissipation rate and hence turbulent transport independent of viscosity and indeed the precise dissipation mechanism, (Kolmogorov).
LES
• resolve large scale energy containing motions responsible for transport.
• model fine scale dissipative motions / replace physical viscosity with sub-grid scale (sgs) viscosity.
• sgs viscosity provides mechanism for dissipation.
• mean profiles, Reynolds stresses, turbulent kinetic energy etcinsensitive to sgs viscosity
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Numerical Requirements
Discretisation:-
• Spatial Derivatives
Convection terms: the use of ‘dissipation free’ schemes is desirable if excessive CPU times/memory are to be avoided.
at least second order accurate central differences. Asymmetric approximation such as QUICK and upwind schemes are too diffusive!
Diffusion and Pressure gradient Terms: Second order accurate central approximations yield reasonable results.
• Time
At least second order accurate: Crank-Nicholson, Adams-Bashforth3-step Runge-Kutta and three-point backward difference approximations have all been used to good effect.
Solution Methods
• If ∆ is linked to the mesh spacing then the solution will not be ‘smooth’ on
the mesh high frequency ‘noise’
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Numerical Accuracy
Grid independence
• If ∆ is linked to the mesh spacing then the only truly grid independent solution will be a DNS.
• Resolve 80-90% of turbulence energy mean quantities, Reynolds stresses etcindependent of mesh size.
Mesh Quality Indicators??
• There is no reliable single mesh accuracy indicator
Mesh size – ‘a rule of thumb’
• Equal mesh spacing, i.e. ∆x≈∆y≈∆z, expansion ratios close to unity; sufficiently fine to resolve mean motion
Energy Spectrum
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LES of an isothermal Jet
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Combustion LES: Approaches
Turbulent transport, e.g. Reynolds stresses, turbulence kinetic energy, scalar fluxes etc., is likely to remain insensitive to the sgs eddy viscosity.
Combustion will take place predominantly within the sub-grid scales. Thus sgs combustion models are likely to exert a more important influence in the LES of turbulent flames.
Approaches:
One/two Scalar Descriptions
Non-premixed Flames
• Conserved scalar/unstrained flamelets/CMC.
Premixed Flames
• Thin flame approach (flame approximated as a propagating ‘material’ surface). Models: flame surface density, level set methods (‘G’ equation), scalar dissipation.
Partially Premixed/Stratified Flames
• Thickened Flame Model, FGM, FSD, Scalar Dissipation,CMC, unsteady flamelets
• Sub-grid scale or Filtered joint scalar pdf transport equation method.
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Non-Premixed Combustion
Conserved Scalar Formalism
Assumptions:
• High Reynolds Number
• Adiabatic Flame
• ‘Fast’ Reaction
Mixture Fraction
• Normalized element mass fraction
• is a strictly conserved quantity
1,01,2,
1,
:stream fuel :stream air ξ
zz
zz
sgs
j
j j sgs j
ut x x x
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Laminar Flamelet Computations
ssYYsTT ,;,;,
A simple model: select a single flamelet at a specified value of
d)(unstraine 15 whereetc, , 1 ssTsTT RR e.g.
Rs
Laminar Flamelets• Flame Thickness < Kolmogorov length Scale• Instantaneous Dependence of Composition,
Density and Temperature on mixture fraction and some measure of Flame Stretch is the same as that prevailing in a laminar flame
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Turbulence-Chemistry Interactions
Probability Density Function
Beta PDF
dP ),(~
1
0
xx
1 11 1
2
; , 1 / 1
1 1where 1 ;
s sr rP t d
r s r
x
22 2 2
2
2
2
2 2
2
or
sgs sgs sgs
j d
j i sgs i sgs i
i
u Ct x x x x
Cx
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Piloted Turbulent Jet Flames
dx
~
dx
dx
dx
dx
T~
4CH
OH2
OH2
2H
U~
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Local Extinction
• In RANs approaches the scalar dissipation,χ is often used to
provide a measure of flame stretch.
• In LES the local value of χ and thus P(ξ,χ) is insufficient to
describe extinction.
Instantaneous mixture fraction contours 20mm
Instantaneous mixture fraction dissipation rate 20mm
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Premixed Flames: Parameters
Energy Containing Motion:
;
t U
41
3
;
21
k
LL S
L
LL S
where is the burning velocity of an unstrained laminar flame
SL
Reynolds number:
DamköhlerNumber:
Karlovitz Number:
Re U
LA
L L
SD t
U
A
k
LA D/ReK 2
1
Energy containing motions:
Kolmogorov scales:
Chemical reaction scales:
Length Time
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Premixed Flames
,ti
i i t i
c c cU t
t x x x
x
Define a reaction progress variable, e.g.
unburnt gas c = 0
burnt gas c =1
2
2 ,
,H O
H O burnt
Y tc
Y
x
View Flame as a propagating material surface – propagating at a speed equal to the local burning velocity, (Bray, Peters, Candel et al, Bradley et al and others)
is obtained from either FSD, G-equation, Scalar Dissipation for which an additional equation is solved. , t x
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Partially Premixed and Stratified Flames
At a minimum both the mixture fraction and reaction progress variable is required.
Reaction Progress Variable becomes:
and
Resulting equation complex; additional terms sometimes neglected.
A solution: solve equations for Y and c(x,t)
Reaction rate obtained as for premixed flames with additional assumptions, e.g.
