Thermodynamic feasibility and flow characteristics of a dual-fuel closed Brayton cycle for wide-envelope TBCC thermal management

High thermal loads and onboard power supply requirements over a wide flight envelope present significant challenges for turbine-based combined cycle (TBCC) engines. In this study, an ammonia/kerosene dual-fuel closed Brayton cycle (CBC) thermal management scheme is proposed. A thermodynamic model applicable to both precooling and ramjet operating modes is established to investigate the effects of key cycle parameters on flow demand, thermodynamic performance, and operational boundaries. Furthermore, the evolution characteristics of CBC flow demand under widely varying thermal environments are analyzed, and the thermodynamic feasible region for cross-mode operation is evaluated under mode transition conditions. The results indicate that, in the precooling mode, the cycle flow demand increases significantly with flight Mach number (Ma) and can be effectively regulated through the fuel mass ratio and precooling temperature. In the ramjet mode, the flow demand is mainly dominated by wall heat absorption load, while the fuel mass ratio and turbine inlet temperature have limited influence on flow demand but significantly affect cycle efficiency and operating boundaries. Further analysis demonstrates that a shared steady-state thermodynamic feasible region exists between the two operating modes at Ma 4, providing a thermodynamic basis for cross-mode parameter matching. Moreover, appropriately increasing the ammonia fuel mass ratio enlarges the overlapping high-pressure-ratio region and increases the available parameter-matching margin between the two modes. This study provides theoretical guidance for wide-flight-envelope thermal-management design and for subsequent dynamic modeling and control development of dual-fuel CBC systems for TBCC engines.