Combustion, Flames and Explosions of Gases PDF
Preview Combustion, Flames and Explosions of Gases
COMBUSTION, FLAMES and EXPLOSIONS f ° GASES Second Edition BERNARD LEWIS, Ph.D., Sc.D. (Cantab.) and GUENTHER von ELBE, Ph.D. (Berlin) Combustion and Explosives Research, Inc. Pittsburgh, Pennsylvania ACADEMIC PRESS INC. · New York and London · 1961 Copyright ©, 1961, by Academic Press Inc. ALL RIGHTS RESERVED NO PART OF THIS BOOK MAY BE REPRODUCED IN ANY FORM, BY PHOTOSTAT, MICROFILM, OR ANY OTHER MEANS, WITHOUT WRITTEN PERMISSION FROM THE PUBLISHERS. ACADEMIC PRESS INC. Ill FIFTH AVENUE NEW YORK 3, NEW YORK United Kingdom Edition Published by ACADEMIC PRESS INC. (LONDON) LTD. 17 OLD QUEEN STREET, LONDON S.W. 1 Library of Congress Catalog Card Number 60-11,267 PRINTED IN THE UNITED STATES OF AMERICA Preface to Second Edition The tempo of combustion research has continued unabated during the past decade. Substantial progress has been made in establishing a com mon understanding of combustion phenomena. However, this process of consolidation of the scientific approach to the subject is not yet complete. Much remains to be done to advance the phenomenological understanding of flame processes so that theoretical correlations and predictions can be made on the basis of secure and realistic models. In this new edition particular emphasis has been placed on the modifi cation of combustion wave propagation due to heat loss to the unburned medium and to localized changes of mixture composition by diffusional processes. Both influences occur as a result of divergent propagation which takes place in a flow field with steep velocity gradients or under conditions of flame initiation from a point source. The heat loss effect imposes stability limits of flames in flow fields and minimal values of flame diameter and ignition energy in flame initiation. Employing a new concept of flame stretch, which refers to the growth of flame surface in divergent propagation, it has been possible by similarity procedure to obtain good correlations of the fundamental, measurable flame param eters of burning velocity and wave width with flame stability limits and with spark ignition data. In this way, for example, the flame sta bility limits on flame holders in high velocity streams have been deduced. Furthermore, it is shown that in all instances of divergent propagation diffusional stratification of mixture composition occurs to a degree de pending upon the relative diffusivities of the fuel and oxidant components of the mixture. This stratification produces effects that are predictable, at least qualitatively. Thus, in an over-rich mixture of a heavy hydro carbon and oxygen, the minimal flame diameter and minimal ignition energy are smaller than predicted from the flame stretch equation for the original mixture because oxygen diffuses into the flame zone more rapidly than fuel. Similar considerations apply to flame stabilization. The concept of flame stretch and diffusional stratification permits an understanding of limits of inflammability. v vi PREFACE TO SECOND EDITION Other parts of this edition have not required the substantial revision given the subject of combustion waves. However, some revisions have been made particularly in the discussion of detonation processes, and much new material has been included where it appears to promote a better understanding of the subject. BERNARD LEWIS GUENTHER VON ELBE Pittsburgh, Pennsylvania February, 1961 Preface to First Edition During the past decade the scope and tempo of combustion research have increased to such extent, and so many new facts and concepts have developed, that preceding treatises appear to be wholly inadequate to meet the present day demands of student and research worker for a mod ern exposition of the subject. Although the authors have borrowed the title of their former book published in 1938, the present book cannot properly be labeled a second edition inasmuch as the text is entirely new with the exception of a few brief sections dealing with subjects that had already been well explored at that time. The purpose of the new volume remains the same however, namely, to provide the chemist, physi cist, and engineer with the scientific basis for understanding combustion phenomena. The terms combustion, flame, and explosion became part of the com mon language long before clear scientific concepts existed, and there fore their usage has remained somewhat arbitrary and flexible. In this book the treatment of the subject matter comprises the theory of reaction chains and the chemical kinetics of reactions between fuel gases and oxygen, the hydrodynamics of combustion waves, detonation waves and jet flames, and the thermodynamics of combustion gases. In