燃烧物理学核心原理与应用（Combustion Physics）
摘要：
本文为《Combustion Physics》第 1、2 章核心内容，系统讲解燃烧热力学与化学反应动力学两大基础板块。热力学部分推导化学平衡判据、平衡常数、生成焓与绝热火焰温度，结合图表分析当量比、压力对火焰温度、产物组分的影响；动力学从质量作用定律切入，介绍可逆 / 多步反应稳态、部分平衡简化手段，详述阿伦尼乌斯公式、碰撞理论、过渡态理论、RRK 单分子反应模型，区分直链与支链链式反应机理，完整给出自由基生成、消耗路径，配套反应速率推导与能量势能曲线，为燃烧数值模拟、反应机理简化提供完整理论框架。
COMBUSTION PHYSICS
In the past several decades, combustion has evolved from a scientific discipline that was largely empirical to one that is quantitative and predictive. These ad- vances are characterized by the canonical formulation of the theoretical foun- dation; the strong interplay between theory, experiment, and computation; and the unified description of the roles of fluid mechanics and chemical kinetics. This graduate-level text incorporates these advances in a comprehensive treatment of the fundamental principles of combustion physics. The presentation emphasizes an alytical proficiency and physical insight, with the former achieved through complete, though abbreviated, derivations at different levels of rigor, and the latter through physical interpretations of an alytical solutions, experimental ob- servations, and computational simulations. Exercises are designed to strengthen the student’s mastery of the theory. Implications of the fundamental knowledge on practical phenomena are discussed whenever appropriate. These distinguish- ing features provide a solid foundation for an academic program in combustion science and engineering.
Chung K. Law istheRobertH. Goddard Professor of Mechanical and Aerospace Engineering at Princeton University. He obtained his doctorate in engineering physics from the University of California at San Diego in 1973. His research interests are in combustion, propulsion, heat and mass transfer, and issues on energy and the environment. For his research accomplishments, he received the Curtis W. McGraw Research Award of the American Society for Engineering Education (ASEE) in 1984 for outstanding early achievement in research, a sil- ver medal of the Combustion Institute in 1990, the Propellants and Combustion Award of the American Institute of Aeronautics and Astronautics (AIAA) in 1994, the Heat Transfer Memorial Award, in science, of the American Society of Mechanical Engineers (ASME) in 1997, the Energy Systems Award and the Pendray Literature Award of the AIAA in 1999 and 2004, respectively, and sev- eral awards for best conference papers. He is an original member of the Highly Cited Researchers database of the Institute for Scientific Information (ISI).
Professor Law is a former president of the Combustion Institute, a Fellow of the AIAA and the ASME, and a member of the U.S. National Academy of Engineering.
Preface
Since the mid-1970s there has been truly significant advancement in combustion sci- ence, spurred by the dual societal concerns for energy sufficiency and environmental quality, and enabled by the rapid increase in the sophistication of mathematical an alysis, computational simulation, and experimental techniques. Consequently, we have witnessed the evolvement of combustion from a scientific discipline that was largely empirical to one that is quantitative and predictive, leading to its useful applications in combustion-related engineering devices and practices.
This text reflects my desire to incorporate these advancesin my lectures on combus- tion. As a result, its preparation has been guided by the three distinguishing themes characterizing recent developments in combustion research, namely the canonical formulation of the theoretical foundation; the strong interplay between experiment, theory, and computation; and the description of combustion phenomena from the unified viewpoint of fluid mechanics and chemical kinetics.
The text also emphasizes an alytical proficiency by presenting complete, albeit ab- breviated, derivations that can be followed by the student with a modest effort. Alternate solutions are sometimes presented to demonstrate that a phenomenon can often be an alyzed using different approaches and at different levels of rigor. I hope that through this gentle guidance the student can acquire the needed confidence to tackle more difficult problems on his or her own.
This text grew out of the lecture material prepared for a one-year graduate course that I have given at several academic institutions. No prerequisite in mathematics, fluid mechanics, and chemistry is expected apart from the usual undergraduate ed- ucation in the physical sciences or mechanical, aerospace, or chemical engineering. The text consists of three parts: Chapters 1 through 4 cover the basic components required to describe chemically reacting flows, namely thermodynamics, chemical kinetics, and transport phenomena; Chapters 5 through 10 cover descriptions of the basic combustion phenomena—those of governing equations, nonpremixed and pre- mixed flames, the limit phenomena of ignition, extinction, and flame stabilization, and the aerodynamics of flames; Chapters 11 through 14 cover combustion in the four major classes of flows, namely turbulent, boundary-layer, two-phase, and supersonic flows. Since the amount of material treated in this text is substantial, the instructor may be more selective in the choice of topics. For example, discussion on reaction mechanisms, especially most of Chapter 3, can be omitted if chemistry is not empha- sized in the course. Similarly, much of the materials that require extensive mathemat- ical derivations, especially those of Chapter 9, can be omitted if strong mathematical experience is not intended. Furthermore, a one-semester course can be structured by abstracting materials from individual chapters, leaving the rest of the text for the enrichment of individual students.
