The Influence of Chemical Inhibitor Addition on Reverse-Jet Flame Stabilization

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1 68 -GT-39 The Society shall not be responsible for statements or opinions advanced in papers or in discussion at meetings of the Society or of its Divisions or Sections, or printed in its publications. $1.50 PER COPY 75C TO ASME MEMBERS Discussion is printed only if the paper is published in an ASME journal or Proceedings. Released for general publication upon presentation Copyright 1968 by ASME The Influence of Chemical Inhibitor Addition on Reverse-Jet Flame Stabilization L. BELLAMY' Monsanto Company, St. Louis, Mo. C. H. BARRON Associate Professor, Chemical Engineering Department, Tulane University, New Orleans, La. J. R. O'LOUGHLIN Associate Professor, Mechanical Engineering Department, Tulane University, New Orleans, La. Assoc. Mem. ASME Flame stability limits produced by a reverse-jet flameholder are experimentally studied with a chemical inhibitor added to the gas stream issuing from the reverse-jet. The result of this addition is a reduction of flame stability limits. Such a reduction indicates the importance of the chemical rate processes in the flameholding phenomenon. 'Formerly, Research Assistant, Chemical Engineering Department, Tulane University, New Orleans, La. Contributed by the Gas Turbine Division of The American Society of Mechanical Engineers for presentation at the Gas Turbine Conference & Products Show, Washington, D. C., March 17-21, Manuscript received at ASME Headquarters, January 15, Copies will be available until January 1, THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS, UNITED ENGINEERING CENTER, 345 EAST 47th STREET, NEW YORK, N.Y

2 The Influence of Chemical Inhibitor Addition on Reverse-Jet Flame Stabilization L. BELLAMY C. H. BARRON J. R. O'LOUGHLIN Many investigators have studied the stabilization of a flame in a high velocity, combustible stream by means of a reverse-jet. Much of this work has been reviewed by Penner and Williams (1) 2 and Cambel (2). Considering the large shift in the blowoff curve (plot of primary stream velocity versus normalized fuel-air ratio at blowoff) for small changes in jet composition, Schaffer and Cambel (3) postulated that the flameholding was determined by a critical zone. The composition of such a zone could be significantly changed by the jet composition even though the jet flow is small since the jet feeds directly into this zone. Noreen and Martin (4) have analyzed the blowoff performance of a reverse-jet flameholder by treating this critical zone as a homogeneous, chemical reactor. This type of analysis assumes that the chemical rate processes occurring in this zone are the most important, from the standpoint of blowoff, of the complex simultaneous processes taking place in the system. The purpose of this study was to examine this assumption by observing the effect of the addition of a chemical inhibitor in the reverse-jet flow. INHIBITION OF HYDROCARBON-AIR FLAMES BY HALOGEN COMPOUNDS Halogen compounds have been used as inhibitors in many investigations of diffusion and premixed flames. The majority of these investigations report only the effect of inhibitors on variables such as burning velocity and flammability limits. The relationship between these quantities and the chemical rate processes is not straightforward. The general influence of the inhibitor is best understood in the framework of a proposed reaction sequence for simple hydrocarbon combustion. The proposed sequence (.5_) consists of two parts. In the first part, the hydrocarbon is converted to carbon monoxide by the following overall reaction. 2 Underlined numbers in parentheses designate References at the end of the paper. Hydrocarbon + H, OH, 0 E, OH, 0 CO + 022, H2O, H2 022 HO, 2 H2 The second part involves the proposed rate limiting step, (1) CO + OH --tsco 2 + H (2 ) The remainder of the reaction sequence consists of the equilibrium reactions of hydrogen and oxygen atoms and the molecules and radicals which result from their combination. A termination reaction is also involved which may be postulated as a general three-body collision process of atomic hydrogen and oxygen and hydroxyl radicals. The contribution of the inhibitor to the reaction system is primarily of the following form (6). + X (3) where X = Cl or Br. Thus the effect of the inhibitor is to reduce the concentration of the atomic species, 0, H, and OH, which participate in both the propagation steps and the termination step of the reaction system. The proposed reduction in the concentration of hydroxyl radicals was verified experimentally by Ibiricu and Gaydon (1) in a spectroscopic study of the effect of chlorine on ethylene-air flames. In an earlier study of the oxidation of carbon monoxide, Palmer and Seery (8) reported a definite inhibition effect on reaction (2) due to the addition of chlorine. Considering the discussion earlier, it is fair to say that there is sufficient evidence to conclude that halogen compounds have a definite inhibition effect on lean hydrocarbon-air flames. EXPERIMENTAL PROCEDURE H OH 0 + HX--> The experiments were conducted in a. 2-in. pipe combustion tunnel equipped with a water H 2 H2 O 2 OH 1

