Onset of secondary flow and enhancement of heat transfer in horizontal convergent and divergent channels heated from below

Experiments for the onset and development of the buoyancy driven secondary air flow and enhancement of heat transfer in a horizontal convergent and a divergent channel have been carried out. The bottom wall of the channel is horizontal and heated uniformly, while the top wall is insulated and inclined with respect to the horizontal plane so as to create a convergence angle of 3° for the convergent channel, or a divergence angle of 3° for the 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 200 to 2000, the buoyancy parameter, Gr/Re2, from 2.5 to 907 and Pr of the air flow is 0.7. Flow structure inside the channel is visualized by injecting smoke at the inlet flowing along the bottom wall. The onset of secondary flow appearing as transverse instability wave and onset of initial protrusion of the bottom heated layer are identified. Secondary flow structures observed are somewhat different from the case in the parallel-plate channel. This is attributed to the destabilization effect of the deceleration in the divergent channel which results in a much earlier initiation of secondary flow and more pronounced enhancement in the heat transfer, and the stabilization effect of the acceleration in the convergent channel which results in a much later initiation of the secondary flow and less pronounced enhancement in the heat transfer. However, the deceleration flow in the divergent channel and the acceleration in the convergent make the average Nusselt numbers approach the results of the parallel-plate channel. Correlation results for the onset of the secondary flow and enhancement of the heat transfer will be presented and discussed.