COMBUSTION BERNARD LEWIS and GUENTHER von ELBE U. S. B U R E A U OF M I N E S , PITTSBURGH 13, P A .
T h e knowledge required for scientific solution of combustion problems is not yet available in sufficient measure] hence much effort is directed to fundamental research. The scope of such research has increased continually in recent years. At present it comprises kinetics of chemical reactions of fuels and explosives; ignition, propagation, and stabilization of cornbustion waves in premixed gases and dust suspensions under conditions of laminar and turbulent Flow, mixing and combustion of fuel jets; decomposition, ignition, and burning of propellants and explosives; detonation waves and transition from burning to detonation; combustion of coal and carbon in individual particles and in bulk; and thermodynamics of combustion gases. Notwithstanding this, practical problems are still largely solved b y empirical testing methods. N e w problems arise continually from changing demands on piston engines, gas turbines, jet and rocket engines, high explosives and propellants, gas burners and furnaces, smoke abatement, and control of explosion and fire hazards.
HIS review of combustion research covers the period from late 1948 to late 1950, as, owing to other commitments, the authors were unable to write a review article for 1949. During this period the trend toward expansion of fundamental research has continued. This reflects the generally felt need for sound scientific concepts in the approach t o combustion problems. With this concentration on the basic aspects one may expect t h a t in time fundamental science can be applied successfully even to those complex practical problems that now permit only empirical investigation. As the volume of publications has grown, the need for contact and direct communication among investigators has become pressing. This is accomplished most satisfactorily by symposia on specific subjects. Outstanding among such gatherings has been the International Conference on Kinetics and Mechanism of Flame Reactions and of Gas-Phase Combustion held in Paris April 20 to May I , 1948, under the auspices of the Centre Nat.ional de la Recherche Scientifique (195) and the Third Symposium on Combustion and Flame and Explosion Phcnomena organized by a newly established Standing Committee on Combustion Symposia and held a t the University of Wisconsin September 7 to 11, 1948 (714).
T
FUNDAMENTALS OF COMBUSTION PROCESSES KINETICS
OF G A S - P H A S E R E A C T I O N S
Although the mechanism of the reaction between hydrogen and oxygen is rather well understood, some work remains to be done to corroborate the occurrence of various elementary reactions comprising the scheme and to determine quantitatively the rate coefficients of these reactions. Lewis and von Elbe (478) have discussed the initiating reaction. They conclude t h a t neither the thermal dissociation of hydrogen, oxygen, or water nor the reaction between hydrogen and oxygen can be responsible for chain initiation, and that the weight of the evidence points to chain initiation via hydrogen peroxide molecules. The latter, being both formed and destroyed principally by chain-carrier reactions, attains a steady-state concentration after a brief induction period and serves as a source of chain carriers by thermal dissociation. An analogous situation occurs in the methane-oxygen reaction where the intermediate formaldehyde attains a steadystate concentration and gives rise t o chain carriers by occasional reaction with oxygen molecules. Experimentally, the role of hydrogen peroxide is further verified by its spectroscopic detection in a slowly reacting mixture of hydrogen and oxygen (378) and by experiments in which hydrogen peroxide vapor has been added to the mixture of hydrogen and oxygen (509). To the
literature on the subject has been added some work by Russian ( 6 1 3 ) and Japanese (648) investigators. An interesting study of the reaction between hydrogen and nitrous oxide was published by Fenimore and Kelso (28s). A wartime investigation in France on reaction of mixtures of hydrogen, carbon monoxide, and oxygen has become available (618). This hydrogen-carbon monoxide-oxygen system, as well as the system phosphine-carbon monoxide-oxygen, was also studied in Russia by DubovitskiI (265’. The mechanism of the carbon monoxide-oxygen reaction was thoroughly discussed by Griffing and Laidler (336). These authors investigated the potential surfaces of the reacting system of oxygen and carbon monoxide and obtained several restrictions on the choice of elementary reactions. The peroxidic molecule COS,whose existence was suggested in a previous paper by von Elbe and Lewis, is now eliminated from the scheme; however, it is noted that the kinetics of the second explosion limit remains esaentially unchanged. An experimental contribution to the subject of hydrogen catalysis in carbon monoxide-oxygen flames was made by Sterling and Arthur (717). Clusius and Huber (200) investigated flames of carbon suboxide, CSOZ, in air. The character of the flame is found to differ in moist and in dry air. Several additional contributions have appeared on the subject of the reaction between hydrogen and chlorine (448,517) or bromine (441-
443). Customarily, the largest crop of papers on combustion kinetics concerns the oxidation of hydrocarbons and related compounds. This is a subject in which interpretation of the experimental facts tends to be controversial oFing to the complexity of the subject. However, in retrospect one notices a broadening area of agreement which a t present comprises the general concept of the chain mechanism and numerous details of the reaction. Valuable contributions to a final understanding of the oxidation of the simplest members of the hydrocarbon and aldehyde families were made by Norrish and his coworkers. Axford and Norrish (70) reported a series of observations on formaldehyde that tend to simplify greatly the kinetics of this system. Harding and Norrish (553) discuss the role of formaldehyde in the oxidation of ethylene, and Norrish (576) contributed new ideas regarding the role of aldehydes in hydrocarbon oxidation which should be helpful in the final elucidation of this crucial aspect of the subject. Pollard and Wyatt (611) studied the reaction between formaldehyde and nitrogen dioxide, McDowell and Thomas (606) wrote of the mechanism of initiation of acetaldehyde oxidation, and Letort and Niclause (473) investigated the oxidation and pyrolysis of acetaldehyde. Letort and Niclause found that a trace of oxygen was sufficient to induce marked pyrolysis by a chain mechanism. Eight investigations are devoted to the oxidation of methane, either alone or in the presence of carbon monoxide or hydrogen (64,671,678,630,635,7..@,770,869). In the oxidation of higher hydrocarbons, where the reaction is self-accelerating and steady-state kinetics fails, Prettre has shown that it is possible t o study quantitatively the dependence of the
