Experimental flow visualization and temperature measurement have been conducted here to explore the effects of the sidewall converging and the duct inclination on the buoyancy induced vortex flow and return flow in mixed convection of air in a horizontal flat duct, modeling horizontal MOCVD processes. In the experiment the Reynolds number is varied from 5 to 50 and Rayleigh number from 7,200 to 21,000 for various degrees of sidewall converging and duct inclination. The major conclusions obtained from the results obtained here can be summarized as follows:
(1) Due to the acceleration of the main flow by the converging of the sidewalls, and the reduction of normal buoyancy component by the duct inclination, the buoyancy driven unstable L-rolls and T-rolls can be effectively stabilized. Besides, the onsets of longitudinal and transverse vortex rolls are delayed to a significant degree.
(2) The main flow acceleration in the convergent duct and the duct inclination noticeably delay the onset of the return flow and weakens it to some degree.
(3) Empirical equations have been proposed to correlate the onset conditions of the return flow.
(4) The sidewall converging and duct inclination also greatly suppress the temporal
oscillation of the flow. At a high converging angle of the sidewalls or a small inclined angle of the duct the buoyancy driven unstable flow oscillation can be completely suppressed.
(5) The reduction in the size of the return flow zone by the sidewall converging and the duct inclination are more effective at a higher Re.
(6) When the duct is inclined slightly, the onsets of thermals and longitudinal and transverse vortex rolls are significantly delayed.
During the course of this investigation, it is recognized that the elimination of the return flow is still very important in the MOCVD processes. Simple methods to delay and even eliminate the return flow need to be further developed and explored in the future.
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