INTRODUCTION TO THE THIRD SYMPOSIUM ON COMBUSTION, FLAME AND EXPLOSION PHENOMENA

Madison, Wisconsin, September 7-1 1, 1948 By BERNARD LEWIS1

Eleven years ago the second comprehensive symposium on combustion to be held in the United States took place a t Rochester, New York, under the auspices of the Gas and Fuel Division of the American Chemical Society, with the cooperation of the Divisions of Industrial and Engineering Chemistry, Petroleum Chemistry, and Physical and Inorganic Chemistry.

At that time we were just beginning to under- stand the ramifications of combustion research and were able to divide the field into three main divisions: (1) chemistry and kinetics of slow and explosive oxidation reactions; (2) ignition and propagation of flame; (3) state of the burned gas. The subjects to be presented in this symposium can, in the main, still be grouped under these three divisions. However, enlargement of scope is noted in the field of ignition and flame propagation. A decade ago the research program on flame propagation was confined essentially to the propa- gation of plane combustion waves in quiescent gases or in laminar streams. There was a lack of concepts that would permit approach to problems of quenching and stabilization of flames, the effect of turbulence on flame propagation, and even of the mechanism of ignition by a local source. The list of papers for this symposium shows that im- portant progress has been made. There are ten papers that treat the subject of stabilization of flames in low and high velocity streams, and dis- cuss relationships between stabilization and quenching of flame by solid surfaces. Turbulence is being actively studied experimentally and theoretically, as far as the resources of the theory permit. I t remains to be seen whether the most significant contribution of turbulence is the in- crease of the transport rates of heat and chain carriers, or whether these transport rates remain comparable with those of laminar flow and the increased rate of flame propagation can be ac- counted for by increased flame surface resulting from turbulence. Results of flame-area measure-

'Physical Chemist, Chief, Explosives Branch, Bu- reau of Mines, Pittsburgh, Pa.

ments will, undoubtedly, be forthcoming in future research.

Ignition has always been a subject of lively interest. In this symposium ignition by a variety of sources is reported, namely, compression waves, hot wires, hot rods, capacitance sparks and in- ductance sparks. Perhaps this meeting will con- tribute toward a deeper understanding of the threshold conditions for ignition and their relation to other critical flame data which determine sta- bility and quenching limits.

The problem of the propagation of the plane combustion wave is still being vigorously studied. Theoretical aspects will be discussed in great detail and it is perhaps not too extravagant to hope that future research will develop methods of experi- mentation for implementing these fundamental relationships with data on the identity and con- centration gradients of various molecular species in the combustion wave, thus permitting studies of the kinetics of combustion wave reactions. Many individual systems must be studied under various conditions. Such detailed work is repre- sented by a number of papers in the section on flame propagation in explosive gas mixtures as contrasted to papers on the general subject of flame propagation and flame structure.

A subject of research, which has been more or less stagnant for 20 years, is that of diffusion flames. Three papers in the present symposium cover flames of fuel jets in laminar and turbulent flow, representing a marked advance in the the- oretical and experimental development of the subject.

Ten years ago it was possible to predict con- fidently that great strides would be-made in the kinetics and mechanism of combustion reactions. A glance a t the 21 papers on this subject readily confirms this. Oxidation of hydrocarbons con- tinues to be, and for a long time will be, the subject of major research effort, whereas work on reactions of the simpler fuels like hydrogen and carbon monoxide may be expected to attain some finality. I n fact, this stage seems to have been reached for

2 INTRODUCTION

the reaction between hydrogen and oxygen, a t least as far as the mechanisnl is concerned.

Spectroscopy as applied to flame is represented by a number of papers and is demonstrated as a valuable tool n-hich can provide specialized in- formation on the chemistry and heat release in the combustion nave. I t is applicable, therefore, to the study of kinetics and thermodynamics of flames.

Thermodynamics has long been the most readily approachable phase of flame research. We ob- serve the ever increasing use of thermodynamic functions deri\-ed from spectra which completely replace values obtained by older experinlental methods. Hand computations are giving way to mechanical methods Ivhich vastly increase the output and the accuracy of data. It is believed that in the not distant future the thermodynamic properties of hot gases and the corresponding equl- librium compositions will be available for a lmos~ any conceix able combination of fuel systems.

Eleven years ago there seemed to be nothing new to report on the subject of detonation, and no paper appeared on the symposium a t that time. The n a r stimulated renewed interest in the sub- ject, embracing both initiation and steady-state propagation and extending to consideration of the details of the detonation process, such as the pres- sure profile of the wave and the structure of the reaction zone. The interest extends to solid ex- plosives and includes the milder types, such as nitrocellulose which is the main constituent of

most propellants. -4 synthesis of the hydro- dynamic and kinetic approaches to detonation seems to be in the making, as it is quite evident that kinetics must play an important role in the initiation of steady-state detonation.

I n the field of engine combustion, the problems of knock and fuel rating in reciprocating engines formerly engaged tfie attention of investigators almost entirely. This has become less urgent, primarily because practical problems have found ready solutions without requiring complete phys- ico-chemical analysis. Fuel rating, for example, has become a highly developed art which serves its purpose without contributing, however, to the science of fuel combustion. That is not to say that research on combustion in reciprocating en- gines is not continuing, for valuable contributions are still being made, but the greatest effort is cen- tered on combustion research in jets and rockets. I t is in the nature of the latter research that much inforn~ation is necessarily restricted and the papers being presented in this symposium are only a small sample of the activity in this field. From a prac- tical viewpoint, one wonders whether the de- emphasis of combustion research in reciprocating engines has not gone too far.

As in the past, we shall again profit from the present gathering, and the impetus provided by the work presented will undoubtedly stimulate further research along both experimental and theoretical lines.