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Extinction of low-stretched diffusion flame in microgravity
Extinction of Low-Stretched Diffusion Flame in Microgravity KAORU MARUTA,* MASAHARU YOSHIDA,* H O N G S H E N G GUO, YIGUANG JU,* and TAKASHI N I I O K A Institute of Fluid Science, Tohoku Unit'ersity, Katahira, Aobu-ku, Sendai 980- 77, Japan tDepartment of Aeronautics and Space Engineering, Tohoku Unil'ersi~. Aramaki Aoba. Aoba-ku. Sendai 9,~¢0.Japan
Extinction of counterflow diffusion flames of air and methane diluted with nitrogen is studied by drop tower experiments and numerical calculation using detailed chemistry and transport properties. Radiative heat loss from the flame zone is taken into consideration. Experimental results identified two kinds of extinction at the same fuel concentration, that is, in addition to the widely known stretch extinction, another type of extinction is observed when the stretch rate is sufficiently low. Consequently. plots of stretch rates versus fuel concentration limits exhibit a "C-shaped" extinction curve. Numerical calculation including radiative heat loss from the flame zone qualitatively agreed with the experimental results and indicated that the mechanism of counterflow diffusion flame extinction at low stretch rates was radiative heat loss. "c: 1908 by The Combustion Institute
INTRODUCTION Since the presentation of the "S-curve" of stagnation-point diffusion flames by Fendell [1], essential discussion has been based on the first D a m k 6 h l e r number [2]. Diffusion flames in a stagnation-point region have been frequently examined, and a n u m b e r of theoretical [3-7], numerical [8-10], and experimental [4, 9, 11-14] studies focused on understanding the fundamental characteristics of diffusion flames have been carried out. The structure, extinction limit, and effect of heat loss have been investigated in these studies. In general, however, the effect o f radiative heat loss has not been considered as being remarkable [13-16]. In fact, Sohrab et al. [14] analyzed the effect o f radiative heat loss from the flame zone on diffusion-flame extinction and concluded that the critical condition for extinction remains defined by a balance between the rate o f heat generation and that of conductive heat loss, although radiative heat loss contributes to reduced reaction rates through reduced temperature. Also, Sohrab and Law [15] studied the combined effects o f stretch and radiative heat loss, and concluded that stretch exerts a stronger influence than radiative heat loss on flame extinction. Consequently, the influence
*Corresponding author. *Present address: Ishikawajima-Harima Heavy Industries Co., Ltd., 3-5-1, Mukodai, Tanashi, Tokyo 188, Japan.
of radiative heat loss on extinction is often neglected when small laboratory-scale flames are studied. On the other hand, several studies in which special attention was given to the low stretch region showed that radiative heat loss plays a significant role. Liu et al. [17] measured temperature profiles of counterflow premixed flame and showed appreciable effects of radiation on small laboratory-scale flames. T'ien [18] and Chao and Law [19] investigated extinction of diffusion flames in the stagnation region of a condensed fuel and predicted the extinction due to the radiative heat loss from the solid surface would occur in the low stretch region. Subsequently, Platt and T'ien [2(I] and Daguse et al. [21] suggested that the existence of extinction is due to the radiative heat loss from the gas phase. Furthermore, two kinds of extinction of m e t h a n e / a i r premixed flames at low stretch rates were recently observed by microgravity experiments [22], and based on numerical calculation, extinction at lower stretch rates was estimated to be induced by the radiative heat loss [23]. Accordingly, the effect of radiative heat loss not only on large scale flames but also on small laboratory-scale flames must be examined. In the present paper, extinction of diffusion flames at low stretch rates is measured by the counterflow method under microgravity [22], and the mechanism of such extinction is examined by numerical calculation using detailed
COMBUSTION A N D FLAME 112:181-187 (19981
© 1998 by The Combustion Institute Published by Elsevier Science Inc.
1101(1-218{I/98/$19.(111 Pll SIIOII)-218tK97)0011)7-7
182
K. M A R U T A El" AL.
chemistry and transport properties, taking radiative heat loss from the flame zone into consideration. MICROGRAVITY EXPERIMENTS All tests were performed under conditions of a microgravity field at the Japan Microgravity Center (JAMIC) in Hokkaida, Japan. At this facility, an experimental module experiences microgravity ( 1 0 - 4 - 1 0 -5 G) of 10 s during a free fall in the 490-m drop shaft. Experimental technique is basically the same as that employed in our previous work [22] on the counterflow premixed flame. The apparatus has been modified for measurement of counterflow diffusion flame. The fuel concentration limits for the counterflow diffusion flame of air and methane diluted with nitrogen were measured at various stretch rates. To obtain fuel concentration limits of fuel flow at small stretch rates within I0 s, the counterflow burner system employed in this study was designed so that the fuel concentration of the fuel flow could be gradually decreased until extinction, maintaining a constant mean flow velocity at the burner exit. A schematic of the experimental apparatus is shown in Fig. 1. The apparatus consists of a counterflow burner, an air and fuel flow supply system, an igniter, a video system, and a sequencer. The counterflow burner is made of brazen circular pipe to minimize the dead volume of the burner itself as shown in Fig. 2. The depth of the sintered metal was fixed at 5 mm from the burner exit. The air and fuel flow supply system is composed of electric mass-flow controllers, a notebook computer, and digitalto-analog and analog-to-digital converters. Several seconds before the drop, conditions of air and fuel flow such as mean flow velocity
Fig. I. Schematicof the experimentalapparatus.