While a serious attempt was made to make the text comprehensive in its cover- age, it is nevertheless inevitable that some important topics were either excluded or inadequately presented. Feedback from readers on possible improvements in future editions will be very much appreciated. Similarly, because of the extensive literature in existence, it is also unavoidable that important references were inadvertently left out. Forbearance of the authors of these articles is requested.
In the preparation of this text I have been ably assisted by many of my present and former graduate students and research associates. In particular, I acknowledge with appreciation the following who have contributed substantially in this effort: John K. Becktold, Beei-Huan Chao, Peck Cho, Suk-Ho Chung, Fokion N. Egolfopoulos, Hong G. Im, Tianfeng Lu, Atsushi Makino, Matei I. Radulescu, Chih-Jen Sung, Hai Wang, Heyang Wang, and Delin Zhu. The manuscript was read in part or in whole by Professor Craig T. Bowman of Stanford University, Professor Sau-Hai Lam of Princeton University, and Professor Forman A. Williams of the University of California at San Diego. Their comments have been substantial and most useful, and I thank them sincerely for their collegiality and generosity.
It was by chance that I became a student of Professor Forman A. Williams in the spring of 1970. His influence on my intellectual and professional development has been profound. I am immensely thankful for his mentorship.
I reserve my most heartfelt appreciation for my wife, Helen Kwan-mei, for having transcribed the first drafts of this text, for constantly encouraging me to bring it to fruition, and for her patience and love over the years.
Chung K. Law
Princeton, New Jersery January 2006
Introduction
ustion science and technology and, as such, covers not only the basic laws and phenomena related to the physics and chemistry of combustion, but also the implications of the fundamental understanding gained therein to the principles behind the practical combustion phenomena affecting our daily lives. It presents the diverse knowledge required of combustion scientists and engineers, the challenges they face, and the satisfaction they derive in providing the proper linkage between the fundamental and the practical.
In Section 0.1 we identify the major areas of practical combustion phenomena, illustrated by some specific problems of interest. In Section 0.2 we discuss thescientific disciplines comprising the study of combustion, and in Section 0.3 we present the classifications of fundamental combustion phenomena. An overview of the text is given in Section 0.4.
0.1. MAJOR AREAS OF COMBUSTION APPLICATION
It is fair to say that the ability to use fire is an important factor in ushering the dawn of civilization. Today our dependence on the service of fire is almost total, from heating and lighting our homes to powering the various modes of transportation vehicles. Useful as it is, fire can also be menacing and sometimes deadly. Wildland and urban fires cause tremendous loss of property and lives every year; the noxious pollutants from automotive and industrial power plants poison the very environment in which we live; and the use of chemical weapons continues tobe an agent of destruction with ever greater efficiency. Combustion is certainly one branch of science that affects almost every aspect of human activities.
Practical combustion problems can be roughly divided into the following five major categories, in each of which we cite some examples of current interest.
Energy and Combustion Devices: Despite the large variety of alternate energy sources available, such as nuclear, solar, wind, hydroelectric, geothermal, and OTEC (ocean thermal energy conversion), chemical energy derived from burning fossil fuels supplies a disproportionately large fraction of the total world energy needs—around 85 percent at present. This trend will continue in the foreseeable future because of its convenience, high-energy density, and the economics.
Combustion energy is mainly used to generate heat and power. Examples of this application are domestic heating, firing of industrial furnaces, and the operation of automotive engines and gas turbines. Hence the design and operation of heat and power devices and engines is closely related to the issue of efficient energy utiliza- tion. Because of the importance of transportation vehicles as a major consumer of petroleum fuels and contributor of air pollution, there has been extensive develop- ment since the early 1970s for more efficient and cleaner burning internal combustion engines for automobiles. For example, the diesel engine offers substantial advantage over the more widely used gasoline engines, for several reasons. First, even though its combustion cycle efficiency is less than that of the gasoline engine for the same compression ratio, it is more efficient overall because it operates at higher compres- sion ratios. Furthermore, unlike the gasoline engine, which requires highly refined fuels with narrow specifications, the diesel engine is very fuel tolerant. Thus diesel fuel requires less refining than gasoline and, consequently, results in a net saving in processing energy at the refinery stage. This property of fuel tolerance also implies that the diesel engine is a good candidate for the use of unconventional or low-grade fuels. The diesel engine, however, does have the potential disadvantages of being relatively noisier and a heavy emitter of soot and oxides of nitrogen (NOx ); both problems have their origin in its operational principle and therefore require funda- mental combustion research. It is nevertheless gratifying to note that much progress has been made recently in alleviating these problems.
