Emulating the Combustion Behavior of Real Jet Aviation Fuels by Surrogate Mixtures of Hydrocarbon Fluid Blends: Implications for Science and Engineering

We have demonstrated previously that a (surrogate fuel) mixture of known pure hydrocarbon species that closely matches four combustion property targets (the derived cetane number (DCN), the hydrogen to carbon molar ratio (H/C), the threshold soot index (TSI), and the average molecular weight) of a specific jet fuel, displays fully prevaporized global combustion kinetic behaviors that are closely consistent. Here, we demonstrate a similar result can be obtained by formulating surrogate hydrocarbon fluid mixtures from distillation cuts of molecular class hydrocarbons or even real gas turbine fuels (for which the specific molecular species classes are no more than qualitatively known). Fully prevaporized chemical reactivities of hydrocarbon fluid surrogate mixtures and real jet fuels are compared using a high pressure flow reactor at 12.5 atm pressure, over the temperature range 500–1000 K, at stoichiometric conditions, and for the same fixed molar carbon content. Results are reported for two different real target jet fuels, a global average Jet-A (POSF 4658) used in numerous publications as a reference target fuel, and a military JP-8 (POSF 5699), both of which contain about 25–30% cycloalkanes (by weight). The surrogate mixture to emulate the Jet-A property targets was formulated from three (normal-paraffinic, iso-paraffinic, and alkyl-aromatic) commercial narrow distillation cut hydrocarbon fluids containing few cycloalkanes. The surrogate mixture for the JP-8 sample was formulated using two different (paraffinic, alkyl aromatic) hydrocarbon solvent cuts and a synthetic, iso-alkane jet fuel (IPK POSF 5642). The composition of the paraffinic fluid was nearly half cyclo paraffinic components (by weight) with similar fractions of normal and iso-paraffinic species. Experimental results for the Jet-A surrogate mixture closely parallel the data for the fully prevaporized Jet-A real fuel, as well for prior surrogate mixtures of n-decane/iso-octane/toluene and n-dodecane/iso-octane/n-propylbenzene/1,3,5-trimethyl benzene, all formulated to share the same combustion property targets. The reactivity comparisons for the JP-8 surrogate mixture and real fuel were of similar quality to the Jet-A comparisons but were improved in the reactivity transition characteristic of hot ignition to chemically branched kinetic conditions. The improvement is hypothesized to be mostly a result of cyclo alkane content differences between the specific surrogate mixtures compared to the real fuels. Sooting properties for the JP-8 hydrocarbon fluid surrogate and fuel are compared in fundamental diffusion flames and a model gas turbine combustor elsewhere.1 The collective results support that surrogate fuels, each with similar fully prevaporized global combustion behaviors but different physical properties (viscosity, surface tension, class composition, distillation curve) and even different functional class distributions across their distillation curve, can be formulated using mixtures of hydrocarbon fluids. This ability can be used to advantage to investigate experimentally the relative importance of physical and chemical kinetic properties in both fundamental and applied multiphase combustion experiments. Surrogates formulated from varied sources of hydrocarbon fluids of differing molecular classes, and even fractional distillation cuts from fluid streams can facilitate unraveling the fuel property sensitivities of combustion/emissions performance for specific combustor configurations and operating conditions