Combustion and Flame I

Outlook in Combustion Research

At the end of 1969 Combustion and Flame will have completed its thirteenth year as a publica- tion of important basic research conducted in various parts of the world in the area of its titte. It began, as it remains, the official journal of The Combustion Institute. With the encouragement of the Institute and under a number of able editors and editorial and advisory boards it has broadened in scope and content until today it is regarded by research workers in the field as the definitive publication medium of research in combustion science.

In an editorial prefacing Volume I, Number I, Sir Alfred Egerton, its first editor-in-chief, wrote. "'The new Journal is a journal of combus- tion science and is not intended to publish papers on plant or process or such matters of a technical and engineering character which do not rolate to the underlying scientific principles but are merely descriptive or are concerned with details of operation. However, methods of experimen- tation and analysis which help to elucidate the scientific principlo, s of combustion or flame would not be outside the range.

"We shall hope to receive papers from m~ny laboratories which are interested in the applic~.~- tion of scientific principles of combustion and are carrying out research on the subject, whether it be in relation to aircraft, to boilers, to furnaces. to burners or whatever it may be. Likewise. papers on the structure and properties of flames -- their propagation, spectroscopic analysis, constitution, ionization and so on--will be wel- come, together with those on the kinetics of

oxidation in the vapour state and other combus- tion reactions. Combustion within liquids and at the surfaces of solids are also within the range and would be fit subjects for papers in the Journal."

As Combustion and Flame enters its four- teenth year, we envisage some new combustion interests at~d. laopefully, the development of new concepts and their applications. With 1970 the journal will pass from its first publisher. Butter- worth & Co., to American Elsevier Publishing Company. The Combustion Institute recognizes its great indebtedness to Butterworths for its foresight in undertaking publication of the journal in the face of uncertain financial out- come and for the continued indulgence which has contributed so much to its success. The new publisher faces a future full of challenge. For one thing, as in all active research areas, the number of papers dealing with aspects of com- bustion for which publication is sought has in- creased very considerably, making the task of editor and publisher an exciting but difficult one. Although the journal will be ever more selective on its already high plane of standards, we are very thankful for the foresight of the new pub- lisher in envisaging a growth in size. Up to 1000 pages or more in its present lbrmat can be made available, to facilitate early publication of the outstanding papers in world combustion t'esearch. Combusth~n and Flame will be pub- lished in six issues per year in two volumes. All members of the Institute and others studying

Cmnbu.~tlvn & Fh;mt,. M, 1-4 (1970) Copyright c' 197,, by Th~ Combustion Institute

Published by Amcrlc~,n Elsevier Publi~hlng Compnny. htc,

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combustion are invited to make use of the journal as their publication medium.

Here, it is in order to acknowledge our great debt to The Combustion Institute. Through its various activities and sponsorship of the bien- nial international combustion symposia, it has for many years succeeded in bringing together scientists and engineers from various parts of the world, with interests in a variety of disci- plines, for the purpose of a common goal--the sharing of combustion information, thus achiev- ing an almost unmatched level of international cooperation in science. At the beginning of the development of turbojets and rockets, when there was great need for combustion informa- tion, the Institute was responsiblt~ for an orderly

' scientific growth of the field, thus avoiding a purely empirical, ad hoc development.

It was not many years ago that research on combustion was confined essentially to the chemical processes involved. Although such studies still go on apace, the inclusion of !he word flame introduced emphasis on the physical aspects of combustion. The combustion wave concept that was developed permits the identifi- cation of both chemical and flow aspects of flames, which is the basis of the successful analysis of flames in a flow field. The consid- eration of turbulent flow brought into being the fascinating subject of turbulent flames. which plays such an important role in modern combustion technology. In fact there are aero- dynamicists who have been fascinated with the potentialities of this subject and who believe that combustion is a branch of aerodynamics. Be this as it may, each of the disciplines that have been brought to bear on the solution of combus- tion problems--chemistry, physics, mathemat- ics, thermodynamics, aerodynamics, fluid mech- anics----might consider similar claims. Certainly it is a fact that the several disciplines have been responsible for combustion science as we know it today. And since combustion is and will con- tinue to be the driving force in our civilization. it is appropriate to consider combustion science

Bernard Lewis

a prime discipline of its own. Nevertheless, it is often at the boundary of two disciplines that new discoveries are made, so that a multidiscipline discipline should have a bright future in this regard.

For many years, and particularly since World War II. we have been exhausting the raw mate- rims (basic concepts) of combustion in the at- tempt to make them useful to mankind. It has been a highly successful enterprise. Consider. for example, combustion in the jet engine. Heat releases of 4-6 x 106 Btu per cubic foot of com- bustion space per hour per atmosphere are now common with efficiencies approaching 100°/o. and heat releases greater by up to an order of magnitude are now in view. This has been made possible by the application of principles of tur- bulence and flame stabilization and by the con- cept that a turbulent flame is nonetheless com- posed of discrete combustion waves fluctuating randomly in space and time.

