| 摘要: | 本研究利用不同紊流產生器產生不同紊流強度於管道背階入口，實驗探討紊流強度在三維背階流之流場與熱傳特性。測試管道使用背階高度1.75cm，展弦比為4的壓克力管道，以圓柱體放置於風洞管道入口作不同排列方式，形成4柱、6柱紊流產生器與無紊流產生器下探討紊流強度對流場與熱場影響。流場方面，選取雷諾數500至17000，包含層流至紊流數種流速，使用都卜勒雷射測速儀進行背階管道內再接觸位置、迴流區流場、邊界層內對流效應、二次流效應與紊流效應等量側；熱場方面，製作一7.2×20 cm2之加熱板，並埋設T型熱電偶線量測溫度。
研究顯示，背階管道內再接觸長度變化為層流時，隨雷諾數增加而增加，過渡流時隨雷諾數增加而減少，但紊流時不隨雷諾數增減，其中以6柱紊流產生器的紊流強度最大，並使得過渡流與紊流提早發生、再接觸長度變小，且加裝紊流產生器後使得流場中迴流區更為明顯。在熱傳方面低雷諾數下，加裝6柱紊流產生器使得整體熱傳效應皆有所提升；高雷諾數時，加裝紊流產生器使得迴流區內熱傳效應有明顯的增益現象。近壁面流場中，主要為紊流效應主導熱傳分佈，僅有在低雷諾數下，迴流區外由對流效應主導。
The research experimentally investigates the effects of turbulence 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. Cylinder-type turbulators are placed inside the inlet duct, and used to generate different levels of turbulence on the inlet free-stream. The Reynolds number are 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 regime, decreases with Reynolds number in the transitional flow regime, and almost remains constant with Reynolds number in the turbulent flow regime. The turbulators, especially the six-cylinder turbulator, much reduce the reattachment length, and the Reynolds numbers for the onset of the transitional and turbulent flows. The study also indicates that the turbulators generally increase the convective mean velocity, and turbulent kinetic energy near the heat transfer surface, and causes the increase in heat transfer rate. The flow turbulent kinetic energy plays the most important role in heat transfer distributions. |