Diffusion Flame
Air =~_ ~
Sintered Metal
,J
__
CH,+N2
~ ._L.~ Sintered Metal
Fig. 2. Structure of the counterflowburner, where U,, and Ue are mean fP:gwvelocitiesof the oxidizerand fuel flowat the burner exit. respectively,and L is the spacing of the burner exits. at the burner exit and appropriate fuel concentration of the fuel flow are established. Then the mixture is ignited by a hot wire which is introduced from the side of the counterflow burner into the stagnation region. Just after the commencement of the drop, the hot wire is removed and the fuel concentration of the fuel flow is gradually decreased until extinction, with the fuel flow velocity at the burner exit being kept constant. The fuel concentration of the fuel flow at extinction is considered to be the fuel concentration limit at the stagnationpoint velocity gradient under consideration. The measurement of velocity distribution under microgravity is very difficult. For the sake of simplicity, the stagnation-point velocity gradient is calculated by the relation [Uo + Uf( pJpo)°'Sl/L [24], where U is the mean flow velocity at the burner exit, subscripts f and o are the conditions in the fuel and the oxidizer flow, respectively, p is the density, and L is the spacing between burner exits. Also for the purpose of simplicity, the difference between potential flow and the flow field generated by the plug-flow-type burner [25] is not taken into consideration, because the change of the fuel concentration limit can be neglected when either velocity at the burner exits or spacing between the burner exits is changed, with the velocity gradient kept constant. The stagnation-point velocity gradient is changed at every drop test, and the fuel concentrations limits are plotted as a function of the stagnation-point velocity gradient. To obtain the fuel concentration limits, responses of the air and fuel supply system and the delay time of the pipe arrangement are taken into consideration [22]. Also, the response of the whole system including the counterflow flame itself is quick enough compared to the change of the fuel composition as
EXTINCTION OF DIFFUSION FLAMES
i described in detail in our previous study [22]. The present experiments were conducted under quasisteady conditions that are indispensable under the conditions used. The temperature of the sintcred metal plate surface was measured by a thermocouple and heat loss from the counterflow flame to the burner was shown to be negligible in our previous report [22]. NUMERICAL CALCULATION To investigate the mechanism of the extinction, the experimental conditions were numerically simulated. The mathematical model and numerical code employed here are basically the same as those employed elsewhere [26]. The only difference is the energy equation, in which the radiation sink term, q,, was retained. An optically thin radiation model is employed to consider the radiative heat loss from the flame zone. Radiation from the gaseous species CO2 and H 20 is taken into consideration. The governing equations for mass, momentum, chemical species, and energy are solved in a cylindrical coordinate. The CHEMKIN code [27-29] and the C1 elementary reaction mechanism, which involves 58 reactions and 18 species, given by Kee et al. [29], were used. Details of the present code were given in our previous work [23]. The distance between burner exits was 8 cm in the calculation.
Fig. 3. Direct p h o t o g r a p h o f a eounterflow diffusion flame u n d e r mierogravity. Uo = Uf = 5 c m / s and L = 1.5 era. T h e n u m b e r s in the p h o t o g r a p h are time(s) from the c o m m e n c e m e n t of the drop.
Iocity gradient near the limit, as seen in the results in normal gravity, the results of microgravity experiments clearly show the existence of a turning point. When the stagnation-point velocity gradient is larger than about 7 s- ~, the fuel concentration limits increase with the increase of the stagnation-point velocity gradient. However, when the stagnation-point velocity gradient is smaller than 7 s- I, the tendency is reversed. In other words, the stagnation-point velocity gradients at extinction are doublevalued for a certain fuel concentration.
T
20
. . . . . .
A,,,,
. . . . . . .
:-.------and" Ishizuka RESULTS AND DISCUSSION
Microgravity Experiments A stable, near-flat counterflow diffusion flame was established in microgravity, as shown in Fig. 3. Figure 4 shows the relationship between the stagnation-point velocity gradient and the fuel concentration limits. For reference, the results of supplementary experiments conducted in normal gravity by using the same apparatus and experimental data obtained by Ishizuka and Tsuji [30] are also presented in the same figure. The counterflow burners were arranged vertically when the supplementary experiments were carried out in normal gravity. Although the fuel concentration limits seem to be independent of the stagnation-point ve-
:
.=_
Tsuji
5~ i , . ~ N o r r n =
"
gravity
0 "6 o
10
.~.~
o
o o._.......~ Microgravity
"
o
o
o ,'
b~5"'. Fuel
0
°
' o ' 2 " ' ' ' " .0.25 concentration
Fig. 4. Experimental results of the relation between stagnation-point velocity gradient and fuel concentration at extinction. Dotted line shows experimental data from Ref. 30.
