Thermodynamic Characteristics Research And Optimization Of Helical Heat Exchangers In Large Helium Cryogenic System

Controllable nuclear power generation technology is one of the most popular direction of future energy development.The operation of superconducting tokamak magnetic confinement devices currently used in China is inseparable from large-scale helium cryogenic systems.As an important part of cryogenic systems,heat exchanger is a key component that drops the equipment to liquid nitrogen or even supercritical helium temperature at room temperature.Further exploration of the heat transfer characteristics of the helium fluid at low temperature in the helical coil heat exchanger of the large helium cryogenic system has important theoretical significance and application value for the design and application of the helical coil heat exchanger in the CFETR cryogenic system of the future fusion reactor.In this paper,the method of numerical simulation is used to study the heat transfer characteristics of the helium fluid in the helical coil heat exchanger.The main contents of the work are as follows:?1?Based on the numerical calculation theory of convective heat transfer in the tube,the physical and mathematical model of the low-temperature helium gas flow and heat transfer of the helical coil heat exchanger is established,and the boundary conditions of the model are simplified according to its actual application background.ANSYS ICEM is used to divide the geometric model of the helical coil tube into a structural grid,and the grid of the near-wall area is encrypted,and the Y⁺value is controlled to<1 to investigate the details of the flow heat transfer in the near-wall area of the tube.Comparing the numerical simulation results of several commonly used turbulence models with the test results of related literature,it is found that the error between the calculated value of the RNG k-?two equation model and the literature value is smaller,which verifies the simulation result of the RNG k-?two equation model Reliability.?2?The flow and heat transfer of low-temperature helium in the helical tube were numerically simulated,and the flow parameters such as fluid velocity,vortex and turbulent kinetic energy in the helical coil heat exchanger tube and the fluid density,temperature,local Nusselt number,the average Nusselt number and other heat transfer parameters in the tube were analyzed.The results show that due to the existence of centrifugal force,secondary flow is generated inside the helical tube,the radial velocity distribution is separated,the intensity of the outer turbulence is enhanced,the velocity is significantly higher than the inner,the thickness of the boundary layer is reduced,and the heat transfer is strengthened,while the inner is manifested as The intensity of turbulence is weak,and the phenomenon of heat exchange deterioration occurs.?3?Change the structural parameters of the helical tube such as the helical tube curvature diameter D,the helical tube diameter d and the pitch H,as well as the operating parameters such as mass flow rate,inlet temperature and pipeline pressure.And investigate the effect of changing the parameters to the convection heat transfer characteristics.The results show that increasing the curvature diameter of the helical tube reduces the heat transfer efficiency and the corresponding pressure drop.Reducing the diameter of the helical tube significantly increases the heat transfer performance of the helical tube;the change of the pitch has little effect on the flow characteristics and heat transfer efficiency of the helical tube;Increase the mass flow,heat transfer is enhanced,and the pressure drop is significantly increased;increase the inlet temperature,there is no significant change in heat transfer and flow characteristics,increase the pipeline pressure,the resistance coefficient increases,and there is no significant change in heat transfer performance.?4?According to the influence of structure and thermodynamic parameters on the flow and heat transfer characteristics of the helical tube heat exchanger,the influence weight ratio of each parameter is analyzed based on the PEC method.The results show that the weight of the comprehensive effect on the flow heat transfer performance is d>m>D>P>H>T_in.Reducing the pipe diameter and increasing the inlet mass flow can significantly enhance the heat transfer performance.According to the analysis results,the helical tube structure parameters were optimized as helical tube diameter d=8mm,curvature diameter D=200mm,and the pitch H=10mm.Comparing it with the benchmark working condition,the comprehensive evaluation factor?was 1.545,indicating that the comprehensive performance was significantly enhanced.In addition,the comprehensive numerical analysis results obtained the fitting relationship of heat transfer and resistance coefficient,and verified that its accuracy is within±10%.