1925
1926
INDUSTRIAL AND ENGINEERING CHEMISTRY
acceleration on the experimental variables, temperature, pressure, mixture composition, and vessel factors. The author published a summary (617) of his work over a number of years; the range of investigation concerns the low temperature regime in which, as is widely agreed, the reaction is manifestly controlled by chain branching due to peroxide scission. The separation of this regime from the high temperature regime in which the chains are probably unbranched has been sharply emphasized by experiments in which mixtures of hydrocarbon and oxygen have been ignited by rapid adiabatic compression (110,406, 649, 737). Rogener's experiments show two distinct stages of the induction period, which may be referred to as r1 and r2and which are associated with the low temperature and high temperature regimes, respectively. Empirical equations, which may apply to the engineknock problem, express the lengths of periods T~ and ra as a function of pressure, temperature, and mixture composition with or without addition of tetraethyllead. Valuable addition to the knowledge of two-stage combustion is furnished by the work of Townend and his collaborators. The latest paper by Spence and Townend (710) contains a summary of this field and new data on combustion waves awociated with the r1 and 7 2 regimes. The waves of the 71 regime are identical with cool flames t h a t have been so frequently described in the literature. Several other papers on cool flames have also appeared (899, 686). A series of interesting papers from the laboratory of Egerton in England and elsewhere deals a i t h peroxide formation during the slow oxidation of hydrocarbons (861,364, 466). From the laboratory of Hinshelwood, a series of papers has appeared t h a t deals with both the low and high temperature oxidation of higher hydrocarbons and related compounds and brings out several important facts that should guide future speculations on details of the mechanism (83, 819-83,368, 666,667, 697). Ethers have been studied extensively by Chamberlain and Walsh (189). They investigated the ignition limits corresponding to the r1 and r2 regimes and contributed extensive discussion to the mechanism. Other investigations contain additional data on hydrocarbon oxidation and deal with the problem of reactivity of hydrocarbon molecules (131,289, 763,808). A series of papers was published by French investigators t h a t deals with the explosion limits of mixtures of hydrocarbons or related compounds and oxygen (966,966, 996, 297, 806, 637). They bring out clearly the difference between ignition produced by the reaction between hydrocarbon and oxygen and ignition produced by the reaction between oxygen and carbon monoxide, which appears as an intermediate product. A number of other papers by Russian and Japanese investigators (191, 414, 416, 446,610, 648, 614, 699, 749, 763, 767, 8gO) deal with various phases of the oxidation of hydrocarbons and related compounds, including kinetics and cool-flame phenomena. A series of studies of hydrocarbon oxidation catalyzed by hydrogen bromide was reported (86,108, 109, 670, 669). Badin ( 7 3 ) investigated the oxidation of metal alkyls and related compounds, Gray and Yoffe (334) the explosion limits of methyl nitrate and other explosive vapors, and Jorissen (4.04) the effect of the nature of the wall of the reaction vessel on the relative rates of hydrocarbon oxidation. Some data are also reported on the liquid-phase oxidation of naphthene ( g o ) and a theory of the effect of low temperature oxidation of hydrocarbons on the nature of rubber has been published (760). A number of investigations appeared regarding the effect of light on slow oxidation of hydrocarbons (99, 990,669,676, 746). Several papers describe experiments with atoms and free radicals in various systems of reacting gases. Melville and Robb (631,638) developed a technique of studying quantitatively atomic hydrogen interactions involving diffusion of the atoms to molybdenum and tungsten oxide surfaces and observation of the luminosity emitted from these surfaces. Other hydrogen-atom studies comprise reactions with hydrocarbons (64,661,761), reactions in mixtures of hydrocarbons and oxygen ( 78),and recomhi-
Vol. 43, No. 9
nation with another hydrogen atom or with a hydroxyl radical (696, 700). Other papers deal with reactions of hydroxyl radicals and oxygen atoms with organic compounds (68, 69, 637), recombination reactions involving hydroxyl and oxygen atoms ( l 7 6 ) , chemical and spectroscopic evidence for the existence of the radicals HOa (640) and CH2 (586)in the gas phase, the mass spectroscopic detection of free radicals (879), and the capture of free radicals by organic reagents ( I 14). Other miscellaneous kinetic investigations were reported; these concern oxidation 6f hydrogen bromide (733), sulfur dioxide (797),aluminum borohydride vapor (74),explosive decomposition of ethylene oxide (168,161), the catalytic and noncatalytic reaction between hydrazine and hydrogen peroxide in the liquid phase (386),formation of carbon in flames (694), and inhibition of carbon formation by sulfur dioxide (689). FREE RADICALS, ATOMS, AND IONS IN FLAMES
Flame Spectra. Gaydon and his coworkers continued their investigations of spectra of burner flames in order to identify chemical species in combustion waves and to investigate the structure of the combustion wave in terms of temperature distribution and concentrations of various molecular and atomic species. A digest of this and other work is found in Gaydon's monograph (314). Since publication of this book, numerous other papers have been published by this group (6@, 313, 316, 766, 816-817). The studies include observations of free radical emission from flames at low pressure a t which the width of the wave is greatly increased, thus facilitating selection of specific wave regions by the spectroscope; detection of the Runge oxygen bands in carbon monoxide and hydrogen diffusion flames; detection of radicals OBr and CBr in bromine-containing flames; and investigation of aluminum powder flames in which emission is ascribed to small droplets of aluminum oxide. American investigators reported on the spectra of flames of carbon monoxide and oxygen, hydrogen or deuterium and oxygen, and also hydrocarbon and oxygen (364,373-376, 704). Miscellaneous papers include a study of emission from methane (804)and cyanogen (688)flames with discussions of the implication of the data for the mechanism; absorption spectra in the combustion of heptane in the presence of halogenated hydrocarbons and tetraethyllead (641); acetyleneair flames (381, 610); total light emission from flames of various hydrocarbons and related compounds (816); ionization produced by metal compounds in flames (107, 706); and electrical conductivity of flames (776). Laidler (469)discussed the origin of the Runge oxygen bands in carbon monoxide-oxygen flames. Other theoretical discussions comprise attainment of equilibrium between molecular translation, rotation, dissociation, and ionization during combustion (666),and the suppression of Cz bands by nitric oxide or sulfur added to hydrocarbon flames (106). An improved spectrograph of high light sensitivity and response has been described and recommended for use in combustion studies (3). IGNITION, PROPAGATION, AND STABILIZATION OF FLAMES IN PREMIXED GASES