3 FLAME FRONT PROPANE-AIR MIXTURE REVERSE JET STREAM AFTERBODY MANIFOLD 2" PIPE \ Fig.l Schematic of test section cooled reverse-jet. The test section is shown schematically in Fig.l. An aerodynamic afterbody was attached to the 1/2 in. od jet manifold to prevent this manifold from acting as a bluff body flameholder. The manifold contained a cooling water passage which surrounded the reverse-jet flow, thereby preventing heating of the latter. The reverse-jet pressure was 20 psig for all runs. The inside diameter of the reverse-jet tip was in. This tip threaded into the manifold. The test section was attached, on the upstream side, to a 3-ft straight run of pipe. The downstream end of the test section exhausted to the atmosphere. The fuel used was propane. The air flow in the primary stream was measured with an orifice plate built to ASME standards. Other flow rates were measured with calibrated rotameters. Temperatures were taken with copperconstantan thermocouples and a null reading potentiometer. Pertinent pressures were read with calibrated Bourdon tube pressure gages and inclined manometers. The jet stream temperature, pressure, humidity, and mass flow rate as well as the primary stream temperature and humidity were essentially constant during any given pair of experiments (i.e., experiment with inhibitor addition and experiment without inhibitor addition at a fixed primary stream velocity). The experimental procedure was as follows. The primary stream mixture was ignited at a low velocity by use of an automotive spark plug supplied with 12,000 v. In the inhibitor experiments, the chlorine was added to the reverse-jet stream immediately after the mixture was ignited. The primary stream mass flow rate of air and propane were increased simultaneously until the desired velocity was obtained. The primary fuel flow rate was then increased or decreased very slowly until the flame blew off the flameholder. All pertinent data were recorded at blowoff. To test the reproducibility of the data, each experiment was conducted at least twice. In order to minimize the effect of any extraneous variables on the results, L PRIMARY STREAM EQUIVALENCE RATIO Fig.2 Effect of inhibitor addition to reverse-jet stream on stability limits. Data are for various weight percent chlorine in the jet stream as indicated. a blowoff point without chlorine in the jet was followed by the identical experiment with chlorine in the jet. RESULTS AND DISCUSSION The stability curves with and without inhibitor addition are shown in Fig.2. The weight percent chlorine for the points marked 4 percent actually varied between 3.80 and 4.10 percent. This is 1.61 to 1.73 mole percent. These percentages are in the same range as those used by Palmer and Seery in their study of the inhibition of carbon monoxide flames by chlorine and those used by Garner et al (2E) in their study of the inhibition of propane-air flames by carbon tetrachloride. The critical zone composition was calculated in the manner suggested by Schaffer (10). This cal- 2