In our zeal to apply the turbulence process we must not overlook other concepts that are awaiting application. The usefulness of the con- cept of the combustion wave has been greatly extended by the concept of flame stretch de- scribed by the characteristic nondimensional number

~o/U × dU/¢(r

where r/o is the width of the combustion wave: U, the flow velocity: and dU/dy, the velocity gradient. With the aid of this concept it is now possible to understand such diverse phenomena as minimum ignition energy, flame stabilization, limits of flammability, instabilities, and theo- retical upper limit for heat release rate. A num- ber of papers on this subject have appeared in Combustion and Flame. the lnstitute's combus- tion symposia volumes, and elsewhere. This concept, well supported in the laboratory, has been applied only sparsely, however, to practi- cal problems. So, while we arc ever seeking an elusive new concept, we must not overlook existing concepts waiting to embrace new ap- plications.

Outlook in Combustion ResearCh

In a modern world with complex interacting forces and social interests it is no wonder that we see a marked upsurge in fire research and in research directed toward countering the pollu- tion of the atmosphere, In the first area contin- ued use of aerodynamical principles can be envisaged, and possibly even the application of the flame stretch concept. In any case we can look forward to very strong activity in fire research to help reduce the great annual loss in lives and property. In the area of pollution, where there is a strong social responsibility, it should be no special feat to burn materials completely. Success will often be a mattter of funds and engineering design. In a world that seems to be bent on smothering itself the dollar sign must not be the barrier to survival. Anti-air- pollution studies should engage research work- ers very effectively over the next decade.

Although much of the interest in combustion has shifted to ~he physical aspects, there is still ample room for chemistry. Witness the solution of the smoke problem in turbojets by the appli- cation of simple chemical kinetics of the oxida- tion of hydrocarbons coupled with good engi- neering sense, a solution that could readily be applied to other smoke-emitting systems. In addit.ion to oxidation and combustion kinetics, other chemical areas in combustion can be seen in the extension of studies of the past ten years on flame structure, insofar as it concerns distri- bution of chemical species throughout the com- bustion wave. Work in this area is still in its infancy and is awaiting the development of instrumentation that will allow detection with- out disturbing the medium. The implications of this work for chemical indust!y are not to be underestimated.

Th.e problem of sell-acceleration of flame has lately engaged the attention of a few workers. This general problem, of which transition from a deflagration wave to a detonation wave is but one aspect, will, 1 believe, become increasingly important both as a subject of inquiry and as an applied area involving a variety of industrial

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problems. One flame-accelerating source, name- ly gravitational or centrifugal field, should prove of direct value to turbojets and other combus- tion systems.

A great deal of work remains to be done in the field of combustion in high subsonic and supersonic flow before we can understand the intricate problems associated with ram and scram jets and turbojets in high-speed flight. The combustion of premixed gases at various pressures (altitude) and the effect of turbulent flow on the flame blowout limits (flammability limits) are still subjects of much practical conse- quence. In the area of not premixed systems the behavior of a fuel droplet (heat and mass transfer of a burning droplet) in forced convection and under supercritical conditions will no doubt oc- cupy combustion scientists for many years to come. The same can be said for the burning of solid particles (e,g.. metal combustion).

I f there is one area that continues to need development it is methods of measurement-- the discovery of new and better ways of measur- ing the parameters of combustion. Although a less glamorous enterprise than the study of combustion phenomena themselves, instrumen- tation deserves the attention of combustion people who understand the need. Two proce- dures that would bear considerable effort are the application of laser technology and holo- graphy. Among other things the laser could conceivably be applied to the measurement in situ of turbulence parameters in a combusting system by utilizing Doppler shift in scattering from a seeded dust particle or floating impurity. Scattering on a molecular level and utilizing the different characteristics of various molecular species would enable one to distinguish between such species. This procedure would have the advantage of avoiding the introduction of a disturbing object into the medium. Holography. applied to burning particle~, would seem to offer the opportunity to measure in sequence pictures tbe change in size and shape 0f the particle, and thus to obtain from the time interval a measure-

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ment of the drag of the burning particle. Other procedures, such as the application of the ioniza- tion probe to the detection of the active combus- tion wave and its ~efinement at elevated temper- atures and pressures, will also find many useful applications in existing problems. The continued use of spectroscopic techniques is to be a~. ',ci- pated.

No one can predict where such studies will

Bernard Lewis

lead, but we can at least strive to attain the goal of knowing essentially everything about com- bustion itself, so that problems are reduced to engineering development and analysis. It is gratifying that this journal will now have suf- ficient space available for the publication of new results in these fields.

DeceJnber 1969 BERNARD LEWIS