Analysis of the characteristics of scramjet mode and ramjet mode of axisymmetric dual-combustion ramjet

Dual-combustion ramjets have the advantages of both ramjets and scramjets, and further research is required to understand the combustion flow state in supersonic combustors and improve the overall performance of engines. Based on the computational fluid dynamics method, the objective of this study was to explore whether the two distinct combustion flow modes, scramjet and ramjet modes, can emerge in a dual-combustion ramjet under the same operating conditions when it was burned with a constant area. If so, the combustion flow characteristics and comprehensive engine performance under the two modes would be compared. Kerosene C₁₂H₂₃ was used as fuel and the equivalence ratio was 0.71. A 6-species 4-step chemical reaction mechanism was adopted, and the RNG k−ε turbulent flow model was selected. The altitude was 35 km and the Mach number was 7. The results show that, under the same operating conditions, the scramjet and ramjet modes can be achieved when the dual-combustion ramjet burns with a constant area. The generation of the two modes is related to the ignition efficiency, and the ramjet mode requires higher ignition efficiency. In the supersonic combustor, type X-shock waves with a first Mach stem length of 20 mm were generated under the scramjet mode, and a C-shock wave with a Mach stem length of 52 mm was generated under the ramjet mode. The core principle of mode transition is to control the rate of heat released in the supersonic combustor. The engine performance was better under the ramjet mode in this case. Specifically, the thrust, specific impulse, and specific thrust of the ramjet mode were all 1.4 times those of the scramjet mode. This is because, compared to the scramjet mode, the total pressure loss of the ramjet mode was slightly higher by 1.4%, but the combustion efficiency increased by 31.5%, significantly improving the engine performance. The ramjet mode can improve the combustion process and combustion efficiency. The reason is that, on one hand, the recirculation zone formed by the C-shock wave not only has a larger number of vortices, but also has a longer length, which is conducive to the efficient mixing of high temperature gas and secondary flow. On the other hand, the airflow velocity in the ramjet mode is lower, which is equivalent to an increase in the residence time of the airflow.