Laminar Flow. Studies of the minimum ignition energies of electric sparks, which is of great theoretical as well as practical interest, have been continuing. Blanc, Guest, von Elbe, and Lewis (118) published additional systematic data of the minimum spark ignition energies of hydrocarbon, oxygen, and inert gas mixtures. As in previous publications, these investigators determined simultaneously the quenching distance, which is the distance of separation of the electrodes above which the size and shape of the electrodes does not affect the minimum ignition energy. The experimental minimum energies agreed well with the theoretical minima calculated from a model of the combustion wave in which enthalpy transport is assumed to occur predoniinantly by heat conduction and only to a negligible extent by dif-
September 1951
INDUSTRIAL AND ENGINEERING CHEMISTRY
fusion of reachnta and reaction products. This model has been challenged by Zeldovich and Semenov (832)and Friedman and Burke ( S o l ) ,who advance the opposite hypothesis t h a t the transport of heat from the burned to the unburned gas is more or less compensated, and according t~ the Russian investigators generally overcompensated, b y transport of chemical enthalpy because of diffusion of reactant molecules from the unburned to the burned gas. Further developnients on this subject can be expected. Theoretical correlations of minimum spark-energy data were attempted by Morris (661). On the experimental side, Broihan (162) attempted to measure the fraction of the discharge energy of a condenser-spark circuit t h a t is imparted t o the gas between the electrodes and is thus available for the ignition proce%. According to his data this fraction is very small, so t h a t minimum ignition energies based on total discharge energy would seem to give a distorted picture of the actual energy required in the ignition process. However, the validity of the measurements is doubtful, and more will be said about this later. Linnett and coworkers (303, 483) studied spark ignition of several explosive mixtures using the criterion of least igniting pressure, which was introduced into the older literature but which has no easily recognizable theoretical significance, being dependent on the gap length, among other parameters. Measurements of minimum spark ignition energies in flowing explosive gases have been reported by Swett ( 7 3 0 ) . For zero and low stream velocities his data are consistent with those of other investigators. Ignition by arcs has been studied by Allsop and Guknault (5) from the standpoint of safety of electrical machinery in coal mine atmospheres. Mullen, Fenn, and Irby (559) reported interesting and suggestive data on the ignition of high velocity streams of explosive gas mixtures by heated cylindrical rods. Stout and Jones (722) extended earlier measurements on the ignition of solid explosive media by hot wires to e~plosivegas mixtures. Shepherd (696) reported on the ignition of evplosive gas mixtures by impulsive pressures produced by means of a bursting diaphragm. It is remarkable that mixtures known to be relatively insensitive can be ignited in this manner by fairly low burst pressures. To improve the usual Bunsen burner method of measuring burning velocities, Mache and Hebra (608) and Bartholorn6 (96) replaced the Poiseuille velocity distribution in the flowing gases by a uniform velocity distribution produced by a contracting nozzle. However, the inconsistencies that are noted in the data of burning velocities obtained by various investigators do not appear to be resolved by such simplification of the flow. In fact, at present no reliable precision method seems to be available for measuring burning velocity, and the literature that has appeared on this subject in this period is as yet inconclusive (11, 12, 60, 307, 309, 338, 766). An interesting burner in which the combustion is a flat surface normal to the stream has been described by Powling (616). The method is restricted to slow-burning niivturee but appears well suited to determine burning velocities near the limite of flammability, and limits that evidently represent the true limits of flammability of the plane adiabatic combustion wave undisturbed by effects of tube confinement. Comparative studies of the burner and soap-bubble methods were reported by LiRnett (481),arid Gaudry (312)reported on the problem of determining burning velocities in a spherical vessel with central ignition. A number of papers appeared dealing with combustion wave theory that comprises hydrodynamics, diffusion theory, and chemical kinetics. Boys and Corner (140, WOQ), Hirschfelder and collaborators (362, 363, 369), Bartholom6 and coworkers (97, 98), and Bechert (104)made only moderate use of simplifying assumptions, whereas Manson (514, 615), van Tiggelen (747), Shorin (698), and Strickland-Constable (724) introduced speculative amumptions of various kinds. The idea t h a t the burning velocity is largely determined by diffusion of atoms and free radicals has been actively advocated by Tanford (734) and appears to receive support from a number of other investiga-
1927
