Combustion chemistry of propane: A case study of detailed reaction mechanism optimization

Detailed chemical mechanisms describing hydrocarbon combustion chemistry are conceptually structured in a hierarchial manner with H-2 and CO chemistry at the base, supplemented as needed by elementary reactions of larger chemical species. While this structure gives a logical organization to combustion chemistry, the degree to which this organization reflects actual reactive fluxes in flames is noe known. Moreover, it has not been tested whether sets of rate parameters derived by optimizing fits to small-hydrocarbon combustion data are secure foundations upon which to optimize the rate parameters needed for modeling the combustion data are secure foundations upon which to optimize the rate parameters needed for modeling the combustion of larger hydrocarbons. In this work, a computer modeling study was undertaken to discover whether optimizing the rate parameters of a 258-reaction C-3 combustion chemistry mechanism that was added to a previously optimized 205-reaction C-<3 mechanism would provide satisfactory accounting for C-3 flame speed and ignition data. The optimization was done with 21 optimization targets, 9 of which were ignition delays and 12 of which were atmospheric pressure laminar flame speeds; 2 of the ignition delays and 2 of the flame speeds, all for methane fuel, had served as optimization targets for the C-<3 rate parameters. It was found in sensitivity studies that the coupling between the C-3 and the C-<3 chemistry was much stronger than anticipated. No set of C-3 rate parameters could account of the C-3 combustion data as long as the previously optimized (against C-<3 optimization targets only) C-<3 rate parameters remained fixed. A reasonable match to the C-3 targets could be obtained without degrading the match between experiment and calculation for the C-<3 optimization targets, by reoptimizing six of the previously optimized and three additional C-<3 rate parameters.