Investigation the thermodynamic performance of closed Brayton cycle with CO₂-based binary mixtures for hypersonic vehicle power generation systems under finite cold source

Closed Brayton cycle (CBC) is a potential airborne power generation (APG) technology for hypersonic vehicles, but finite cold source limits its power output. To achieve power enhancement and weaken the constraint of the critical properties of the working medium on the minimum cycle conditions, this study assessed and compared the thermodynamic performance for nine types of CO₂-based binary mixture simple recuperated CBC. Results indicate that for enhanced power output, the average specific heat of the binary mixtures should be greater than CO₂, while their maximum enthalpy should be lower than CO₂. Moreover, their critical temperature should be above CO₂ and critical pressure should be below CO₂. There is optimal fuel temperature difference in precooler for electric power of CO₂-based binary mixture CBCs. The electric power under per unit mass flowrate of fuel for CO₂-SO₂ is higher than CO₂ at the maximum circulating pressure (P_2max) of 15 MPa (159 kJ/kg vs. 130 kJ/kg). Furthermore, the CO₂-SO₂ exhibits a lower optimal compressor inlet temperature (327 K vs. 352 K), which can simplify compressor manufacture and design. The thermodynamic performance of CO₂-SO₂ CBC is better with the higher P_2max. In summary, given system performance, component design, and operating conditions, CO₂-SO₂ appears more suitable for APG systems under finite cold source. This research can provide a new way to improve the power level of hypersonic vehicle APG systems.