Large Eddy Simulation Of Turbulent Mixed Convection Of LBE In Heated Upward Pipe Flows

Due to the difficulty in carrying out experimental research on liquid metal fluids,the existing RANS model is not suitable for the research of low-Prande number fluids,and the direct numerical simulation has the problem of large calculation.Therefore,the large eddy simulation method is adopted in this paper.The turbulent mixed convective heat transfer process of liquid metal lead bismuth in a vertically upward flow tube is simulated at a higher Reynolds number(Reynolds number 37700).In this paper,the turbulent heat transfer of liquid lead bismuth(Prandt number is 0.026)under six different floating lifts is simulated.The Richardson numbers are 0,0.05,0.1,0.15,0.2 and 0.4,respectively.The turbulence statistics in the circular tube under various lifting forces are given and analyzed,including the velocity average and its pulsation,temperature average and its pulsation,Reynolds stress,turbulent kinetic energy and its budget,temperature kinetic energy budget,friction coefficient and heat transfer.Coefficients,etc.The results show that buoyancy has different effects on the momentum field and temperature field of liquid lead bismuth in vertical tubes.With the increase of buoyancy,the fluid in the near wall area of the circular tube is accelerated by buoyancy,and the flow velocity in the central region of the circular tube is reduced.When the buoyancy is increased to a certain extent,the velocity in the entire tube exhibits an M-shaped distribution;The Reynolds stress gradually decreases with the increase of buoyancy.When the buoyancy is large enough,the S-type distribution occurs,that is,there is a negative value in the central region,but the absolute value is restored,which indicates that the turbulence intensity in the tube is dependent on the buoyancy.After weakening and then enhancing,through the analysis of the turbulent energy,it is found that buoyancy changes the turbulent flow field in the tube by changing the turbulence generation term.For the liquid lead-bismuth temperature field,as the buoyancy increases,the average temperature decreases first and then increases near the wall surface,but increases first and then decreases in the central region,while the temperature pulsation is first with buoyancy.After the reduction is enhanced,it is found that the buoyancy changes its temperature kinetic energy by changing the temperature kinetic energy generation item by analyzing the temperature kinetic energy.The calculation results also show that the dimensionless friction coefficient increases with the increase of the lift force,while the dimensionless heat transfer coefficient decreases first and then increases with the increase of the lift force.The reason can be visualized by a new FIK decomposition method.The FIK decomposition indicates that the change of the dimensionless friction coefficient mainly depends on the relative contribution of the contribution of the turbulence term and the contribution of the lift force term,and the change of the dimensionless heat transfer coefficient mainly depends on the change of the contribution of the turbulent term.Finally,this paper also uses the quadrant analysis method to analyze the mechanism of turbulence in mixed convection.The research shows that the turbulent coherent structure near the wall caused by the lift force(slow thermal fluid emission and rapid cold fluid sweep event)is weakened.It is the main cause of the weakening of turbulence.