| Summary: | 碩士 === 國立交通大學 === 機械工程系所 === 96 === An experiment is carried out in the present study to investigate the vortex flow patterns resulting from a round air jet impinging over a large confined heated horizontal circular disk of 16 inch in diameter. In the present experiment the jet flow rate is varied from 0.4 to 10.0 (standard liter per minute) for the jet Reynolds number ranging from 27 to 676 with the injection pipe diameter Dj = 20.0 mm and jet-disk separation distance H = 12.5mm and 25.0 mm. The temperature difference between the disk and the air injected at the inlet of the jet is varied from 0 to 14.0℃ for the Rayleigh number Ra ranging from 0 to 7,340.
The results from the flow visualization for H=25.0 mm indicate that the vortex flow is characterized by the primary and/or secondary inertia-driven circular rolls along with several buoyancy-driven rolls. The number and strength of the buoyancy induced rolls increase with the buoyancy-to-inertia ratio, which can vary from 1 to 5. Besides, at a low Gr/Rej2 slightly deformed circular buoyancy rolls prevail in the test section. For slightly higher Gr/Rej2 the flow is dominated by the highly deformed curved rolls. The roll pattern becomes somewhat irregular for a further increase in Gr/Rej2 . For all cases examined here the vortex flow is unsteady at long time after the initial transients have died out and is irregular to a certain degree since the buoyancy effect is rather strong for the large impinging plate tested here. In addition to the roll deformation, the mutual pushing, merging and splitting of the buoyancy rolls occur nonperiodically in time. This unsteady and irregular vortex flow is also reflected in the data for the air temperature variations with time. Some quantitative buoyancy-driven vortex flow characteristics such as the size and location of the buoyancy rolls are summarized and correlated empirically. Moreover, a flow regime map is provided to delineate various induced vortex flow patterns and the boundaries separating various vortex flow patterns are also correlated empirically.
When the jet-disk separation distance is reduced to 12.5 mm for ΔT varied from 8.0℃ to 14.0℃ ( corresponding to the Rayleigh number ranging from 1,470 to 2,670 ), circular waves traveling in the radial direction and radial vortex rolls appear in the confined impinging jet flow. These waves are somewhat deformed and are not axisymmetric. The radial rolls originate in the stagnation region of the impinging jet and grow in size in the radial direction. The size of the radial rolls is also nonuniform in the circumferential direction. Besides, the moving waves and radial rolls can coexist in the flow. Moreover, the ranges of the jet Reynolds number and Rayleigh numbers leading to the new vortex flow patterns are determined. Flow regime maps delineating various vortex flow patterns including all inertia- and buoyancy-driven roll and wave patterns found here are given. And the boundaries among various vortex flow patterns are empirically correlated.
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