184
K. M A R U T A E T AL. TABLE 1
The Conditions of MicrogravityExperiments
v,,v,
L
~u + % p,,j
(cm/s)
(cm)
L (s-t)
11.5
1.5
14.9
I I .(1
1.5
14.3
9.0 7.0 5.0 7.0 5.11 2.5 2.8
1.5 1.5 1.5 2.5 2.5 1.5 2.5
11.7 9. l 6.5 5.5 3.9 3.2 2.2
,, ......
~
v
~.15~ E
0.168 o. 164 11.155 0.149 0.150 0.159 (I.170 0.210 0.270
Furthermore, the fuel concentration limits obtained in microgravity are leaner than those obtained in normal gravity. We can suppose the difference between them is due to the effect of buoyancy, since a stable flame could not be established in normal gravity in the small velocity gradient region when the burners were arranged horizontally. The conditions of the microgravity experiments are listed in Table 1 for reference. Numerical Calculations
Figure 5 shows the relation between maximum flame temperature and stretch rate by the computation which takes radiative heat loss into consideration. When the stretch rate is large, the maximum flame temperature decreases with an increase of the stretch rate, and stretch extinction limits induced by stretch, which are normally found, are identified. In addition, when the stretch rate is small, the maximum flame temperature decreases with the decrease of the stretch rate, and the other extinction limits due to the radiative heat loss are observed. We define these limits as radiative extinction limits. Figure 6 shows that numerical results of the relation between the stagnation-point velocity gradients and the fuel concentration limits. Also, the results of microgravity experiments are plotted in the same figure. The numerical results qualitatively agree with the results obtained in the microgravity experiments. AI-
E
1 , ¢t t E 1~~
,
,
, .....
I
. . . . . . . .
CH4,%
15 14 |
. . . . . . .
10 0 101 Stretch rate, sec -1
10 2
Fig. 5. Relation between maximum Rame temperature and stretch rate for various fuel concentrations.
though the numerical and experimental results do not quantitatively agree with each other, the existence of the turning point on the extinction curve due to the radiative heat loss is clearly verified in a way similar to that in the case of counterflow prcmixed flames [22, 23]. To find any difference between the two kinds of extinction, distributions of major species such as CH4, N2, 0 2 , CO2, H 2 0 , CO, and H 2 at a fuel concentration of 0.17 were examined. Their distributions were found to be qualita-
+ .~10 +
/ " • • Experiment
\"
•
•~.
$
"
'
' b.~5'
' ' '0'.2'
' ' 'o.~,5'
'
Fuel concentration
Fig. 6. Calculated results of the relation between stagnation-point velocity gradient and fuel concentration at extinction.
EXTINCTION OF DIFFUSION FLAMES
185
tively the same as those obtained by DixonLewis et al. [8], Smooke et al. [9], and Keyes and Smooke [10]. Although the flame zone near the radiative extinction (stretch rate = 0.48 s - ' ) is much thicker than that near the stretch extinction (stretch rate = 11.7 s ~), the profiles of species distributions of these flames are similar to each other as shown in Fig. 7a, b. Temperature distributions near the stretch extinction and the radiative extinction are shown in Fig. 8. Figures 7 and 8 show that the only difference between the flames near the stretch extinction and those near the radiative extinction is the thickness of the flame zone.
,00 o..
+,o-,
10"6"-
'
o2
/YA
I]
J
o z-:£1
I / .~~a,=11"7, Sec-I 0 1 2 X, cm
-2
0 2 x, cm Fig.8. Temperaturedistribution.
The thickness of the flame zone is widened up to 6 cm in the calculation. However, the distance between the burner exits was 2.5 cm in the experiment. This difference appears to be due to very low radiative extinction limits in the calculation curve. For instance, at CH4 concentration of 17%, experimental radiative extinction occurred at the velocity gradient of 3.0 s i, but radiative extinction by the calculation occurrcd at 0.48 s- t. Therefore, the calculated flame region thickness at the radiative extinction should be much larger than that of tb.c real flame. "It) estimate the effect of the radiative heat loss quantitatively, the ratio of the radiative heat loss to the heat release, called the radiation fraction [23], r I, written as
(a) -I./2
2 Ck = I
lo 0
CH4 02o= lo -2
. 10-4
.-e
10--2
a=0.48 sec-' i I -1 0
I I
i
1
x, ¢m
H2~\ i 2
~.\
3
(b) Fig. 7. Major species distributions: (a) stretch rate = 11.7 s-'; (b) stretch rate = 0.48 s-'.
