Influence Of Turbulence And Surface Roughness On Flow And Heat Transfer Of An Elliptical Cylinder

The flow and heat transfer in the process of flow around elliptical cylinder are affected by many factors.This paper focuses on the two common factors:free-stream turbulence and surface roughness.Firstly,the numerical simulation method is used to calculate the flow and heat transfer of four elliptical cylinders with different axis ratios in the subcritical region(Re=10~5)and the critical region(Re=10~6).The axis ratios of the four elliptical cylinders are AR=0.4,0.6,0.8,and 1.The?-Re?t transition model based on local variables was used in the simulation.This model have better performance in predicting the boundary layer transition.By analysis of numerical calculation results,the skin-friction coefficient,boundary layer transition point,separation point,drag coefficient,local heat transfer and average heat transfer change laws under different free-stream turbulence and surface roughness are obtained.The main conclusions are as follows:(1)When Re=10~5,the flow around the elliptical cylinder is in the subcritical region,and the laminar boundary layer separation occurs during the flow,but no transition occurs.When Re=10~6,the flow around is in the critical region,and there is a transition from laminar flow to turbulent flow in the boundary layer,accompanied with separation bubble.(2)In the subcritical region,when the turbulent flow increases,the boundary layer separation point moves downstream,the friction coefficient increases,and the average heat transfer increases.In the critical region,the transition to the supercritical region occurs when the turbulence intensity reaches the critical turbulence level.Below the critical turbulence level,the boundary layer transition position,separation position,and average heat transfer does not change with turbulence;above the critical turbulence,as the turbulence continues to increase,the boundary layer transition point moves upstream and the average heat transfer increases.The separation position stays the same.(3)In the critical region,when the surface roughness increases,the boundary layer separation position does not change,the friction coefficient increases slightly,but the total drag coefficient is basically unchanged,and the average heat transfer has a small increase trend.In the critical region,the transition to the supercritical region is also achieved when the roughness is greater than the critical roughness.Below the critical roughness,the boundary layer transition position,separation position,and average heat transfer do not change with roughness;at the critical roughness point,the boundary layer transition position moves upstream to the front stagnation point,friction coefficient and average heat transfer efficiency Increase rapidly.As the surface roughness continues to increase,the transition position and heat transfer no longer change.By analyzing numerical data above,this paper proposes a modification coefficient of the turbulence and roughness of the nusselt number in the subcritical and critical regions.The modification coefficient can correct the heat transfer empirical relationship under different turbulence and roughness.By correcting the relationship in the literature,the ideal results are obtained,and the validity of the correction coefficient is verified.Finally,the heat transfer experiment of naphthalene sublimation based on the principle of thermal mass comparison is carried out,and the heat transfer characteristics of elliptical cylinders under two turbulence levels and roughnesses height in the subcritical region is studied.The heat transfer empirical relationship under specific turbulence level and surface roughness is obtained after processing the experimental data.The empirical relationship is modified by the turbulence and roughness correction coefficient.Although the curve deviation before and after the correction is reduced,the result is still not ideal.The source and limitation of the experimental error are analyzed.