Study Of Cooling Process Of JT Cooler In Liquid Helium Temperature Zone

Pre-cooled JT cryocooler technology is currently the dominant technology for obtaining 4-6 K temperature region in space and has a promising application in space.However,the cooling rate of JT cryocoolers is slow compared with that of pulse tube cryocoolers and Stirling cryocoolers,and there is still no definite solution for the auxiliary cooling of JT cryocoolers.The bypass tube technology is widely used in foreign countries in the current space JT cryocoolers,the lack of low-temperature shutoff valve,complex structure and low space utilization were found during the domestic research.And the current lack of quantitative calculations for the cooling process and the possible shortcomings of the bypass tube solution are not fully understood.In this paper,the following work was carried out in this regard.1.a cooling model of the final stage heat exchanger of the JT cryocooler is built.Two models are included.Model Ⅰ calculates the enthalpy of the entire system at various points.The quantitative analysis of the cooling capacity that the system can get from the fluid during the cooling process of the final stage heat exchanger in the low temperature zone.And the relationship between the temperature before throttling and the sequence of the evaporator into the liquid helium temperature zone is explained.After adding the entropy production to the model,the entropy production during the cooling process is quantified and the final stable temperature of the system under different heat loads is analyzed.Model Ⅱ numerically simulates the final heat exchanger of the JT system and discretizes the continuity equation and energy equation of the casing using the finite difference method.The model quantifies the time required for bypass tube assisted cooling for different structure sizes,flow rates and inner casing materials.The structural reality is simulated and compared with the experiments.An improved solution to increase the cooling rate when using a bypass tube is proposed.2.Other feasible cooling solutions were investigated.A solution using a helium thermal switch to assist the JT system for cooling was proposed.Two aspects of work were included.The design boundary of the thermal switch was clarified in conjunction with the JT cryocooler.A helium air gap type thermal switch with a switching ratio greater than 1000 at 4K was designed according to the requirements.And the experimental investigation of the thermal switch was carried out to make it meet the demand for cooling the JT cryocooler.By improving Model II,a numerical computation model using thermal switchassisted cooling was constructed.The feasibility of the scheme was analyzed by simulation and experimental investigation was conducted.Finally,the system was cooled down to 4 K temperature zone after 14.5 hours at the three-stage high pressure inlet cooling time of 5 hours.The lowest temperature measured during the regulation of the compressor low pressure was 3.8 K.