Flame studies of conventional and alternative jet fuels and their surrogates

Conventional and alternative jet propellant (JP) fuels contain a large number of compounds, which makes the modeling of their combustion a rather daunting task. Fuel surrogates, typically mixtures of a small number of neat hydrocarbon compounds, can be used to mimic the physical and chemical characteristics of realistic fuels. The fuel surrogates must be selected carefully in order to reproduce accurately a variety of combustion characteristics of realistic fuels.;Towards the goal of developing a combustion kinetic model for jet fuel surrogates, archival experimental data on global flame phenomena such as propagation and extinction are essential. The counterflow configuration was used to determine laminar flame speeds and/or extinction strain rates experimentally over a wide range of fuel surrogate candidates including normal alkanes (5 ≤ carbon number ≤ 12), branched alkanes (C8 isomers), cyclic alkanes (cyclohexane and mono-alkylated cyclohexanes), and aromatics (cyclopentadiene, benzene, and alkylated benzenes). Experiments were conducted for both premixed and non-premixed flames at ambient pressure and elevated unburned reactant temperatures. The experimental data were simulated using detailed description of chemical kinetics and molecular transport. Analysis of the computed flame structures provided insight into the physical and chemical processes that control the overall oxidation process. The effects of two possible transport formulations and the boundary velocity gradients realized in the experiments were investigated numerically. The results indicated that both the type of transport formulation and boundary velocity gradient have little or no influence on the propagation of vigorously burning stretched flames, however, they do affect the computed state of extinction.;Detailed sensitivity and reaction path analyses were performed to illustrate the oxidation chemistry of the fuels components in the present study. Compared to flame propagation, flame extinction is in general more sensitive to chemical kinetics and molecular diffusion, especially under non-premixed conditions.;The high temperature oxidation of normal, branched, and cyclic alkanes was determined to be largely sensitive to H2/CO and C1-C 4 small hydrocarbon chemistry, with little or no influence from the fuel-related reactions. The variations of the values of laminar flame speeds and/or extinction strain rates for branched and cyclic alkanes are mainly attributed to differences in the production of C2-C4 intermediates. On the other hand, fuel-specific reactions were found to restrict the overall oxidation rates of aromatics compounds. Intermediates directly produced from the initial consumption of aromatics compounds, such as benzyl, and methylbenzyl radicals, are considered to be stable and more likely to recombine with H radical. They can be consumed largely through the reaction with O atoms.;The propagation of flames of fuel blends was then studied and found to be mostly sensitive to the flame temperature through its influence on the main branching reaction H + O2 → OH + O. The kinetic couplings appear to have minor effect on flame propagation.;Finally, the propagation and extinction and conventional and alternative jet fuels were investigated.