Fluid flow and heat transfer in a horizontal channel with divergent top wall and heated from below

Secondary flow structure and its enhancement on the heat transfer in a horizontal divergent channel have been studied. The bottom wall is horizontal and is heated uniformly while the opposite top wall is insulated and inclined with respect to the horizontal plane so as to create a divergent angle of 3 deg. At the entrance of the channel, the aspect ratios for the width to the height and the channel length to the height are 6.67 and 15, respectively. The Reynolds number ranges from 100 to 2000 and the buoyancy parameter Gr/Re² from 0 to 405. Both flow visualization and temperature fluctuation measurements at different locations are made to indicate the flow structure and oscillation of the secondary flow. The adverse pressure gradient in the divergent channel causes a thicker heated layer in the bottom and earlier initiation of secondary flow. Interaction between neighboring vortices and plumes becomes more severe and highly unstable. This precludes the formation of steady two-dimensional longitudinal vortex rolls in the downstream and leads to an earlier and larger enhancement of the heat transfer than the case of the parallel-plate channel. The effects of the buoyancy parameter and the divergence of the channel on the secondary flow structure and the Nusselt number are presented and discussed.