Flame the Hy
agation Studies Using eaction STONE D. COOLEY AND ROBBIN C. ANDERSON
Department of Chemistry and t h e Defense Research Laboratory, T h e University of Texas, Austin, Tex.
ECENT developments in the theory of flames have eniphasized the fact that flame propagation may be a function of diffusion and reaction of active chemical particles as well as of heat flow, but there is still much uncertainty as t o how these concepts can be used in calculating or predicting flame properties. For example, Lewis and von Elbe (9) calculated burning velocities for ozone flames on the basis of chemical energy and thermal energy. Zel’dovich and Frank-Kamenetsky ( 1 4 ) in Russia developed general equations which take into account diffusion of reactants and the velocity of the chemical reaction along with heat flow. More recently Tanford and Pease ( 1 3 ) have derived an equation for burning velocity which involves the assumption that flames are propagated by diffusion of active particles such as hydrogen atoms. Manson (10) also considers active particles as essential but assumes that flame propagation results from projection of these particles as a result of a pressure gradient in the flame. Bartholom6 (12), on the other hand, contends that free atoms can be of significance only in L‘high-temperature” flames (above 2500” C.) and that “10.iv-temperature~’flames must involve essentially a thermal mechanism of propagation. These, and other equations, raise questions concerning the rates and mechanism of the reactions in a flame front and the resulting concentrations of atoms and free radicals-especially those such as hydrogen and hydroxyl which may diffuse rapidly However, ordinary combustion systems involve reaction mechanisms so complex that data are not available on the rates and mechanisms of individual steps or on actual concentrations of atoms or free radicals, It is of interest, therefore, to studv systems which may be sufficiently simple so that theoretical calculations of flame properties from independent data may perhaps be feasible. The reaction of hydrogen and bromine is one system which is promising for such purposes. It involves reagents quite different from those in ordinary combustion, and heat effects which are relatively low, the standard heat of formation for hydrogen bromide being 8.7 kg.-cal. per mole as contrasted to 57.8 kg.-cal. per mole for water and 94 kg.-cal. per mole for carbon dioxide. Furthermore, the kinetics and mechanism of the reaction under ordinary conditions are well known and less complicated than those for oxygen reactions, since they involve straight chains only, The system offers promising possibilities for parallel experimental measurements and theoretical calculations of flame properties and thus for making some tests of various theories of flames. Experiments in static gas mixtures and preliminary calculations (3) showed the range of conditions and rates to be expected for flame propagation. These also indicated that maximum flame temperatures should be such that the mechanism of reaction in flames should be similar to that for the ordinary gas phase combination. Measurements of burning velocities were then begun using a Bunsen-type burner. A recent report ( d ) described the apparatuEi and procedure in detail and gave burning velocity data for mixtures containing excess hydrogen. The present paper is a report of further measurements of burning velocities and further calculations on the mechanism of flame propagation.
Since this work was begun, Kokochashvili ( 8 ) has reported results of a study of the ignition of hydrogen-bromine mixtures and of flame propagation in tubes. Hirschfelder and coworkers are now making calculations on the application of a general theory of flames to the hydrogen-bromine system ( 5 ) . EXPERIMENTAL hlATERIALS, APP.4RATUS, AND P R O C E D U R E . These have aiready been described ( 2 ) . The apparatus consists of a borosilicate glass tube buiner mounted in a brass housing so that an inert gas (nitrogen 01’ carbon dioxide) may be used as atmosphere. The bromine vapor is supplied from a heater which vaporizes liquid bromine, and the hydrogen from a tank supply. The hydrogen line is equipped with an orifice-type flowmeter and the bromine line with a special rotameter. All connecting lines are jacketed and heated to avoid condensation of bromine. For the measurements at increased temperatures, a jacketed heating unit of borosilicate glass tubing was connected in the line just ahead of the burner proper, and the burner tube itself was equipped with a heating jacket extending up to the burner tip. To measure burning velocities, adjustments were made to give a mixture which could be readily ignited. This was ignited by a spark, the spark gap was removed, and the hydrogen and bromine flows were then readjusted to give a mixture of desired composition, The inlet gas temperature was determined with a thermocouple just above the burner tip. Measurements were made just before and after each burning velocity test. The composition of the mixtures and the gas flow velocities were calculated from the flowmeter readings, the temperature of gas entering the burner, and the atmospheric pressure. Photographs of the flames were taken. The burning velocity was then evaluated from the gas flow velocity and either the area of the flame or the angle of the flame cone taken a t a point corresponding approximately to the midpoint between the burner mall and the center of the burner port. The two methods give results in close agreement for mixtures containing less than 40 mole % ’ bromine. Above that range the angle method gives lower values and is probably less reliable than the area method because of irregular flame shapes. The data reported here have all been determined from the surface area of the flames. GENERALNATUREOF FLAMES. In color the flames range from colorless (for mixtures with minimum percentages of bromine) to yellow to reddish orange (in mixtures with excess bromine). This is in accord with the earlier experiments on static gas mixtures ( S ) , and with the work of Kitagawa (7), which showed that the distinctive color of flames in ordinary mixtures close to stoichiometric proportions was a result of excitation of bromine. The color and shape of the hydrogen-bromine flames was described earlier ( 2 ) . With minimum percentages of bromine, the flames are commonly simple, flat disks. As the percentage of bromine is increased, very low “cones,” almost hemispherical, are formed, gradually shifting to ordinary cones with slightly rounded tips. At about 45 mole % ’ bromine, an extended tip
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INDUSTRIAL AND ENGINEERING CHEMISTRY
June 1952
begins to appear on the cone. With mixtures containing excess bromine, this tip may be extremely elongated. The flames in mixtures in the lower percentage range burn very smoothly and steadily, but above 45 mole %, where the queerly elongated cones appear, the flames are likely to flicker markedly and may easily blow off. The observations raise a question as to the range of bromine content for which a continuous flame front can occur, particularly with excess bromine present. Limited ignition or flickering has been observed in
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been observed in different series with various sizes of burner tube, with the burner vertical, inclined, and horizontal, and with nitrogen or carbon dioxide as atmosphere. The measured burning velocities under these varied conditions are changed slightly, but the trend toward a maximum is found in the same region for all. EFFECTOF INITIAL TEMPERATURE.Results of experiments in which the temperature.of the gas entering the burner tube was varied are shown in Figures 1 and 2. The range of temperature used was determined by the problems of condensation of bromine which might occur a t too low temperatures and that of formation of hydrogen bromide in the incoming gas which might occur appreciably a t higher temperatures. Figure 1 shows a comparison of burning velocities a t various compositions for the highest and lowest temperature ranges used. In general, the effects of changes in composition are much the same a t the two temperatures but stable flames could be obtained with somewhat lower mole yo bromine a t increased temperatures. There is also some indication that the maximum burning velocity may also be shifted somewhat toward mixtures of lower mole per cent bromine, but the refiults are not conclusive. Figure 2 shows the variation in burning velocity with inlet gas temperature for a 0 bromine. Measurements on mixture containing 39 mole 7 other mixtures (36, 43, and 44.5'35) gave similar results. THEORETICAL CALCULATIONS
34
Figure 1.
