lecture 5. laminar premixed flames structures and propagation · premixed fuel/air mixture: flame...
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Lecture 5. Laminar Premixed Flames
Structures and Propagation
X.S. Bai Laminar Premixed Flames
Premixed flames
X.S. Bai Laminar Premixed Flames
Premixed flames causing coal mine explosion
X.S. Bai Laminar Premixed Flames
A mine explosion in the Ukraine resulted in 36 dead, 14 missing, and this guy looking very terrible indeed. The explosion was methane gas, and they were mining coal. August 2001. http://cellar.org/showthread.php?t=452
X.S. Bai Laminar Premixed Flames
Most cars run with SI engine
X.S. Bai Laminar Premixed Flames
SGT-750 37 MW, 40% Launched nov 2010
Gas turbine engines
X.S. Bai Laminar Premixed Flames
Premixed fuel/air mixture: auto-ignition
Premixed fuel/air mixture
Auto-ignition sites
X.S. Bai Laminar Premixed Flames
Hydrogen/air auto-ignition
Initial temperature
Ignition delay time H2 + M = H + H + M
Low T0
High T0
thermal runaway and radical runaway
Induction period
X.S. Bai Laminar Premixed Flames
Hydrogen/air auto-ignition
Ignition delay time of H2/air mixture
X.S. Bai Laminar Premixed Flames
Structures and propagation of laminar premixed flames
• Structures of laminar premixed flames • Burning velocity of lamniar premixed flames • Propagation of laminar premixed flames in flow field • Stabilization of premixed flames • Laminar premixed flame instability
X.S. Bai Laminar Premixed Flames
Laminar premixed flames: Bunsen burner
X.S. Bai Laminar Premixed Flames
Laminar premixed flames
Reaction zone
Hot gas radiation
X.S. Bai Laminar Premixed Flames
Experimental observations of flames
X.S. Bai Laminar Premixed Flames
CH2O CH photo
Vo=0.45 m/s, phi=1.17; Vin=11m/s, phi=1.1
X.S. Bai Laminar Premixed Flames
Structure of premixed flames
X.S. Bai Laminar Premixed Flames
Structure of premixed flames
X.S. Bai Laminar Premixed Flames
Premixed fuel/air mixture: flame propagation
Premixed fuel/air mixture
Preheat zone
Reaction zone
Post flame zone
X.S. Bai Laminar Premixed Flames
Structure of premixed flames
• Preheat zone: heating up the reactants
• Inner-layer: radical formation
• Oxidation-layer: finishing the remaining reactions
• Post-flame zone: hot products, NOx formation
X.S. Bai Laminar Premixed Flames
Analysis of flames: the post-flame zone
Flame front
Unburned fuel/air mixture
Burned hot products
YF,u / YA,u =! / "A, and YF,u +YA,u =1
YF,u =!
!+ "A
, YA,u ="A
!+ "A
, and YO,u =0.233"A
!+ "A
, YN,u =0.767"A
!+ "A
,
F + A = P 1 γA 1+γA
X.S. Bai Laminar Premixed Flames
Analysis of flames: the post-flame zone
! >1
YF,b =YF,u "YA,u / #A = (!"1)YA,u / #A =YF,u!"1!
=!"1!+ #A
,
YO,b = 0, YN,b =YN,u =0.767#A
!+ #A
YP,b =1"YF,b , P includes N
Flame front
Unburned fuel/air mixture
Burned hot products
X.S. Bai Laminar Premixed Flames
Analysis of flames: the post-flame zone
Flame front
Unburned fuel/air mixture
Burned hot products
,
, , , , ,
, ,
10
1 1(1 ) ,
1
F b
A b A u A F u A u A F u AA
P b A b
Y
Y Y Y Y Y
Y Y
γ γ γγ
Φ <
=
−Φ −Φ= − = −Φ = =
Φ Φ +
= −Both A and P include N
X.S. Bai Laminar Premixed Flames
Analysis of flames: the post-flame zone
Flame front
Unburned fuel/air mixture
Burned hot products
0 0, , , ,
0 0 0, , , , , ,
[ ( )] [ ( )]
[ ( )] [ ( )] [ ( )]u F u F p F u ref A u A p A u ref
b F b F p F b ref A b A p A b ref P b P p b b ref
h Y h c T T Y h c T T
h Y h c T T Y h c T T Y h c T T
= + − + + −
= + − + + − + + −
X.S. Bai Laminar Premixed Flames
Analysis of flames: the post-flame zone
0
T
Tu
Ф=1 Ф
( )rb u
p A
HT T
c γ−Δ Φ
= +Φ +
( ) 1rb u
P A
HT TC γ−Δ
= +Φ +
( ) 11
rb u
P A
HT TC γ−Δ
= ++
X.S. Bai Laminar Premixed Flames
Laminar burning velocity
• How fast can a laminar flame propagate? • What is the mechanism of flame propagation? • What are the influencing factors?
