Study Of Flow And Heat Transfer Of Liquid Metal In Microchannel Under The Magnetic Field

In this paper,the plasma facing component(PFC)structures of Tokamak device is taken as the research background.The flow and convection heat transfer of liquid metal in microchannel under the influence of strong magnetic field are studied by experimental method and numerical simulation method.The main research content is the effect of magnetic field on the flow and heat transfer of liquid metal in microchannels.In the Tokamak device,the PFC module needs to withstand ultra-high intensity heat load and high density neutron bombardment.The known solid materials are not suitable for this extreme situation,so the liquid metal is used as the PFC material.As the PFC module,liquid metal needs to achieve the functions of energy transportation and protecting the solid wall of the device from erosion.In order to achieve the expected functions,it is necessary to realize the uniform spreading and stable flow of liquid metal on the solid wall.PFC modules in a strong magnetic field,there is MHD effect when the liquid metal moves in the magnetic field,which will affect the flow and make it difficult to achieve even spread or stable flow.In order to solve the technical problem,people usually study from the perspective of wall structure,expecting to improve the spreading effect of liquid metal by designing different wall structure.In this paper,the surface of microchannels structure is designed,the flow and heat transfer characteristics of liquid metal in microchannel under strong magnetic field are studied by experiment and numerical simulation.In the part of experimental research,the microchannel experimental plate is designed,and the experimental platform is built using the electromagnet device equipped in the laboratory.In the experiment,the liquid metal Galinstan alloy was used as the working fluid to study the effect of magnetic field on the flow and convective heat transfer of liquid metal in microchannel.K-type thermocouple was used to measure the temperature,a high-speed camera was used to capture the flow pattern of the liquid metal film,liquid metal flow is driven by argon pressure.In the experiment of spreading,it is proved that the surface structure of microchannel is helpful to the uniform spreading and stable flow of liquid metal.The microchannel has the function of flow diversion,when the liquid metal flows into the experimental plate,it can spread evenly and flow stably under the guidance of the microchannel.The flow pressure drop as a measure to analyzed the influence of magnetic field on the flow resistance of liquid metal.It is found that magnetic field can increase the flow resistance of liquid metal.A high-speed camera was used to shoot the flow pattern of the liquid metal film from directly above.By comparing the flow pattern of the liquid metal film under the influence of different magnetic field strengths,it can be seen that the magnetic field has two different effects on the flow pattern of the liquid metal film.When the magnetic field strength is small,the fluctuation in the back-end region of the liquid film increases with the increase of the magnetic field strength,and the ripple shape becomes more regular,when the magnetic field strength is large,the back-end region of the liquid film will change from a large fluctuation pattern to a steady flow,and the middle region will change from a small random fluctuation pattern to a regular large amplitude fluctuation.There is a turning point between the two effects.In the experiment of forced convection heat transfer under the condition of no magnetic field,it is found that liquid metal has the basic properties as a coolant.Through the analysis of experimental data,the relationship curve between the Nu and the Re was fitted when the flow velocity was large,and the relationship curve between the Nu and the We was fitted when the flow velocity was small.The relationship between Nu and Re and the relationship between Nu and we are all exponential,and the Nu increases with the increase of Re or We.In the experiment of forced convection heat transfer under the influence of magnetic field,it is found that the flow rate of liquid metal decreases gradually with the increase of magnetic field intensity.There is a power exponential relationship between the mean Nu and Ha,and the mean Nu decreases with the increase of Ha.It shows that the magnetic field has an inhibitory effect on the flow and convective heat transfer of liquid metal.In the part of numerical simulation,the flow and heat transfer of liquid metal in a rectangular duct with microchannels under the influence of a perpendicular magnetic field are simulated.The calculation model is designed as a rectangular duct.One side wall of the duct is microchannel surface structure,which works as heating plate,and the wall temperature is constant.The other walls are adiabatic,and all the walls of the duct are conductive.It is found that the velocity distribution of liquid metal is m-shaped under the action of magnetic field.The flow in the central area of the square tube will be inhibited,and the liquid metal will be squeezed to the sidewall by Lorentz force,so that the flow rate of the side walls area will increase or even form a jet.With the increase of magnetic field strength,the temperature gradient of liquid metal near the heat plate will increase,and the temperature boundary layer will gradually become thinner.The mean Nu between the liquid metal and the microchannel heating wall is 32% higher than that of the plate heating wall,which indicates that the microchannel heating wall is helpful to enhance the convective heat transfer.With the increase of the external magnetic field intensity,the mean Nu of convective heat transfer between liquid metal and microchannel heating wall will increase gradually,which indicates that the magnetic field strengthens the convective heat transfer.Magnetic field affects heat transfer by affecting the flow velocity of liquid metal.Because the magnetic field causes the formation of jet flow in the sidewall area,which increase the velocity of the sidewall area,so the convective heat transfer is strengthened.The local Nu number of convective heat transfer between liquid metal and microchannel heating wall will be affected by the location.The closer to the entrance,the greater the local Nu number,and the farther away from the entrance,the smaller the local Nu number.And the local Nu number will increase with the increase of magnetic field strength.In this paper,the topic of the flow and convective heat transfer characteristics of liquid metal in microchannel under external magnetic field are studied,and many research results are obtained,which is of great significance to the study of magnetohydrodynamic(MHD)problems in tokamak device,and has practical engineering reference value for the design of PFC modules.