Numerical Investigation On Combined Heat Loss Characteristics In Partially Side-Opened Cylindrical Cavity Based On Constant Wall Heat Flux

The heat loss characteristics in a cavity with one vertical side open have a widerange of industrial applications, some applications are solar thermal receivers, coolingof electronic devices, energy-saving household refrigerators and building energyconservation etc. The heat losses of the cavity mainly include the radiation heat lossamong the interior wall surfaces of the cavity, the convection heat loss from the apertureto the ambient and the conduction heat loss through the insulation. It is of greatimportance to analyze these three heat losses and hence improve the thermalperformance of the cavity.In the case of considering conduction heat loss through the insulation, a3Dnumerical investigation of the combined natural convection and radiation heat lossesunder two different heating boundary conditions in one side open cylindrical cavity withconstant heat flux has been carried out, the numerical procedure is validated by ourlatest experimental measurements. The two heating boundary conditions：both the sidewall and bottom wall are heated (Case1), only side wall is heated (Case2).Visualization results, i.e., temperature contours，velocity vectors on the symmetry planeand velocity vector field on the aperture plane are provided to reflect the air flow andheat transfer characteristics. Moreover, based on the variable air thermophysicalproperties, the influence of relevant parameters, such as aperture ratio AR, tilt angle φ,surface emissivity ε as well as heat flux q on the convection heat loss Nusselt numberand radiation heat loss Nusselt number have been quantitatively analyzed. In addition,based on a large set of numerical data, separate empirical Nusselt number correlationsfor natural convection heat loss and radiation heat loss under two different heatingboundary conditions for a given range of parameters of practical interest have beenproposed.Results show that the radiation heat loss and natural convection heat losscharacteristics under the two different heating boundary conditions have similarvariation trend as the parameters change. That is, under a given aperture ratio，as the tiltangle changes from0°to90°, the radiation heat loss Nusselt number Nu_rincreasesslightly, while the natural convection heat loss Nusselt number Nu_cdecreases relativelysharply. For a given wall heat flux, both the natural convection heat loss Nusselt numberNu_cand the radiation heat loss Nusselt number Nu_rincrease with the increases of the aperture ratio. When the tilt angle is constant, the radiation heat loss Nusselt number Nu_rincreases tremendously and the natural convection heat loss Nusselt number Nu_cincreases relatively slowly as the wall heat flux increases. Moreover, variations ofradiation heat loss Nusselt number Nu_rand natural convection heat loss Nusselt numberNu_cwith surface emissivity ε are different when ε changes from0to0.3and0.3to1.While there are some different points, when the wall heat flux, aperture ratio, tiltangle and surface emissivity are all the same, the temperature distribution in the cavityunder Case2is more even, but the average temperature of the inner wall of the cavityand the air speed in the same position of the aperture plane under Case1are higher thanthose under Case2. Moreover, the percentages of conduction heat loss and radiationheat loss under Case1are higher than those under Case2, but the percentage ofconvection heat loss under Case2is higher than that under Case1. It should be notedthat when the tile angle is90°(the opening facing vertically downward), under case1,no matter how aperture ratio and wall heat flux change, natural convection heat lossNusselt number always fluctuates in a small value, but under case2, natural convectionheat loss Nusselt number increases with the increase of aperture ratio and wall heat flux.