The influence of stretch and preferential diffusion on premixed flame extinction and stability was investigated via two model flame configurations, namely the stagnation flame and the bunsen flame. Using a counterflow burner and a stagnation flow burner with a water-cooled wall, the effect of downstream heat loss on the extinction of a stretched premixed flame investigated for lean and rich propane/air and methane/air mixtures. It was demonstrated that extinction by stretch alone is possible only when the deficient reactant is the less mobile one. When it is the more mobile one, downstream heat loss or incomplete reaction is also needed to achieve extinction. The local extinction of bunsen flame tips and edges of hydrocarbon/air premixtures was investigated using a variety of burners. Results show that, while for both rich propane/air and butane/air mixtures tip opening occurs at a constant fuel equivalence ratio of 1.44 and is therefore independent of the intensity, uniformity, and configuration of the approach flow, for rich methane/air flames burning is intensified at the tip and therefore opening is not possible

Law, C. K.

English

The concept of flammability limits in the presence of flame interaction, and the existence of negative flame speeds are discussed. Downstream interaction between two counterflow premixed flames of different stoichiometries are experimentally studied. Various flame configurations are observed and quantified; these include the binary system of two lean or rich flames, the triplet system of a lean and a rich flame separated by a diffusion flame, and single diffusion flames with some degree of premixedness. Extinction limits are determined for methane/air and butane/air mixtures over the entire range of mixture concentrations. The results show that the extent of flame interaction depends on the separation distance between the flames which are functions of the mixtures' concentrations, the stretch rate, and the effective Lewis numbers (Le). In particular, in a positively-stretched flow field Le  1 (  1) mixtures tend to interact strongly (weakly), while the converse holds for flames in a negatively-stretched flow. Also established was the existence of negative flames whose propagation velocity is in the same general direction as that of the bulk convective flow, being supported by diffusion alone. Their existence demonstrates the tendency of flames to resist extinction, and further emphasizes the possibility of very lean or rich mixtures to undergo combustion

NASA Technical Reports Server

LEAN LIMIT PHENOMENA
* C.K. Law
Northwestern University
I Extensive theoretical and experimental research have been accomplished on the
i various aspects of the structure and extinction of premixed flames. The• presentation will focus on only one specific topic of the lean/rich limit phenomena,
!
i namely the concept of flammability limits in the presence of flame interaction, and
the existence of negative flame speeds. Attention is called to other topics
4 reported in the publications Listed; contributions from them are no less significant
j but will not be presented due to the lack of time.
To appreciate the importance of flame interaction, we first note that although
predictions of complex flame phenomena are frequently based c3 understanding of
isolated flames, it is clear that in most situations the com Justion flow field is
composed of an ensemble of flamelets, of different intensity and extent, which
i continuously interact with each other and thereby can cause significant
modifications of the bulk combustion behavior.
In the present investigation downstream interaction between two counterflowt
premixed flames of different stoichlometrles are experimentally studied. Various | ,
flame configurations are observed and quantified; these include the binary system of
two lean or rich flames, the triplet system of a lean and a rich flame separated by
a diffusion flame, and single diffusion flames wi_h some degree of premixedness.
Extinction limits at_ determined for methane/air and butane/alr mixtures over the
entire range of mixture concentrations.
The results show that the extent of flame interaction depends on the separation
distance between the flames which are functions of the mixtures' concentrations, the
stretch rate, and the effective Lewis numbers (Le). In particular, in a positively-
stretched flow field Le<I (>l) mixtures tend to interact strongly (weakly), while
the converse holds for flames in a negatively-stretched flow.
i
An important consequence of flame interaction is the modification of the
flammability limits of a mixture when it is stratified. If the mixture is only i
weakly stratified, then the conventional flammability limits for uniform mixtures
may still apply. H_wever, with more extensive stratification the stronger portion I
of the mixture may burn independent of the weaker portion, while the weaker portion
can also burn by receiving support from the stronger flame. The mixture is made
more flammable as a whole.
I
Our study has also established the existence of negative flames whose
propagation velocity is in the same general direction as that of the bulk convective
flow, being supported by diffusion alone. Their existence oemonstrates the tendency
of flames to resist extinction, and luther emphasizes the possibility of very lean
or rich mixtures to undergo combustion.
