市面上常見離心風機均存有噪音問題，為了達到降噪要求，本研究利用風機性能及氣流噪音結合之數值模擬，並加裝λ/4減噪器評估其對特徵頻率與噪音量之影響；藉由數值模擬與實驗量測兩項利器之結合，來了解其流場與噪音的相關性，進而確認可能之流場與噪音缺失，以做為未來改善風機性能與噪音依據。首先量測風機之性能曲線，再根據文獻中之結果與建議，針對開孔直徑、開孔率及減噪器腔體長度，並根據本研究之風機大小(1.31m×1.52m×0.8m)，選定不同之開孔直徑與開孔率。除此之外，考慮風機並不會只於額定轉速下操作，因此將增加三個轉速進行聲壓頻譜量測。首先進行無減噪器之離心風機，於不同轉速下之聲壓頻譜量測；之後便將繪製好之不同參數減噪器，於製作後安裝於風機舌部，同樣進行不同轉速下之聲壓頻譜量測，此外亦同時建立相關之數值模型，以進行不同轉速下配合不同減噪器之風機流場與聲壓頻譜模擬。
實驗結果顯示，於1,175rpm時開孔10mm配合開孔率40%時，可有效降低第一特徵頻率點噪音約5 dBA，而1,100rpm時則是開孔10mm開孔率50%下，可有效降低第二特徵頻率噪音達9.5 dBA；然而，於1,000rpm時則無明顯之降噪效果，但於900rpm時開孔5mm開孔率30%，可降低第二特徵頻率點噪音約3 dBA。將所得之量測結果與數值模擬相互比較後，顯示模擬與實驗之性能曲線相當，且於特徵頻率點位置，其模擬與實驗之聲壓頻譜趨勢亦相同；雖然於噪音值有些許誤差，但仍在可接受之範圍內，因此證實數值模擬具有一定的準確度。而模擬中所預測之降噪結果，於1,175rpm、1,100rpm與900rpm分別為降低4.6 dBA、4.7 dBA及3.6 dBA；與實驗結果比較後可發現，其誤差值都在5 dBA以下。綜合上述之研究成果顯示，本文所設計之λ/4減噪器，已達到降噪效果之目標；此外透過流場分析，便能更進一步探討減噪器內之氣動性對其降噪之影響，同時規劃出完整之設計方法可供業界作為參考。

This study intends to analyze and reduce the flow-induced noise for the centrifugal fan with a quarter-wave resonator by integrating efforts of numerical simulation, mockup fabrication, and experimental test. At first, a high-performance centrifugal fan is formed with the redesigned spiral housing and the radial impeller, which is constructed with the aids of the cascade theory and NACA airfoils. Also, four rotational speeds covering different operation conditions are considered here in the aerodynamic and acoustic evaluations, which are executed inside the AMCA test chamber and the semi-anechoic room, respectively. Later, several quarter-wave resonators are designed, fabricated, and installed with the centrifugal fan to diminish its noise level, which is attained from the FFT analyzer and served to evaluate the reduction effect of quarter-wave resonator. In the meantime, to save the mockup cost and cut the R&D time, the flow and acoustic fields associated with the centrifugal fan are simulated and assessed by using numerical software Fluent on various resonators to identify an appropriate resonator design.
Consequently, the experimental outcomes show that the resonator with 10mm-diameter holes and 40% open-area ratio can result in a 5 dBA noise reduction on its fundamental frequency operating under 1,175 rpm. And the maximum 9.5 dBA reduction on the second harmonic frequency appears for the resonator with 10-diameter holes and 50% open-area ratio running at 1,100 rpm. However, noise reduction is not obvious at 1,000 rpm while a 3 dBA noise reduction on the second harmonic frequency is found on the resonator with 5mm-diamter holes and 30% open-area ratio at 900 rpm. Regards the CFD predictions, the calculated results indicate that the noise reductions at 1,175, 1,100, and 900 rpms are 4.6 dBA, 4.7 dBA and 3.6 dBA, respectively. Therefore, it is concluded that the noise deviation between numerical and experimental results are within 5 dBA while a good correlation over the entire frequency domains is found between these two outcomes. Furthermore, a comprehensive parametric study on the quarter-wave resonator is carried out and summarized to attain a design guideline for its application on centrifugal fan. In summary, the accomplishment of this study provides a systematic and complete scheme on noise reduction for a centrifugal fan with the addition of quarter-wave resonator design.

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