An important concept in engine development is that of stratified charge combustion. The basic idea is that the combustion of lean mixtures has the potential of simultaneously increasing the combustion efficiency and reducing the formation of most pollutants. Lean mixtures, however, are hard to ignite. Therefore, the con- cept of stratified charge combustion is to stratify an overall fuel lean mixture from relatively rich to ultra lean. Since the relatively rich portion can be ignited easier, the hot combustion products so generated can in turn ignite the ultra lean portion of the charge. Thus by combining the merits of high-pressure combustion, direct fuel injection for uniform cylinder-to-cylinder charge distribution and controlled fuel vaporization, spark ignition for controlled ignition event, and stratified charge combustion, there has been considerable development on high-compression-ratio, direct-injection, spark-assisted, stratified charge engines.
In contrast to stratified charge engines, there is also considerable interest in the development of HCCI (homogeneous charge compression ignition) engines. Conceptually, by having reaction taking place homogeneously within the entire en- gine cylinder, instead of being confined to localized, high-temperature regions con- stituting the flames, the formation of soot and NOx can be substantially reduced.
Furthermore, higher compression ratios and hence higher efficiency can be attained with compression ignition.
The fact that improvements in the engine performance can be pursued through the opposite concepts of stratified and homogeneous charges not only demonstrates the complexity of the combustion phenomena underlying such technological processes, but it also highlights the richness of the possible avenues that can be explored for optimization.
Fuels: Combustion needs fuel. Furthermore, the satisfactory operation of different heat and power engines usually depends critically on the compatibility of the fuel used. Examples are the unsuitability of diesel fuel for use in gasoline engines because it is relatively less volatile, and the narrow compositional specifications of gases which can be used in domestic gas stovesin order to maintain flame stabilization by avoiding blowoff and flashback.
The importance of fuel in combustion has been receiving increased interest be- cause of the concern over the shortage and reliability of petroleum supply. Thus “energy crisis” is simply a “fuel crises.” Since the world’s petroleum supply is pro- jected to be severely depleted within this century, the long term solution for the next few centuries in terms of fossil fuels appears to largely depend on the burning of coal, either through direct utilization or as coal-derived fuels. Two approaches for direct coal utilization are being actively pursued. The first is fluidized-bed combustion, in which air is introduced through the bottom of a bed of coal particles at a sufficiently fast rate such that the particles are levitated, that is, fluidized. This approach has the advantages that the coal particles are in direct contact with the oxidizing air such that their burning rates are maximized, that neutralization of oxides of sulfur (SOx ) can be facilitated by mixing limestone with the coal particles, and that the production of NOx can be minimized by controlling the fluidization rate. The second approach for direct coal utilization is the burning of coal–water slurries. Here, finely crushed coal particles of sizes ranging between 40–70 μm are mixed in water and sprayed directly into the combustion chamber of industrial furnaces. The advantages are that the physical processes of coal crushing and mixing are less energy expensive than the chemical process of coal liquefaction, and that the slurries can be transported through pipelinesand subsequently directly burned in conventional oil-fired combus- tors. This requires minimum hardware modification, and thereby capital outlay and combustor downtime. Slurries up to 70 percent coal content have been successfully burned.
Oil can also be derived from coal. These coal-derived oils have higher boiling points, wider boiling point ranges, and higher contents of aromatics and nitrogen- containing compounds. Consequently, they tend to produce more soot and NOx . Various alternate and hybrid fuels have also been formulated. Prominent among these are methanol, ethanol, and mixtures of ethanol with oil. Methanol can be derived from natural gas and coal, while both methanol and ethanol can be produced from biomass. Alcoholshave smaller heats of combustion because of the extra oxygen atom in the molecule. However, they have higher knock ratings in gasoline engines and produce less NOx and soot. Blends of ethanol and gasoline, and methanol and gasoline, have been successfully marketed.
Coal, of course, can also be gasified in the presence of air, with or without steam, to produce a combustible gaseous fuel that consists of hydrogen and carbon monoxide. Coal gasification becomes progressively more attractive as a source of clean fuel with the dwindling supply of natural gas.
Pollution and Health: The major pollutants from combustion are soot, SOx , NOx , unburned hydrocarbons (UHC), and carbon monoxide (CO). As just mentioned, soot is expected to be a serious problem with the burning of coal-derived fuels and the large-scale deployment of high-compression engines such as the diesel. Soot not only is unsightly but can also be carcinogenic due to the condensation and thereby presence of carcinogenic liquid combustion products on the particle surface.