243
®
I | I
1984012457-237
https://ntrs.nasa.gov/search.jsp?R=19840012484 2020-03-22T08:16:31+00:00Z
{!
: Flame interaction is also of relevance to the modeling of turbulent flames.
Within a turbulent flow field stratifications in temperature and reactant
concentrations invariably exist, caused by mixing inhomogenelties or, more subtly,
through the coupling between stretch a._d preferential diffusion even for an ,
initially homogeneous mixture. Thus a flame kernel initiated _rlthln a turbulent
' eddy will manifest effects of downstream Inteactlon between flames of different
intensities.
+ Publications
? i. "Effects of Heat Loss, Preferential Diffusion, and Flame Stretch on Flame-Front
Instability and Extinction of Propane/Air Mixtures," by S. Ishlzuka, K.
Miyasaka, and C. K. Law, Combustion and Flame, Vol. 45, pp. 293-308, (1982).
2. "On the Opening of Premixed Bunsen Flame Tips," by C. K. Law, S. Ishizuka, and
P. Cho, Combustion Science and Technology, Vol. 28, pp. 89-96, (1982).
3. "On Stability of Premixed Flames In Stagnatlon-Polnt Flow," by G. I.
Sivashlnsky, C. K. Law, and G. Joulln, Combustion Science and Technology,
Vol. 28, pp. 155-159, (1982). !
i
4. "An Experimental Study of Extlnctlon and Stability of Stretched Premixed
Flames," by S. Ishizuka and C. K. Law, Nineteenth Symposium on Combustion,
_ the Combustion Institute, Pittsburgh, Pa., pp. 327-335, (_983).
5. "Heat and Mass Transfer in Combustion: Fundamental Concepts and Analytical
Techniques," Proc. of ASME-JSME Joint Thermal Engineering Confer@nee , (Y.
Mori and W. J. Yang, Ed.), Vol. 2. pp. 535-559, (1983). ** Plenary Paper**
6. "An Invarlant Derivation of Flame Stretch," by S. H. Chung and C. K. Law, t
Combustion and flame, Vol. 55, pp. 123-125 (1984).
7. "Extinction of Premlxed Flames by Stretch and Radiative loss," by S. H. Sohrab
and C. K. Law, to appear in International Journal of Heat and Mass Transfer.
8. " On the Determination of Laminar Flame Speeds from Stretched Flames," by C. K.
Wu and C. K. Law, submitted.
9. "An Experimental Investigation on Flame Interaction and the Existence of
_ Negative Flame Speeds," by S. H. Sohrab, Z. Y. Ye, and C. K. Law, submitted.
" I0. "Effects of Preferential Diffuolon on the Burning Intensity of Curved Flames,"
by M. Mizomoto, Y. Asdka, S. Ikat, and C. K. Law, submitted.
|
L
!
,
1984012457-238
1OVERALL OBJECTIVES
4
o TO UNDERSTAND THE FUNDAMENTAL PRESENT ACCOMPLISHMENTS
" MECHANISMS GOVERNING EXTINCTION OF
PREHIXED FLAMES. o IDENTIFIED AND QUANTIFIED THE
_- IMPORTANCE OF FLAME INTERACTION.
o TO QUANTIFY FLAMMABILITY LIMITS OF ,,
MIXTURES AND IDENTIFY DOMINANT o INDENTIFIED THE EXISTENCE OF LAMINAR
MECHANISMS IN OPERATION AT THESE FLAMES WITH NEGATIVE FLAME SPEEDS•
LIMITS.
o STUDIED EFFECTS OF FLAME CURVATURE ON
o TO IDENTIFY POSSIBILITIES OF BURNING INTENSITY.
EXTENDING THESE EXTINCTION/
J
FLAMMABILITY LIMITS. o CRITICALLY RE-EXAMINED EXISTING
METHODS IN LAMINAR FLAME SPEED
DETERMINATION; PROPOSED NEW
METHODOLOGY •
2 o THEORETICAL STUDIES ASSOCIATED WITH
POTENTIAL EXTINCTION MECHANISMS THE ABOVE TOPICS.
