Dynamic simulation and analysis of transient characteristics of a thermal-to-electrical conversion system based on supercritical CO2 Brayton cycle in hypersonic vehicles

The supercritical CO2 Brayton cycle has promising prospects for application in hypersonic vehicles owing to its performance and compactness. However, the extreme thermal environment in aircrafts and limited cold sources make the transient characteristics of the Brayton cycle unclear. In this study, dynamic models of supercritical CO2 Brayton cycle were established. The heat exchanger model was based on the supercritical moving boundary method proposed in our previous work and the finite volume method, whereas the modeling of the turbomachinery was based on the performance map approach. Proportional-integral-derivative (PID) controller modules were introduced to enable closed-loop control of various parameters to meet different working conditions. The transient characteristics of the heat exchanger and Brayton cycle model were validated using literature data. Dynamic models for both simple and recuperated layouts were developed to study the transient behavior in aerospace scenarios with sudden thermal load increases, cold-source limitations, and combined disturbances. Under the given conditions, results indicate that both sudden increases in thermal load and cold-source limitations cause a decrease in the thermodynamic performance, with a reduction in thermal efficiency of 0.7 and 2.2%, respectively. When these conditions were combined, the performance further deteriorated, with the thermal efficiency decreasing from 14.4 to 9.5%. This condition can result in compressor over speeding and failure of the PID controller. The simulation results for the two layouts show that the recuperated layout has a 34.8% higher power output at the cost of increasing the total weight by 29.7%. The dynamic models proposed in this study provide valuable insights into the behavior of Brayton cycle systems in hypersonic vehicles, aiding system design, evaluation, and control strategy development.