Numerical Simulation Of Gas Flow And Heat Transfer In Minor-scale By Lattice Boltzmann Method

With the advancement of science and technology and the development of modern industry,mechanical equipment is gradually developing towards miniaturization and intelligence.Micro-electromechanical systems(MEMS)emerged as the times require.In recent years,it has been widely used in aerospace,biomedical,automotive engineering,optoelectronics and other fields,and it has a good application prospect.In the field of household air conditioners,small-diameter heat exchangers have been extensively studied due to relatively low manufacturing costs and high compactness.However,when the diameter of the tube is gradually reduced,some negligible microscale effects at the macroscopic scale need to be considered.As a numerical simulation method in the mesoscopic field,the lattice Boltzmann method has the advantages of simple boundary processing and easy implementation of the program.It is widely used in the field of gas micro-flow and heat transfer.In this paper,firstly based on the minor-scale heat exchangers,the double distribution function thermal model is used to study the flow and heat transfer phenomenon of the single tube and bundle flow.For the single tube simulation,as the Re number increases,the flow of the fluid around the tube from steady state to the gradual out-of-body to the last unsteady flow,resulting in the Karman vortex street,and as the Re number increases,the heat exchange gradually increases.For the aligned tube bundle and the staggered tube bundle,the tube spacings of 1.5D,2D and 2.5D are simulated.When Re=40,the average Nu number increases with the increase of the tube spacing,and the heat exchange trend is almost the same when the tube spacing is 2D and 2.5D,and the difference between the Nu numbers is small.The staggered tube bundle has better heat transfer effect than the aligned tube bundle because there is no direct shielding of the front tube bundle.When the pipe diameter is less than 65?m,the micro-scale effect occurs,and the flow and heat transfer laws are different from the conventional scale.At this time,velocity slip and temperature jump must be considered.Then the gas flow and heat transfer in the microchannel slip zone are analyzed,and a new boundary processing format is adopted,which is,the boundary of equilibrium and specular reflection,aims to capture the velocity slip and temperature jump at the solid boundary,and this boundary format are used to simulate and analyze two-dimensional Poiseuille flow and Couette flow.The results show that the velocity and temperature of the gas at the wall surface deviate from the velocity and temperature of the wall in the smooth microchannel in Poiseuille flow and the Couette flow,so the phenomenon of velocity slip and temperature jump are occurred,and with the increase of the Knudsen number,the rarefied effects of gas is more significant.The velocity slip and temperature jump at the gas-solid interface are gradually increased,and the relative slip length and temperature jump length are also increased,the frictional resistance are gradually decreased.At the microscale,even if the roughness is small,the disturbance caused at the boundary affects the flow and heat transfer of the entire channel.Finally the Poiseuille flow of rough wall are simulated and compares it with the smoothness,the presence of roughness increases the strength of the interaction with the gas and the wall,so that the energy exchange is enhanced,the velocity slip and temperature jump at the wall surface are weakened,and the relative velocity slip and the temperature jump length are reduced as the roughness element height increases.