"" o HEAT LOSS.
o KINETIC TERMINATION.
! !.o FLAME STRETCH: IMPORTANCE OF FLAME INTERACTION
• t
FLOW NONUNIFORMITY
o PRACTICAL COMBUSTION FLOW FIELDS
FLAME CURVATURE CONSIST OF ENSEMBLES OF FLAMELETS.
FLAME ACCELERATION _
o INDIVIDUAL FLAMELETS BURN WITH
DIFFERENT INTENSITY AND EXTENT.
o INTERACTION BETWEEN THEM CAN EITHER
3 CONTRACT OR WIDEN THE EXTINCTION
"J PREVIOUS CONTRIBUT IONS -- LIMIT.
• o FLAME STRETCH, COUPLED WITH o ESPECIALLY RELEVANT TO TURBULENT
! PREFERENTIAL DIFFUSION (NON-UNITY FLAME MODELING AND FLAME PROPAGATION "
LEWIS NUMBER) EFFECTS, WAS IDENTIFIED IN FLOWS WITH CONCENTRATION ,
TO BE THE DOMINANT MECHANISM IN STRATIFICATION OR POOR MIXING
• OPERATION AT THE FLAMMABILTY LIMITS. CHARACTERISTICS
'_ o FLAMMABILITY LIMITS, DETERMINED BY
USING THE COUNTERFLOW FLAME, AGREE
WELL WITH EXISTING DATA•
!
245
®:
1984012457-239
METHODOLOGY PRACTICAL CONCLUS IONS --
o ESTABLISH TWO PREMIXED FLAMES OF o METHANE-AIR MIXTURES LEANER THAN 5%
UNEQUAL STRENGTH BY THE COUNTERFLOW REMAINS NON-FLA_LE IF
• OF TWO STREAMS WITH DIFFERENT MIXTURE CONCENTRATION STRATIFICATION (OR
: CONCENTRATIONS. MIXING INHOMOGENEITY) IS WITHIN 4-6%;
¢ A HIGHER STRATIFICATION WILL INDUCE
_" o MAP THE FLAME CONFIGURATIONS AND BURNING.
EXTINCTION LIMITS AS FUNCTIONS OF
STRETCH AND FUEL CONCENTRATIONS OF
o MIXTURES ABOVE 6% WILL ALWAYS BURN,
- THE UPPER (_U) AND LOWER (_L) HENCE AUGMENTED FLAMMABILITY LIMIT.
STREAMS.
o INTERACTION IS STRONG (WEAK) FOR LEAN
o MEASURE THE TEMPERATURE PROFILE FOR (RICH) MIXTURES•
THE INTERACTING FLAMES.
_ o FLAMMABILTY LIMITS LESS (MORE)
SUSCEPTIBLE TO CONCENTRATION
STRATIFICATION FOR LEAN (RICH)
• MIXTURES.
7 o PROPANE-AIR AND BUTANE-AIR MIXTURES
,, EXTINCTION LIMITS (METHANE-AIR BEHAVE OPPOSITE TO METHANE-AIR
MIXTURES) MIXTURES.
o FOR RL < 4%, RU " 6% AND IS o MIXING INHOMOGENEITY PROMOTES i ._
INSENSITIVE TO RL; WEAK INTERACTION; BURNING• i' 1
STRONG FLAME BURNS INDEPENDENTLY, o EXISTENCE OF NEGATIVE FLAME SPEEDS
WEAK FLAME PARASITIC DEMONSTRATES THE RESISTENCE OF FLAMES : i
TO EXTINCTION.
o FOR 4% < (£L' RU) < 6%, o FLAM% STRETCH AND PREFERENTIAL iDIFFUSION ARE CRITICAL FACTORS IN
(RL + RU)/2 " 5%; STRONG INTERACTION; DETERMINING FLAME CHARACTERISTICS, ,
SYMBIOTIC COMBUSTION. ESPECIALLY EXTINCTION.
_, o FRACTIONAL DEVIATION OF LEWIS NUMBER
o INTERACTION IS STRONG (WEAK) FOR LEAN FROM UNITY CAN CAUSE SAME EXTENT OF
_ DEVIATION OF FLAME TEMPERATURE.
