Evaluation on the operating boundaries and performance of sCO2 closed-Brayton cycles for hypersonic vehicles

Supercritical CO₂ closed Brayton cycles have demonstrated significant advantages in thermal management systems for hypersonic vehicles, that require thermal protection and power generation. Due to the limitations in temperature and temperature difference, the limited cold source provided by the propellant has limited the operating range of SCBCs. This investigation examines the operating boundaries and performance characteristics of various SCBC configurations in the presence of these limitations. A zero-dimensional thermodynamic analysis model in conjunction with a quasi-one-dimensional cooling channel model is appropriately established to determine the operating boundaries of three typical SCBC configurations, including simple layout, recuperative layout, and recompression layout. The influences of compressor inlet temperature (T₀), inlet pressure (p₀), maximum cycle pressure (p₁) are methodically analyzed, along with the effects of recuperator effectiveness and split ratio. First, the upper and lower boundaries of the operating range are defined, and the obtained results reveal that lower T₀ leads to a reduction of the operating range in all layouts, while p₀ exhibits a similar impact on layouts with recuperators. An optimal combination of recuperate effectiveness and T₀ that maximizes the net work output is also identified in the RL configuration, however, no optimal split ratio is detected in the RCL configuration. The layouts are then suitably optimized and compared under the concept of maintaining a certain level of operational flexibility. The RL configuration achieved the highest cycle net work of 40.61 kW in the presence of a flow margin of ±5 %, making it the optimal layout for onboard supercritical CO₂ closed Brayton cycles systems with a finite cold source.