On the inhibition of low pressure quenched flames by CF3Br

COMBUSTION A N D F L A M E 24, 401- 403 (1975)

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BRIEF COMMUNICATIONS On The Inhibition of Low Pressure Quenched Flames By CF3 Br JOAN C. BIORDI, CHARLES P. LAZZARA, and JOHN F. PAPP Fires and Explosions Group, Pittsburgh Mining and Safety Research Center, Bureau o f Mines, Pittsburgh, Pennslyvania

We have been studying the microstructure of low pressure methane flames to which small amounts of CF3 Br have been added [1 ]. CFaBr is a known flame inhibiting agent [2], and the purpose of these studies is to learn more about the chemical mechanisms occurring in inhibited flames. Low pressures are commonly used in these types of studies in order to gain sufficient spatial resolution to permit quantitative evaluation of rate data [3]. The usually accepted criterion for inhibition is a reduction in burning velocity upon addition of the agent in question, and relative inhibitor effectiveness is judged on the basis of relative reduction of burning velocity by equivalent amounts of inhibitor. The fact that the effectiveness of a given inhibitor may be reduced at low pressures is well established [4, 5, 6]. The extent of this reduction, at a given low pressure, varies with the agent, and the pressure dependence of this reduction, at a given concentration of an agent, also varies with the agent [4, 6]. This situation has led to expressions of caution in the interpretation of results of inhibition studies on low pressure flames with respect to the "real-life" application of 1 atm systems [7]. There has also appeared in the recent literature, a description of a measure of inhibition for quenched (i.e., nonadiabatic) flames [8]. It is a rise in flame temperature upon inhibition at constant

burning velocity, in contrast to the usual reduction in burning velocity at constant (adiabatic) temperature. The authors demonstrated the reality of the former effect, but did not relate it, quantitatively, to the latter. The purpose of this communication is to show that similar behavior may be expected with quenched flames at low pressure and to present data that relate the two criteria for inhibition. It is then demonstrated that the effectiveness of CF3Br is indeed reduced at 1/20th atmosphere, but not to such an extent that the flame may be viewed as noninhibited. Kaskan [9] has shown, for a variety of fuels burning with air on a cooled porous metal burner at atmospheric pressure, that the logarithm of the mass burning velocity is a linear function of the reciprocal of the maximum flame temperature. He also found that extrapolation of this line to the adiabatic flame temperature gave reasonable values for adiabatic burning velocities. In order to determine whether this behavior is the same at low pressures, we measured the final flame temperature of slightly lean (~ = .95) CH4/02/Ar flames stabilized on a porous plug flat flame burner at 32 Torr as a function of the mass burning velocity. The apparatus and procedures were essentially the same as those described previously [ 1 ]. Silica coated Pt-Pt- 10%-Rh thermocouples were used to measure the final flame temperature, Copyright © 1975 by the Combustion Institute Published by American Elsevier Publishing Company, Inc.

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and these temperatures were corrected for radiation loss. Thermocouples constructed from 10 mil as well as the usual 1 mil thermocouple wire could be used since at this low pressure the temperature prorde has a rather broad m a x i m u m and so the spatial resolution requirement for determining T(max) is much less severe than for measuring the reaction zone temperature profile. The results o f these measurements are shown in Fig. 1 where V2s is the linear flow velocity o f the unburned gas measured at 25 °C and 0.042 atm. The relationship described b y Kaskan not only applies, but the temperature dependence is similar with E (as defined b y Eq. (6) in [9] ) being 54 kcal mole "1 here. The dashed lines in this figure are drawn after the description in [8], the vertical dashed line being the locus o f inhibited adiabatic flames and the horizontal dashed line the locus of inhibited quenched flames. The other points in this figure will be discussed later.

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Two other types o f measurements (at P = 3 2 Tort) have been made: (1) Determination o f the temperature rise at constant burning velocity as a function o f the initial CF3Br concentration, and (2) determination o f the velocity dependence o f the final flame temperature for a ~b = .95, CHa/O2/ Ar flame containing 1% CF3Br initially. The former is the suggested measure o f inhibition for quenched flames [8], and Fig. 2 shows the results for a flame burning at 80 cm sec "1 . The data shown in this figure include measurements made on this flame over a period o f about 18 months. On the basis of this criterion for inhibition there can be little doubt that the flame is in. hibited by CFaBr at 0.042 atm. The latter measurements provide the relationship between the temperature rise criterion o f inhibitor and the burning velocity reduction criterion. The open circles in Fig. 1 are the results, and the lines describing the inhibited and clean quenched flames are nearly parallel. Some recent results by Hayes and Kaskan on CHaBr inhibited CH4-air flames suggest that at atmospheric pressures ana-

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Fig. 1. Mass burning velocity (expressed as a linear velocity at 25 °C, .042 arm) as a function of the maximum flame temperature for a methane/oxygen/argon flame (~b= .95) burning at 32 Torr •, the clean quenched flame; O, the quenched flame containing 1% CF3Br initially; [i~, adiabatic (2379 °K) burning velocity at 32 Torr calculated from the expression given in [ 13 ].

