Study on the coupling performance of a turboexpander compressor applied in cryogenic reverse Brayton air refrigerator

A small cryogenic reverse Brayton air refrigerator with turboexpander compressor (TEC) is presented in this study. Because of stable process, simple matching between expander and brake blower, and easy regulation, a turboexpander with brake blower is usually used in small reverse Brayton refrigerator. However, a turboexpander with brake blower just consumes and wastes the output energy during the enthalpy drop. In contrast, the output energy of TEC is absorbed by its coupled compressor for recycling. Thus when employing a TEC, the reverse Brayton refrigerator will achieve lower refrigeration temperature, larger cooling capacity and more effective energy use. TEC overall performance, which has an important impact on the refrigerator thermal performance, is mainly determined by the coupling between expander and compressor. In a TEC, the compressor and expander should seek balance among energy, rotating speed, mass flow rate and pressure, though restricted by individual working characteristics. The coupling relations among compressor efficiency, expander efficiency, compressor pressure ratio and expander expansion ratio are quite complex. In this study, theoretical coupling analysis between expander and compressor was conducted. The aerodynamic performances of compressor and expander were calculated using CFX simulation with SST model. The performance curves of compressor and expander were obtained through simulation results, which were validated by experimental data. Based on the coupling analysis and numerical simulations, the automatic coupling model between compression process and expansion process was established via Matlab code. The refrigerator was tested under varied coupling parameters (compressor inlet pressures and expander inlet temperatures). The calculated coupling performance was validated by experimental data. With good coupling, the cooling capacity of refrigerator reached 48.0 W at 99.6 K. The coupling performance was calculated through the coupling model, and mutual relations and interactions among coupling parameters were quantitatively described and clarified. The coupling model could effectively predict the coupling performance and therefore allow for the further design and optimization of TEC.