Combustion, Flames and Explosions of Gases PDF
Preview Combustion, Flames and Explosions of Gases
Combustion, Flames and Explosions of Gases Third Edition Bernard Lewis Ph.D., Sc.D. (Cantab.) Guenther von Elbe Ph.D. (Berlin) Combustion and Explosives Research, Inc. Pittsburgh, Pennsylvania 1987 ACADEMIC PRESS, INC. Harcourt Brace Jovanovich, Publishers Orlando San Diego New York Austin Boston London Sydney Tokyo Toronto COPYRIGHT © 1987 BY ACADEMIC PRESS, INC. ALL RIGHTS RESERVED. NO PART OF THIS PUBLICATION MAY BE REPRODUCED OR TRANSMITTED IN ANY FORM OR BY ANY MEANS, ELECTRONIC OR MECHANICAL, INCLUDING PHOTOCOPY, RECORDING, OR ANY INFORMATION STORAGE AND RETRIEVAL SYSTEM, WITHOUT PERMISSION IN WRITING FROM THE PUBLISHER. ACADEMIC PRESS, INC. Orlando, Florida 32887 United Kingdom Edition published by ACADEMIC PRESS INC. (LONDON) LTD. 24-28 Oval Road, London NW1 7DX Library of Congress Cataloging in Publication Data Lewis, Bernard, Date Combustion, flames, and explosions of gases. Includes index. Ί. Combustion. 2. Flame. 3. Explosions. 4. Gases. I. Von Elbe, Guenther, Date . II. Title. QD516.L38 1987 665.7 86-20590 ISBN 0-12-446751-2 (alk. paper) PRINTED IN THE UNITED STATES OF AMERICA 87 88 89 90 9 8 7 6 5 4 3 2 1 Preface to the Third Edition This third edition of Combustion, Flames and Explosions of Gases adheres to the basic purpose of this treatise as stated in the preface to the first edition, namely, to provide the chemist, physicist, and engineer with the scientific basis for understanding combustion phenomena. The principal revision of the text concerns the oxidation kinetics of carbon mon oxide and the much more ramified and complex oxidation kinetics of hydrocarbons. The carbon monoxide-oxygen system is found to be so completely interlinked with the well-understood hydrogen-oxygen system that in the total absence of hydrogenous compounds no reaction path is discernible by which the latent enthalpy of the system can be released in rapid reaction. This insight furnishes interpretations of phenomena such as the steady and oscillatory glow described by Peter Gray and his colleagues. The treatment of hydrocarbon oxidation kinetics has been completely revised pri marily on the basis of re-examination of the excellent data that are found in earlier literature. Newer investigations support and amplify the deductions drawn from this earlier research. This confirms that careful and methodical investigations retain validity, unaffected by the passage of time, even though the then available methods of experimentation and data reduction were far removed from present techniques of computerization and a plethora of modern sensing and recording devices. In the past two decades computer programs have come into use to manage complex problems of reaction kinetics. This is especially appropriate for high-temperature reactions in hydrocarbon combustion such as occur in experimental flow systems and in combustion waves. In such systems the overall reaction tends to go to completion and in the course of this process virtually all imaginable reactions between the reactants and the numerous intermediate products may take place. Whereas, we fully agree that such problems are manageable only by means of computer techniques, we have reservations with respect to other problems such as initiation of explosive reactions in the stagnant gas of a reflected shock wave. Here, the initial concentration of free radicals is virtually zero. Hence the initial reaction rate is very small or imperceptible but builds up during an induction period to explosive proportions at a self-accelerating rate. In the hydrocarbon chapter of this text, the treatments of such xi xii PREFACE TO THE THIRD EDITION problems are given, which are based on the concept that the development of the reaction rate from initially zero to explosive rates is governed by only a few chain- branching and chain-breaking reactions and that reactions involving species that appear only in the later stages of the overall reaction require no consideration. Accordingly, no extensive catalogue of reactions is required, and the problem of the induction period is approached by reaction-kinetic analysis rather than by computer programs. The reader will observe that a new concept has been developed in the interpretation of the low-temperature oxidation of hydrocarbons, which is marked by the appearance of the much-studied cool flames. We now discern two chain-branching reactions referred to as chain branching by double peroxidation and by alkoxyperoxide dis sociation. In conjunction with Benson's explanation of what was formerly referred to as the "negative temperature" coefficient in hydrocarbon oxidation, it is now possible to describe the whole range of phenomena occurring in this complex reaction regime. A vast number of investigations have been recorded in the literature that specializes in combustion. Since 1962, 66 volumes of the Combustion Institute's Journal of Combustion and Flame, 13 volumes of the International Combustion Symposia published by the Institute, and numerous volumes of other journals, notably Com bustion Science and Technology and Progress in Energy and Combustion Science have made available an enormous number of generally excellent studies on details of every aspect of combustion. It seems, however, that the conceptual basis of the discipline had already been sufficiently established at the time of the appearance of the previous edition of this book to make unnecessary a revision of the subject of flame propagation in Part Π, except for some specific points meriting