COMBUSTION AND FUELS
FLAME AERODYNAMICS
COMBUSTION AND FUELS
IMPORTANCE OF AERODYNAMICS IN COMBUSTION
Flow reactor
Combustion chamber, furnace
Fuel
Air
Flue gas
Heat
Heat
COMBUSTION AND FUELS
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.
COMBUSTION AND FUELS
BASIC OBJECTS OF AERODYNAMICS
OF COMBUSTION
Basic objects:
- burners,
- boiler furnaces,
- combustion chambers.
COMBUSTION AND FUELS
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.
COMBUSTION AND FUELS
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.
COMBUSTION AND FUELS
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.
COMBUSTION AND FUELS
NEAR BURNER AERODYNAMICS (NBA)
NBA influences:
- flame pattern,
- fuel burnout,
- pollutants,
- heat transfer.
COMBUSTION AND FUELS
JETS
COMBUSTION AND FUELS
FORMATION OF FREE JET Streamlines
orifice
COMBUSTION AND FUELS
STRUCTURE OF FREE JETS (fully separated)
orifice
transition
boundary layer
initial range
main range
COMBUSTION AND FUELS
VELOCITY PROFILES OF FREE JETS (fully separated)
Axial profile of velocity Radial profile of velocity
temperature, concentration
velocity
COMBUSTION AND FUELS
MIXING PATTERN DUE TO VELOCITY GRADIENT
y
x
0
Boundary layer
COMBUSTION AND FUELS
MIXING BY JET PENETRATION
widok z góry
s
hD c
u1 u2
View fromthe top
COMBUSTION AND FUELS
JETS IN CO-FLOWING PARALLEL STREAMS ANNULAR
AND COAXIAL JETS
transitionrange
initialrange
mainrange
orfice
y
x
COMBUSTION AND FUELS
SWIRLED FLOWS
SWIRLED JETS
COMBUSTION AND FUELS
FREE SWIRLED JETS
Week swirl Strong swirl
1 – parallel strem in the pipe , 2 – swirler, 3 – swirled stream,4 – recirculation zone
COMBUSTION AND FUELS
STRUCTURE OF SWIRLED JETS
steam
lines
recirculation
radi
aldi
stan
ce,
cm
distance, cm
COMBUSTION AND FUELS
CONFINED SWIRLED JETS
Recirculation
zone
COMBUSTION AND FUELS
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
COMBUSTION AND FUELS
RECIRCULATION ZONE OF SWIRLED FLOW
Swirl number S
Recirculation zone
swirl generator
oilburner
COMBUSTION AND FUELS
SWIRLER
β
2Rh
2R
COMBUSTION AND FUELS
TURBULENCE
COMBUSTION AND FUELS
BASIC FUTURES OF TURBULENT FLOWS
u = um + u’, T = Tm + T’
COMBUSTION AND FUELS
GENERATION OF TURBULENCE
Boundary
layer
A wake
Free jet
COMBUSTION AND FUELS
LAMINAR AND TURBULENT MIXING
laminar
turbulent
COMBUSTION AND FUELS
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
COMBUSTION AND FUELS
„SURFACE” MECHANISM OF TURBULENT
COMBUSTION
AT AL
ST
SL
T
L
L
T
A
A
S
S=
COMBUSTION AND FUELS
‘ISLAND’ MECHANISM OF TURBULENT COMBUSTION
uo
c = 0 c = 1
COMBUSTION AND FUELS
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
COMBUSTION AND FUELS
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
COMBUSTION AND FUELS
FLAME STABILIZATION
COMBUSTION AND FUELS
FLAME POSITION
Su = U ∗cos(α)
Flame front
Direction of flame propagation
αSu
U
Direction of mixture flow
COMBUSTION AND FUELS
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
COMBUSTION AND FUELS
METHODS OF FLAME STABILIZATION
�in boundary layer
�by a pilot flame
�by recirculating flow
COMBUSTION AND FUELS
FLAME STABILIZATION BY HOT FLUE GAS
Stabilization by hot gases:
- pilot flame,
- recirculating flows.
Recirculation:
- outer,
- inner.
COMBUSTION AND FUELS
RECIRCULATION ZONE OF CONFINED JETS
Zone of recirculation
COMBUSTION AND FUELS
FLAME STABILIZATION BY INNER RECIRCULATION
Inner recirculation can be generated by:
- bluff bodies,
- strong swirl.
COMBUSTION AND FUELS
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
COMBUSTION AND FUELS
FLAME STABILISATION ON A FLAME HOLDER
COMBUSTION AND FUELS
STABILITY LIMITS FOR BLUFF BODY STABILIZERS
Stability limit
Stableflame
Unstableflame
Fue
l/air
ratio
Airflow, kg/s
COMBUSTION AND FUELS
STABILITY LIMITS OF SWIRLED PULVERIZED
COAL FLAME
Influence of swirl no. S
on the stability limits for
pulverized coal flame
stable
unstable
load
kW