tions (75,95,318, 482, 504, 663). Camel, das Gupta, and Guruswamy (186) discussed factors t h a t determine flame propagation through dust clouds, pointing out the effect of energy transfer by radiation. Other investigators report data of burning velocities of carbon monoxide, ethylene, and propane-air mixtures (226), nitrogen peroxide and formaldehyde mixtures ( 6 1 9 ) , hydrogenbromine mixtures (810); the effect of pressure on burning velocity (308, 316, 389, 421, 484, 794); the effect of molecular structure of hydrocarbons (638); and the effect of additives acting as promoters (363). A number of studies on flame propagat,ion in tubes have been published (94, 330, 331, 340, 342,383, 673, 793). Several studies deal with effects of additives and the condition of the tube wall on limits of flammabilit,y in a number of mixtures ($64, 869, 468, 690, 591). Bechert (103) proposed a theory of the limits based on an approximate equation of burning velocity and assuming arbitrarily the limit t o occur a t a burning velocity of 1 em. per second. Spontaneous disintegration of combustion waves into cells or flamelets has been reported by hfarkstein (518) and Bohm and Clusius (126). Burgoyne and Thomas (166) made the unusual observation that fine iron particles can displace the lower limit of flammability of hydrogen and air from 4 to 3.5'35 and of ethylene and air from 3.4 t o 2.7%. Effects of sound waves (350, 493, 619) and electrical fields (178) on burners are reported. H a r t (357) noted t h a t hydrogen peroxide vapor can be exploded with a hot wire or spark and can support a stable flame. The stabilization of burner flames and the quenching of flames by solid surfaces have engaged the attention of a number of workers. In order that a combustion wave may remain stationary in a gas stream, it is necessary t h a t a mechanism operate that maintains equality of gas velocity and burning velocity somewhere in the stream. Various such mechanisms have been investigated by Wohl, Kapp, and Gazley (813). They comprise the mechanism previously described by Lewis and von Elbe (479) which lrads t o the concept of critical boundary velocity gradient for blowoff and flashback and a mechanism, peculiar to the phenomena of flame lift and flame blowout, which deprnds on the change of f l o ~pattern in a free jet and takes place in tirrhulent as well as in laminar flow. The blowoff and flash-back limits are intimately related to the phenomenon of quenching. Hence, a quantitative theory of quenching developed by Lewis and von Elbe (267) t h a t relates quenching distance to burning velocity and other measurable data also relates the blowoff and flash-back limits to these latter data. The volume of data on quenching distances and critical velocity gradients has greatly increased (300, 302, 366, 634, 780). Correlations of such data by means of dimensionless numbers have been proposed by Putnam and Jensen (620). Other d a t a and discussions based on various considerations have been published (160, 224, ,035, 291, 607,680,740). The development of a combustion wave from an ignition source in a gas stream has been derived theoretically by Dery (237). The construction of the combustion wave surface a t successive stages shows the gradual development of the inner cone of the burner flame. FLAMES
IN
PREMIXED GASES
Turbulent Flow. Williams and Bollinger (129, 808) investigated the effect of turbulence on burning velocity in order to test theories proposed by Damkohler and Shchelkin. Their results appear to be in contradiction to these theories. It is noted, however, that these investigators did not use the reference surface of the turbulent combustion wave for their measurements proposed by Damkohler. Within the period covered by this review the theory of turbulent burning velocity a s well as the problem of approach to measurement of turbulent flames remains unsettled. Additional publications concern observations of the self-acceleration of a flame passing through grids or nozzles (376, 977), meas-
1928
INDUSTRIAL A N D 'ENGINEERING CHEMISTRY
urements on Bunsen burners ( 175),and some theoretical considerations of the effect of small-scale turbulence (233). Wohl, Kapp, and Gazley (813) extended the concept of critical boundary velocity gradient for burner flames to the turbulent flow range. They show bhat the value of the gradient does not change on transition from laminar t o turbulent pipe flow. This was also found by Bollinger and Williams (130). Flame stabilization in streams of explosive gas mixtures by baffles has been studied by a number of investigators (491, 574, 809, 810). MIXING AND COMBUSTION OF GASEOUS FUEL JETS AND LIQUID FUELS
The earlier investigation of laminar diffusion flames in the low gas-velocity range by Burke and Schumann (1928) has been extended t o higher gas velocities and the turbulent flow range in independent investigations by Hottel, Hawthorne, and Weddell (360, 3Y6), Wohl, Gazley, and Kapp (812),and Yagi (819). For the interpretation of the data on flame length as function of experimental variables (jet orifice, gas flow, nature of fuel gas, admixture o€ primary air or inerts to the fuel gas), these treatments combine theoretical concepts of laminar and turbulent diffusion with semiempirical corrections. Some basic features of such flames can be understood from simple dimensional analysis which predicts that for a cylindrical orifice the length of a laminar flame is proportional t o t h e gas velocity and the square of the orifice diameter, whereas for a turbulent flame the length is independent of gas velocity and proportional to the first power of diameter. This is in good agreement with experimental facts. A paper by Zeldovich (830) on the theory of combustion of unmixed gas has not been available t o the reviewers. The combustion of droplets of liquid fuel injected into hot air diluted with combustion products has been studied by Mullins and Barr (86, 88, 660). Their observations cover the stages of gasification and chemical reaction, the latter exhibiting the wellknown cool flame phenomenon of hydrocarbon oxidation. Frank-Kamenetski1 and Minskir (294) investigated the theory t h a t the scale of turbulence is smaller than the average size of the fuel droplets. Articles on the general aspects of atomization of liquid fuels for combustion have been published by Joyce (408) and Godsave (322); Rupe (668) reported on a technique for investigating the spray characteristics of nozzles. A number of papers by Spalding (YO9)deal with the combustion of liquid fuels evaporating from a heated surface. KINETICS O F THERMAL DECOMPOSITION OF EXPLOSIVES AND PROPELLANTS
Rideal and Robertson (644) examined the spontaneous ignition
of nitrocellulose samples exposed t o temperatures above 180O C. According t o these authors, the thermal decomposition leads t o liquefaction of the material after an induction period, accompanied by the evolution of a reactive gaseous mixture of nitric oxides and aldehydes. The rate of decomposition is accelerated not only by the temperature rise due to the exothermicity of the reaction b u t also by the liquefaction of the condensed phase, and these two factors together determine the transition from slow reaction t o explosion. The gas phase may also participate in this process if the reactive gaseous products are allowed t o accumulate around the sample. Robertson (64Y) extended such studies of decomposition and spontaneous ignition to a number of explosives with similar results. Other authors report on influence of pressure on the rate of thermal decomposition of explosives (660),thermal decomposition of mercury fulminate (7YI), and mechanism of explosion of ammonium nitrate (232). IGNITION AND BURNING OF SOLID AND LIQUID EXPLOSIVES AND PROPELLANTS