°" (RICH) MIXTURES.
t
o THEORY SUBSTANTIATES THE ABOVE _
BEHAV IOR.
!
1984012457-240
IPF PF PF PF ORIGINAL PAGE "-i:_
I
° o
(a) (b)
J
PF PPF LPF DF RPF
ilr ,Tll
o o :i
(c) (d)
PDF RPF PDF
:/J_L_,l" T
o F ;
F 0
(e) (f) i
F£gure 1. Possible modes of doqanstream _nteraction between two
premixed (lames, IF, Prem£xed Flame; DF: Dlffumion
FI_mP; LPF/RPF" l.ean/Rfch Premzxed Flame; PPF' Partially
Fre_,:xed Flame; PDF" Partial Diffuston Flame.
• , , ,--i • -_--'_ , , ,_--
'°°L -i I oo__ _ ..... q.
_ ** IV
• ?" /
12 v / /
0 = 4 6 S Io 12 14 ts _ Zo
'_'L (% CH4 ) _ ( % C4Hlo |
Figure 2. .t4applng of tl,e flame Figure 3. Mapptn8 of the flame
configurations and extinction confl&uratione and extLnctton ,
bounderle.q tot mLthane/alr boundariel for butlne/atr "
mixtures, mlxL urea. "
247
1984012457-241
ORIG)NAL PAGE IS
OF POOR QUALITY /
_4001 1400 I
_U =_L'57 % _U,63 %
I)OO F STAGNATION 1300 F UPPER STAGNATION _L '5 7 %|
PLANE { FLAME PLANE '
FL"_LT CL_E 1200 I, , LOWER
_-I000 _ , - _-
900 ' 900J
i
' eO0800 b
, ; A , i J
, ; _ I008 ' ' 2 4 6 B
DISTANCE FROM _eTAGNATION PLANE (MM) OISTANCE FROM STAGNATION PLANE (MM}
(0) (b)
1400 i
_R
FLAME LOWER _u ,60 %
1200 _ -- FLAME _I.,4 0 %
,000 __ STAGNAT,ON
-- 80C i
60C I
t '400 1
200[ I
6 5 4 3 2 I 0 I 2 3 4 5 %
DISTANCE FRCM STAGNATION PLANE (MM) a_
(C)
,too 14oo F,_,_
[ D_rFuS,ON flU,, 60Z % _2-PART,AL !
IlL ,150 % //_
.... // _ \,?rrus,oN
"5 _PER / ., V
,,. _.ANF_ I ,.DOiZo0 aoc i|
i_00 ZOO ,,
1
a i
DISTANCE FROM DIFFUSION FLAME (MM } OISTANCE FROM FLAME SHEET IMM|
(d) (,e) . !
Figure 4. Temperature profiles of methane/air flames: (a) Symmetric flames;
(b) Asymmetric flames; (c) Negative flame; (d) Triple flames; (e) !
l. Diffus_un flame; 2, Partial diffusion flame, concenCra¢ions are
fuel + 107. stoich. 02 In upper flea and air " 107. stoich, fuel Ln .,lower flow. c
!
248
®,
1984012457-242
T
?
7
6 o o e o _oeo,_.8
-%5
,}% _. 8
, "; 4 og - 6 _:
z _ '_U '
0
" _ EXTINCTION _ o. 4
= g _ 2
c; 2 o 5ILl
_ 0
I o W
=E -2o <;.
.-J;%
0 I 2 3 4 5 6 ? v I 2 3 4 5 6 7 •8
,Q.L ( % CH4 ) ,.¢'_L( % CH4 )
Flsure _. The eztlnctlon boundar_ for two leau
prem/xed methane/alr _l_es. _[8ure 6. Diltsnce be_een lover fla_ and stagnatlon
surface, _, as a function of _L"
i
• o
I
i
' i
• i
t!
i
L I
249
,®
1984012457-243


Lean limit phenomena

LEAN LIMIT PHENOMENA 
C. K. Law 
Northwestern University 
Evanston, Illinois 60201 
The influence of stretch and preferential diffusion on premixed flame 
extinction and stability have been investigated via two model flame 
configurations, namely the stagnation flame and the bunsen flame. The results 
are separately summarized in the following. 
