Emulating the combustion behavior of real jet aviation fuels by surrogate mixtures from solvent blends

Surrogate fuel mixtures that can emulate the physical and chemical properties of specific real fuels are important tools for understanding fuel property effects on the performance and emissions characteristics of existing combustion hardware and in optimizing the design of new energy conversion devices. Recently, surrogate fuel mixtures of small numbers of pure molecular species have been studied in many laboratory scale experiments, comparing their fully pre-vaporized combustion properties with those of real jet fuels. However, the cost of similarly studying these same surrogates in applied combustion studies, e.g. combustor rigs, is prohibitive given the expense of typical high purity, single species materials. As a result, large scale rig testing has been historically pursued using jet fuels and, in a few cases, much less expensive mixtures of commercially available hydrocarbon solvents. The utility of these solvent mixture studies depends strongly on the conceptual formulation techniques utilized to develop the appropriate solvent surrogate blend to simulate a jet fuel, considering both their physical and chemical properties. Here we investigate the fully pre-vaporized kinetic behavior of surrogate mixtures formed from solvent blends by utilizing a combustion property target matching concept. A three-component surrogate mixture of normal-paraffinic, iso-paraffinic and alkyl-aromatic commercial solvents is formulated to closely match four combustion property targets (the derived cetane number (DCN), the H/C molar ratio, the threshold soot index (TSI) and the average molecular weight) of a specific jet fuel sample (Jet-A POSF 4658). Similar techniques were used previously to determine mixtures of three and four specific molecular species that similarly replicated the same jet fuel sample. Chemical reactivity data determined using the Princeton Variable Pressure Flow Reactor are compared for the fully pre-vaporized solvent surrogate, the earlier surrogate component mixtures and the real fuel at 12.5 atm pressure, over the temperature range 500-1000 K, at stoichiometric conditions and for the same fixed carbon content in the reactor carrier flow. Experimental results for the solvent surrogate mixture closely parallel the data for the fully pre-vaporized real fuel and prior surrogates across all temperatures studied, including the low and negative temperature coefficient, and intermediate temperature range. Surrogate mixtures having different chemical composition and physical properties but the same combustion property target values are shown to yield the same fully pre-vaporized chemical kinetic behavior for a wide range of combustion conditions. The results further emphasize the importance of considering functional group interactions controlling the radical pool dynamics influencing combustion rather than the specific real fuel molecular class distribution in selecting surrogate components and formulating surrogate fuel mixtures. As a result, formulating surrogate fuels with the same combustion property targets, but different physical properties and organic structure distributions across their distillation curve can be used to investigate experimentally the relative importance of physical and chemical kinetic properties on multi-phase combustion in applied environments. In a separate paper at this meeting, comparisons are made of commercial solvent surrogate similarly constructed and a real jet fuel in terms of sooting behavior in a multiphase, high pressure combustor [1], further supporting the use of solvent surrogate mixtures in elucidating fuel property effects in multi-phase, larger scale, applied combustion experiments relevant to gas turbines.