Structural Design,Heat Transfer Calculation And Performance Testing Of A Load-carrying Solid Nitrogen Cryostat

The second-generation high-temperature superconductor（HTS）,which has excellent properties of high critical temperature,high critical current density and high critical stress,has been used in superconducting magnetic levitation train currently.It is operated under liquid nitrogen（LN₂）circumstance or directly cooled by cryocooler.However,LN₂ has the drawbacks of limited cooling temperature,insufficient cold energy storage,meanwhile under LN₂ temperature the performance of HTS does not achieve its best level.In addition,the HTS are poor in thermal uniformity and can not work without power supply if cooled by the cryocooler.Furthermore,the weight of the entire cooling system will increase a lot with accessories.As an alternative option,solid nitrogen（SN₂）is an inexpensive solution with a wide temperature range,easy manufacture access,light weight,large heat capacity and good electrical insulation.Besides,SN₂ can absorb more heat and suppress the temperature rise for the HTS when undergoing the solid-solid phase transition,leading to a better thermal stability.This thesis takes the on-board SN₂ cooled ultra-low temperature system as the research object.In this work,the research methods including analytical calculation,finite element simulation and experimental test as well.Besides,the structural design,structural modeling and heat transfer calculation of pluggable load-carrying SN₂ cryostat applying to electric maglev train have been carried out.And the improvement of the thermal leakage and warm-up time of SN₂ cryostat when using the structure of pluggable cryocooler was analyzed.To reduce the total thermal load and extend the warm-up time of SN₂,this thesis implemented structures optimization like,LN₂ vessel,two radiation shields,super insulation layers and shrink bellows in the process of structural design for pluggable load-carrying SN₂cryostat.This thesis placed high value on the structure of pluggable cryocooler to not only prevent the conduction heat and radiation heat,but also reduce the total weight of load-carrying SN₂ cryostat,thereby it can achieve the purpose of light-weight design of on board ultra-low temperature system.Next,this thesis built a 3-D model of load-carrying SN₂ cryostat to calculate the conduction heat load,radiation heat load,convection heat load as well as Joule heat load,and studied the impact of total heat load of cryostat on the volume and warm-up time of SN₂.Using the structure of pluggable cryocooler to reduce the total heat load of on board SN₂cooled ultra-low temperature system,the HTS magnets can work stably more than 8 hours from 20 K to 40 K.Meanwhile,this thesis performed the finite element simulation for load-carrying SN₂ cryostat to simulate the deformation of outer vessel in a high vacuum environment.Both the analytical and simulated results have proved the rationality of the 3-D model of load-bearing SN₂ cryostat.Finally,this thesis have completed the prototype and performance test of the load-carrying SN₂ cryostat.Then,this thesis used a two-stage G-M cryocooler,which has a cooling capacity of 4.2 K,to cool down the LN₂ by conduction method.Meanwhile,this thesis have investigated the cooling time and warm-up time of SN₂ effected by the LN₂vessel and pluggable cryocooler,respectively.The experimental results further verify the accuracy of analytical calculation of SN₂ cryostat and the importance of LN₂ vessel as well as pluggable cryocooler structure to reduce the heat load.Therefore,it is a better choice to use SN₂ to cool down the HTS magnets.Also,to apply it to the ultra-low temperature system of superconducting electrodynamic suspension train will be a new application direction.