A Comprehensive Modeling Study of n-Heptane Oxidation H. J. CURRAN,* P. GAFFURI, W. J. PITZ, AND C. K. WESTBROOK Lawrence Livermore National Laboratory, Livermore, CA A detailed chemical kinetic mechanism has been developed and used to study the oxidation of n-heptane in flow reactors, shock tubes, and rapid compression machines. Over the series of experiments numerically investi- gated, the initial pressure ranged from 1– 42 atm, the temperature from 550 –1700 K, the equivalence ratio from 0.3–1.5, and nitrogen-argon dilution from 70 –99%. The combination of ignition delay time and species composition data provide for a stringent test of the chemical kinetic mechanism. The reactions are classed into various types, and the reaction rate constants are given together with an explanation of how the rate constants were obtained. Experimental results from the literature of ignition behind reflected shock waves and in a rapid compression machine were used to develop and validate the reaction mechanism at both low and high temperatures. Additionally, species composition data from a variable pressure flow reactor and a jet-stirred reactor were used to help complement and refine the low-temperature portions of the reaction mechanism. A sensitivity analysis was performed for each of the combustion environments. This analysis showed that the low-temperature chemistry is very sensitive to the formation of stable olefin species from hydroperoxy-alkyl radicals and to the chain-branching steps involving ketohydroperoxide molecules. © 1998 by The Combustion Institute INTRODUCTION There is continued interest in developing a better understanding of the oxidation of large hydrocarbon fuels over a wide range of operat- ing conditions. This interest is motivated by the need to improve the efficiency and performance of currently operating combustors and reduce the production of pollutant species emissions generated in the combustion process. This study particularly focuses on the effect of elevated pressures on the oxidation of n-heptane. Many important practical combustion systems such as spark-ignition, diesel and gas-turbine engines operate at pressures well above 1 atm. n-Hep- tane is a primary reference fuel (PRF) for octane rating in internal combustion engines and has a cetane number of approximately 56, which is very similar to the cetane number of conventional diesel fuel. Therefore, a better understanding of n-heptane oxidation kinetics is useful in studies of engine knock and autoigni- tion. Recent experimental studies of n-heptane oxidation have focused on shock tubes [1–5], jet-stirred reactors [6 – 8] performed under sta- tionary conditions, rapid compression machines [9 –12], engines [13–19], plug flow reactors [20 – 22], and jet-stirred flow reactors [23, 24] in which a dynamic behavior is observed. All of these systems exhibit phenomena including self- ignition, cool flame, and negative temperature coefficient (NTC) behavior. Furthermore, vari- ation in pressure from 5– 40 bar changes the temperature range over which the NTC region occurs. Recent modeling studies of the premixed systems such as stirred reactors and shock tubes cited above [2– 6] have helped in the develop- ment of detailed chemical kinetic mechanisms that describe n-heptane oxidation. These publi- cations have been complemented by the work of Chevalier et al. [25, 26], Muller et al. [27], Foelsche et al. [28], and Lindstedt and Maurice [29]. In addition, Bui-Pham and Seshadri [30] carried out a numerical study of an n-heptane diffusion flame. More recently, Ranzi et al. [32] have used a semi-detailed chemical kinetic model to simulate n-heptane pyrolysis and oxi- dation. In addition, this semi-detailed model was used to simulate the oxidation of primary reference fuel (n-heptane and 2,2,4-trimethyl- pentane) mixtures [22]. Co ˆme et al. [33] have used a computer package to generate chemical kinetic mechanisms for n-heptane and iso-oc- tane. In this study we include all of the reactions known to be pertinent to both high- and low- temperature kinetics. We show how the detailed Corresponding author. Current address of H. J. Curran, L-407, Lawrence Liver- more National Laboratory, Livermore, CA 94550. COMBUSTION AND FLAME 114:149 –177 (1998) © 1998 by The Combustion Institute 0010-2180/98/$19.00 Published by Elsevier Science Inc. SSDI 0010-2180(97)00282-4