4 culation is simply a mass balance of fuel and air flowing into the critical zone. Such a balance allows the equivalence ratio 3 in the critical zone to be calculated. The inhibitor content of this zone varied from 0.58 to 1.10 weight percent which is the equivalent of 0.24 to 0.47 mole percent. The inhibitor content of the critical zone was much less than that of the jet stream since the primary stream contribution to the critical zone is at least twice that of the jet. The inhibitor content of the critical zone increased as the primary stream velocity increased since the relative contribution of the jet stream to the critical zone increases as the primary stream velocity increases. This effect of the primary stream velocity on the variation of the relative contributions f the primary stream and the jet stream to critical zone has also been investigated by Schaffer (10). It is important to note that the reduction in stability limits increased as the inhibitor content of the critical zone increased. This result is in agreement with the concept that the critical zone can be described as an adiabatic, homogeneous chemical reactor. Thus any increase in inhibitor content of the critical zone would tend to reduce the overall reaction rate which would result in a reduction in stability limits. A few additional experiments were conducted with 2.0 and 8.0 weight percent inhibitor in the jet stream. These results indicate that an increase in inhibitor content of the jet stream at a fixed primary stream velocity generally decrease the stability limits. Note that the critical zone inhibitor content increases directly as the jet stream content increases if the primary stream velocity is constant. The increase in inhibitor content of the critical zone was accompanied by a decrease in stability limits which is in agreement with the previous results, Thus both the results with variable primary stream velocity and those at constant velocity show that the stability limits decrease as the inhibitor content of the critical zone increases. Calculated values of the equivalence ratio in the critical zone are greater than 1.0 for primary stream equivalence ratios greater than Since the effect of inhibitors on rich hydrocarbon flames (i.e., equivalence ratios greater than 1.0) is not well understood at the present ime, discussion is limited to primary stream equivalence ratios less than There was a slight decrease in the mass flow rate of air through the jet due to the addition of chlorine. The variation was less than 2 percent for any given pair of experiments. This is within the combined accuracy of the rotameters used to measure the chlorine and air flow rates. The weight percent chlorine in the jet stream varied from 3.80 to 4.10 for the nominal 4 percent data. The resulting variation in the momentum transport rate 4 at the jet exit was less than 4 percent. Thus it was assumed that the slight variation in jet air mass flow rate did not contribute to the reduction in stability limits with chlorine addition. CONCLUSIONS The proposal that the observed reduction in stability limits was due to the chemical inhibition effect of chlorine in some type of critical zone is based on the following points. 1 The variation in the reduction of the stability limits agrees qualitatively with that predicted by consideration of the change in chlorine concentration in the critical zone as a result of the variation in the primary stream contribution. 2 When chlorine was added to the jet stream, neither the jet air flow rate nor the jet momentum transport rate varied to such an extent as to explain the observed reduction in stability limits. s ACKNOWLEDGMENTS This research was supported by the U.S.Army Research Office - Durham. One of the authors (L. Bellamy) held a National Defense Education Act Fellowship, and this assistance is also gratefully acknowledged. The assistance of the Tulane Computer Laboratory is appreciated. REFERENCES 1 S. S. Penner and F. Williams, "Recent Studies on Flame Stabilization of Premixed Turbulent Gases," Applied Mechanics Reviews, vol. 10, 1957, pp A. B. Cambel, "A Review of Flame Stabilization by Means of Gaseous Jets," Combustion and Propulsion - Third AGARD Colloquium, Pergamon Press, London, 1958, pp A. Schaffer and A. B. Cambel, "The Effect of an Opposing Jet on Flame Stability," Jet Propulsion, vol. 25, 1955, pp A. E. Noreen and W. T. Martin, "Opposed 3 4 The equivalence ratio is a normalized fuel- The momentum transport rate is defined as the air ratio. It is defined as the actual fuel-air product of the mass flow rate and the corresponding ratio divided by the stoichiometric fuel-air ratio, velocity. 3

5 Jet Flameholding," American Rocket Society Paper R. M. Fristrom and A. A. Westenberg, Flame Structure, McGraw-Hill, New York, N. Y., 1965, Chapter XIV. 6 H. Wise and W. A. Rosser, "Homogeneous and Heterogeneous Reactions of Flame Intermediates," Ninth Symposium (International) on Combustion, Academic Press, New York, N. Y., 1963, pp M. M. Ibiricu and A. G. Gaydon, "Spectroscopic Studies of the Effect of Inhibitors on Counterflow Diffusion Flames," Combustion and Flame, vol. 8, 1964, pp H. B. Palmer and D. J. Seery, "Chlorine Inhibition of Carbon Monoxide Flames," Combustion and Flame, vol. 4, 1960, pp F. H. Garner, R. Long, A. J. Graham, and A. Badakhshan, "The Effect of Certain Halogenated Methanes on Pre-mixed and Diffusion Flames," Sixth Symposium (International) on Combustion, Reinhold, New York, N. Y., 1957, pp A. Schaffer, "Phenomenological Analysis of the Opposing Jet Flameholder," PhD Dissertation in Mechanical Engineering, Northwestern University,

The Influence of Tailpipe Length on Flame Stability

The Influence of Tailpipe Length on Flame Stability 67-GT-4 The Society shall not be responsible for statements or opinions advanced in papers or in discussion at meetings of the Society or of its 64-GT-4 Divisions or Sections, or printed in its publications.