( , ) with ( , ) ( ( , ))
( , )b b
b
Y tc Y t Y t
Y t
xx x
x
d d d
dt dt dt
c c Y c
Y
( , ) ( ) ( )P c P c P
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Finite Rate Chemistry Effects
• Fully coupled flow and chemical reaction required
• Detailed but reduced chemical mechanism
required
• Currently available approaches:
• Thickened Flame Model
• CMC – with at least double conditioning
• PDF transport equation methods
• Lagrangian stochastic particles
• Eulerian Stochastic fields
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Combustion: Sub-Grid Pdf Equation Method
Fine grained pdf
Sub-grid Pdf
; , ', ; ', ( '; ) 'sgsP t t t G dxx x x x xF
2
1 1
1
1
,
j
N Nsgssgs
i i i s
Nsgs sgs sgs
gs i
sgs
j j
s
N
j
d
sg
u
P P
x x
P P P P
t x x x
t
x x
CPx
The modelled sub-grid Pdf Equation
1
; , ,F
x xsN
t t
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Stochastic Field Solution Method
Represent PDF by N stochastic fields
Configuration field methods, M. Laso and H.C. Öttinger (1993); Stochastic Fields,
Valino (1998), Sabel’nikov (2005)
1 2
12 0.5
sgs
sg
n nn
i
i i i
nn n n n n
i d sgs
s
sgs
sgs i
d u dt dtx x x
dW t C dt dtx
, n n
i i
n
i
n N dW dt
where 1
and is a -1,+1 dichotomic vector
, :is advanced from to according to n t t t dtx
Ito formulation
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Pdf Equation/Stochastic Fields: Applications
Simulated Flames
• Auto-ignition – Hydrogen and n-heptane
• Lifted flames : Cabre - Hydrogen & methane
• Forced ignition: methane,
• Local extinction – Sandia Flames D, E & F
• Premixed swirl burner (Darmstadt)
• Darmstadt stratified flame
• Lean burn (natural gas) industrial combustor
• Premixed baffle stabilised flame
• Cambridge/Sandia stratified flames
• Methanol Spray flames
• Axisymmetric swirl combustor
• FAUGA Combustor,
• Sector combustor
• Genrig combustor Kerosene spray
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SGT100 Combustor: Snapshot of Heat Release Rate
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Stratified Premixed Turbulent Flames
• The Cambridge Stratified
Swirl Burner
Ui(m/s)
Uo(m/s)
Uco (m/s)
Ʌ(mm)
Re_i Re_o φ_g
8.7 18.7 0.4 1.9 6000 11500 0.75
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Computational Setup
• BOFFIN-LES
• 13.5M cells & 610 blocks
3.7M cells & 163 blocks
0.8M cells & 27 blocks
• Synthetic turbulence generation
• Dynamic Smagorinsky model
• Constant Prandtl/Schmidt number = 0.7
• Reduced GRI 3.0 with 19 species and
15 reactions
• 8 stochastic fields
~ measurement locations
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Results – SwB5
Velocity Magnitude Methane Temperature
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Results – SwB5 Velocities
Mean Velocities RMS Velocities
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Results – SwB5 Scalars - Mean
10 mm
30 mm
50 mm
70 mm
Temperature CH4 O2 CO2 CO H2
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Results – SwB5 Scalars - RMS
10 mm
30 mm
50 mm
70 mm
Temperature CH4 O2 CO2 CO H2
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Methanol Spray Flame
Gas Phase
• Stochastic Fields
18 species-84 reaction steps
for methanol-air combustion
Dispersed Phase
• Lagrangian stochastic particles
(droplets) with models for:
Sub-grid dispersion
Abramson-Sirignano
evaporation
Stochastic breakup model
The instantaneous gas-phase velocity field with droplet motions and (b) a detailed view of breakup processes near the atomiser. Note that the appearance of droplets is scaled according to their size.
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Methanol Spray: Profiles of SMD and mean and rms
droplet axial velocity
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Reacting Spray: Instantaneous gas-phase temperature
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Reacting Spray: Profiles of mean and radial Droplet
velocity snd SMD
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Instantaneous OH mass fraction and droplets
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LES with Conditional Moment Closure: ignition, blow-off,
emissions
Mastorakos, Cambridge
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Large Eddy Simulation of Swirl Stabilized Combustor
2
0
0
3a x xE
t t
Modelling of fuel injection (DNS)
24 combustors
24 combustors+ outer casing
1 combustor
Dong-hyuk Shin, Edinburgh University
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High-Hydrogen Content Alternative Fuel Blends for
Stationary and Motive Combustion Engines
Large eddy simulations of non-premixed syngas jet flames and swirling flames
1. Ranga Dinesh, Jiang, van Oijen et al. Proc. Combust. Inst. , 2013 2. Ranga Dinesh, Jiang, van Oijen et al. Int. J. Hydrogen Energy, 2013 3. Ranga Dinesh, Luo et al. Int. J. Hydrogen Energy 20134. Ranga Dinesh, van Oijen et al. Int. J. Hydrogen Energy 2015
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Conclusions
Large Eddy Simulation is capable of reproducing the velocity field in the majority of
inert and combusting flows.
Providing that the Reynolds number, is large and the flow is adequately
resolved then results are relatively insensitive to the sub-grid stress and scalar-flux
models.
Near Wall Flows and the Viscous sublayer
• models needed if DNS like requirements are to be avoided.
• no accurate and reliable wall models currently exist.
Re sgsu L
Velocity
Combustion
Thin flame LES combustion models give good results when applied appropriately.
The LES Stochastic field pdf method together with detailed but reduced chemistry
has been applied to a wide range of flames – non-premixed, partially premixed,
premixed and spray flames - to good effect.
Practical liquid fuel systems limited by chemical reaction mechanisms.