The ignition of solid explosive media by hot wires has been studied by Jones (399s)who presents data on the critical heating
Vol. 43, No. 9
time required for ignition, the variable being the electric c u ~ , r t ~ t , the length and diameter of the wire, the wire material, and thc. nature of the explosive medium. The data are in accord with H plausible theory of the ignition process, and their scientific intclrest extends beyond the immediate goal of this investigation, which was directed toward a n understanding of the behavior of bridge wires in detonator matchheads. Solutions of the equation of heat flow from a heat source in an explosive medium have I w i i given by Frazer and Hicks (296), who used a model of reaction mechanism in which the reaction rate is a function of tempvlature only. Solutions of the steady-state equation of burning of propellant strands have been given by Rice and Gincll (640) and Crawford and coworkers (216, 696). Several other papc*i,r deal with the experimental and theoretical aspects of propellant burning (14, 16, 616, Y l l , 788, 806). A method of detcrniiniiig the temperature profile of the combustion wave in propellant strands by means of fine thermocouples has been described t)y Klein, Mentser, von Elbe, and Lewis (435). CARBON COMBUSTION AND FUEL BEDS
The kinetic aspects of carbon and coal combustion continue t o interest many investigators. The papers t h a t have appeared during the review period are too numerous t o be listed individually. They deal with the surface chemistry of carbon oxidation and the transport processes in the gas phase adjacent to the burning carbon (121, 173, 174, 190, 193, 231,281, 324, 4Y4, 488, 492, YO2, 723), the combustion of pulverized coal (62, 120, 4SO, 446, 682, 608, 639), the reactivity of coke (192, 878, SQO), and the oxidation of coals which involves intermediate formation of peroxidic complexes (lY0, 188, 400-402, 681, 684, 823). Another kinetic aspect of carbon combustion t h a t has received much attention is the gas-phase reaction between carbon monoxide and oxygen, which is held responsible for a large fraction of the heat release in carbon combustion and whose course can be greatly influenced by catalysts or inhibitors in the gas phase (SS, 56, 57, 68, 137, 14.4, 634). Several investigatoi-a also deal with the combustion of coal in bulk in fuel beds (66,66, 69, 618).
FUNDAMENTALS OF DETONATION One of the unsolved problems of the hydrodynamic theory of detonation waves has been the absence of valid proof for the existence of a steady-state detonation velocity. I n the earliest formulation of the theory, Chapman (1899) introduced the hypothesis t h a t the steady-state detonation velocity equals the sum of particle velocity and sound velocity in the burned gas. SUI+ sequently, Jouguet (1906) pointed out t h a t if a rarefaction 11R V ~ formed behind the detonation wave it would pursue the lattci with a velocity equal t o the sum of particle velocity and sound velocity; therefore, the detonation wave could be stable only a t velocities equal t o or less than this sum. Becker (1922) argucd t h a t for reasons of thermodynamic probability the detonat ion wave should travel a t velocities exceeding the sum of particle :tiid sound velocity; b u t as the wave is mechanically unstable a t buch velocities i t can only travel a t a rate equal t o the sum. Honever, thermodynamics cannot predict t h a t during some process the state of greatest statistical probability is actually attained and hence the argument does not have the force of proof. Such proof, based exclusively on hydrodynamic theory, has been furnished by Brinkley and Kirkwood (146). These authors show t h a t the isentropic expansion process behind the wave operates in such manner t h a t the wave accelerates or decelerates when its velocity is smaller or larger, respectively, than the sum of particle and sound velocity, and hence the wave travels a t a steady-state velocity equal to the latter sum. A paper by Eyring and coworkers (2880) presents theories for detonation in solid charges ; i t is concluded t h a t chemical reaction in a detonation wave starts at load-bearing contact points, proceeds along the surfaces of grains, and is probably diffusion-controlled. Other papers deal
September 1951
INDUSTRIAL AND ENGINEERING CHEMISTRY
with theoretical and experimental aspects of the transition from burning t o detonation (47,454, 678, 765). Striking discoveries have been reported by Bowden and others o n the mechanism of initiation of explosives by impact (132, 133, 532, 333, 335, 556, 822). Minute gas bubbles in liquids such as nitroglycerin initiate detonation by adiabatic compression; in solid explosives, friction between particles similarly produces localized sources of heat that may initiate detonation. Impact sensitivity caused by formation of localized hot spots also has been discussed by other investigators (113, 645), and Ubbelohde (764) advances a general discussion of the theoretical and practical aspects of initiation. Detonation waves in e.xplosive gas mixtures continue t o engage the attention of many research workers. Such waves permit measurement of pressure as well as velocity (50),and as the pressures are low enough for application of the perfectrgas law to reactant- and reaction products, the data lend themselves to ready com2arison with hydrodynamic theory (115, 245, 328, 438, 832). The phenomenon of spinning detonation has received further attention (341, 513, 625) and Rivin (646) points t o a possible role of heat conduction in the propagation mechanism. In explosions of soap bubbles of acetylene-oxygen mixtures, Rakipova, Troshin, and Shchelkin (66%’) observed spherical detonation waves for mixtures of 47.6 t o 92.0% acetylene. The extremely high pressures developed in solid explosives make the perfect-gas law inapplicable and introduce the problem of the equation of state of the detonation products. This has been the subject of investigations by Jacobs (586), Cook (205), Jones (398), and Morris and Thomas (550). Numerous data have been published that show the detonation velocity t o be dependent on charge density, charge diameter, and grain size (111, 206, 505); occasionally it is observed that an explosive may possess a low and a high detonation velocity depending on the strength of initiation (48, 467, 757). A new method for photographic studies of detonations is provided by the electronic image converter (214).