is evaluated. In the equation, hk represents the specific enthalpy of the kth species, tok is the molar rate of production of the kth species per unit volume, and Mk is the molecular weight of the kth species. The radiation fraction near the stretch extinction is only about 1%, as shown in Fig. 9. Howe-,er, it increased with the decrease of the stretch rate and reached a level of more than 20% near the radiative extinction. It follows that the mechanism of the radiative extinction is mainly due to the increase of the radiation fraction, as further described in the following text. First, the flame zone thickens with the decrease of the stretch rate [31] and this thickening leads to a decrease of the heat release rate
186
K. M A R U T A E T AL. i
i
J
i
i
their assistance in carrying out the present experiment. This work was performed under the management o f the Japan Space Utilization Promotion Center as a part o f the research and decelopment project o f Advanced Furnaces and Boilers supported by the New Energy and Industrial Technology Development Organization.
0.2 .Co
GH41796
~
~cliative extinction
4= .~ 0.1
Z= fl:::
~
0
i
stretch extinction Ii
i 4 8 Stretch rate, see-1
REFERENCES
'
12
Fig. 9. Relation between radiation fraction and stretch
rate.
per unit volume of the flame zone. O n the other hand, the radiative heat loss, which is proportional to the volume fraction of the radiant species, is not significantly affected by the stretch rate. As a result, the ratio of the radiative heat loss to the heat release increases and flame temperature decreases with the decrease of the stretch rate. The increase of the flame zon~ thickness is the ii~dircct cause of the radiative extinction. CONCLUSIONS Extinction of counterflow diffusion flames of air and methane diluted with nitrogen has b e e n investigated by drop tests and numerical calculations. The relation between stagnation-point velocity gradient and fuel concentration at extinction exhibits a G-shaped curve, indicating that the flammable region is the widest at the stagnation point velocity gradient around 7 s - I. That is, two distinct kinds of extinction of counterfiow diffusion flames were observed experimentally: stretch extinctions and another type of extinction thought to be radiative extinction. The results of the computation taking radiative heat loss from the flame zone into consideration qualitatively agreed with those of the experiments, indicating that the experimental C-curve is caused by the radiative heat loss. The authors would like to express their thanks to Susumu Hasegawa and Atsutaka Honda for
I. Fendall,F. E., .l. Fhdd Mech., 21:281 (1965). 2. Linan,A., Acta Astronautica 1:1007(1974). 3. Krishnamurthy,L., and Williams, F. A., Acta Astronautica i:711 (1974). 4. Puri,1. K., Combust. Flame 65:137 (1986). 5. Paczko,G., Lefdal, P. M., and Peters, N., Twenty-First Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1986,p. 739. 6. Peters, N., and Kee, R. J., Combust. Flame 68:i7 (1987). 7. Seshadri, K., and Peters, N., Combust. Flame 73:23 (1988). 8. Dixon-Lewis,G., David, T., Gaskell, P. H., Fukutani, S., Jinno, H., Miller,J. A., Keeo R. J. Smooke, M. D., Peters, N., Effelsberg, E., Warnatz, J., and Behrendt, I% Iwentwth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1984, p. 1893. 9. Smooke,M. D., Purl, !. K,, and Seshadri, K., TwentyFirst Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1986,p. 1783. 10. Keyes, D. E., and Smooke, M. D., J. Comput. Phys. 73:267 (1987). 11. Tsuji,H., and Yamaoka, 1,, Twelfth Symposium ( International) on Combustion, The Combustion Institute, Pittsburgh, 1969,p. 997. 12. Tsuji, H., and Yamaoka, 1., Thirteenth Symposium (International) on Combustion, The Combustion institute, Pittsburgh, 1971,p. 723. 13. Seshadri, K., and Williams, F. A., J. Polym. Sci. Polym. Chem. Ed. 16:1755(1978). 14. Sohrab, S. H., Linan, A., and Williams, F. A., Cornbust. Sci. Technol. 27:143 (1982). 15. Sohrab, S. H., and Law, C. K., J. Heat Mass Trans. 27:291 (i984). 16. Egolfopoulos,F. N., Twenty-FifthSymposium (huernational) on Combustion, The Combustion Institute, Pittsburgh, 1994, p. 1375. 17. Liu, G. E., Ye, Z. Y., and Sohrab, S. H., Combust. Flame 64:193 (1986). 18. T'ien, J. S., Combust. Flame 65:31 (1986). 19. Chao, B. H., and Law, C. K., Combust. Flame 92:1 t 1993). 20. Platt, J. A. and T'ien, J. S., The Proceedings of the Chemical and PhysicalProcessesin Combustion, East-
EXTINCTION
21. 22.
23. 24. 25.
