an important tool in the analysis of combustion ... analysis of practical energy conversion systems such ... important kinetic problems including particularly the.
0360-I285/84SO.OO+ -50 Pcrgamon Press Ltd.
Prog. EntrgyComlnut. Set. 1984.Vol. 10, pp. 1-57. Printed in Great Britain.
CHEMICAL KINETIC MODELING OF HYDROCARBON COMBUSTION Charles K. Westbrook* and Frederick L. DRYERf 'Lawrence Litermore National Laboratory, University of California, Lkermore, California 94S50, U.S.A. i Departmentof Mechanicaland Aerospace Engineering, Princeton University, Princeton, New Jersey 08544, U.S.A.
Abstract—Chemical kinetic modeling of high temperature hydrocarbon oxidation in combustion is re viewed. First, reaction mechanisms for specific fuels are discussed, with emphasis on the hierarchical structure of reaction mechanisms for complex fuels. The concept of a comprehensive mechanism is de
veloped, requiring model validation by comparison with data from a wide range of experimental regimes. Fuels of increasingcomplexity from hydrogen to n-butane are described in detail, and further extensions of the general approach to other fuels are discussed. Kinetic modification to fuel oxidation kinetics is considered, including both inhibition and promotion of combustion. Simplified kinetic models are then described by comparing their featureswith those of detailed kinetic models. Finally, application of kinetic models to study real combustion systems are presented,
beginning with purely kinetic-thermodynamic applications, in which transport effects such as diffusion of heat and mass can be neglected, such as shock tubes, detonations, plug flow reactors, and stirred reactors. Laminar flames and the coupling between diffusive transport and chemical kinetics arc then described, together with applications of laminar flame models to practical combustion problems.
1. INTRODUCTION
In recent years, chemical kinetic modeling has become an important tool in the analysis of combustion systems. Availability of large amounts of elementary kinetic data, improved techniques for estimating specific reaction rates, development of efficient "stiff equation" solution techniques, and continual growth in the size, speed, and availability of large computers have contributed to the increasing application of
detailed chemical kinetic modeling. For at least twenty years this approach has been employed in the simu lation of controlled laboratory experiments. More recently, kinetics models have become useful in the analysis of practical energy conversion systems such as internal combustion engines. The influences of
higher temperature conditions encountered in flames and explosions (T > 1000K). High temperature com bustion studies are complicated by the fact that typical time scales are very short, often of the order of micro seconds. As a result, spatial scales are also very small, making experimental studies very difficult. On the other hand, high temperature reaction mechanisms can be conceptually simpler than those for combus tion below 700 K.
We will first discuss in detail the kinetic mechanisms
which are used to describe the combustion of hydro
carbon fuels. Applications of these kinetic models to the analysis of selected types of problems will then be examined. 2. PROBLEM FORMULATION
kinetic factors on other combustion fields such as the
assessment of safety factors involved with large scale storage, transportation and use of liquid and gaseous fuels are only beginning to receive attention. Other important kinetic problems including particularly the oxidation of practical fuels in turbulent flows also need a great deal of examination. Although many fuel types are encountered in com bustion environments, hydrocarbons comprise the vast majority. In this review we will devote most of our attention to the combustion of hydrocarbon
The general mathematical formulation of the prob lem of chemically reactive flow systems8"" consists of equations for conservation of mass, momentum, energy, and concentration of each chemical species, together with equation of state and other thermo dynamic relationships. Chemical kinetics provides the coupling among the various chemical species concen trations, and with the energy equation through the heat of reaction. In many combustion problems the kinetics terms determine the characteristic space and
fuels, but it should be pointed out that the same types of analyses can be applied to other fuels such as ammonia, hydrazine, carbon disulfide, and many others based on N-H, C-S, N-C-H, or other com binations rather than C-H systems. We will only briefly discuss the kinetics of the formation ofchemical pollutants, a subject which has been treated in detail
time scales over which the equations must be solved. When spatial transport effects can be neglected, the conservation equations become a coupled set of ordinary differential equations (ODE's) for the species
elsewhere.' "J Detailed reviewsof hydrocarbonoxida tion have appeared quite recently,4"7 emphasizing those processes which are dominant at temperatures below about 700 K. Here we will deal primarily with JPBCS 10:1-*
concentrations an 1 the energy (or temperature) with time as the independent variable. If transport terms must be considered, then the equations are coupled partial diflerential equations involving derivatives with respect to both time and space. Examples of both types of systems will be presented in the second half of this paper.
