| Summary: | 碩士 === 淡江大學 === 航空太空工程學系碩士班 === 95 === The research experimentally investigates the effects of vortex intensity on the flow and heat transfer characteristics in three-dimensional backward-facing step flows. A wind tunnel system is used to generate a uniform flow at an inlet duct. Another test duct of different height is connected to the inlet duct to form a backward-facing step duct of aspect ratio 4, expansion ratio 1.33, and step height of 1.75 cm. A 0.1 mm thick stainless steel foil is attached to the step wall to serve as the heat transfer surface. Vortex generator is placed inside the inlet duct, and used to generate different levels of vortex on the inlet free-stream. The Reynolds number is between 500 and 17000, which cover the laminar, transitional and turbulent flows. The velocity and temperature measurements were conducted using laser Doppler velocimetry, and Type-T thermocouples, respectively. Specifically, the measurements include (1) the reattachment length, and the flow structures at different duct cross-sections downstream of the step wall; (2) the flow characteristics near the heat transfer surface, such as the convective mean velocity, the secondary flow and the turbulent kinetic energy; (3) the heat transfer distributions on the heat-transfer surface. Results of this study show that the reattachment length increases with Reynolds number in the laminar flow region, decreases with Reynolds number in the transitional flow region, and almost remains constant with Reynolds number in the turbulent flow region. The vortex generator much reduce the reattachment length, and the Reynolds numbers for the onset of the transitional and turbulent flows. This study also indicates that the vortex increase the convective mean velocity, and turbulent kinetic energy near the transfer surface, and cause the increase in heat transfer rate. The flow turbulent kinetic energy plays the most important role in heat transfer distributions. The maximum heat transfer rate occurs near in the reattachment point.
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