OF
DIFFUSION
FLAMES
ern Section of the Combustion Institute. 19911 Fall Technical Meeting. Daguse, T., Croonenbroek, T., Rolon, J. C., Darabiha, N., and Soufiani, A., Combust. Fhmw 1116:271 (19961. Maruta, K., Yoshida, M., Ju, Y., and Niioka, T., Twenty-Sixth Symposium (h~ternational) on Combustion, The Combustion Institute, Pittsburgh, 1996. Guo, H., Ju, Y., Maruta, K., Niioka. T., and [,iu, F., Combust. Flame, to appear. Seshadri, K., and Williams, F. A., J. th'at Mass Trans. 21:251 (19781. Chelliah, H. K., Law, C. K., Ueda, T., Smooke, M. D., and Williams, F. A., Twot~.'-Third ,~)'mposinm (b~ternational) on (~mlbustion, The Combustion Institute, Pittsburgh, 1'9911,p. 503.
187 26. Giovangigli. V., and Smt~)ke. M. D., Combust..~i. TechnoL 53:23 (1987). 27. Sm~ke, M. D., J. Compm. Phys. 48:72 (19821. 28. Giovangigli, V., and Smt~ke, M. D., AppL Numer. Math. 5:3115 (1989). 29. Kee, R. J., Grcar, J. F., Smooke, M. D., and Miller, J. A., Sandia National Lal~ratories Rel~rt No, SAN D85-82411, 1994. 30, ishizuka. S., and Tsuji. H., E(ghteenth S)'mposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1981, p. 695. 31. Williams, F. A., Combust. Theol.', 2nd ed., BenjaminCummings, Redwtmd City, CA, 1985, p. 417. Recciced q Octohcr 1996: acx'epted 21 FebnlaD, 1997
Extinction of counterflow diffusion flames of air and methane diluted with nitrogen is studied by drop tower experiments and numerical calculation using detailed chemistry and transport properties. Radiative heat loss from the flame zone is taken into consideration. Experimental results identified two kinds of extinction at the same fuel concentration, that is, in addition to the widely known stretch extinction, another type of extinction is observed when the stretch rate is sufficiently low. Consequently. plots of stretch rates versus fuel concentration limits exhibit a "C-shaped" extinction curve. Numerical calculation including radiative heat loss from the flame zone qualitatively agreed with the experimental results and indicated that the mechanism of counterflow diffusion flame extinction at low stretch rates was radiative heat loss. "c: 1908 by The Combustion Institute
INTRODUCTION Since the presentation of the "S-curve" of stagnation-point diffusion flames by Fendell [1], essential discussion has been based on the first D a m k 6 h l e r number [2]. Diffusion flames in a stagnation-point region have been frequently examined, and a n u m b e r of theoretical [3-7], numerical [8-10], and experimental [4, 9, 11-14] studies focused on understanding the fundamental characteristics of diffusion flames have been carried out. The structure, extinction limit, and effect of heat loss have been investigated in these studies. In general, however, the effect o f radiative heat loss has not been considered as being remarkable [13-16]. In fact, Sohrab et al. [14] analyzed the effect o f radiative heat loss from the flame zone on diffusion-flame extinction and concluded that the critical condition for extinction remains defined by a balance between the rate o f heat generation and that of conductive heat loss, although radiative heat loss contributes to reduced reaction rates through reduced temperature. Also, Sohrab and Law [15] studied the combined effects o f stretch and radiative heat loss, and concluded that stretch exerts a stronger influence than radiative heat loss on flame extinction. Consequently, the influence
*Corresponding author. *Present address: Ishikawajima-Harima Heavy Industries Co., Ltd., 3-5-1, Mukodai, Tanashi, Tokyo 188, Japan.
of radiative heat loss on extinction is often neglected when small laboratory-scale flames are studied. On the other hand, several studies in which special attention was given to the low stretch region showed that radiative heat loss plays a significant role. Liu et al. [17] measured temperature profiles of counterflow premixed flame and showed appreciable effects of radiation on small laboratory-scale flames. T'ien [18] and Chao and Law [19] investigated extinction of diffusion flames in the stagnation region of a condensed fuel and predicted the extinction due to the radiative heat loss from the solid surface would occur in the low stretch region. Subsequently, Platt and T'ien [2(I] and Daguse et al. [21] suggested that the existence of extinction is due to the radiative heat loss from the gas phase. Furthermore, two kinds of extinction of m e t h a n e / a i r premixed flames at low stretch rates were recently observed by microgravity experiments [22], and based on numerical calculation, extinction at lower stretch rates was estimated to be induced by the radiative heat loss [23]. Accordingly, the effect of radiative heat loss not only on large scale flames but also on small laboratory-scale flames must be examined. In the present paper, extinction of diffusion flames at low stretch rates is measured by the counterflow method under microgravity [22], and the mechanism of such extinction is examined by numerical calculation using detailed
COMBUSTION A N D FLAME 112:181-187 (19981
© 1998 by The Combustion Institute Published by Elsevier Science Inc.
