Onset of secondary flow and heat transfer enhancement for convergent or divergent channel heated from below

Experimental studies of secondary air flow structure and heat transfer in horizontal parallel-plate, convergent and divergent channels have been carried out. The bottom wall is horizontal and heated uniformly, which the opposite wall is insulated and inclined with respect to the horizontal plate so as to create a convergence and divergence angle of 3° for the convergent and divergent channel. The aspect ratio (width to height) and the ratio of channel length to height at the entrance of the channel is 6.67 and 15, respectively. The Reynolds number ranges from 100 to 2,000 and the buoyancy parameter, Gr/Re², from 0 to 907. Secondary air flow structure inside the channel is visualized by injecting smoke at the inlet flowing along the bottom wall and the locations of the onset of secondary flow are recorded at once. Correlations are suggested for the heat flux and for the Reynolds number, and for the locations of the onset of secondary flow, the onset of heat transfer enhancement, the maximum Nusselt number. The adverse pressure gradient in the divergent channel causes a thicker heated layer in the bottom and earlier onset of secondary flow due to the destabilization effect of deceleration. The interaction between neighboring vortices and plumes becomes highly unstable and the flow transfers to turbulent immediately. For the convergent channel, the favorable pressure gradient causes a thinner bottom heated layer which results in much later onset of secondary flow due to the stabilization effect of the acceleration. The interactions between neighboring vortices and plumes are suppressed, and results in less pronounced enhancement of heat transfer, later onset of heat transfer enhancement and a much stable flow.