Effect of a Baffle on Pseudosteady-State Natural Convection inside Spherical Containers

Abstract

Pseudosteady-state natural convection within spherical containers with and without thin baffles was studied computationally. Insulated or isothermal baffles were considered for passive management of the flow and thermal fields. For Rayleigh numbers of 104, 105, 106 and 107, baffles with 3 lengths positioned at 5 different locations were investigated. Elaborate grid size and time step size independence tests were performed. The solution of the governing equations was obtained by use of a commercial computational fluid dynamics (CFD) package. For the case of no baffle, computational results were validated successfully to previous data available in the literature by comparing the heat transfer correlations, temperature distribution and streamline patterns. Both thermally stable and unstable layers are present in this problem and for the higher Rayleigh numbers, the onset of instabilities was observed in this system. Regardless of the thermal status of the thin baffle, placing it on the inner wall of the spherical container directly leads to modification of the velocity field. It can generally be stated that the resulting --Y´confinement¡ or ´compartmentalization¡ causes the fluid above the baffle to be characterized by stable constant-temperature layers that are slow moving and dominated by heat conduction. In contrast, the fluid below the baffle is subjected to strong natural convection currents. Regardless of the Ra number, the modifications of the flow and temperature fields for short baffles are limited to the vicinity of the baffle and a possible interaction with the eye of the primary clockwise rotating vortex. The modifications of the flow and thermal fields were more pronounced for the longest baffle for which two clockwise rotating vortices are clearly observed when the baffle is positioned at or in the vicinity of the mid-plane. The Nusselt numbers and maximum stream function of the primary vortex were generally lower than the reference cases with no baffle. The degree of degradation of the Nusselt number has a strong dependence on the position and length of the insulated baffle. In contrast to the general reduction of heat transfer trends exhibited by the insulated or isothermal baffles, placing a baffle near the top of the sphere for high Ra number cases can lead to heat transfer enhancement in comparison to the reference case with no baffle. The extra heat that is brought in the fluid through the surface of the sphere is linked to the disturbance of the thermal boundary layer by the thin baffle. Some differences are observed due to the thermal status of the baffle. Due to the extra heating afforded by a thin isothermal baffle, the velocity and temperature fields were more complicated than the case with a thin insulated baffle. In addition to confinement, a strong counterclockwise rotating vortex was created due to the extra heating of the baffle for high Ra numbers and baffle positions on or below the mid-plane. The hot fluid in this vortex was observed to be transported toward the center of the sphere, thus disturbing the stable stratified layers. In contrast to insulated baffles, placing isothermal baffles near the bottom for high Ra number cases also gave rise to heat transfer enhancement due to disturbance of the stratified layers by the CCW rotating vortex that is energized by the heated baffle.