1101(1-218{I/98/$19.(111 Pll SIIOII)-218tK97)0011)7-7
182
K. M A R U T A El" AL.
chemistry and transport properties, taking radiative heat loss from the flame zone into consideration. MICROGRAVITY EXPERIMENTS All tests were performed under conditions of a microgravity field at the Japan Microgravity Center (JAMIC) in Hokkaida, Japan. At this facility, an experimental module experiences microgravity ( 1 0 - 4 - 1 0 -5 G) of 10 s during a free fall in the 490-m drop shaft. Experimental technique is basically the same as that employed in our previous work [22] on the counterflow premixed flame. The apparatus has been modified for measurement of counterflow diffusion flame. The fuel concentration limits for the counterflow diffusion flame of air and methane diluted with nitrogen were measured at various stretch rates. To obtain fuel concentration limits of fuel flow at small stretch rates within I0 s, the counterflow burner system employed in this study was designed so that the fuel concentration of the fuel flow could be gradually decreased until extinction, maintaining a constant mean flow velocity at the burner exit. A schematic of the experimental apparatus is shown in Fig. 1. The apparatus consists of a counterflow burner, an air and fuel flow supply system, an igniter, a video system, and a sequencer. The counterflow burner is made of brazen circular pipe to minimize the dead volume of the burner itself as shown in Fig. 2. The depth of the sintered metal was fixed at 5 mm from the burner exit. The air and fuel flow supply system is composed of electric mass-flow controllers, a notebook computer, and digitalto-analog and analog-to-digital converters. Several seconds before the drop, conditions of air and fuel flow such as mean flow velocity
Fig. I. Schematicof the experimentalapparatus.
Diffusion Flame
Air =~_ ~
Sintered Metal
,J
__
CH,+N2
~ ._L.~ Sintered Metal
Fig. 2. Structure of the counterflowburner, where U,, and Ue are mean fP:gwvelocitiesof the oxidizerand fuel flowat the burner exit. respectively,and L is the spacing of the burner exits. at the burner exit and appropriate fuel concentration of the fuel flow are established. Then the mixture is ignited by a hot wire which is introduced from the side of the counterflow burner into the stagnation region. Just after the commencement of the drop, the hot wire is removed and the fuel concentration of the fuel flow is gradually decreased until extinction, with the fuel flow velocity at the burner exit being kept constant. The fuel concentration of the fuel flow at extinction is considered to be the fuel concentration limit at the stagnationpoint velocity gradient under consideration. The measurement of velocity distribution under microgravity is very difficult. For the sake of simplicity, the stagnation-point velocity gradient is calculated by the relation [Uo + Uf( pJpo)°'Sl/L [24], where U is the mean flow velocity at the burner exit, subscripts f and o are the conditions in the fuel and the oxidizer flow, respectively, p is the density, and L is the spacing between burner exits. Also for the purpose of simplicity, the difference between potential flow and the flow field generated by the plug-flow-type burner [25] is not taken into consideration, because the change of the fuel concentration limit can be neglected when either velocity at the burner exits or spacing between the burner exits is changed, with the velocity gradient kept constant. The stagnation-point velocity gradient is changed at every drop test, and the fuel concentrations limits are plotted as a function of the stagnation-point velocity gradient. To obtain the fuel concentration limits, responses of the air and fuel supply system and the delay time of the pipe arrangement are taken into consideration [22]. Also, the response of the whole system including the counterflow flame itself is quick enough compared to the change of the fuel composition as
EXTINCTION OF DIFFUSION FLAMES
i described in detail in our previous study [22]. The present experiments were conducted under quasisteady conditions that are indispensable under the conditions used. The temperature of the sintcred metal plate surface was measured by a thermocouple and heat loss from the counterflow flame to the burner was shown to be negligible in our previous report [22]. NUMERICAL CALCULATION To investigate the mechanism of the extinction, the experimental conditions were numerically simulated. The mathematical model and numerical code employed here are basically the same as those employed elsewhere [26]. The only difference is the energy equation, in which the radiation sink term, q,, was retained. An optically thin radiation model is employed to consider the radiative heat loss from the flame zone. Radiation from the gaseous species CO2 and H 20 is taken into consideration. The governing equations for mass, momentum, chemical species, and energy are solved in a cylindrical coordinate. The CHEMKIN code [27-29] and the C1 elementary reaction mechanism, which involves 58 reactions and 18 species, given by Kee et al. [29], were used. Details of the present code were given in our previous work [23]. The distance between burner exits was 8 cm in the calculation.
Fig. 3. Direct p h o t o g r a p h o f a eounterflow diffusion flame u n d e r mierogravity. Uo = Uf = 5 c m / s and L = 1.5 era. T h e n u m b e r s in the p h o t o g r a p h are time(s) from the c o m m e n c e m e n t of the drop.
Iocity gradient near the limit, as seen in the results in normal gravity, the results of microgravity experiments clearly show the existence of a turning point. When the stagnation-point velocity gradient is larger than about 7 s- ~, the fuel concentration limits increase with the increase of the stagnation-point velocity gradient. However, when the stagnation-point velocity gradient is smaller than 7 s- I, the tendency is reversed. In other words, the stagnation-point velocity gradients at extinction are doublevalued for a certain fuel concentration.
