An optimization method for formulating model-based jet fuel surrogate by emulating physical, gas phase chemical properties and threshold sooting index (TSI) of real jet fuel under engine relevant conditions

Abstract

Two jet fuel surrogates (S1 and S2) were proposed in this work, aimed at improving the quantitative accuracy of fuel properties that affect both premixed and spray-guided combustion modes under engine relevant conditions by emulating real jet fuel properties including physical, gas phase chemical properties and threshold sooting index (TSI). An intelligent optimization approach was employed to calculate the composition that inherently satisfies both the physical and gas phase chemical characteristics as well as sooting tendency. The jet fuel surrogate S1 was composed of five components including decalin, n-dodecane, iso-cetane, iso-octane and toluene (0.005/0.4011/0.1249/0.098/0.371 by mole fraction), while surrogate S2 is a mixture of the same components but different in proportions (0.1449/0.3706/0.2059/0.0195/0.2591 by mole fraction). Based on the newly proposed surrogates, a skeletal jet fuel surrogate chemical reaction mechanism was developed by describing the chemistries for the oxidation of large molecules C4Cn and smaller H2/CO/C1 molecules respectively. The skeletal jet fuel surrogate mechanism was significantly compacted into 74 species and 189 reactions, making it practical to be used in 3-dimensional (3-D) engine combustion simulations. This newly developed mechanism was verified against the experimental results of ignition delay times, species concentrations and laminar flame speed under a wide range of conditions, while 3-D validations were conducted for spray liquid and vapor penetrations in a constant volume chamber. As a result, the proposed fuel surrogate is capable of predicting the spray and combustion characteristics and main species profiles under engine relevant circumstance. Due to the stringent target properties used in this work, the surrogate S1 performed best in assessing ignition characteristics attributed to its elaborate chemical properties, making it the most suitable candidate for chemical dominated combustion like premixed engine combustion. Alternatively, S2 displayed outstanding spray-guided combustion behavior because of the stringent chemical and physical target properties assigned. It is worthwhile to note that this new method of formulating surrogates for different applications was efficient and time-saving. Finally, we aim to perform experimental validation tests for CN, density, viscosity, surface tension, TSI, sooting tendency and particle size distribution to further support the validity of the current proposed jet fuel surrogates (S1 and S2) in the future.