this respect we have followed a plan used in the former book, which we believe has proven valuable for delineating the field. Comparison of the state of knowledge 13 years ago with that of today shows a considerable advance in the field of chemical kinetics, a very large number of new facts and concepts in the field of ignition and combustion wave propagation, and substantial progress in the understanding of diffusion flames and detona tion waves. On the other hand, the thermodynamic aspects of combus tion had already attained a degree of finality many years ago and there fore no significant conceptual advances are to be noted, though the volume of reliable data has increased markedly. The reader will find new chemical mechanisms which have been de duced from the latest available evidence and which should be regarded vii viii PREFACE TO FIRST EDITION as the present best judgment of the authors. These mechanisms are recommended as a basis for further experimental work and discussion. There already exists a considerable area of agreement among workers and it is hoped that the new analyses will contribute to its enlargement. The systems that have been treated are hydrogen and oxygen, carbon monoxide and oxygen, and hydrocarbons and oxygen. The system best understood is that of hydrogen and oxygen. In the carbon monoxide- oxygen system the reaction mechanism appears to be approaching some degree of clarification although quantitative data are still lacking. Con siderable progress has been made in the understanding of the hydrocarbon- oxygen system and an early clarification of the oxidation mechanism, at least for the lower members, appears to be distinctly possible. There is also discernible a skeleton of consistent facts and theories of the oxida tion of higher hydrocarbons. Some interlinking and interdependence of the reaction mechanisms of these systems are noted, and as further studies of selected mixtures are made the requirement of interconsistency of reaction mechanisms will become a powerful factor in the final eluci dation of elementary chemical reactions. In the second part of the book dealing with flame propagation, empha sis shifts from reaction kinetics to hydrodynamics as the branch of science applicable to combustion and detonation waves and the combustion of fuel jets. It is on the subject of ignition and propagation of combustion waves that the greatest progress has been made in recent years. For the first time one understands the relationship between stabilization of com bustion waves in burner flames and quenching in channels of critical diameters. An understanding is also provided of the various phenomena of flame propagation in tubes. A particularly significant advance is the integration of the data on spark ignition with the concept of minimum ignition energy which is derived from theoretical considerations of the development of combustion waves from an ignition source. It is shown that the hydrodynamic equations can be simplified to permit solutions which lead to correlation of the quenching distance with burning velocity and other measurable quantities. Important advances have been made by the recent theoretical and experimental investigations of Karlovitz on the interaction between com bustion waves and turbulent motion in explosive gas mixtures. Very considerable progress is noted in the field of combustion of laminar and turbulent fuel jets since the early work of Burke and Schumann on laminar diffusion flames. A theory of burner performance is made possible by combining the theory of air entrainment of gas jets with the theory of flame stabiliza tion. It is expected that this development will find useful application PREFACE TO FIRST EDITION ix to the problem of fuel interchangeability in the gas industry. A discussion of this subject is included. The theory of detonation waves, while already far advanced by the early work of Chapman, Jouguet, and Becker, has been developed further by von Neumann and by Brinkley and Kirkwood. New explanations are suggested for the frequently observed discontinuous and spiral propa gation of detonation waves. An insight is gained into the wave structure and of the interaction between shock front and reaction zone. Fundamental research should ultimately contribute toward improved understanding and control of technical combustion processes, particu larly in engines. To accomplish this it is necessary to analyze the engine process in terms of the fundamental physical and chemical processes that occur in the various phases of starting and operation. At present this has been pursued only to a very small extent. Too often engine studies are confined to observations of the effect of fuel and engineering variables on over-all performance