T
20
. . . . . .
A,,,,
. . . . . . .
:-.------and" Ishizuka RESULTS AND DISCUSSION
Microgravity Experiments A stable, near-flat counterflow diffusion flame was established in microgravity, as shown in Fig. 3. Figure 4 shows the relationship between the stagnation-point velocity gradient and the fuel concentration limits. For reference, the results of supplementary experiments conducted in normal gravity by using the same apparatus and experimental data obtained by Ishizuka and Tsuji [30] are also presented in the same figure. The counterflow burners were arranged vertically when the supplementary experiments were carried out in normal gravity. Although the fuel concentration limits seem to be independent of the stagnation-point ve-
:
.=_
Tsuji
5~ i , . ~ N o r r n =
"
gravity
0 "6 o
10
.~.~
o
o o._.......~ Microgravity
"
o
o
o ,'
b~5"'. Fuel
0
°
' o ' 2 " ' ' ' " .0.25 concentration
Fig. 4. Experimental results of the relation between stagnation-point velocity gradient and fuel concentration at extinction. Dotted line shows experimental data from Ref. 30.
184
K. M A R U T A E T AL. TABLE 1
The Conditions of MicrogravityExperiments
v,,v,
L
~u + % p,,j
(cm/s)
(cm)
L (s-t)
11.5
1.5
14.9
I I .(1
1.5
14.3
9.0 7.0 5.0 7.0 5.11 2.5 2.8
1.5 1.5 1.5 2.5 2.5 1.5 2.5
11.7 9. l 6.5 5.5 3.9 3.2 2.2
,, ......
~
v
~.15~ E
0.168 o. 164 11.155 0.149 0.150 0.159 (I.170 0.210 0.270
Furthermore, the fuel concentration limits obtained in microgravity are leaner than those obtained in normal gravity. We can suppose the difference between them is due to the effect of buoyancy, since a stable flame could not be established in normal gravity in the small velocity gradient region when the burners were arranged horizontally. The conditions of the microgravity experiments are listed in Table 1 for reference. Numerical Calculations
Figure 5 shows the relation between maximum flame temperature and stretch rate by the computation which takes radiative heat loss into consideration. When the stretch rate is large, the maximum flame temperature decreases with an increase of the stretch rate, and stretch extinction limits induced by stretch, which are normally found, are identified. In addition, when the stretch rate is small, the maximum flame temperature decreases with the decrease of the stretch rate, and the other extinction limits due to the radiative heat loss are observed. We define these limits as radiative extinction limits. Figure 6 shows that numerical results of the relation between the stagnation-point velocity gradients and the fuel concentration limits. Also, the results of microgravity experiments are plotted in the same figure. The numerical results qualitatively agree with the results obtained in the microgravity experiments. AI-
E
1 , ¢t t E 1~~
,
,
, .....
I
. . . . . . . .
CH4,%
15 14 |
. . . . . . .
10 0 101 Stretch rate, sec -1
10 2
Fig. 5. Relation between maximum Rame temperature and stretch rate for various fuel concentrations.
though the numerical and experimental results do not quantitatively agree with each other, the existence of the turning point on the extinction curve due to the radiative heat loss is clearly verified in a way similar to that in the case of counterflow prcmixed flames [22, 23]. To find any difference between the two kinds of extinction, distributions of major species such as CH4, N2, 0 2 , CO2, H 2 0 , CO, and H 2 at a fuel concentration of 0.17 were examined. Their distributions were found to be qualita-
+ .~10 +
/ " • • Experiment
\"
•
•~.
$
"
'
' b.~5'
' ' '0'.2'
' ' 'o.~,5'
'
Fuel concentration
Fig. 6. Calculated results of the relation between stagnation-point velocity gradient and fuel concentration at extinction.
EXTINCTION OF DIFFUSION FLAMES
185
tively the same as those obtained by DixonLewis et al. [8], Smooke et al. [9], and Keyes and Smooke [10]. Although the flame zone near the radiative extinction (stretch rate = 0.48 s - ' ) is much thicker than that near the stretch extinction (stretch rate = 11.7 s ~), the profiles of species distributions of these flames are similar to each other as shown in Fig. 7a, b. Temperature distributions near the stretch extinction and the radiative extinction are shown in Fig. 8. Figures 7 and 8 show that the only difference between the flames near the stretch extinction and those near the radiative extinction is the thickness of the flame zone.
,00 o..
+,o-,
10"6"-
'
o2
/YA
I]
J
o z-:£1
I / .~~a,=11"7, Sec-I 0 1 2 X, cm
-2
0 2 x, cm Fig.8. Temperaturedistribution.