( 1 )  Extinction and Stability of Stretched Premixed Flames in the Stagnation 
Flow 
Using a counterflow burner and a stagnation flow burner with a 
water-cooled wall, the effect of downstream heat loss on the extinction 
of a stretched premixed flame has been systematically investigated for 
lean and rich propane/air and methane/air mixtures. Based on results of 
the concentration limits and flame separation distances at extinction, it 
is demonstrated that, in accordance with theoretical predictions, extinc- 
tion by stretch alone is possible only when the deficient reactant is the 
less mobile one. When it is the more mobile one, downstream heat loss or 
incomplete reaction is also needed to achieve extinction. A variety of 
unstable flame configurations have been observed; the mechanisms for 
their generation and sustenance are discussed. 
(2 )  Opening of Premixed Bunsen Flame Tips 
The local extinction of bunsen flame tips and edges of hydrocarbon/ 
air premixtures has been experimentally investigated using a variety of 
burners. Results show that, while for both rich propane/air and butane/ 
air mixtures tip opening occurs at a constant fuel equivalence ratio of 
1.44 and is therefore independent of the intensity, uniformity, and 
configuration of the approach flow, for rich methane/air flames burning 
https://ntrs.nasa.gov/search.jsp?R=19830020958 2020-03-22T00:49:05+00:00Z
is intensified at the tip and therefore opening is not possible. These 
results substantiate the concept and dominance of the diffusional strati- 
fication mechanism in causing extinction, and clarify the theoretical 
predictions on the possible opening of two-dimensional flame wedges. 
(3) Publications 
(a) "Lean-Limit Extinction of PropaneIAir Mixtures in the Stagnation- 
Point Flow," by C. K. Law, S. Ishizuka, and M. Mizomoto, Eighteenth 
Symposium on Combustion, pp. 1791-1798 (1981). 
(b) "Effects of Heat Loss, Preferential Diffusion, and Flame Stretch on 
Flame-Front Instability and Extinction of ~ropane/Air Mixtures ," by 
S. Ishizuka, K. Miyasaka, and C. K. Law, Combustion and Flame, Vol. 
45, pp. 293-308 (1982). 
(c) "On the Opening of Premixed Bunsen Flane Tips ," by C. K. I a w ,  S. 
Ishizuka, and P. Cho, Combustion Science and Technology, Vol. 28, 
pp. 89-96 (1982). 
(d) "On Stability of Premixed Flame in Stagnation-Point Flow," by G .  I. 
Sivashinsky, C. K. Law, and G. Joulin, Combustion Science and 
Technology, Vol. 28, pp. 155-159 (1982). 
(e) "An Experimental Study of Extinction and Stability of Stretched 
Premixed, Flames, " by S. Ishizuka and C. K. Law, to appear in 
Nineteenth Symposium on Combustion, 1983. 
O B J E C T I V E S  
@ T O  STUDY E F F E C T S  O F  
1. PREFERENTIAL D I F F U S I O N  ( L e # l )  
2. AERODYNAMIC STRETCHING (FLOW NON-UNIFOWIITY , 
UNSTEADINESS, AND FLAME CURVATURE) 
3. UOWNSTREAM HEAT LOSS 
.ON 
A. FIAME E X T I N C T I O N  
B. F LAEIE-FRONT INSTAB I L I T Y  
METHO DOLOGY 
1. PKEFERENTIAL D I F F U S I O N  E F F E C T S  S T U D I E D  BY U S I N G  
METHANEIAIR PROPANE/AIR 
LEAN L e < l  L e > l  
R I C H  L e > l  Le < 1 
2. AERODYNAMIC STRETCHING S T U D I E D  BY USING STAGNATION 
FLOW WHICH HAS WELL-DEFINED VELOCITY GRADIENT 
3. DOWNSTREAM HEAT LDSS S T U D I E D  BY USING 
(a) STAGNATION .FLOW WiTH WATER-COOLEE SURFACE 
( b) SYMMETRICAL COUNTERFLOW 
209 
STAGNATION SURFACE 
/ / / / / / / , / / / / . / /  / / / / / / / , /  / ,  
Control Volume 
Mass Diffusion 
S c h e m a t i c  of Stagnat ion-Point  F l o w  I l l u s t r a t i n g  t h e  
D i r e c t i o n s  of H e a t  and m a s s  D i f f u s i o n .  