TEMPERATURE AND EQUILIBRIUM STATE OF C O M B U S T I O N GASES The development of methods and equipment for measuring flame temperatures by optical means has engaged the attention of numerous investigators, Applications of the well-known sodium line-reversal method were described by Barret (89) and by Strong, Bundy, and Larson (725); Pcnner (603) discussed twocolor and line-reversal techniques, reversal methods for nonisothermal flames, a two-path method, a compensated hot-wire method, and methods based on measurement of line intensities. Gaydon and Wolfhard (317) reported temperature data obtained from rotational band spectra and reversal of iron lines; Silverman (703) and Sobolev (706) described the determination of flame temperature by infrared radiation and atomic spectra lines, respectively; and Kandyba (411) described a method of photoelectric spectrometry for the purpose. Several papers deal n i t h optical measurement of temperatures in flames containing incandescent soot particles (106, 651, 707, 818). The use of thermocouples in streaming exhaust gas has been discussed by Fiock, Olsen, and Freeze (287). Increasing use is being made of calculations of thermodynamic equilibrium for computing the temperature, pressure, and composition of combustion gases. An extensive program of such calculations, comprising any combination of the elements carbon, hydrogen, oxygen, and nitrogen, a pressure range from 0.1 to 100 atmospheres and a temperature range t o 5000’ K., and utilizing high speed electronic computing machines has been announced by Brinkley and Lewis (146). Numerous charts have appeared on working fluids in internal-combustion engines (275, 377, 503, 526, 631, 655, 677) and on furnace combustion (282, 417, 632, 686, 773). Several methods of calculation are described (380,
1929
662, 671, 811).
New thermochemical data include an improved value of the energy of dissociation of the hydroxyl radical (260, 580), of fluorine (798) and methane (460); data on hydrazine (687), and estimated data on free radicals in combustion processes (785). The question of unreleased energy in combustion processes is raised by Leah (469, 470) and Weeks (790).
PISTON-TYPE ENGINES OTTO ENGINES
Now t h a t the principks of knocking combustion have become common knowledge, i t was t o be expected t h a t engine designers should attempt t o root out the problem b y preventing the formation of a n adiabatically compressed and heated fuel-air mixture in the combustion chamber. This principle has indeed been known since the invention of the Diesel engine, It merely remained t o substitute spark ignition for autoignition of the fncil spray. Thus a revolutionary new engine has finally been announced which, if reliability of mechanical performance can be maintained, may become a powerful competitor of the present conventional Otto engine (82,247,789). The engine is reported to be capable of using a wide range of fuels and therefore promises substantial reduction of fuel costs. The fuel is injected as in the Diesel cycle into a swirling stream of air; the mixture is i g n i d by a spark plug and burned just downstream of the injection point. Another less radical method of improving engine performance consists in quenching the incipient knock reaction by introdueing antiknock liquids into the chamber at the proper point of the cycle. This is the function of the “Vitameter” (’7, 66,186,577, 615, 769, 803); however, the attention of the engine designer and the fuel industry is still focused on the present mode of operation of the engine. Extensive service tests are reported on engines of various designs (80, 151, 180, 660, 689). Correlations are sought between engine design and operation and knock (211, 658, 708, 735, 755, 775, 807) and efforts are being continued t o improve fuels by blending and by addition of antiknock compounds (39, 44, 79, 101, 250, 251, 352, 407, 579). Compilations of the knocking characteristics of a great number of fuels (298, 495, 726) have become available, t h a t of Love11 (494)being the most extensive. Sulfur is shown t o have a deleterious effect on knock prevention b y tetraethyllead (485, 486‘); tetraethyllead improves the storage stability of gasolines (783). Methods of knock testing are still being studied both as t o instrumentation (4,29,116,122,19.t, 246,.405,587) and procedure (945,436, 758, 774, 821). An attempt is being made t o substitute, a t least in part, calculations based on various empirical and theoretical ronsiderations for testing (81,13‘7,629). A review of rating methods, including Russian methods, has been given by KolomatskiI and Levin (444). Walsh and his coworkers (648, 781, 795), Egerton and Moore (262), and others (102,385,685, 636, 800) studied the chemistry of the reactions leading t o knock, which is a phase of the general problem of hydrocarbon oxidation. An extensive series of investigations was reported by King and his coworkers in Canada on various factors affecting engine and fuel performance. These factors include prevention of preignition and knock (453), oxidation of pentane and effect of iron carbonyl (@id), causes of antiknock properties of rich mixtures (431, 456),cause of preignition (425), power loss coincident with antiknock action of iron and nickel carbonyl in rich mixtures (426),high compression ratio engine and adverse effect of high jacket temperatures on thermal efficiency (428, 429), effect of hydrogen sulfide on critical compression ratio (430), and knock induced in town gas by finely divided carbon (427). In the 1948 review reference was made to high speed schlieirn photographs taken in the National Advisory Committee for hrrcnautics laboratories with camera speeds of 40,000 and 200,000 frames per second. This work has been extended to a speed of 500,000 frames per second showing details of shock-wave development in the actual knocking stage of the process (512).
