本研究改善軸流風扇翼端間隙之逆流現象，以此提升風扇之氣動力性能及降低噪音，首先選定14025風扇在固定葉輪下，於扇框邊緣進行共振器的設計與安裝，藉由彎曲頸部的造型以增加頸部空間，使其能在狹窄空間規劃針對較低特徵頻達到降噪效果，然而其結果不如預期般理想。接著探討以扇框鑽孔方式引進外氣阻斷間隙逆流，以達到性能與噪音之改善目標，藉由CFD模擬軟體分析其流場缺失，據以改善原始風扇以達到優化設計，依據流場計算結果對鑽孔位置做修正，考量參數包括孔徑、數量、位置高度、角落與邊框鑽孔及增加導圓角，經數值計算並與先前之流場及性能作比較。此外依序執行並分析各項鑽孔參數之噪音模擬，透過軟體之流場可視化與噪音模擬功能，探討流場與噪音之關聯以提出風扇性能與噪音的優化建議，而後分析最佳化設計之內部流場變化。彙整結果顯示，扇框鑽孔確實有助於改善風扇翼端間隙之逆流現象，當鑽4個5mm直徑無導圓孔於角落之葉尖中間高度時，可產生最佳之性能效果，最大流量與最大壓力分別提升1.09%與2.04%。於噪音改善方面，透過鑽孔之導圓角與位置高度的配置，使原始風扇之噪音降低2dBA具明顯降噪效果，而本研究之完成可對風扇性能與噪音減低提供另一改良方案。

This research intends to enhance the performance of axial-flow fan by drilling holes on its frame, which can create a vertical incoming airstream to block and diminish the reversed flow inside the tip clearance. At first, a 14025 axial-flow fan widely used in PC products is selected as the reference and demonstration target to analyze and evaluate its flow pattern and aerodynamic performance within the framework of CFD codes Fluent. Note that the numerical outcomes are used to visualize the detailed flow phenomenon associated with the blade tip, the tip-clearance reversed flow, and the frame for appropriately adjusting the hole position. Subsequently, a systematic parameter analysis on the drilling holes is executed to obtain the best hole design. The important geometric parameters considered here include the number, the diameter, the location, and the rounded inlet of holes. As a result, the numerical calculations illustrate that 1.09% and 2.04% increases on the maximum flowrate and pressure are attained by adding four 5mm-in-diameter, unrounded holes on the frame corners. Additionally, the 2-dBA noise reduction is found via an acoustic simulation of this 14025 axial-flow fan with the optimized hole setting. In summary, the accomplishment of this numerical work provides another useful alternative for improving the fan performances, especially the acoustic characteristics.

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