Analysis on thermophysical property and thermodynamic performance of He-Xe Brayton cycle system

[Background] Megawatt-level nuclear reactor combined with helium-xenon Brayton cycle system can effectively meet the energy needs of large-scale deep space explorer, satellite base, deep-sea unmanned underwater vehicle and other special energy power equipment for high power, small size, high reliable power supply, which has wide application foreground and research necessity. Currently, the study of the physical properties of helium-xenon gas mixtures in non-ideal state is not sufficient. [Purpose] This work aims to establish the thermophysical property model and the thermodynamic model of helium-xenon Brayton cycle, and analyze the effect of the non-ideal gas characteristics to the thermal performance of the cycle. [Methods] The second or third order virial expansion is adopted to construct the helium-xenon mixture physical property model to reflect the deviation caused by the non-ideal gas characteristics. The thermodynamic models of turbine, compressor, mixing chamber, and heat exchanger are conducted based on the thermophysical property model. The function models of efficiency and specific work are derived from the thermodynamic models of the above main components. [Results] The correctness and accuracy of the thermophysical property model and the thermodynamic model of helium-xenon Brayton cycle is verified by the submerged subcritical safe space reactor (S4) design. The influence of He-Xe mixing ratio on the He-Xe thermophysical property is revealed under different temperature and pressure. The influence of the thermophysical properties of helium-xenon mixture on thermal performance of helium-xenon Brayton cycle system such as adiabatic coefficient, pressure loss and relative convective heat transfer coefficient at different temperature, pressure and molar fraction of helium is analyzed. The adiabatic coefficient of He-Xe mixture decreases with the increase of temperature and increases with the decrease of helium mole fraction. The increase of adiabatic coefficient can increase the turbine outlet temperature, increase the power consumption of the turbine, and reduce the thermal efficiency of the cycle. The pressure loss of He-Xe mixture increases with the decrease of helium mole fraction. The heat transfer coefficient of He-Xe mixture first increases and then decreases with the decrease of helium mole fraction and reaches the maximum with the helium molar fraction being 0.9118. The He-Xe gas mixture can significantly reduce the number of compressor stages, and reduces the size and weight of the turbomachinery, compared with helium. [Conclusions] The thermophysical properties of He-Xe mixture, including density, specific heat capacity, viscosity, thermal conductivity and Prandtl number can be accurately calculated by the proposed model under different helium molar fraction. The model proposed in this work can be applied to the design and optimization of the He-Xe Brayton cycle systems and direct the device selection of the He-Xe Brayton cycle system.