1930
INDUSTRIAL AND ENGINEERING CHEMISTRY
There are also available simultaneous direct and schlieren photogriaphs of the process a t 40,000 frames per second (685). Such time resolution permits isolation of the T I and 7 2 reaction regimes nieiitioned in this review- under kinetics of gas-phase reactions. liohlke (439) reported on studies of the nature of gas movements in the chamber under knocking conditions. These studies point clearly to the cause of reduction of thermal efficiency and cylinderwall erosions. Deulblein (238) attempted to predict knock theoretically on t h e bash of the physical-chemical processes and reported results t h a t are in good agreement with experiments. Bates and Quinnelly (100) attempted a theoretical treatment of knock-induced vibrations and the temperature-pressure relations in the gas assuming t h a t the knocking part of the charge reacts instantaneously. Watson (786) reviewed the development ef spark-ignition equipment and the conclusions to be drawn from it. Some ideas mith regard t o spark ignition were noted by Cipriani and Middleton (196), and Peroutky (606) discussed high-frequency ignition. Yweral auttiurv have made studies of engine cycle and thermal efficieiiry (179, 184, 468, 797, 777). I n foreign countries interest prmiBts in the use of gaseous fuels such as methane (478,600,609). A revised edition of the monographs on internal-combustion engines by Taylor and Taylor (736) has appeared; Philippovich published a book on fuels in internal-combustion engines (607). A lecture series by Broeze (149) discusses all phases of the internalcombustion engines. A paper by Berruys (693) a t the Fourth World Power Conference discusses the comparative efficiencies of the main types of internal-combustion engines and a review (33) sponsored by the British Motor Industries Research Association describes progress in road vehicsle engines. DIESEL ENGINES
It is now widely recognized that thc pioblems of the Diesel engine concern the engineer rather than thc chemist and it is not surprising therefore to find a much snialler number of papers devoted to Diesel engine combustion. Papers that have appeared within the period covered by this review are concerned with survey and testing of eurrrntly available fuels for high speed Diesels (236, 949, 46S), studies of factors affecting selfignition such as chemical composition (13, 7 l ) , influence of inert gltses (388),the effect of ethyl nitrate (801), the relative merits of additives such as nitrates, peroxides, ethers, and hydrocarbons vcraus pilot injection, particularly for cold starting (66, 884), the practice of pilot injection (40, d l ) , and the effect of combustion on depth of penetration of liquid fuel droplets (422). Measurements of temperature and radiation by means of pyrometers arc' reported (544, 545); pressure indicators are described t h a t serve to detect speedily any disturbance in the operation of the engine such as uneven loading or piston leakage (38, 348, Yi5). A special engine for testing Diesel lubricants and an ignition delay meter for the doterminstion of cetane numbers are dewxibed ($7, 389). American practice in the use of Diesel-electric locomotives (599)and Russian practice in the use of natural gas for Diesel and steam locomotiws (30) are noted. Exhaust-gas studieti looking toward the use of Diesel engines in mines have been made ( 1 7 1 ) and Elliott (268) reviewed Diesel-engine combustion. G A S TURBINES The fuel problem in gas turbines has been discussed by Lloyd (487). Accepting the principle that the gas turbine is omnivorous of fuels, the author proceeded to investigate the range of possible liquid fuels and the fuel characteristics of greatest practiral significance. Though certain properties of liquid fuels, as, for example, ignition \:haracteristies and molecular structure, are so complex t h a t no correlation with cornbustion behavior in the gas turbine has been possible, other properties exist for which such correlations are possible. These include carbon-hydrogen ratio, density, vapor pressure, viscosity, and flammability limits. The
Vol. 43, No. 9
influence of these properties on the combustion process is noted in the fuel injection and stabilization of flame, the burning of the droplets after ignition, and the nature of the exhaust products. The author has arrived a t a number of new and interesting generalizations. Though no hard and fast conclusions may be drawn the author indicates t h a t the main fuel characteristics affecting gas-turbine operation are viscosity, initial and final volatility, carbon-hydrogen ratio of the organic constituents, and physical characteristics of the ash. Two other papers on gasturbine fuels provide information on fuel properties commonly employed for identification and specifications of fuel oils and also on those properties felt to be significant to the fuel-system design and performance of the combustion chamber and turbine (361, $79). Research on use of heavy fuels in connection with marine application of gas turbines (65), on the relative advantages of upstream and downstream injection (198), and on flame holder. (833)were reported. Much attention was given to construction metals for combustion chambers and to means of avoiding excessive temperature and at the same time maintaining satisfactory combustion efficiency and uniform temperature distribution in the exit gas from the combustion chamber to the turbine blades (87,596, 665, 648, 661). Interest prevailed in the use of ceramics for turbine blades (159,254,571) and work continued on development of coal-burning gas turbines (28, 41.2, 416,693) and gas turbines suitable for automobiles and other vehicles (684, 769). Methods of calculating gas-turbine performance were described by Kurochkin (454) and Kestin (419), and the measurement of gas-turbine combustion efficiency by gas analysis was discussed by Richards and Street (641). Two new books on gas turbines appeared, one by Vincent (778) and another by Adams (2) and several reviews describing the principles of gas turbines, their combustion problems, and the progress t h a t has been made in their development (24, 35, 496, 497) were written.
JET AND ROCKET P R O P U L S I O N I n a security-sensitive subject of this type much work is probably unreported. Nevertheless, the open literature has been increased by a number of articles. Longwell (490) discussed combustion problems in ramjet design; he compared fuels with relation to heat of combustion, availability, ease of combustion, and handling. The process of mixing and distribution of fuel and air was investigated-and an equation was derived for fuel concentration downstream from a point source. The stabilization of flame and the spreading of flame over the cross section of the gas stream were discussed also. Other papers deal with the problem of specification of jet fuels (973),the relative merits of direct injection and pre-evaporation (993), the relation of flammability limits t o turbojet combustion (672), and the disturbing effect of fuel nozzle carbon deposition (240). The problem of combustion instability was discussed by Randels (694) and Park (592): Hall (351) made a theoretical comparison of various methods of thrust augmentation for turbojet engines. The methods investigated were tail pipe burning, water injection a t the compressor inlet, a combination of these two methods, and rocket assistance. A paper by Mullen (668)furnishes an introduction to problems in the development of burners for supersonic ramjets. I n current practice the critical problem is flame stabilization a t these extremely high velocities. A solution to this problem is outlined and difficulties encountered in burner development are enumerated. Schmidt (679) discussed the function of impact waves in the ignition process of periodically operating jet engines. Mathematical theories applicable to jet combustion were presented by Hicks (366), Schaaf (676),Kahane and Lees ( d o g ) , and Reid and Herbert (630). An experimental study of flame-holder stabilization in high velocity streams with air preheated by an auxiliary flame in front of the holder was made by Haddock and Weeks (347).
