The significance of intermediate hydrocarbons during wall quench of propane flames

Abstract

A one-dimensional model is used to study end-on wall quench using a detailed chemical kinetics mechanism for propane. Previous models using detailed chemical kinetics mechanisms for methane, methanol, and acetylene revealed that intermediate hydrocarbons exist at much lower levels than unreacted fuel molecules during quench, and thus led to the general conclusion that one-step global chemistry (which accounts only for the rate of disappearance of the fuel) is adequate for studying hydrocarbon evolution during wall quench. However, these fuels are extremely simple, low molecular weight molecules with very limited paths available for forming intermediate hydrocarbons. In the present study, wall quench is studied for propane-air mixtures at equivalence ratios of 0.9, 1.0, and 1.1; pressures of 1, 10, and 40 atmospheres; and wall temperatures of 400 and 500 K. It is shown that intermediate hydrocarbons exist at higher levels and at greater distances from the wall during quench than unreacted fuel. Furthermore, the intermediate hydrocarbons are oxidized less rapidly and persist at significant levels much longer after quench. The persistence of the intermediate hydrocarbons is aggravated by lower wall temperatures, lower pressures, and equivalence ratios both greater than and less than stoichiometric. At lower pressures, the rate of oxidation of the intermediate hydrocarbons is slowed to an even greater extent than is the fuel oxidation rate. The conclusion that intermediate hydrocarbons contribute significantly to wall quench hydrocarbon evolution indicates that one-step global chemistry is inadequate for studying turbulent wall quench, bulk quench, and crevice volume quench of higher hydrocarbon fuels and thus casts doubt on the use of the results of previous theoretical wall quench studies to obtain general conclusions regarding wall quench hydrocarbon evolution in practical engines using practical fuels.