in a manner that excludes the possi bility of recognizing the controlling physical and chemical processes. While development of modern engines has been eminently successful, this success has only been made possible by the accumulation of a very large volume of empirical information and by the continual maintenance of large and costly testing facilities. The question may be asked whether timely fundamental research could not have eliminated a large portion of this testing that has been carried on and is still continuing on a world wide scale, and whether practical developments could not have been materially facilitated by scientific knowledge. In this text some attempt has been made to collect the meager information of fundamental char acter, first, to show that in principle it appears to be quite feasible to estimate the knock-limited performance of an Otto engine in a rather simple manner, and second, to illustrate that in jet engines a close scrutiny of the flame structure might lead to useful information on per formance limits. The authors are grateful to their colleagues in the Explosives and Physical Sciences Division of the Bureau of Mines whose interest in and devotion to their researches have materially assisted in providing the experimental basis of the newer concepts of combustion wave propa gation. BERNARD LEWIS GUENTHER VON ELBE Pittsburgh, Pennsylvania May, 1951 In the preparation of this new edition the authors acknowl edge the support in part by the United States Air Force through the Propulsion Research Division, Air Force Office of Scientific Research of the Office of Aerospace Research, under contract Number AF49(638)-307. List of Principal Symbols PART I D Diffusion coefficient d Vessel diameter F Formaldehyde / (with subscript indicat Mole fraction ing molecular species) K (with subscript refer Rate coefficient of surface reaction (average value per ring to reaction num unit volume) ber) k (with subscript refer Rate coefficient of reaction; temperature-dependent part ring to reaction num of rate coefficient of surface reaction ber) [ΛΠ Total molecular concentration [Mìe Total molecular concentration at explosion limit n Average chain carrier concentration P, v Pressure r Radius of spherical vessel U Average molecular velocity Chain branching coefficient Distance of approach in three-body collisions (with and without sub Chain breaking efficiency of surface script indicating chain carrier) λ Mean free path Effective mean free path T (subscripts 1 and 2 in Induction period or ignition lag dicating regimes in hy drocarbon oxidation) PART II Superscript ° refers to plane and adiabatic combustion wave A Cross sectional area of turbulent jet a CppuSu/μ C Critical capacitance for ignition C Mole fraction of nozzle fluid in air entraining fuel jet c, Cv Specific heat at constant pressure and volume, respec P tively D Diffusion coefficient XV XVI LIST OF PRINCIPAL SYMBOLS D Shock wave velocity; detonation velocity d Diameter of inner duct in mixing and combustion of fuel jets d! Diameter of outer duct in mixing and combustion of fuel jets do Quenching diameter of tubes dp Depth of penetration of quenching d\\ Quenching distance between parallel plates g Boundary velocity gradient ÇB Boundary velocity gradient for blow-off QF Boundary velocity gradient for flash-back H Absolute minimum ignition energy Hi Enthalpy per mole of component i h Excess enthalpy per unit area of combustion wave K A constant governing energy relations between burned and unburned gas in closed vessels K Karlovitz number Ki Rate of change of concentration of component i due to chemical reaction k Coefficient of heat conductivity L Length of flames of fuel jets l Scale of turbulence (observation at one point) t lì Scale of turbulence (synchronized observations along y axis) M Mass flow in combustion wave rrti Molecular weight of component i n Burned fraction of contents of closed vessel n Constant in Poiseuille equation m Number of moles per unit volume of component i P, p Pressure P Pressure at end of combustion in closed vessel e P Pressure in closed vessel before ignition t q Rate of heat evolution per unit volume R Gas constant R Stream radius in cylindrical streams; distance from stream center to boundary in flow between parallel plates Re Reynolds number r Distance from stream center r Flame radius n Radius of combustion wave in spherical vessel with cen tral ignition n Preignition radius of spherical volume of unburned gas in spherical vessel with central ignition S Relative velocity normal to any combustion wave sur face SL Laminar burning velocity ST Turbulent burning velocity S Relative velocity of burned gas normal to combustion b
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