Numerical study on the dynamic process of ramjet/scramjet mode transition in the integrated RBCC full flow path through multistage fuel injection strategies

The rocket-based combined-cycle (RBCC) engine usually completes the transition process from ramjet to scramjet mode under the Ma_∞ = 5.5–6.0. The dual-mode (ramjet/scramjet) transient process is an important technology for engine operation. CFD modeling based on Reynolds average Navier-Stokes was for studying the influence of fuel injection position, high-temperature rocket jet and fuel throttling time on the combustion flow structure and transient characteristics of RBCC engine during the mode transiton. The results showed that whether RBCC engine could successfully achieve stable mode transition was mainly influenced by flame stability in the combustor. When the rocket shuts down, it was more suitable to choose the fuel injection arrangement which distributed downstream of the combustor to realize relatively smooth mode transition. The analysis shows that in the mode transition process, using the synergistic combustion-supporting effect of the cavity and the strut is beneficial to improve the fuel combustion efficiency and reduce the thrust fluctuation. Although the high temperature rocket jet with low mass flow rate cannot maintain the fuel combustion in the isolator at high Mach number, it can expand the high temperature combustion area width in the central flow of RBCC engine and increase the secondary fuel combustion efficiency. Therefore, keeping the rocket running at low mass flow rate can reduce the thrust drop caused by insufficient fuel heat release. When adopting a mode transition strategy that throttles the fuel linearly over time, the engine can go through the thrust dip more smoothly, but this correspondingly increases the time required for the internal flow field to stabilize.