運輸是經濟發展過程中相當重要的基礎設施，其中由鋼軌引導鋼輪行進的軌道運輸系統，由於運輸量大、安全性高、消耗能源少等特性，使其在大眾運輸服務中扮演重要的角色，在都會區的捷運系統就是個典型例子。由於車廂內常載滿旅客，常藉著空調系統使車廂空氣流通維持舒適度，其中風扇的效能是最重要影響因素，故風扇性能提升即成為本研究之目標。本文以葉輪外徑252 mm、高度84.3 mm之後傾式離心扇為對象，探討其安裝於捷運車廂內部環境，在2,800 rpm轉速之流場特性與性能分析，同時使用CFD軟體FLUENT進行模擬計算與流場可視化；首先，針對原始模型進行流場分析，選定扇葉入口角、出口角、外殼尺寸、以及葉片類型作為變數，並將工質設定在室溫25℃、轉速2,800 rpm下，觀察其出口流量以及效率，藉此判斷上述變數對整體效能之影響。
　　本改良研究分成箱體流線化與葉輪改良設計兩部分，在箱體流線化包括空間縮減、流道改良、角落改良等三方向，針對箱體進行導板加裝之規劃；再將各方案的最佳導板在不互相影響下做整合，最終完成的最佳箱體可將流量提升7.01％、效率提升4.61％。至於葉輪改良設計中，分成葉片類型探討與葉片角度優化，透過葉片類型的評估改進，本研究選用NACA-44與NACA-6系列之機翼外形做為新葉輪之葉片，而葉片角度範圍則是出口角從45°至50°、入口角從30°至40°做評估；經由數值模擬結果顯示，以NACA-6508機翼葉片可以產生最高之整體風扇效率，同時，將原始的入口角60°、出口角15°分別修改成入口角35°、出口角50°，則可以提升整體離心扇之流量，將葉輪優化之後，可使整體風扇之流量提升33.54%、效率提升6.57%。最後，將上述兩部分的最佳方案進行組裝，最佳箱體與最佳葉輪之組裝可使整體風扇之流量提升44.52%、效率提升12.43%；綜合而言，本研究成功地建構一套優化風機性能的系統，可據以完成通風系統的離心風扇之性能提升，亦可應用於各種型式之風扇優化。

Mass rapid transit (MRT) system is considered as an essential infrastructure for relieving the traffic challenge in metropolitan city. To ensure a comfortable environment, the passenger cabin is equipped with an effective ventilation system, which is powered by a centrifugal fan. Thus, performance enhancement of radial-flow fan is in great demand and become the target of this work. At first, the original centrifugal impeller (252 mm diameter and 84.3 mm height) is installed inside an untraditional housing, which is a near rectangular space with the airstream inlet and outlet on the neighboring side plates. Next, the associated flow simulation is executed numerically and observed carefully on this ventilation unit running at 2,800 rpm to identify the adverse flow patterns for potential modification. As a result, due to the lack of spiral housing, it is found that the airstream spreads out of rotor in all directions. Also, most of the high-pressure airflows bounce back from wall to proceed into the lower part of case for discharging through the outlet port. Accordingly, adverse flow phenomena are found in all housing corners and lower portion beneath the rotor, especially close to the outlet.
Afterwards, this research proposes several guiding plates for streamlining the casing geometry and enlarging the outflow path to eliminate the serious circulations. Successively, parametric study on the impeller is enforced to enhance the flow rate and static efficiency of this ventilation unit. Subsequently, CFD calculations indicate that streamlined case can increase the flow rate and static efficiency by 7.01% and 4.61%, respectively. Regarding the rotor redesign, 33.54% and 6.57% enlargements on airflow and efficiency are obtained by introducing NACA-6508 airfoil blade and adjusting blade inlet and outlet angles to 35 and 50. Consequently, the above two approaches are integrated to construct an optimized design. Also, numerical outcome shows that this optimized ventilation unit with airfoil blade and streamlined case results in a significant 44.52% improvement on flow rate and a fair 12.43% upgrade on efficiency. In conclusion, this thesis successfully establishes a systematic scheme to analyze the flow pattern and enhance the aerodynamic performance of a ventilation unit powered by a centrifugal fan.

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