The main source of SOx is from burning coal. When combined with water in the atmosphere, the emitted SOx forms sulfuric acid and precipitates as acid rain, with devastating effects on aquatic life and soil erosion.
NOx can be formed from either the N2 in the atmosphere or the nitrogen atoms in the fuel molecules, with the former produced under high-temperature, intense combustion situations because of the need to dissociate the nominally inert N2 in the air. Fuel-bound NOx is less temperature sensitive and could be a major contributor of NOx emission from burning coal or coal-derived oils. When it reacts with UHC and ozone in the presence of sunlight, NOx forms smog that is detrimental to the respiratory system.
A problem of potential concern is indoor pollution. With houses being better insu- lated to conserve energy, the trace pollutants (CO, NOx , UHC), from such domestic heating devices as the gas stove, furnace, and kerosene heater, may exist atsufficiently high levels as to be injurious to health.
There is also interest in applying combustion technology in the management of municipal, munition, and chemical hazardous wastes through incineration. The prob- lems with burning these wastes are the uncertainty of the toxicity of the combustion intermediates and products and the fact that some of the chemicals are halogenated compounds, which can be resistant to efficient burning because of the scavenging of the crucial hydrogen atom by the halogen radicals in the oxidation process.
A serious, and potentially catastrophic, environmental problem is global warming caused by the increased amount of anthropogenic CO2 in the atmosphere. Since CO2 is a by-product of hydrocarbon combustion, suggestions have been made to use hydrogen as the primary fuel source. In the event that hydrogen is derived through the conversion of hydrocarbons, CO2 is still produced during conversion and needs to be sequestered properly in order to prevent its release into the atmosphere.
A discussion on the adverse effects of combustion on health would not be complete without mentioning the well-established cancer-causing consequence of cigarette
Safety: This topic can be divided into three categories, namely fires, explosions, and materials. Fires, both structural and wildland, are costly in terms of human suffering as well as financial loss. Problems of interest include improving fire detection tech- nology and understanding the dynamics of fire propagation in confined spaces such as buildings and aircraft cabins.
Explosions are of concern to safety in mine galleries and grain elevators, as a con- sequence of LNG (liquefied natural gas) spills or rupturing of pressurized hydrogen storage tanks in urban areas, and in nuclear reactor accidents. In the last example, hydrogen gas is generated and could accumulate in sufficient quantity to cause an explosion. This would in turn rupture the reactor containment structure, causing the release of radioactive gases into the environment.
Since the inhalation of smoke and the toxic products of combustion is a cause of fatality in fires, the choice of materials for structure and decoration is also an important consideration in the overall strategy for fire control.
A strategy toward the prevention of fires and explosions in aircraft and combat vehicles, such as tanks, is the development of fire-safe fuels which, while burning well within the engine, will not catch fire upon spillage. For example, diesel oil emulsified with a small amount of water has been found to be fire resistant.
Defense and Space: The various defense establishments are interested in the for- mulation of high-energy munitions and propellants; the suppression of combustion instability within jet engines, rockets and guns; signature and detection vulnerability from the exhausts of jet engines and rockets; and measures at preventing explosion of fuel tanks when being penetrated by projectiles. The development of chemical lasers as an intense power source and of hypersonic aircraft up to Mach 25 are also of interest to the national defense.
Since combustion experiments conducted on earth are frequently complicated by the presence of buoyant flows, there has been much interest to conduct these exper- iments in the weightless environment of a space shuttle or station. The intrusion of buoyancy is particularly problematic when the burning is slow as in the propagation of a flame in a weak mixture, or for long-duration phenomena such as smoldering. The presence of buoyancy can also distort the flame configuration from an other- wise symmetrical one, and hence significantly complicates data reduction as well as theoretical an alysis or computational simulation of the phenomenon of interest.
Fire safety is of paramount interest in space exploration. For example, while earth- bound smoke detectors of incipient fires are placed at the ceiling of a room in or- der to capture the buoyancy-driven, upwardly rising smoke, they are clearly inop- erative in the weightless space environment. Furthermore, flammability standards established on earth may not have much meaning for the fire safety evaluation of a spacecraft.
Recognizing that the environment within a spacecraft is artificial anyway, there has been the suggestion of creating an almost fire-proof living environment so that fire hazard ceases to be a concern. This concept is based on the recognition that whereas ignition and combustion intensity depend on the fractional amount of oxygen in the oxidizing gas, human comfort depends only on the absolute amount of oxygen. Furthermore, it is also empirically known that the combustibility of most organic materials decreases drastically with decreasing oxygen concentration. They become hardly flammable when the oxygen concentration is reduced to less than, say, 15 mole percent. Thus if we can reduce the cabin oxygen concentration to half of its value in air, but increase the cabin pressure to two atmospheres, then a comfortable, but fire-proof environment can be created.
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