Reduced Kinetic Mechanisms for Propane Diffusion Flames

Abstract

15.1 Introduction Propane is an important practical fuel and its high temperature combustion is characterised by the rapid decomposition into smaller C 1 -C 3 intermediates [15.1]. This behaviour is similar to the combustion of more complex hydrocar-bon fuels. From a modelling perspective, a propane combustion mechanism, compared to that of other higher hydrocarbons, requires the smallest num-ber of species and reactions that are necessary for a thorough kinetic study of the CI-C3 species. Previous modelling studies of propane combustion with detailed [15.1,15.2] and simplified [15.3] chemistry have mainly focused on premixed flames with little attention given to non-premixed conditions [15.4-15.5]. There is also a lack of simplified mechanisms based on the systematic reduction of complex chemical mechanisms for non-premixed propane flames. The purpose of the present study is to formulate reduced reaction mecha-nisms based on the systematic theoretical investigation of propane-air diffu-sion flames using a planar counterflow geometry and the detailed chemistry defined in Chap. 1. Propane flames are here computed using rates of strain from lO/s to extinction at pressures ranging from 1 to 10 bar. The deduced mechanisms are also validated against the experimental results obtained by Tsuji and Yamaoka [15.6] for counterflow propane-air flames at strain rates of 150/s and 350/s. Using the results of the above computations the behaviour of non-premixed propane flames is analysed and the most important reaction paths indentified as functions of rate of strain and pressure. The validity of steady-state assumptions for intermediate species are also examined. Subse-quent formulations of the reduced mechanisms are based on the use of steady-state assumptions for most of the species and the elimination of unimportant reactions from the original detailed mechanism. 260 15. Reduced Kinetic Mechanisms for Propane Diffusion Flames 15.2 Main Reaction Paths in Non-Premixed Propane Flames The full mechanism used in the present study is shown in Table 1 of Chap. 1. The reactions (83)-(87), which involve CH20H and CH30H, are neglected in the present study as they are considered to be unimportant for non-premixed propane flames. Computations have been performed in the range from 1 to 10 bar for the entire strain rate range with this mechanism. These computa-tions showed that the removal regions of propane and oxygen are well sepa-rated spatially even in highly strained flames at elevated pressures. For ex-ample, in a flame at 10 bar pressure and with a strain rate of 2000/s, which is close to the extinction point of 2300/s, the maximum rates of consumption of propane and oxygen occurs at mixture fractions of 0.094 and 0.085 respec-tively, with the mixture fraction (Z) obtained according to the definition in