Heat Transfer Characteristics Of Supercritical Methane In U-Type Tube

The heat transfer law of supercritical methane is special and complicated due to the drastic change of physical properties near the pseudo-critical temperature.Therefore,in-depth study on the heat transfer characteristics of supercritical methane near the pseudo-critical temperature point is of reference significance for the design of heat exchangers in the future.In this paper,ICEM software is used to establish the physical model and divide the grids.Fluent software is used to study the turbulent flow and heat transfer characteristics of supercritical methane in horizontal straight tube under the condition of constant heat flux,mass flow inlet and tube inner diameter of10mm,based on the Computational Fluid Dynamics（CFD）principle.On this basis,the convection heat transfer of supercritical methane in U-type tube under the same conditions is continuously studied.The details are as follows:Firstly,the two-equation model Realizable k-εis adopted as the turbulence model in the tube,and the numerical simulation result of supercritical CO ₂ in the straight tube are compared with the experimental research result under the same condition to verify the simulation accuracy of the simulation method and turbulence model.Secondly,the flow and heat transfer characteristics of supercritical methane in horizontal straight tube are numerically simulated,and the effects of mass flow rate,heat flux density,pressure,pipe inner diameter and gravit y on flow and heat transfer are analyzed.The results show that the peak value of convective heat transfer coefficient h increases with the increase of mass flow rate and gravity.With the increase of pressure,heat flux density and inner diameter of pipe line,h peak value decreases.In the calculation range of this paper,the order of the influence of various parameters on heat transfer performance is mass flow rate,pressure,gravity,heat flow density and inner diameter of the pipeline from large to small.The floating force causes secondary flow and temperature stratification in the cross section,which deteriorates the heat transfer on the upper wall and strengthens the heat transfer on the lower wall.Buoyancy effect decreases with the increase of mass flow rate and heat flux density,and increases with the increase of pressure,pipe inner diameter and gravity.The buoyancy factor Gr/Re² can predict the effect of buoyancy on heat transfer.Then,the flow and heat transfer characteristics of supercr itical methane in U-type tube are numerically simulated,and the effects of mass flow,heat flux,pressure,radius of curvature,gravity and arrangement on flow and heat transfer are analyzed.The results show that the existence of elbow has no effect on t he front straight tube but has effect on the subsequent straight tube.Dean vortex deteriorates or intensifies local heat transfer in the elbow.h in the elbow increases with the increase of mass flow,heat flux,gravity and radius of curvature,and decreases with the increase of pressure.The influence of various parameters on heat transfer performance is in the order of pressure,mass flow,gravity,heat flow density and curvature inner diameter from large to small.Heat transfer deterioration occurs in the elbow under the conditions of G=100kg/m²s,P=5.43MPa,R=17mm and horizontal arrangement.Dean vortex strength increases with the increase of mass flow,heat flux and gravity,but decreases with the increase of pressure and radius of curvature.Finally,the effects of mass flow,heat flux,pressure and gravity on the field synergy angle of horizontal straight tube and U-type tube are analyzed.The results show that the field synergy of U-type tube is better than that of horizontal straight tube.The influence of each parameter on the field synergy angle and heat transfer performance is the same from large to small.