Study Of Coupled Heat Transfer Of Turbulent Natural Convection With Surface Radiation In Enclosures With Internal Heat Source

Turbulent natural convection in enclosures,as a process accompanied by fluid flow and heat transfer,is widely used in engineering applications.In this paper,both experiment and numerical simulation are used to focus on the variation of the relevant parameters in the heat transfer process of air turbulent natural convection inside an enclosure having different physical geometries,the regular results not only can improve the cooling efficiency of electronic components in enclosures also can guide practical problems such as structural optimization design.Firstly,a preliminary test was conducted on natural convection of air turbulence in a enclosure using an IFA300 hot wire anemometer and a T-thermocouple.The distribution of vertical average velocity and average temperature in the boundary layer ofhot wall surface of the cavity are mainly obtained.The experimental results show that The temperature stability value outside the temperature boundary layer on the hot wall surface gradually rises upward along the wall and the thickness of the hot-wall surface boundary layer increases along the height of the wall surface.Secondly,ANSYS FLUENT numerical simulation software was used to calculate the air turbulent natural convection in an two-dimensional enclosure with the same size and conditions with experiment.The RNG k-ε model was used to calculate the flow field and temperature field with the Boussinesq hypothesis.The results show that the average temperature and average velocity obtained by the numerical calculation method are in good agreement with the experimental data of the literature.The average vertical velocity and the average temperature change trend in the boundary layer are consistent with the results of this experiment as well as the thickness of velocity boundary layer and temperature.Finally,two-dimensional and three-dimensional numerical study of coupled heat transfer of turbulent natural convection with surface radiation inside an enclosure with internal heat source at a Rα number of Rα=1.58×10⁹ have been calculated by RNG k-ε model and DO model.The mean and fluctuating parameter are presented,i.e.（?）,u_r’_ms,（?）,v_r’_ms,（?）,T_r’_ms.The local and average Nusselt numbers,the wall shear stress,the stream function,the thermal field as well as the turbulent viscosity are also presented.Particular efforts have been focused on the effects of five types of influential factors such as time,horizontal boundary conditions,the internal surface emissivity 0≤ε≤0.9,local heater position and three-dimensional conditions.The calculation results show that the two-dimensional and three-dimensional turbulent natural convection unsteady heat transfer processes gradually enter the quasi-steady state at t=250s,the two-dimension heat transfer and flow fully reach the quasi-steady state at t=1000s.The wall radiation weakens the effect of the horizontal thermal boundary on the structure of the flow field and the structure of the temperature field,compensates the heat dissipated out of the horizontal surface and slightly accelerates the flow along with the cavity level.For the calculation of the average Nusselt number of the heat source wall surface,hot wall surface and cold wall surface,the horizontal plane heat conduction considers the wall radiation,it increases by 39.5%,80.1%,and 55.1%compared with the same conditions without considering the wall radiation,while the horizontal adiabatic plane considers wall radiation,it increased by 41.2%,89.3%and 50.2%compared to no considering the wall radiation,respectively.Therefore,no matter the horizontal plane is insulated or not,the heat source wall surface heat transfer,the cold and hot wall surface heat transfer significantly increased due to the wall radiation.The calculation results of average Nusselt number of heat source wall surface,hot wall surface and cold wall surface are increased by 30.38%,54.5%,and 39.18%respectively underε=0.6 compared withε=0.3 while increased by 63%,127.3%,and 85.12underε=0.9 compared withε=0.3.Therefore,the greater the wall emissivity,the greater the heat transfer due to the radiative heat transfer from the wall surface,and the greatest increase of heat transfer ability from the hot wall surface,followed by the cold wall surface and the heat source wall surface.When the heat source moves from x₀=0.15m to x₀=0.6m,the average Nusselt number of the cold wall surface and the average Nusselt number of the heat source wall surface increase by 1.1%and 1.7%respectively.The average Nusselt number of the hot wall surface reaches a maximum of 39.87 at x₀=0.225m.It shows that the heat transfer of the cold wall surface and the heat source wall surface increase with the amount of x₀,while the hot wall surface heat transfer increases first and then decreases（the maximum at x₀=0.225m）.According to the calculation results of the three-dimensional cavity model,the temperature field and the flow field are symmetrical structures with the heat source as the center along the z direction.The effect of the thermal plume above the heat source is more obvious than that of the two-dimensional model.The heat transfer of the cold wall surface and the heat source wall surface in the three-dimensional enclosure are greatly reduced while the hot wall surface has a smaller reduction.The three-dimensional calculation results（area average）of the average Nusselt number for the hot and cold wall surfaces are reduced by 5.8%and 52.4%respectively compared to the two-dimensional results（line integral average）and the average Nusselt number of heat source surface is reduced by 68.9%.