Secondary flow and enhancement of heat transfer in horizontal parallel-plate and convergent channels heating from below

Experimental studies of secondary air flow structure and enhancement of heat transfer in horizontal parallel-plate and convergent channels have been carried out. The bottom wall is horizontal and heated uniformly, while the opposite wall is insulated and inclined with respect to the horizontal plate so as to create a convergence angel of 3°for the convergent 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 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 complete processes for the formation of the plumes associated with vortices and their transformation into longitudinal convection rolls due to the lateral extension and combination of the vortices are observed. The number of convection rolls formed is much less than those found in the experiments with water. For the convergent channel, the favorable pressure gradient causes a thinner bottom heated layer which results in much later initiation of secondary flow and fewer the number of plume produced. The interactions between neighboring vortices and plumes are suppressed by the acceleration of mainstream, and results in a stable flow and less pronounced enhancement of heat transfer. Temperature fluctuations at different locations are measured to indicate the flow structure and oscillation of the secondary flow. The effects of the buoyancy parameter and the convergence of the channel on the secondary flow structure and the Nusselt number are presented and discussed.