38 42 Bromine, Mole
46
Effect of Composition
mixtures ranging apparently from about 30 up to above 90 mole yo bromine. However, if a close check is made for a continuous flame front across the burner port, the lower limit is found t o be 34 to 35 mole %. The maximum percentage at which it has been possible to observe such a flame front is 55 mole %, but this involves very unsteady flames. In tests on static gas mixtures (S), the lower limit was also found to be 34 mole %. An upper limit around 85% was found, but with large excess of bromine present, the flames extended across only a portion of the tube. Care must be taken, therefore, in the interpretation of burning velocity data for compositions close to 50% bromine and above, because of uncertainty as to the existence of a true, continuous flame front. The changes in shape and height of flames as the percentage of bromine is increased do show clearly that the burning velocity decreases markedly above 45 mole yobromine. Kokochashvili has reported spark ignition for mixtures ranging from 20 to above 95 mole yo. In upward propagation, flames were observed over the entire range, although varying widely in shape; in downward propagation, flames were observed to stop part way, even with as low as 65 mole % bromine. Similar difficulties in stabilizing flames have been reported for other systems of low burning velocity, e.@;.,in the decomposition of ethylene oxide ( 4 ) . No marked changes in appearance or behavior of the flames were observed a t increased temperatures. BURNING VELOCITIES.Some results for mixtures with excess hydrogen a t an average initial temperature of 50" C. and atmospheric pressure (740 to 750 mm.) are shown in the lower curve of Figure 1. The points represent a typical series of measurements, each being t@eaverage of several individual observations. I t is apparent that in the determination of absolute values of burning velocities, measurements in bromine have not reached the precision of those in a few of the older combustion reactions in air or oxygen. However, the trend of variation with composi; tion shown here, with a maximum a little above 4070, has now
While experimental studies of the hydrogen-bromine flames are in progress, burning velocities for various conditions are also being calculated so that calculated and observed values may be compared.
200
u' 160
g@ 120 80
30
40
50
60
70
80
Burning Velocity, Cm./Seo.
Figure 2.
Effect of Initial Temperature
Such calculations involve certain assumptions with regard t o the mechanisms of reaction and, in particular, the existence of steady state conditions. There is always some uncertainty in the rate constants, but in this case sufficient data are available and the system is simple enough so that calculations and analyses can be made for a particular mechanism independent of actual data on flames. It would be premature t o attempt t o say that any one mechanism actually represents the real flames or that the calculated burning velocities are directly applicable. However, the calculations do represent reasonable approximations for certain types of systems which are practicable and the results can give insight into the factors which are of importance in determining flame properties. Various equations for calculating burning velocities are being tested. The numerous types of calculations and the voluminous numerical data involved are summarized in detail elsewhere ( I ), However, certain results of initial steps in calculation can be summarized a t this time.
INDUSTRIAL AND ENGINEERING CHEMISTRY
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I n considering possible mechanisms of flame propagation, three possibilities are of primary importance, either separately or in combination: 1. A thermal mechanism involving heat transfer 2. A mechanism involving diffusion of bromine atoms 3. A mechanism involving diffusion of hydrogen atoms The thermal mechanism in its simplest form depends upon the flame temperature and the heat conductance of the gases. The maximum theoretical flame temperatures have been evaluated bv assuming reaction under adiabatic conditions ( 3 ) . 'Table I shows some typical values for initial gas temperatures of 50" C. Since the heat conductivities do not vary much with composition, the simple thermal concept indicates maximum burning velocities for mixtures close t o stoichiometric proportions. However. actual observations show combustion is slow and flames unstablc in this region.
Vol. 44, No. 6
given somewhat smaller values for hydrogen atom concentrations, but more complete data such as used in Figure 3 show that higher values may be attained in earlier stages of reactionsometimes a t 90%, sometimes a t 95% conversion. Figure 4 shows a comparison of the maximum values for hydrogen and bromine atom concentrations in flames of gas mixtures of varying composition. The data indicate that since diffusion is dependent on concentration, bromine atoms should have maximum effect h
TABLE I. FLAXE TEJfPER.4TURES Bromine, Mole 70 20
30 40
48 50 60
70
Coeff. of H e a t Cond. X 10-6 8.3 6.3 5,G 6 0 5.9
4.9 3.8
Theoletical Flame T e m p . , I
