The structure of nonadiabatic, low-pressure methaneoxygen flames

COMBUSTION AND FLAME 31,325-327 (1978)

325

BRIEF COMMUNICATION The Structure of Nonadiabatic, Low-Pressure Methane-Oxygen Flames* L. DOUGLAS SMOOTt Chemical Engineering Department, Brigham Young University, Provo, Utah 84602

Comparison of measurements of species profiles and predictions for low-pressure, methane-oxygenargon flames have recently been reported [1]. Agreement was very good for two independent sets of measurements [2, 3]. However, computations were made assuming adiabatic flames, while measurements included significant heat losses. Thus, a question on the influence of such heat losses on flame structure was raised. It is also of interest to compare flame propagation model predictions with profile measurements for methane/ oxygen flames where kinetic reactions involving the suppressants are reasonably well known. These comparisons would provide the potential opportunity to evaluate suppressant mechanisms in the absence of complicating particulate suppressant effects. However, such comparisons would also raise the question concerning the influence of heat losses in these low pressure, experimental flames, which often use water-cooled, porous plug burners to achieve flame stabilization. This paper presents comparisons of predictions for adiabatic and nonadiabatic flames with a set of available laboratory measurements [4]. Effects of heat loss on flame structure are suggested by these computations. Results also show that lack of complete chemical equilibrium is partly responsible for observed temperatures that are well below adiabatic equilibrium values. *This work was supported by the Pittsburgh Mining and Safety Research Center, Bureau of Mines, Department of the Interior, Pittsburgh, Pennsylvania under contract H0122052, with Dr. Martin Hertzberg, contract officer. t Professor and Dean, College of Engineering Sciences and Technology. Copyright ©1978 by The Combustion Institute Published by Elsevier North-Holland, Inc.

The computerized code used in making these laminar flame model predictions was discussed by Smoot et al. [1]. The code is a generalized solution of the one-dimensional, unsteady equations of conservation of energy and gas species. Basic differential equations, auxiliary equations, and solution technique are presented in Reference [ I ] . The methane-oxygen reaction mechanism "A" of Table 2, Reference [1] was used in these computations. This reaction scheme involves 28 reactions among 13 chemical species. Predictions were made assuming a Lewis Number of unity. The predictions reported below were made for the CH4-Oz-Ar system studied by Biordi et al. [4]. Characteristics of the 10 cm diameter, watercooled, low-pressure burner and a description of the experimental technique are discussed in Reference [4]. Pressure and initial temperature were 0.032 atm and 290°K, respectively, while the inlet gas composition was 10.3% CH4, 21.6% 09. and 68.1% Ar. Two sets of model predictions were made. The first was for an adiabatic flame. The second was made assuming a uniform thermal volumetric heat loss rate of 0.067 cal/cm a sec. This value, which was assumed to be uniform throughout the flame, was selected to give a final predicted temperature close to the measured value. No attempt was made to estimate or predict the total heat loss rate, nor was any nonuniformity of the heat losses in the flame considered. Results of these two computations are compared with the measured values in Figs. 1-3. Agreement with measurements is generally very good in both cases, but somewhat better for the nonadiabatic case. There is not much difference in the spe-

0010-2180/78/0031-0325 $1.25

326

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Comparison of predicted and measured temperatures and species profiles for the uninhibited CH4-O2-Ar flame.

cies profiles for the adiabatic and nonadiabati cases, even though the final temperatures for the two cases differ by 216°K. Agreement is not a good for the CHa radical as for other specie. However, experimental uncertainty was greatest for this species. Biordi et al. also report approximate maximum molar fractions for CH20, CHO and H02 species. Predicted values agree reasonable well with these measurements, as shown in Table 1 Flame thickness and propagation velocity value are also quite well predicted as indicated in Table 1. Predicted agreement for these new data is quite comparable to that for two independent, low pressure flames previously investigated [ 1]. Heat losses in these lean, low pressure flames do not seem to have a major impact upon the measured species profiles. Thus, a detailed knowledge of heat losses is apparently not critical in interpreting chemical processes in these uninhibited flames. The adiabatic equilibrium temperature for the flame is 144°K higher than the adiabatic tempera ture predicted by the flame program that does not require chemical equilibrium at the flame outlet

BRIEF COMMUNICATION

327 TABLE 1

Comparison of Measurements of Biordi et al. [4] with Flame Propagation Model Predictions Predictions

Property/Units Peak Temp. (K) Final Temp. (K) Velocity, (cm/s) Thickness (cm)

Experimental 1760(1868) b 1760(1868) b 80 1.0

Adiabatic flame

Non-adiabatica flame

2042 2042 87.5 1.36

1831 1826 68.9 1.24

.159(-2) .757(-4) .218(-3)

.149(-2) .690(-4) .196(-3)

Peak Molar Fraction CH20 HCO HO2

.14(-2) .4(-4)c .8(-4) c

a Qs = 0.067 cal/cm3 sec. b First value from Fig. 2. Second value measured in absence of quartz sample probe. e Approximate values as given in Reference [4].

Comparison of predicted equilibrium and flame model species concentrations confirm that the outlet gas composition is not in chemical equilibrium even for the adiabatic case, but is rich in H, O, CO, and H2 and lean in H 2 0 and CO2, when compared to equilibrium. This result suggests that a significant part of the difference in the adiabatic equilibrium temperature and nonadiabatic outlet flame temperature is due to lack of complete reaction, and not entirely to heat losses. This observation is characteristic of low-pressure flames and disappears as the pressure is increased.

2. Fristrom, R. M., Grunfelder, C., and Favin, S.,Methane oxygen flame structure. I. Characteristic profiles in a low pressure, laminar, lean, premixed methane-oxygen flame, Journal Phys. Chem. 64, 1386 (1960). 3. Peeters, J., and Mahnen, G., Reaction mechanisms and rate constants of elementary steps in methane-oxygen flames, Fourteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, Pa (1973) p. 133. 4. Biordi, J. C., Lazzata, C. P., and Papp, J. F., Flame structure studies of CF3Br-inhibited methane flames. II. Kinetics and mechanisms, Fifteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, Pa (1975), p. 917.

REFERENCES 1. Smoot, L. Douglas, Hecker, William C., and Williams, Gerald A., Prediction of propagating methane-air flames, Combust. Flame 26, 323-342 (1976).

Received 27May 1977; revised 4 September 1977