The thickness of the flame zone is widened up to 6 cm in the calculation. However, the distance between the burner exits was 2.5 cm in the experiment. This difference appears to be due to very low radiative extinction limits in the calculation curve. For instance, at CH4 concentration of 17%, experimental radiative extinction occurred at the velocity gradient of 3.0 s i, but radiative extinction by the calculation occurrcd at 0.48 s- t. Therefore, the calculated flame region thickness at the radiative extinction should be much larger than that of tb.c real flame. "It) estimate the effect of the radiative heat loss quantitatively, the ratio of the radiative heat loss to the heat release, called the radiation fraction [23], r I, written as
(a) -I./2
2 Ck = I
lo 0
CH4 02o= lo -2
. 10-4
.-e
10--2
a=0.48 sec-' i I -1 0
I I
i
1
x, ¢m
H2~\ i 2
~.\
3
(b) Fig. 7. Major species distributions: (a) stretch rate = 11.7 s-'; (b) stretch rate = 0.48 s-'.
is evaluated. In the equation, hk represents the specific enthalpy of the kth species, tok is the molar rate of production of the kth species per unit volume, and Mk is the molecular weight of the kth species. The radiation fraction near the stretch extinction is only about 1%, as shown in Fig. 9. Howe-,er, it increased with the decrease of the stretch rate and reached a level of more than 20% near the radiative extinction. It follows that the mechanism of the radiative extinction is mainly due to the increase of the radiation fraction, as further described in the following text. First, the flame zone thickens with the decrease of the stretch rate [31] and this thickening leads to a decrease of the heat release rate
186
K. M A R U T A E T AL. i
i
J
i
i
their assistance in carrying out the present experiment. This work was performed under the management o f the Japan Space Utilization Promotion Center as a part o f the research and decelopment project o f Advanced Furnaces and Boilers supported by the New Energy and Industrial Technology Development Organization.
0.2 .Co
GH41796
~
~cliative extinction
4= .~ 0.1
Z= fl:::
~
0
i
stretch extinction Ii
i 4 8 Stretch rate, see-1
REFERENCES
'
12
Fig. 9. Relation between radiation fraction and stretch
rate.
per unit volume of the flame zone. O n the other hand, the radiative heat loss, which is proportional to the volume fraction of the radiant species, is not significantly affected by the stretch rate. As a result, the ratio of the radiative heat loss to the heat release increases and flame temperature decreases with the decrease of the stretch rate. The increase of the flame zon~ thickness is the ii~dircct cause of the radiative extinction. CONCLUSIONS Extinction of counterflow diffusion flames of air and methane diluted with nitrogen has b e e n investigated by drop tests and numerical calculations. The relation between stagnation-point velocity gradient and fuel concentration at extinction exhibits a G-shaped curve, indicating that the flammable region is the widest at the stagnation point velocity gradient around 7 s - I. That is, two distinct kinds of extinction of counterfiow diffusion flames were observed experimentally: stretch extinctions and another type of extinction thought to be radiative extinction. The results of the computation taking radiative heat loss from the flame zone into consideration qualitatively agreed with those of the experiments, indicating that the experimental C-curve is caused by the radiative heat loss. The authors would like to express their thanks to Susumu Hasegawa and Atsutaka Honda for
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EXTINCTION
21. 22.
23. 24. 25.
OF
DIFFUSION
FLAMES
ern Section of the Combustion Institute. 19911 Fall Technical Meeting. Daguse, T., Croonenbroek, T., Rolon, J. C., Darabiha, N., and Soufiani, A., Combust. Fhmw 1116:271 (19961. Maruta, K., Yoshida, M., Ju, Y., and Niioka, T., Twenty-Sixth Symposium (h~ternational) on Combustion, The Combustion Institute, Pittsburgh, 1996. Guo, H., Ju, Y., Maruta, K., Niioka. T., and [,iu, F., Combust. Flame, to appear. Seshadri, K., and Williams, F. A., J. th'at Mass Trans. 21:251 (19781. Chelliah, H. K., Law, C. K., Ueda, T., Smooke, M. D., and Williams, F. A., Twot~.'-Third ,~)'mposinm (b~ternational) on (~mlbustion, The Combustion Institute, Pittsburgh, 1'9911,p. 503.
187 26. Giovangigli. V., and Smt~)ke. M. D., Combust..~i. TechnoL 53:23 (1987). 27. Sm~ke, M. D., J. Compm. Phys. 48:72 (19821. 28. Giovangigli, V., and Smt~ke, M. D., AppL Numer. Math. 5:3115 (1989). 29. Kee, R. J., Grcar, J. F., Smooke, M. D., and Miller, J. A., Sandia National Lal~ratories Rel~rt No, SAN D85-82411, 1994. 30, ishizuka. S., and Tsuji. H., E(ghteenth S)'mposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1981, p. 695. 31. Williams, F. A., Combust. Theol.', 2nd ed., BenjaminCummings, Redwtmd City, CA, 1985, p. 417. Recciced q Octohcr 1996: acx'epted 21 FebnlaD, 1997