X.S. Bai Laminar Premixed Flames
Laminar burning velocity
Flame front
Unburned fuel/air mixture
Burned hot products
Laminar burning velocity (also called laminar flame speed): the speed at which the flame front propagates towards the unburned mixture
X.S. Bai Laminar Premixed Flames
Derivation of laminar burning velocity
Flame front
Unburned fuel/air mixture
Burned hot products
Assumptions - 1 D - steady state - unity Lewis number - low Mach number
!!!t
+"#!v!= 0
!!!v!t
+ !!v "# !v =#" pI +!( )
!DhDt
!DpDt
="# !""h ! !" 1! 1Lei
$
%&&
'
())hi"Yi
i =1
N
*$
%&&
'
())+!Qr +! :"
!v
!!Yi!t
+"#!Yi v!="#!Di"Yi +"i
X.S. Bai Laminar Premixed Flames
Development of flame speed theory
• Mallard: – Ann Mines 7, 355 (1875)
• Mallard & Le Chatelier: – Ann Mines 4, 274 (1883)
• Bernard Lewis and G Von Elbe (1937)
• Zeldovich, Frank-Kamenetskii (1938) and Semenov (1940)
• Peters, Seshadri, Williams (1990s)
X.S. Bai Laminar Premixed Flames
Derivation of laminar burning velocity
Flame front
Unburned fuel/air mixture
Burned hot products
Assumptions - 1 D - steady state - unity Lewis number - low Mach number
!"!t+#$"v
!= 0
!"Yi!t
+#$"Yiv!=#$"Di#Yi +%i
X.S. Bai Laminar Premixed Flames
Derivation of laminar burning velocity
Flame front
Unburned fuel/air mixture
Burned hot products
Assumptions - 1 D - steady state - unity Lewis number - low Mach number 0
!"!t+#$"v
!= 0
!"Yi!t
+#$"Yiv!=#$"Di#Yi +%i
X.S. Bai Laminar Premixed Flames
Derivation of laminar burning velocity
Flame front
Unburned fuel/air mixture
Burned hot products
Assumptions - 1 D - steady state - unity Lewis number - low Mach number
!"!t+d"udx
= 0
!"Yi!t
+d"Yiudx
=ddx
"DidYidx
#
$%
&
'(+)i
0
X.S. Bai Laminar Premixed Flames
Derivation of laminar burning velocity
Flame front
Unburned fuel/air mixture
Burned hot products
Assumptions - 1 D - steady state - unity Lewis number - low Mach number
!"!t+d"udx
= 0
!"Yi!t
+d"Yiudx
=ddx
"DdYidx
#
$%
&
'(+)i
0
X.S. Bai Laminar Premixed Flames
Derivation of laminar burning velocity
Flame front
Unburned fuel/air mixture
Burned hot products
Assumptions - 1 D - steady state - unity Lewis number - low Mach number
!"!t+d"udx
= 0
!"u ="uSL = constant0 !"Yi!t
+d"Yiudx
=ddx
"DdYidx
#
$%
&
'(+)i *"uSL
dYidx
=ddx
"DdYidx
#
$%
&
'(+)i
SL
X.S. Bai Laminar Premixed Flames
Derivation of laminar burning velocity
!uSLdYidx
=ddx
!DdYidx
"
#$
%
&'+(i ) !uSL
dYidx
"
#$
%
&'dx =
*+
++
, ddx
!DdYidx
"
#$
%
&'+(i
"
#$$
%
&''dx
*+
++
,
)!uSL(YP,b *0) =(P-L ) SL ./-L
x!"#
YP = 0,dYPdx
= 0
x = 0
x!+"
YP =YP,b ,dYPdx
= 0
δL
X.S. Bai Laminar Premixed Flames
Derivation of laminar burning velocity
!uSLdYidx
=ddx
!DdYidx
"
#$
%
&'+(i ) !uSL
dYidx
"
#$
%
&'dx =
*+
*,L /2
- ddx
!DdYidx
"
#$
%
&'+(i
"
#$$
%
&''dx
*+
*,L /2
-
)!uSL(YP,b / 2*0) =!DYP,b *0,L
) SL,L .D
x!"#
YP = 0,dYPdx
= 0
x = 0
x!+"
YP =YP,b ,dYPdx
= 0
X.S. Bai Laminar Premixed Flames
Derivation of laminar burning velocity
Flame front
Unburned fuel/air mixture
Burned hot products
SL ! D!, D : mass diffusion cofficient [m2 /s]
"L ! D /! ! : reaction rate [1/s]
X.S. Bai Laminar Premixed Flames
Dependence on fuel/air ratio
X.S. Bai Laminar Premixed Flames
Dependence on flame temperature
X.S. Bai Laminar Premixed Flames
Dependence on flame temperature
X.S. Bai Laminar Premixed Flames
Dependence on flame temperature
X.S. Bai Laminar Premixed Flames
Dependence on reactivity
