flame aerodynamics -...

COMBUSTION AND FUELS

FLAME AERODYNAMICS


IMPORTANCE OF AERODYNAMICS IN COMBUSTION

Flow reactor

Combustion chamber, furnace

Fuel

Air

Flue gas

Heat

Heat


MEDIA

Oxidizer:

- air: primary, secondary sometimes tetriary

- enriched air: [O2] > 21%, oxygen, ...

Fuel:

- gas (mixing), oil (dispersion), dust (transport)

Flue gas:

- mixing (recirculation), heat exchange, errosion and corrosion.


BASIC OBJECTS OF AERODYNAMICS

OF COMBUSTION

Basic objects:

- burners,

- boiler furnaces,

- combustion chambers.


REQUIREMENTS FOR FURNACE AND BURNER

- fuel delivery to the furnace assuring required thermal

power,

- air delivery to the furnace assuring required stoichiometric

ratio λ,

- mixing of air and fuel to get proper flame pattern (shape)

- keeping fuel in furnace to complete burnout.


MIXING IN COMBUSTION PROCESSES

1. A direct contact between fuel and oxygen is

necessary for burning.

2. Mixing is a process which secures a contact

between fuel and air.

3. Two mixing patterns are distinguished in the

combustion processes:

a. molecular mixing,

b. turbulent mixing.


MIXING AND FLAME PATTERN

1. Depending on the flow pattern two types of

flames can be distinguished:

� laminar flames,

� turbulent flames.

2. In laminar flames mixing has molecular

character.

3. In turbulent flames mixing has turbulent (also

molecular on small level) character.


NEAR BURNER AERODYNAMICS (NBA)

NBA influences:

- flame pattern,

- fuel burnout,

- pollutants,

- heat transfer.


JETS


FORMATION OF FREE JET Streamlines

orifice


STRUCTURE OF FREE JETS (fully separated)

orifice

transition

boundary layer

initial range

main range


VELOCITY PROFILES OF FREE JETS (fully separated)

Axial profile of velocity Radial profile of velocity

temperature, concentration

velocity


MIXING PATTERN DUE TO VELOCITY GRADIENT

y

x

0

Boundary layer


MIXING BY JET PENETRATION

widok z góry

s

hD c

u1 u2

View fromthe top


JETS IN CO-FLOWING PARALLEL STREAMS ANNULAR

AND COAXIAL JETS

transitionrange

initialrange

mainrange

orfice

y

x


SWIRLED FLOWS

SWIRLED JETS


FREE SWIRLED JETS

Week swirl Strong swirl

1 – parallel strem in the pipe , 2 – swirler, 3 – swirled stream,4 – recirculation zone


STRUCTURE OF SWIRLED JETS

steam

lines

recirculation

radi

aldi

stan

ce,

cm

distance, cm


CONFINED SWIRLED JETS

Recirculation

zone


THE SWIRL NUMBER S

Gx – is the axial thrust

Gφφφφ – is the annular momentum

( ) ,π20∫=R

rdruwrG ρφ

∫ ∫+=R R

x rdrPrdruuG0 0

π2π2ρ

S = Gφφφφ/(0.5 Gxdo)Swirl number S is a non-dimensional characteristic of rotating flow


RECIRCULATION ZONE OF SWIRLED FLOW

Swirl number S

Recirculation zone

swirl generator

oilburner


SWIRLER

β

2Rh

2R


TURBULENCE


BASIC FUTURES OF TURBULENT FLOWS

u = um + u’, T = Tm + T’


GENERATION OF TURBULENCE

Boundary

layer

A wake

Free jet


LAMINAR AND TURBULENT MIXING

laminar

turbulent


STRUCTURE OF TUBULENT FLAME

1 102 104 106 108

1

102

104

106

Da = 1 Ka = 1

Re = 1

Stosunek l/lF

Sto

sune

k u’ k

w/S

L

Reaktor doskonałego wymieszania

Strefa spalania

rozproszonego

Strefa płomyków

Płomyki pofałdowane

Płomyki pomarszczone

Ka < 1

Re < 1

Da < 1

Da > 1 Ka > 1

perfect chemical reactor

Rat

io

folded flamelets

Ratio l/lF

wrinkled flamelets

flameletszone

zone of dispersed

combustion


„SURFACE” MECHANISM OF TURBULENT

COMBUSTION

AT AL

ST

SL

T

L

L

T

A

A

S

S=


‘ISLAND’ MECHANISM OF TURBULENT COMBUSTION

uo

c = 0 c = 1


MECHANISM OF TURBULENT COMBUSTION

Freshmixture

Products ofreactions

Mixing andreactions

THE TRANSITION FROM LAMINAR TO

TURBULENT FLAME

Variation of flame height and

structure with the outlet velocity.

stream velocity

stream velocity

heig

ht

laminarflame

heig

ht

trasitionrange

turbulentflame

envelope of flame length


CRITICAL REYNOLD NUMBERS FOR TRANSITION

FROM LAMINAR TO THE TURBULENT FLOW ReCR

Gas ReCR

– hydrogen: 2000

– town gas: 3000–4000

– CO: 5000

– hydrogen + air: 5500–8500

– town gas + air: 5500–8500

– propane, acetylene: 9000–10000

– methane: 3000


FLAME STABILIZATION


FLAME POSITION

Su = U ∗cos(α)

Flame front

Direction of flame propagation

αSu

U

Direction of mixture flow


NECESSARY CONDITION OF FLAME STABILIZATION

The condition of flame stabilization in a flow field of non-uniform velocity is that there is a point in the flow field where the flow velocity is equal and opposite to thevelocity of the combustion wave.

atmospherewallof burner

front

of f

lam

e

gasv

eloci

typr

ofile

, v


METHODS OF FLAME STABILIZATION

�in boundary layer

�by a pilot flame

�by recirculating flow


FLAME STABILIZATION BY HOT FLUE GAS

Stabilization by hot gases:

- pilot flame,

- recirculating flows.

Recirculation:

- outer,

- inner.


RECIRCULATION ZONE OF CONFINED JETS

Zone of recirculation


FLAME STABILIZATION BY INNER RECIRCULATION

Inner recirculation can be generated by:

- bluff bodies,

- strong swirl.


AERODYNAMICS METHODS

OF FLAME STABILZATION

Examples of recirculating flows generators.

BLUFF BODY STABILIZER

Flow in bluff body recirculation.

flame

of reaction

subs

trat

es a

nd p

rodu

cts

of r

eact

ion

products

flame

flamestabiliser

recirculation

prod

ucts

of c

ombu

stio

nflammable

mixture

swirl layer

ignition zone

mixing zone


FLAME STABILISATION ON A FLAME HOLDER


STABILITY LIMITS FOR BLUFF BODY STABILIZERS

Stability limit

Stableflame

Unstableflame

Fue

l/air

ratio

Airflow, kg/s


STABILITY LIMITS OF SWIRLED PULVERIZED

COAL FLAME

Influence of swirl no. S

on the stability limits for

pulverized coal flame

stable

unstable

load

kW

flame aerodynamics -...

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