| The long-term zero boil-off(ZBO)technology of liquid hydrogen in space puts forward an urgent demand for cryocoolers working at liquid hydrogen temperature with a large cooling capacity.Through the scheme comparison,it is found that the precooled Joule-Thomson(JT)cryocooler working at liquid hydrogen temperature has the potential to achieve the goal of providing a large cooling capacity.However,JT cryocoolers for space use can only provide a small cooling capacity at liquid hydrogen temperature at present.Meanwhile,the bypass structure is needed at cool down process for typical JT cryocoolers which increases the system complexity.For a JT cryocooler with a large cooling capacity at liquid hydrogen temperature,its internal mechanism and cooling characteristics under different working conditions are not clear yet.To solve these problems is crucial for improving the cooling capacity and efficiency of the JT cryocooler.Aiming at the shortages in this research filed,we carry out the following work in this study.1)A new scheme as a direct throttling cryocooler working at liquid hydrogen temperature is proposed.Based on the analysis of the typical throttling cooling mechanism,a new scheme as a direct throttling cryocooler working at liquid hydrogen temperature is proposed.The reason for the existence of the critical working condition of the direct throttling JT cryocooler is explained.Based on thermodynamics analysis,isothermal compression work characteristics of these two kinds of JT cryocoolers under different precooling temperatures and high pressures are studied.With a given cooling capacity,taking the isothermal compression work as the optimization objective,the difference and internal mechanism of the optimized isothermal compressor work and the corresponding high pressure of two kinds of JT cryocoolers are clarified.It is pointed out that,with a precooling temperature lower than 32 K,the direct throttling JT cryocooler has a similar performance compared to the typical JT cryocooler.The direct throttling JT cryocooler has the potential to be used for space applications.2)Cool down process and cooling characteristics of direct throttling JT cryocooler with a large cooling capacity at liquid hydrogen temperature are studied.An experimental prototype of the direct throttling JT cryocooler working at liquid hydrogen temperature is built of which design parameters are obtained by the optimization model.The goal of providing a cooling capacity more than 10 W at liquid hydrogen temperature is achieved.Cool down process is analyzed based on the experimental data.Temperature changing regularities of the JT cryocooler at cool down process are obtained.The cooling characteristics of the direct throttling JT cryocooler under different heat load patterns are studied.Temperature and mass flow rate changing regularities under different heat load patterns are clarified.Cooling capacity characteristics of the direct throttling JT cryocooler under different precooling temperatures are studied experimentally.Effects of precooling temperature on cooling capacity is revealed.3)The feasibility of driving JT cryocoolers working at ultra-low temperatures by scroll-type compressors is verified.An oil-free floating scroll-type compressor is used to drive the JT cryocooler working at liquid helium temperature.Stable cooling performance data at liquid helium temperature of the closed-cycle JT cryocooler is obtained.It is pointed out that the back pressure of the oil-free floating scroll-type compressor has an important influence on its exergetic efficiency.It is found by experiments that the oil-free floating scroll-type compressor and linear compressor need to be modified before used for hydrogen compression.Based on the successful application experience of hydrogen compression by an oil lubricated scroll-type compressor,the compressor is used to drive the JT cryocooler working at liquid hydrogen temperature.Stable cooling performance data at liquid hydrogen temperature is obtained which lays a foundation for the research of closed-cycle JT cryocoolers working at liquid hydrogen temperature. |