X.S. Bai Laminar Premixed Flames
Dependence on preheat temperature
X.S. Bai Laminar Premixed Flames
Dependent on transport properties
X.S. Bai Laminar Premixed Flames
Dependence on pressure
X.S. Bai Laminar Premixed Flames
Syngas/air (CO/H2/air) flames – effect of reactivity and diffusivity
Author's personal copy
the laminar burning velocity of the biomass gases. Forhydrogen rich mixtures (with hydrogen mole fraction largerthan 20%, such as GG-H, GG-V, GG-L1, GG-L2, GG-S, PG-D andSG-50 listed in Table 1), co-firing with methane would lead toa decrease in the laminar burning velocities. In such cases, co-firing improves the burning velocity of methane and naturalgas rather than the BDGs, and this may help improving theperformance of natural gas based gas engines.
Co-firing hydrogen with BDGs would certainly improve theburning characteristics of the BDGs. Recent studies onhydrogen addition to methane (equivalent to purified landfillgas) have shown that the laminar burning velocity increasesmonotonically with the hydrogen volume fraction in the fuelblends [32], and the ignition of methane can be significantlyimproved with hydrogen addition [33]. For stretched flameswith curved flame front, hydrogen addition to methane can
not only increase the burning velocity (similar to the freelypropagating un-stretched flames), but also improve theextinction limits. It can also affect the flame instabilities[34–36], due to the low Lewis number of hydrogen.
3.3. Correlation between laminar burning velocity andBDG gas composition
To gain further insight into the inter-correlations between thelaminar burning velocity and BDG compositions, numericalsimulations are carried out for various BDGs reported in theliterature. The simulated laminar burning velocity, laminarflame thickness, and adiabatic flame temperature aresummarized in Table 3, with the compositions of the BDGsand their LHV given in Table 1. All flames are under
Fig. 10 – Laminar burning velocity and adiabatic flame temperature of syngas flames at atmospheric pressure and unburnedmixture temperature of 300 K.
Fig. 9 – Laminar burning velocity and adiabatic flame temperature as a function of equivalence ratio for gasification gas/airflames. GG-S: a model producer gas generated during the gasification of biomasswastes [3], GG-Va: gasification gas from theVarnamo plant [9].
i n t e r n a t i o n a l j o u rn a l o f h y d r o g e n en e r g y 3 5 ( 2 0 1 0 ) 5 4 2 – 5 5 5 551
Diffusivity increases
X.S. Bai Laminar Premixed Flames
Propagation of laminar premixed flame
y , ey
z , ez
x , ex
r
v p
SL , n
Unburned G<0 Burned
G>0
G(x,y,z,t)=0
Flame front (reaction zone)
!G!t
+!v "#G = S L #G
X.S. Bai Laminar Premixed Flames
Planar flame in a pipe
• Stable flame • Flash back • Blow-off • Wall effect
u SL Unburned x< 0 burned
x> 0 x= 0 (at t=0) x
X.S. Bai Laminar Premixed Flames
Bunsen flame
• Conical shape • Flame length vs R, u, SL
R r
u
G(x,r,t)=0
x
( , , ) ( ) 0G x r t x f r= + =
( )2 1Lu S f ʹ′= +
2 2
2( ) constantL
L
u Sf r rS−
= +
2 2
2( ) L
L
u Sx R rS−
= −
X.S. Bai Laminar Premixed Flames
Bunsen flame stability
• Rim effect • Lifted flame • blowout
Stable flame position B Stable flame
position A rim
X.S. Bai Laminar Premixed Flames
Stabilization of premixed flames
• Rim stabilization • Pilot flame stabilization • Bluff body stabilization