Mode transition analysis of a turbine-based combined-cycle considering ammonia injection pre-compressor cooling and variable-geometry ram-combustor

Mode transition (MT) is a significant process in the turbine-based combined-cycle (TBCC) involving the co-operation of high-speed and low-speed channels. To improve the thrust pinch caused by the deviation of the turbine and ramjet from the design point, a novel TBCC engine with ammonia mass injection pre-compressor cooling (Ammonia MIPCC) and variable-geometry ram-combustor (VGRC) is used to extend the operation envelope of the MT. CFD simulations and shock loss calculations are used to construct combined dual-channel inlet model. Through simulation, we find that the Ammonia MIPCC can reduce the pre-compressor temperature, improve the compressor efficiency, and increase the turbine's specific thrust. Meanwhile, increasing the ramjet combustor expansion ratio (CER) helps to alleviate the inlet unstart and extend the low-speed operating boundary of the ramjet. Furthermore, the extension of the operating envelope for TBCC under the effects of ammonia injection and CER is investigated. Based on specific thrust and specific impulse, the optimal starting Mach number field for MT process is studied, and the corresponding TBCC thrust is given. Overall, the results show that the optimal starting Mach number field for the MT of the proposed TBCC is 2.85–3.2, and two-stage sub-engines have high thrust performance in this range. •A TBCC model with pre-compressor cooling and variable-geometry ramjet is established.•Ammonia injection pre-compressor cooling extends turbine's upper operation limit.•Variable-geometry ram-combustor extends ramjet's lower operation limit.•The optimal starting Mach number field for the mode transition process is investigated.