Development of reduced kinetic models for petroleum-derived and alternative jet fuels

The surrogate fuel concept to replicate the detailed gas phase combustion behaviors of conventional and alternative jet aviation fuels in numerical combustion models is extended and tested in specific examples of synthetic jet fuels derived from coal and natural gas, and also to the pressure and equivalence ratio dependences of the combustion responses of conventional Jet–A fuel. The formulation of surrogate fuels for Syntroleum S-8, Shell SPK and Sasol IPK, is described. Assuming these compositions, a detailed chemical kinetic model construction previously elaborated upon is extended and tested against reference data sets of shock tube ignition delay and laminar burning velocity. Calculations with the detailed kinetic model, containing 3147 species correctly represent the experimentally measured reactivity of the target fuels for shock tube ignition delay. The model also captures trends in the ignition delay for a reference Jet-A as a function of pressure and equivalence ratio.

The earlier reported detailed model is expanded to encompass a range of n-alkane carbon numbers up to C16 and iso-cetane. The expanded model is validated against available shock tube ignition delay in detailed form and against laminar burning velocity datasets using a series of numerically reduced models of decreasing dimension for n-hexadecane, iso-cetane, and their mixtures.

Though the detailed model reproduces the general kinetic behavior for the ignition delays of each jet fuel, the predicted values are generally longer than experimental results. A series of reduced models of the order of 100 species in size, are produced for simulation of flame environments. Calculations for laminar premixed flames for each jet fuel are similar with burning velocities for IPK flames marginally lower than those for the conventional Jet-A which in turn are marginally lower than those for S-8.

The requirement for severely reduced, but high fidelity chemical kinetic numerical schemes that retain predictive capacities for the combustion behaviors of real liquid transportation fuels is addressed through the introduction of a strategy to produce “compact” models of the order of 35 species. The strategy utilizes calculations of the detailed model construct as a fundamental and scientific standard, to which engineering approximations achieved through adjusting reaction rates and omitting or diverting the fate of select reaction pathways at high carbon numbers are applied. The strategy is tested for the exemplar real fuel test case of the S-8 ignition delay and laminar burning velocity data sets. The calculations of a compact model comprising only 36 species is demonstrated to retain quantitative fidelity to the calculations produced using the benchmark detailed model. Thus, a procedure to couple the real fuel description through combustion property target characterization, to the production of compact models for use in computational fluid dynamic platforms is demonstrated.