Supersonic Combustion and Mixing Characteristics of Hydrocarbon Fuels in Screamjet Engines

The combustion characteristics of gaseous propane in supersonic airflow using the rearward-facing step that is swept inward from both end sides is studied. The effect of sweeping the step on the flow field features of propane combustion is investigated. The study of the supersonic combustion of ethylene is carried out using different combustor configurations, different main fuel equivalence ratios, and different pilot fuel equivalence ratios. The swept step shows the ability to hold the propane flame in the supersonic air stream without extinction. It was found that the side sweeping of the combustor exhibits the high temperature and combustion products concentration in the far field domain while the area downstream of the normal injection location characterizes lower temperature and products concentration. It is recommended to optimize the combustor length to ensure the complete combustion and consequently the full liberation of the chemical energy stored in the fuel before the fuel exits the combustor. The main findings from the ethylene study can be summarized in the following points. The step configuration with no pilot injection can afford the flame holding mechanism in the supersonic air stream by creating the flow recirculations in the step base area and featuring permanent high temperature regions surrounding the normal fuel injection. The step configuration showed good mixing capabilities in the far field domain. The wedge configuration proved superiority over the generic rearward-facing step configuration in holding the ethylene flame in the supersonic airstreams, producing overall higher temperature medium throughout the combustor, and exhibiting lower flow losses and higher combustor efficiency. The increase in the equivalence ratio of the ethylene normal fuel injection enhances the general flow field features and energy field characteristics in the combustor except in the step base area where the lower equivalence ratio features better temperature distribution and higher combustion efficiency. Although the wedge with no pilot injection configuration presents the highest level of temperature distribution in the cavity and downstream regions, the 0.02-pilot equivalence ratio increases the temperature of the upstream face of the normal injection and enhances the flame holding mechanism. The 0.02-pilot equivalence ratio presented the optimum pilot injection case that can promote the flame holding mechanism and keep good combustion and flow field qualities. While further increase of the pilot injection equivalence ratio quenches the high temperature gases in the cavity region, which leads to the deficiency in the flame holding mechanism, the excessive pilot fuel injection shows its positive effect by increasing the average flow field static temperature and absolute pressure in the far field domain.