Roles of fuel composition on the ignition process of endothermic hydrocarbons

Abstract

Due to the characteristics of heat absorption and decomposition, endothermic hydrocarbon fuels (EHFs) have been widely used in scramjets for thermal protection and heat recirculation. The understanding of ignition characteristics of EHFs is of great importance for their safe and efficient utilization. In this paper, the ignition processes of EHFs were numerically simulated at atmospheric pressure and with an initial temperature of 500 K. Three different ignition stages were identified based on the chemical heat release and flame kernel propagation. A 3-component kerosene surrogate model composed of n-dodecane, methyl cyclohexane and m-xylene was adopted, as well as the corresponding chemical kinetic model with 369 species and 2691 reactions. Results showed that the discrepant decomposition characteristics of n-alkanes and cycloalkanes affected the chemical heat release and propagation during the ignition process. Two-stage exothermic characteristic was observed in the time evolutions of chemical heat release rate and fuel decomposition. The mass production of molecules and accumulation of radicals dominated the first and second exothermic peaks, respectively. Furthermore, the minimum ignition energies (MIEs) of EHFs with various methyl cyclohexane were determined to quantify the effect of fuel composition on ignition performance. Characteristically, the MIE dramatically decreased from 10.2 to 2.15 mJ when 20% n-dodecane was replaced by methyl cyclohexane. However, it was slightly increased as methyl cyclohexane continued to increase. Analyses from both physical and chemical aspects were conducted to elaborate the dependence of MIE on fuel composition. The dominant effects of flame-dynamic and chemical effects on different ignition stages were analysed. The faster propagation speed and stronger endothermic ability of methyl cyclohexane led to the nonlinear variation of MIEs. The results in this study provide useful guidance for composition optimization and safety evaluation of EHFs.