Blockage of Natural Convection Boundary Layer Flow in a Multizone Enclosure

Two separate mechanisms can be responsible for natural convection flow between the hot and cold zones of a multizone enclosure: (1) bulk density differences created by temperature differences between the fluid in the hot and cold zones (bulk density driven regime) and, (2) thermosyphon 'pumping' generated by boundary layers or plumes (boundary layer driven regime). This paper reports the resultsof an experimental study that examines the transition between flow regimes, as a function of aperture size, in a two-zone enclosure with heated and cooled?end walls. A constant heat flux boundary condition was maintained on one vertical end wall, and an isothermal cold temperature sink was maintained on the opposite vertical end wall. All of the remaining surfaces were highly insulated. Thetransition between the boundary layer driven regime and the bulk density driven regime was established as a function of the geometry of the aperture in the partition that separated the hot and cold zones. The results demonstrate that transition from the boundary layer driven regime to the bulk density driven regime is caused by blockage of the boundary layer flow, when the area of the flowaperture is reduced below a critical value. A simple flow model has been developed which predicts that the critical aperture area for the onset of flow blockage is directly proportional to the number of active heat transfer surfaces and inversely proportional to the Rayleigh number which characterizes the level of heating and cooling provided to the active heat transfer surfaces.