The main purpose of this article is to shed more light on the cause of flow reversal in vertical concentric annular channels for buoyancy-aided flows under isothermal boundary conditions. The concept used to quantify the critical values of the modified buoyancy parameter (Gr/Re)_(critical) at which the adverse pressure gradient first occurs is outlined and applied to analytically estimate these critical values. The conditions for flow reversal are also analytically obtained for buoyancy aided and buoyancy-opposed flows. The analytical solutions show that pressure buildup takes place only for buoyancy-aided flows while flow reversal occurs for both buoyancy-aided and buoyancy-opposed flows. The analytical solutions reveal also that values of the buoyancy-parameter required to initiate the pressure buildup for buoyancy-aided flows in the vertical annuli are smaller than those required to initiate the flow reversal. Thus, for buoyancy aided flows, pressure buildup would precede the flow reversal. A numerical scheme is developed and used to simulate the development of pressure and pressure gradient in the entrance region of the vertical concentric annuli. The numerical scheme is validated by the analytical solution as well as the previously published pertinent results. Numerical results that show the pressure buildup in the developing entry region of the annuli for values of the modified buoyancy parameter (Gr/Re) greater than its critical values (Gr/Re)_(critical) along with other flow and heat transfer parameters of importance are also presented and discussed. The locations of pressure build up as well as those of flow reversal have been obtained and reported. The numerical solutions show clearly that pressure buildup for buoyancy-aided flows takes place prior to the flow reversal onset.
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