Differential turbulent supersonic combustion of hydrogen, methane, and ethylene, without assisted ignition

Although hydrogen has the desired ignition properties for supersonic combustion in a scramjet, it has the disadvantage of low energy density, thereby motivating the interest in alternate, mostly hydrocarbon, fuels, such as methane, ethylene, and kerosene. Because the hydrocarbon fuelsdonotignite easily, their usein scramjet combustion, where the strain rate is large and flame stability is difficult to maintain, depends on assisted ignition. However, a comparative evaluation of the combustion characteristics of these fuels under realistic turbulent supersonic conditionsin ascramjet engine, and within the framework of advanced and highly efficient combustion modeling with detailed chemical mechanism, has not received enough attention. This study has the objective of addressing this issue. The effects of superposing ignition assistance can then be clearly delineated in subsequent studies. Three questions pertaining to the comparative combustion characteristics of hydrogen, ethylene, and methane under laminar flame condition, the significance of progress variable modeling, and turbulent supersonic combustion conditions are answered in this paper. To accomplish this, detailed analysis of opposed-jet flame is carried out for the fuels for the purpose of selecting the kinetic mechanisms, comparing the combustion characteristics, and studying the bifurcation curves for the three fuelsas a function of pressure. Turbulent supersonic combustion simulations are then carried out using large-eddy simulations, from which many interesting results have been obtained. For example, of the three fuels, only hydrogen exhibits the back pressure phenomenon, whichis tracedto combustion-generated choking, rather than to high mechanical pressure from injection.