Buoyancy Driven Vortex Flow Patterns in Mixed Convection of Air through a Blocked Horizontal Flat Duct Heated from below
林 清 發
研究生： 許丁士 指導教授：林清發 教授
國 立 交 通 大 學 機 械 工 程 研 究 所
而本研究得知方塊的位於上游時對流場有明顯的影響，且會使得縱向渦流（longitudinal roll）提前發生，而對於混合對流流場（mixed vortex flow）管道中心部分得規則橫向渦流，由於方塊在上游的出現使得橫向渦流被抑制而形成縱向渦流，且當方塊高度增加時其效應愈加明顯。在低浮慣比（buoyancy-to-inertial ratio）時的規則橫向渦流（transverse roll），則由於橫向合併的力量被方塊給破壞，且在方塊後方形成結構較為薄弱的縱向渦流，且隨方塊高度的增加，縱向渦流結構會更趨明顯。
Buoyancy Driven Vortex Flow Patterns in Mixed Convection of Air through a Blocked Horizontal Flat Duct Heated from below Student: Ding-Shi Shu Advisor: Prof. Tsing-Fa Lin Institute of mechanical Engineering National Chiao Tung University ABSTRACT Experimental flow visualization combined with transient temperature measurement are carried out here to study the possible stabilization of the buoyancy driven vortex flow in mixed convection of air in a bottom heated horizontal flat duct by placing a rectangular solid block on the duct bottom. Two acrylic blocks having dimensions 40x20x5 mm3 (block A) and 40x20x10 mm3 (block B)are tested. The blocks are placed on the longitudinal centerline of the duct bottom at selected locations. How the location and orientation of the rectangular block affect the stability of the vortex flow is investigated in detail. An open loop mixed convective appratus established earlier by Yu et al. was chosen in this investigation and the test section is a high aspect ratio (A=12) rectangular duct. Experiments are conducted for the Reynolds number varying from 3 to 30 and Rayleigh number from 3,000 to 6,000, covering a wide range of the buoyancy-to-inertia ratio. For longitudinal vortex flow, the presence of the blocks placed near the duct entry causes the onset points of the longitudinal rolls to move significantly upstream especially for the roll pair directly behind the block. Besides, the longitudinal vortex flow in the exit portion of the duct is destabilized by the block. The transverse vortex flow is found to be only slightly affected by the block when it is placed in the exit half of the duct. There is significant deformation of the transverse rolls as they pass over the block. However, they restore to their regular shape in a short distance. Significant decay in the flow oscillation is noted in the region right behind the block. Elsewhere the flow oscillates at nearly the same frequency and amplitude as that in the unblocked duct. When the block is placed near the duct entry the vortex flow is significantly changed. Stabilization of the vortex flow behind the block is more pronounced. This flow stabilization is more prominent for block B with its height being twice of block A. Placing the block with its longsides normal to the flow direction can also enhance the flow stabilization. More specifically, behind block B we have steady longitudinal rolls. For mixed vortex flow, placing the block near the duct inlet causes the transverse rolls to change to regular or deformed longitudinal rolls in the duct depending on the buoyancy-to-inertial ratio and orientation of the block. The flow stabilization by the block is substantial. Again the flow stabilization can be enhanced by increasing the block height and placing the block with its longsides normal to the forced flow direction.
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