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Fig. 2. Temperature rise on inhibition as a function of initial CF3Br concentration for a 32 Torr flame burning at constant velocity, e, A, [] are points taken at different times for V2s = 80 cm sec-1 : O, shows the average temperature difference between inhibited and clean flames containing 1% CF3Br and having burning velocities from 32-96 cm sec-1 (see Fig. 1).

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logous behavior may be expected [10]. Extrapolating both the lines (Fig. 1) to the calculated adiabatic flame temperature (2379 °K) suggests that at 32 Torr, 1% CF3Br would reduce the burning velocity of the adiabatic flame by 33%. Furthermore, if we assume that the log V2s vs ( l / T ) curves are nearly parallel for other CFa Br concentrations, we can construct a curve showing the expected reduction of the adiabatic burning velocity as a function o f CF3 Br concentration at this pressure. Figure 3 shows this curve together with that deduced from flame speed measurements made at 1 atm for a CH4/air flame o f similar stoichiometry to which up to 0.3% CF3Br was added [2]. A comparison of these curves with Figs. 1 and 2 of [6] suggests that the relative influence of pressure on CF3Br effectiveness is similar to that for other monohalogenated (i.e., excluding F) compounds, even at pressures four times lower than reported there.

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the agency of halogenated species first arose from consideration of the results of microstructure studies on inhibited flames burning at reduced pressure [11] and more recent studies have demonstrated the feasibility of those ideas for atmospheric pressure flames [12]. Our present degree of understanding of the chemistry of inhibited flames precludes generalities about mechanisms, except possibly the one that states that the inhibition observed is a chemical rather than a physical effect. The hope, of course, is that eventually such generalizations will be possible, but in the meantime considerably more data o f a microscopic nature is required. Microstructure studies o f flames at any pressure are a rich source of such information, and the data presented here show that the measure of inhibition suggested by Iya et al. [8] may be used to provide a link between burning velocity reduction criteria and the results o f low pressure studies of inhibited flames. References 1. Biordi, J. C., Lazzara, C. P., and Papp, J. F., Fourteenth Symposium (International) on Combustion,

The Combustion Institute, Pittsburgh, 1973, p. 367. 2. Rosser, W. A., Wise, H., and Miller, J.,Seventh Symposium (International) on Combustion, Butterworth, London, 1959, p. 175. 3. Wilson, W. E., O'Donovan, J. T., and Fristrom, R. F.,

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The Combustion Institute, Pittsburgh, 1965, p. 929.

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4. Bonne, F., Jost, W., and Wagner, H. Gg., Fire Res. Abstr. a n d R e v 4, 6 (1962). 5. Miller, Wm. J., Combust. Flame 13, 210 (1969).

6. Homann, K. H., and Poss, R., Combust. Flame 18,

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Fig. 3. The relative reduction of burning velocity of methane flames as a function of initial CFaBr concentration at low and atmospheric pressure. The 1 atm curve is from data given in [2]. The fact of a pressure effect of inhibition suggests the possible significance of termolecular reactions in addition to bimolecular radical scavenging reactions [3]. The idea that more efficient radical recombination reactions may occur through

300 (1972). 7. Hastie, J. W., J. Res. Nat. Bur. Standards 77A, 733 (1973). 8. iya, K. S., Wollowitz, S., and Kaskan, W. E., Combust. Flame 22,415 (1974). 9. Kaskan, W. E., Sixth Symposium (International) on Combustion, Reinhold, N. Y., 1957, p. 134. 10. Hayes, K., and Kaskan, W. E., Combust. Flame 24, 405-407 (1975). 11. Fenimore, C. P., and Jones, G. W., Combust. Flame 7, 323 (1963).

12. Day, M. J., Stamp, D. V., Thompson, K., and DixonLewis, G., Thirteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh,

1971,p. 705. 13. Andrews, G. E., and Bradley, D., Combust. Flame 18, 133 (1972). Received 7November 1974