Thermodynamic modeling and performance analysis of bypass-driven mixing turbofan: A novel variable cycle engine for high Mach numbers

Conventional turbine-based combined cycle (TBCC) engines suffer from inlet spillage losses at low Mach numbers and degraded thrust performance in the Mach 2.8–3.2 regime. To address these limitations, this paper proposes a bypass-driven mixing turbofan (BDMT) engine that extends the operating envelope of a mixed-flow turbofan through internal flow redistribution. A component-level model is developed to investigate the variable-cycle characteristics and operational feasibility of the proposed concept. The BDMT integrates a rear variable area bypass injector (RVABI) and a low-pressure turbine guide vane bleed ring (GVBR) to regulate fan efficiency and turbine power balance at high Mach numbers. Above Mach 2.9, RVABI adjustment improves fan efficiency by up to 27.6%. At Mach numbers exceeding 3.5, activation of the GVBR enhances bypass–core mixing near the low-pressure turbine inlet, resulting in an average installed thrust increase of 7.2%. Compared with a tandem TBCC engine with equivalent takeoff thrust, the BDMT achieves installed thrust improvements of up to 11.71% at subsonic and transonic conditions and up to 16.51% in the Mach 2.8–3.2 regime, accompanied by specific fuel consumption reductions of up to 15.25%. These results demonstrate the potential of bypass-driven mixing as an effective variable-cycle strategy for high-Mach-number propulsion systems.