本研究主要目的為在有限的馬達規格，以系統化的數值模擬工具改良葉輪、導流外罩、導流葉片及葉面紋路，來達到目標要求之最大流量與出口風速，接著加裝亥姆霍茲共振器以降低其特徵頻及整體運轉噪音，最後藉由實際量測與模擬相互比對以驗證改良成效，同時檢驗共振器應用於大型軸流風機之可行性。利用數值方法對基準風機進行評估，發現葉面的有效面積集中在第二到第三截面間，且流體撞擊導流外罩反彈形成無效區，加上不佳的導流葉片入口角度使流體分離產生渦流；因此確立改良項目為葉輪、導流外罩與導流葉片，並於葉面加裝紋路導正氣流出口方向，最後將四者結合找出最適化參數設計組合。最適化參數設計與原始風機相比流量由8,671提升至11,682 CFM(上升34.7%)，而距離出口5、10與15米處之風速分別為9.0、4.8與3.7m/s(增幅30~50%)，且整體噪音值與原始風機相近。在瞭解流場與噪音之連動關係後，將亥姆霍茲共振器裝於噪音源處進行降噪，模擬結果顯示第一與第二特徵頻皆明顯下降，其中第二特徵頻最高降低28dB，而整體噪音約降1~1.5dBA。
為了確認性能改善與共振器降噪的效果，將改良設計作實體化製作並且組裝進行實驗量測，經由實驗結果得知，在氣動力性能方面，最佳化風機之出口風速比起原始風機增加35%，且實測值與模擬的趨勢相符。至於噪音部分，由於數值模擬只計算氣動噪音並未包括馬達振動噪音所導致，故測試噪音值比模擬計算值還要高，但其趨勢與模擬結果大致相同。經由實測發現加裝3個針對第一特徵頻設計之共振器，所呈現之減噪效果最好，於第二特徵頻可下降20dB，而整體噪音約降1.5dBA，此結果雖與模擬趨勢相符，但減噪量仍有所誤差。探討後發現原因應歸因於製作與組裝誤差，對於大型軸流風機而言，些微誤差即造成葉輪之翼端間隙的嚴重不均勻性，這讓共振器於第一特徵頻的降噪效果消失，同時容易使體積小且頸部直徑大的共振器增加風切噪音。綜合歸納上述結果，本文所設計的最佳化軸流風機在馬達最大負載下，成功地達到目標要求之出口風速，並驗證了共振器運用於大型軸流風機的可行性，以及分析改善共振器設計之重要參考。

This research intends to enhance the performance of a vaneaxial fan under the limit of the motor specifications. With the aids of numerical simulation and experimental test, a systematic design scheme is established by considering the concerns on both of aerodynamic and acoustic aspects. Based on customer’s requirement, the goal of aerodynamic performances for this axial fan, with a 60cm-in-diameter impeller and operating under 1,425 rpm, is set to be capable to deliver at least 11,500 CFM with air speeds above 11, 8, and 5 m/s at 5, 10, and 15 meters from its outlet, respectively. Besides the concerns on flow rate and air speed, this work also plans to improve the acoustic characteristics of fan by using Helmholtz resonator to reduce the noise, especially on the harmonic frequencies. At first, a commercial fan with the same size is selected as the reference fan for executing the flow optimization. Also, CFD codes Fluent is used to simulate and calculate numerically on this reference fan for providing information of flow field and to estimate torque needed under the limit of motor specification to be 11 N-m. Later, a comprehensive parametric study is performed over impeller, housing, guiding van, and motor tail body in sequence along the flow path. As a result of this optimization procedure, the appropriate design fan, which consists of a impeller with NACA airfoil blade and proper setting angle, a streamlined housing, a well-designed guide vane, and an effective Helmholtz resonator, is attained successfully to deliver a satisfactory aerodynamic performance at 11,682 CFM (a 34.7% increase) and a minor 1~1.5 dBA decrease on the overall acoustic output with a significant 28 dB reduction on its second harmonic frequency. For validating this performance improvement and the numerical model used here, the mockups of appropriate fan and Helmholtz resonator are manufactured and assembled for testing its air speed and acoustic performances for confirming the actual noise-reduction effect. It is found that a rough 35% increase on air velocity at all the measuring locations is observed. Also, the acoustic measurements indicate that a similar noise reduction trend is attained while the deviation between numerical and test results is near 5 dBA, which is reasonable and expected due to the ignorance of motor operating and structure-induced noises in CFD prediction. Consequently, the design and analysis tool established here offers a rigorous and systematic scheme for a complete set of vaneaxial fan.

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