E X T I N C T I O N  MECHANISMS 
Le > 1 F l a m e s  
1. STRETCH ALONE CAN CAUSE EXTINCTION;  DOWNSTREAM HEAT LDSS 
M I N I M A L  E F F E C T  
2. INCREASING STRETCH DECREASES F IAPIE TEMPERATURE 
3. AT EXTINCTION,  FLAME LOCATEC AWAY FROM STAGNATION SURFACE 
4. AT E X T I N C T I O N ,  D E F I C I E N T  REACTAM' COMPLETELY CONSUMED 
Ze < 1 F l a m e s  
1. INCREASING STRETCH INCREASES FLAME TEMPERATURE, THEREFORE 
STRETCH ALONE CANNOT CAUSE E X T I N C T I O N  
2. E X T I N C T I O N  CAN BE ACHIEVED THROUGH 
(a) DOWNSTREAM HEAT LOSS,  WITH FLUE AWAY FROM WALL 
(b )  INCOMPLETE COMBUSTION WITH FLAME AT WALL 
N 
t-' 
t-' 
Various Flame Configurations for Propane/Air Mixtures 
in the Stagnation-Point Flow (See Publication No. b) 
Various Flame Configurations for PropaneIAir Mixtures 
in the Stagnation-Point Flow (See Publication No. b) 
Flame Separatedness at Extinction in the Counterflow Geometry. 
(a) Lean MethaneIAir, (b) Rich MethaneIAir, (c) Lean Propane/Air, 
(d) Rich Propane/Air (See Publication No. e) 
~ethanefAir L =  15 rnm 
V = l O  cm/sec 
EXTINCTION LIMIT 
L =  15 mm 
V = 10 cm/sec 
EXTINCTION LIMIT 
Loca t ion  o f  t h e  Binary  Flames I l l u s t r a t i n g  Flame Separa tedness  a t  E x t i n c t i o n  
LEAN ~ 3 ~ 4  
/- I RICH CH. 
RICH C3H8 
0.5 I I I I I I I 1  
R a t i o  of  t h e  E x t i n c t i o n  Concen t ra t ion  L i m i t s  w i t h  and w i t h o u t  
Downstream Heat Loss 
- - 
% Methane % Propane 
Author Method Defini t ion 
Lean Rich Lean Rich 
Zabetakis Propagating flame ( tube)  Extinct ion 5.0 15.0 2 . 1  9.5 
Andrevs and Bradley Propagating flame ( v e s s e l )  Ext inct ion 4.5 15.5 - - 
Egerton and Thabet F l a t  flame Burning ve loc i ty  5 .1  - 2.01 - 
Sorenson, Savage. 
and Strehlow Tent flame Cone angle 
Yamaoka and Tsuj i Double flame F lameloca t ion  4.7 15.3 - - 
Ishizuka and Law Binary flame Extinct ion '4.8 15.8 2.0 9.7 
Comparison o f  t h e  Flammabi l i ty  L i m i t s  o f  ~ e t h a n e / A i r  and p r o p a n e / ~ i r  
Mixtures  Determined by D i f f e r e n t  Methods 
(a) (b) 
Schemat i c  o f  Closed  and Open Bunsen Flame T i p s  
Set-Up o f  t h e  Bunsen Flame Exper iment  
216 
Tip Intensification of Rich MethaneIAir Bunsen Flame with 
Increasing Methane Concentration (See Publication NO. c) 
Tip Opening of Rich Propane1Ai.r Bunsen Flame with Increasing 
Propane Concentration (See Publication No. c )  
T U B E  N O Z Z L E  
F u e l  C o n c e n t r a t i o n s  a t  T i p  Opening a s  Funct ion o f  Flow V e l o c i t y  f o r  a  V a r i e t y ' o f  Burners  


Lean Limit Phenomena

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