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 hea t transfer in horizontal parallel-plate and convergent channels have been c arried out. The bottom wall is horizontal and heated uniformly, while the o pposite wall is insulated and inclined with respect to the horizontal plate so as to create a convergence angel of 3 degrees for the convergent channe l. The aspect ratio (width to height) and the ratio of channel length to he ight at the entrance of the channel is 6.67 and 15, respectively. The Reyno lds number ranges from 100 to 2000, the buoyancy parameter, Gr/Re-2, from 2 .5 to 907 and PI of the air flow is 0.7. Flow structure inside the channel is visualized by injecting smoke at the inlet Bowing along the bottom wall. The complete processes for the formation of the plumes associated with vor tices and their transformation into longitudinal convection rolls due to th e lateral extension and combination of the vortices are observed. The numbe r of convection rolls formed is much less than those found in the experimen ts 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 enhancemen t of heat transfer. Temperature fluctuations at different locations are mea sured to indicate the flow structure and oscillation of the secondary flow. The effects of the buoyancy parameter and the convergence of the channel o n the secondary flow structure and the Nusselt number are presented and dis cussed. (C) 1999 Elsevier Science Ltd. All rights reserved.