Study Of The Turbulent Heat Transfer Of Liquid Metals By Direct Numerical Simulation

In present dissertation,direct numerical simulation(DNS)method is adopted to study turbulent heat transfer for lead-bismuth eutectic(LBE),as the representative for liquid metals(LMs).Mechanisms of the heat transfer in turbulent mixed convection in pipe flow are analyzed.Influences of Prandtl number(Pr)on turbulent heat transfer in backward-facing step(BFS)are studied.Differences of buoyancy-aided and-opposed flow in BFS are discussed.Turbulent heat transfer is weakened first and then enhanced with the increase of buoyancy in studying the buoyancy-aided flow in pipe.When the buoyancy is weak,the acceleration of the warm fluid disturbs the ejection-sweep cycle in the turbulent boundary layer.This effect attenuates the turbulent fluctuations and re-laminarization would occur in certain circumstances.As buoyancy increases further,the "M-shaped"velocity profile is prominent and the near-wall shear stress changes its sign and increases gradually.Thus,the production of turbulent kinetic energy increases and turbulence gets enhanced.The variation of the near-wall turbulence responsible for the first decrease and then increase of heat transfer for aided flow.Compared to the air mixed convection,the velocity field of LBE is more susceptible to buoyancy effect.Studies of forced convection of different Pr fluids in BFS show that peak location of Nusselt number(Nu)coincides with the reattachment point.As Pr increases,both of the temperature fluctuations and Nu enhance and their peak location moves upstream.The impingement of the shear layer which entrains intense vortices on the reattachment zone results in the augmentation of Nu.Turbulent heat transport in the recirculation zone is important even for low Pr fluids.Non-uniform turbulent Prandtl number Prt profiles in the recovering zone challenge the Reynolds-averaged Navier-Stokes(RANS)simulation of turbulent heat transfer for LMs flow in BFS.Buoyancy has an important influence on turbulent heat transfer of LMs flow in BFS.In aided flow,the acceleration effect makes the secondary vortex prevail over the main vortex and the wall friction coefficient(Cf)increases.The recirculation zone is reduced in size and the shortened shear layer weakens the turbulent intensity.Nu is increased due to the less entrainment of heat upstream driven by the main vortex.For Richardson number(Ri)equals 0.2,turbulence enhancement induced by buoyancy near the outlet can be seen.In opposed flow,a little Ri could enlarge the recirculation zone quickly.The main vortex almost occupies the whole recirculation zone.The elongated shear layer strengthens the turbulent intensity.Oppositely,Nu in the recirculation zone is decreased due to the increase of heat entrainment by the main vortex.Besides,based on our BFS DNS data and experimental correlations for bundle sub-channel heat transfer,some efforts have been made to improve the turbulent heat flux model in RANS.Constant Pr,0.85 is replaced by different Prt models.Their applicability has been evaluated in simulating the BFS flow and bundle sub-channel flow for LMs.The model of Kays which is based on the local flow parameters has an outstanding performance in comparison with others.