September 1951
INDUSTRIAL AND ENGINEERING CHEMISTRY
The problem of optimum propellant combinations for rockets studied by Rcinhardt and Pisclli (633). FVith respect to specific impulse the ultimate in performance seems t o be the hydrogen-fluorine systrm. However, performance alone can Iiardly be considcred the prime criterion for the selection of rocket propellants. Sacrifices must be made in performance for logistic, safety, availability, handling, and economic reasons. A ?iimilar review was presented by Bowman (134). Zaehringer (866)discussed cost, sources of supply, containers, and transportation of numerous propellant combinations and chemical and physical propcrtics of some inexpensive compositions. Other papers treat propellants such as nitromethane (67, 110), ammonium nitrate (7391, nitrogen dioxide derivatives (136),nitrcgen tetroxide ( 6 6 2 ) , hydrides and organometallic. compounds (I%'), aluminum borohydride (619), fluorine (141), liquid oxygen ( 5 6 6 ) ,hydrogen pcroxide ( I d $ ) , hydrazine (688),some propellant combinations of oxidizers with inexpensive carbonaceous fuels (279, 620, 872, 674), solid propellants (164), and nuclear energy (199). A paper by Altman and Penner ( 6 ) deals with chemical reaction during adiabatic flow through a rocket nozzle; the temperature change is so rapid that some equilibrium reactions can be neglected. Several papers discuss engineering and construction problems of rocket motors (1, 666, 7B9, 761). An apparatus for the measurement of ignition delays of self-igniting fuels such as used in rockets (148) and spectroscopic methods of measuring velocity and pressure of gases in rocket flames (167)arc described. A monograph by Finch (886)deals mainly with the engineering aspects of turbojets. Fiock (286) collected a bibliography on gas turbines, jet propulsion, and rockets. The Guggenheim Jet Propulsion Center and the Inyokern Naval Te,st Station reported 011 their activities (673, 769). Canright (185) wrote a survey paper on liquid propellant rocket motors. Five theoretical papers have appeared regarding mechanical and mathematical aspects of rocket propulsion (217, 330, 327, 664, 728). Xwicky (854)presrnted the view t h a t increased knowledge of reaction kinetics may lead to greatly improved rocket-propulsion devices. Sellers (6'91) investigated the influence of earth gravitation on the requirements of the vertical trajectory rocket with special reference to escape. Can we fly to the moon? is answered in the negative by Himpan and Reichel (567),who point to fantastic proportions of the take-off mas8 of the rocket carrying a human passenger around the moon. Man must evidmtly continue t o suffer ennui on earth.
\vas
HIGH EXPLOSIVES, GUN PROPELLANTS, INCENDIARIES. AND PYROTECHNIC MATERIALS Jouguet (406) discussed the nature of shock waves produced in a gas by the detonation of solid explosives and pointed out that the velocity of the shock frequently exceeds the velocity of the detonation wave. Measurements on such shock waves have been reported by Dubois (268). Jones and Miller (399) reported data of detonation velocity and detonation products of T N T as a function of loadingdensity, and Cybulski, Payman, and Woodhead (827)observed that cast T N T may undergo stable or unstable detonation depending on the size of the crystals. Kenner (418) proposed a theory of detonators such as lead azide and mercury fulminate. According t o this theory a detonator contains an anion t h a t is converted to a radical which decomposes, releasing available energy. Only a small amount of energy is required for radical formation because of a covalent union with a heavy-metal ion. Other papers deal with the effect of pressure on several priming explosives (115),the properties of cobalt(II1) and chromium(II1) roordination compounds containing oxidizing and oxidizable groups ( 7 5 2 ) , measurement of electrostatic charge and ignition sensitivity of various primary and detonating explosives (6B6), ignition of cordite by hot gases (ICs), detonation of common explosives by high velocity impact (169), effect of moist.ure and
1931
sririliglit on the propertics of T S T (230), specific energy and detonation velocity of explosives containing aluminum (628), uses of tetranitromethane and a method of manufacture (849), and the preparation of explosives of fixed particle size and shape ( 1 87). Muraour and his coworkers continued their studies of the force and covolume of colloidal posxders and the burning rate law (663-664). Additional data on propellant burning rates as a function of pressure are reported by Pekkarinen (601). Ballistic mortar operation for determining the power of high explosives wae discussed by Taylor and Cook (738). Other papers on experimental methods include flash radiography applied t o ordnance problems (197), shock-wave collisions as a source of illumination (636, 666), and flame photography of gunpowder (827). Some spectroscopic observations on pyrotechnic flames were reported (96) and the phosphorus content and danger of incendiaries described (51s )
GAS BURNERS The problem of interchangeability of fuel gases continues to be of great interest to the gas industry. A method is sought for predicting the performance of gas appliances in a community when the composition of the send-out gas is changed. I n an aerated gas burner such &s the Bunsen burner, the fuel-air mixture is formed by air entrainment of the turbulent fuel jet from the gas orifice; a stable flame is formed if composition and gas velocity correspond to a point within the stable flame region bounded by regions of blowoff and flash back in the flame stability diagram. The theory of aerated gas burners therefore must consider the processes of both air entrainment and flame stabilization, and the coniplicated interdependence between these two processes renders it highly improbable that the practical problems of the gas industry can be solved by methods such as empirical correlations of numerous data. This is indeed borne out by the very restricted validity of the results of such methods and the failure to discover any grncrally applicable relations (8-10, 32,46, 138, 259,866, 449, 543, 688, 607). On the theoretical side, Silver (701) and Westerdijk and Lantzius (7
