Optimization Design Of Multi-Channel Cryogenic Transfer Line And Research On Multilayer Insulation Heat Transfer

The multi-channel cryogenic transfer line is a key component of large-scale helium cryogenic systems,which serves to transport the cold fluid of different media and thermodynamic states generated by the refrigerator to the user-side equipment.In recent years,the number and scale of helium cryogenic systems have been increasing day by day,and the cost of cold loss at low temperatures has become increasingly high as the scale grows.Thus,it has become more and more important to reduce the loss of cold energy during the transportation of the cold fluid over long-distance transfer lines.The heat leakage in multi-channel transfer lines is mainly caused by support and multilayer insulation,and the leakage caused by the large wrapping area of multilayer insulation accounts for the main part.The variable density multilayer insulation,proposed nearly 30 years ago,is an optimized form that can be easily manufactured and applied on existing equipment by manufacturers.However,the current optimization for multilayer insulation still uses the calculation equation of ordinary multilayer insulation,which limits the breakthrough of variable density form for a long time.In addition,the gas heat transfer item that has been ignored in multilayer insulation calculation for a long time has become important due to the increase of outgassing rate caused by the increase of spacer in the variable density form.This paper focuses on optimizing the design of multi-channel cryogenic transfer line and studying the heat transfer mechanism of variable density multilayer insulation through theoretical and experimental research,including modifications to the heat transfer equation,gas heat transfer measurements,and optimization of support and structure.In order to address the challenge of quantifying the impact of spacers in optimizing variable density multilayer insulation,a thorough analysis is conducted on both the Lockheed equation and layer by layer model utilized for theoretical calculations of such insulation.Furthermore,incorporating optical properties of spacers into radiation heat transfer term is achieved through application of three heat transfer equations within said layer by layer model.By converting the layer density parameter into the compressive pressure,the complex experimental values of contact thermal resistance and layer by layer compressive pressure are introduced into the layer by layer solid heat transfer equation.The derived interlayer gas pressure distribution equation is substituted into the free molecular gas heat transfer equation.The optimal distribution configuration of spacers in multilayer insulation with variable density is iteratively calculated using a greedy algorithm.The heat leakage of cryogenic transfer lines is primarily divided into support heat leakage and multilayer insulation heat leakage.Due to the large surface area,multilayer insulation dominates the transfer line’s leakage heat,with its performance dependent on the apparent vacuum degree.In order to enhance the performance of the multi-channel transfer line,a thermodynamic analysis was conducted on the support structure,multilayer insulation and thermal shiled during cooling and operation processes based on a sample section of a large-scale six-channel cryogenic transfer line.The test flow and implementation plan for different temperature were designed,followed by experimental testing of the apparent vacuum degree of the sample section.In order to accurately measure the interlayer gas pressure in variable density multilayer insulation,a device for continuously calibrating the pressure-sensing tube and measuring the internal gas pressure of the multilayer insulation under a continuous temperature range was designed and constructed.Based on the principles of vacuum calibration and the characteristics of low-temperature vacuum,the device was designed from four aspects:temperature control,vacuum measurement control,calibration chamber design,and vacuum container design.The temperature stability of the calibration chamber and thermal shield as well as the temperature distribution of the measuring tube were analyzed using numerical analysis software,and the required heating power at different temperatures was simulated.The assembly process,experimental instruments,and calibration steps of the device were designed,and the uncertainty of the device was evaluated.Finally,the device was calibrated at 150K and 220K,and the feasibility of using the pressure-sensing tube to measure low-temperature vacuum was verified.Based on the research results of optimizing cryogenic transfer line structures and utilizing multilayer insulation with variable density,a general method for optimal design of multi-channel cryogenic transfer lines was proposed.The optimal design of a four-channel helium cryogenic transfer line was conducted.According to the transfer line scale,the overall structure scheme is determined.The thermal shiled temperature is determined based on analysis results and refrigerator temperature distribution.The correlation programming method and derived transfer line dynamic heat leakage calculation method are used to determine the temperature and size of inner pipe and thermal shield,calculate thermal shield cooling pipe flow.Finally,the optimal configuration scheme for variable density multilayer insulation in a multi-channel transfer line is calculated.