| Summary: | 博士 === 國立交通大學 === 機械工程系所 === 94 === In this study an experimental flow visualization combined with temperature measurement are conducted to investigate how the sidewall converging and duct inclination affect the buoyancy induced return flow structure and stabilization of vortex flow in mixed convection of gas in a horizontal rectangular duct. The buoyancy driven secondary flow including the return flow and vortex flow is driven by a heated circular disk embedded in the bottom plate of the duct, simulating that in a horizontal longitudinal MOCVD reactor. Specifically, in the first part of the present study the sidewalls of the duct are inclined toward the duct core so that the gas flow in the duct is accelerated, causing the buoyancy-to-inertia ratio to decrease in the main flow direction. While in the second part of the study the duct is inclined upwards with its exit end above the entry end and the component of the buoyancy force normal to the heated plate is reduced. In the experiment the Reynolds and Rayleigh numbers of the flow at the duct inlet are respectively varied from 5 to 50 and from 7,200 to 21,000. In the first part of the study the duct aspect ratio is reduced from 20 at the inlet to 16 or 12 at the exit. The duct is slightly inclined from the horizontal in the second part. Particular attention is paid to delineating the spatial changes of the return flow structure with the sidewall converging and to explore how the duct inclination possibly suppresses and stabilizes the secondary flow.
The results obtained in the study show a substantial delay in the onset of the return flow and the effective suppression of the buoyancy driven unstable longitudinal and transverse vortex flows by the sidewall converging and the duct inclination. Besides, the sidewall converging and the duct inclination can weaken the return flow more effectively at slightly higher Reynolds numbers. An empirical equation is provided to correlate the present data for the onset condition of the return flow in the duct. Some preliminary results from the second part of the study indicate that the slight inclination of the duct at 2° can significantly weaken the return flow. The reduction in the size of the return flow zone and the intensity of the return flow is prominent. Besides, the onsets of longitudinal and transverse vortex rolls are delayed substantially.
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