further attention. For example, a new insight is offered on the relation between the experimental minimum ignition energy of electric sparks and experimentally determined quenching distances. Another example is the theoretical understanding of the observed increase of the burning velocity with increasing pressure whenever the burning velocity exceeds the order of 50-100 cm/sec and of the reversal of this effect when the burning velocity is below this range. The explanation of this effect illustrates the understanding that has been achieved concerning the role of chemistry in flame propagation. The state of the burned gas in Part ΙΠ had already been adequately treated in the first edition. This also applies to the subject of internal combustion engines in Part IV. The authors acknowledge their indebtedness to Leland A. Watermeier, of the Ballistic Research Laboratory, Aberdeen Proving Grounds, and to Robert S. Levine of the National Bureau of Standards for a grant that enabled them to undertake a study of the kinetics of oxidation of carbon monoxide on which Chapter HI is based. In the preparation of Chapter VI the authors acknowledge with appreciation the PREFACE TO THE THIRD EDITION xiii advice and contributions of Béla Karlovitz. The Atlantic Research Corporation, through the good offices of Edward Τ McHale, was most helpful in making available graphic and computer facilities. Thanks are due to Hartwell F. Calcote of AeroChem Research Laboratories, Inc., for fruitful discussion of the subject of ions in flames. BERNARD LEWIS Pittsburgh, Pennsylvania GUENTHER VON ELBE Alexandria, Virginia May 1986 Preface to the Second Edition The tempo of combustion research has continued unabated during the past decade. Substantial progress has been made in establishing a common 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 correla tions and predictions can be made on the basis of secure and realistic models. In this new edition particular emphasis has been placed on the modification of combustion wave propagation due to heat loss to the unburned medium and to localized changes of mixture composition by diffusion 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 parameters of burning velocity and wave width with flame stability limits and with spark ignition data. In this way, for example, the flame stability limits on flame holders in high velocity streams have been deduced. Fur thermore, it is shown that in all instances of divergent propagation diffusional stratification of mixture composition occurs to a degree depending 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 hydrocarbon 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. Other parts of this edition have not required the substantial revision given the subject of combustion waves. However, some revisions have been made particularly XV xvi PREFACE TO THE SECOND EDITION 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 the 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 modern 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, physicist, and engineer with the scientific basis for understanding combustion phenomena. The terms combustion, flame, and explosion became part of the common language long before clear scientific concepts existed, and therefore 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 detonation waves. On the other hand, the thermodynamic aspects of combustion had already attained a degree of finality many years ago and therefore 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 deduced from the latest available evidence and which should be regarded 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 xvii xviii PREFACE TO THE FIRST EDITION 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 reactive mechanism appears to be approaching some degree of clarification although quan titative data are still lacking. Considerable progress has been made in the understand ing 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 oxidation 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 elucidation of elementary chemical reactions. In the second part of the book dealing with flame propagation, emphasis 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 combustion 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 combustion 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 stabilization. It is expected that this development will find useful application 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 propagation of detonation waves. An insight is gained into the wave structure and to the interaction between shock front and reaction zone. Fundamental research should ultimately contribute toward improved understanding and control of technical combustion processes, particularly in engines. To accomplish PREFACE TO THE FIRST EDITION xix 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 possibility of recognizing the control ling physical and chemical processes. While development of modern engines has been erninently successful, this success has only been made possible by the accu mulation 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 worldwide scale, and whether practical developments could not have been materially facilitated by scientific knowl edge. In this text some attempt has been made to collect the meager information of fundamental character, 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 performance 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 propagation. BERNARD LEWIS GUENTHER VON ELBE Pittsburgh, Pennsylvania May 1951
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