Visualization of return flow structure in mixed convection of gas over a heated circular plate in a horizontal flat duct

Abstract

Experimental flow visualization is conducted here to investigate the buoyancy-induced return flow structure in mixed convection of gas over a heated circular plate mounted on the bottom of a horizontal flat duct, simulating that in a horizontal longitudinal flow reactor. In particular, how the return flow induced in the upstream portion of the duct is related to the vortex flow in the downstream is delineated. The results show that at a low buoyancy-to-inertia ratio only steady longitudinal vortex rolls (L-rolls) are induced in the exit half of the duct and no return flow appears in the duct. At an intermediate buoyancy-to-inertia ratio a small flow recirculation exists around the duct inlet near the top wall and the L-rolls become unsteady. When the buoyancy-to-inertia ratio is high and the Reynolds number is small with Reo10; the upstream return flow is strong and occupies a large region. There is another return flow zone around the exit end of the duct. Over the heated circular plate the flow is dominated by the moving transverse rolls (T-rolls). Moreover, the return flow around the duct inlet at steady or statistically stable state is in the form of a semicircular roll around the upstream edge of the circular plate. Flow visualization also reveals that the splitting of the downstream tip of the return flow zone and the rising of the buoyancy-driven spanwisely extended thermal under the tip generate a pair of counter-rotating transverse rolls in the entry portion of the duct. In addition, the criterion for the onset of the return flow near the duct inlet, the size and the center position of the return flow, based on the present experimental data, are correlated empirically.

Delayed onset of return flow by substrate inclination in model horizontal longitudinal MOCVD processes

“…2-5. This roll is in fact the buoyancy-induced return flow which was already discussed in our earlier study for f=01 [3]. Note that the return flow driven by the heated circular disk is 3-D.…”

“…can also appear in the duct above the heated disk [3,20]. The longitudinal and transverse vortex flows are, respectively, characterized by the rolls with their rotation axes parallel and perpendicular to the main flow direction [22].…”

“…The experimental system established in the previous study [3,20] is also used here with a slight modification of the test section. Fig.…”

“…But the details on how the vortex and return flows are affected by the substrate inclination remain largely unexplored. In a recent model experiment [3], we examined the buoyancy-driven 3-D return flow pattern in a gas flow over a heated circular disk embedded in the bottom plate of a horizontal rectangular duct, simulating that in a horizontal longitudinal MOCVD reactor. Here in the present study, we move further to investigate how the buoyancy-driven return flow is affected by the substrate inclination.…”

“…They concluded that for a large Gr/Re 2 the return flow appeared at the leading edge of the susceptor, and it caused an increase in the growth rate of the film at this region and a decrease in the downstream of the susceptor. Recently, the 3-D return flow structure driven by a heated circular disk embedded in the bottom wall of a rectangular duct was examined by Tuh and Lin [3] through experimental flow visualization.…”