| Summary: | 碩士 === 國立交通大學 === 機械工程系 === 89 === An experiment combining flow visualization and transient temperature measurement is carried out here to study the possible stabilization and elimination of the buoyancy driven unstable longitudinal, transverse and mixed vortex flow in mixed convection of air in a bottom heated horizontal flat duct by the top plate heating. Systematic variations in the top plate temperature were performed to examine its effects on the spatial and temporal flow structures. How the top plate temperature and the Reynolds and Rayleigh numbers of the flow affect the vortex flow characteristics is investigated in detail. An open loop mixed convection apparatus established earlier was employed in this investigation and the test section is a high aspect ratio (A=12) rectangular duct. Experiment is conducted for the Reynolds number varying from 1 to 50, Rayleigh number from 4,000 to 8,000 and the nondimensional top plate temperature from 0 to 1 at an interval of 1/8, covering a wide range of the buoyancy-to-inertia ratio.
The results from this study indicate that the top plate heating substantially stabilizes and eliminates the longitudinal, transverse, mixed longitudinal and transverse, and irregular vortex flows. It shows significant effects of stabilizing the vortex flow induced by the buoyancy resulting from the heated bottom plate of the duct. At the high top plate temperature even the entire irregular vortex flow can be eliminated and the flow is unidirectional in the duct. Obviously the transient flow oscillation in the flow is completely suppressed. An unfamiliar vortex structure consisting of stationary transverse rolls in the duct inlet and longitudinal rolls in the downstream is noted in the present study. They appear only at high buoyancy- to-inertia ratios. Various vortex flow patterns observed in the present study are summarized in Table 3.2.
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