Research On GM＿JT Helium Refrigeration System Of Maglev Train And Its Cold Head Heat Exchanger

In recent years,with the continuous development of superconducting technology,research on superconducting maglev trains has become popular again;and the application of superconducting technology has always been inseparable from the guarantee of cryogenic engineering.Among them,the small helium refrigerator system is applied to the cooling of vehicle-mounted superconducting magnets.Based on the application background of the relatively mature low-temperature superconducting magnetic levitation train,this paper conducts research on the G-M/J-T helium refrigerator system used for cooling the low-temperature superconducting magnet on the vehicle.The main research contents of this paper are as follows:Perform static heat leakage calculation and dynamic heat load calculation on the inner container of the superconducting magnet,the proportions of which are55% and 45%,respectively.It is determined that the designed 8W/4.2K cold capacity helium refrigerator can meet its low temperature requirements;Perform thermodynamic analysis of the closed GM/JT helium refrigerator system,analyze the influence of the parameters of the compression stage,pre-cooling stage,and throttling stage on the performance of the refrigerator,and determine the best thermodynamics in the two cases of balanced flow and unbalanced flow State parameters;the system process model is established through HYSYS software,and the process and the state parameters of each point in the refrigeration mode and mixed mode of the GM/JT helium refrigerator are obtained respectively,And determine the efficiency of low temperature heat exchangers at all levels and the heat exchange temperature difference.According to the design and research of the cold head heat exchanger of the key equipment in the G-M/J-T helium refrigerator,a new type of cold head heat exchanger is proposed.This type of heat exchanger has a high heat exchange specific surface area to meet the compact and efficient design requirements of low-temperature heat exchangers.First,this type of cold head heat exchanger is designed using traditional design methods;then the heat transfer control equation is numerically solved to obtain the dimensionless temperature distribution of the heat exchanger and the helium fluid;finally,the lowest temperature at the outlet of the heat exchanger is taken as optimize the objective function to optimize the design of the cold head heat exchanger;determine the optimal values of the two parameters of the heat exchanger height and diameter to make the heat exchanger as compact as possible while ensuring the heat exchange requirements of the heat exchanger.In the COMSOL finite element software,a numerical model of the actual size of the three-dimensional cold head heat exchanger is established according to the design results,and the heat exchange process of the cold head heat exchanger is further simulated,and the flow field distribution and temperature field distribution of the heat exchanger are respectively simulated.The heat transfer temperature difference of the cold head heat exchanger and the average temperature of the helium fluid outlet are determined,and the heat transfer results consistent with the analytical solution above are obtained.Finally,the effects of heat exchanger area(number of layers),heat exchanger tube diameter,and surface roughness on the heat transfer effect are studied.The results show that under the condition that the working characteristics of the cryogenic refrigerator are determined,there are the best design parameters to make the cold head heat exchanger the most compact and economical;at the same time,the above optimized design results are verified.