隨著科技進步與技術的蓬勃發展，人們對生活品質的要求也逐漸提升，空氣濾清器已是家庭中不可或缺的電器產品；且因為工業過度發展，空氣品質逐漸惡劣，許多家庭中小孩的過敏也開始產生，因此空氣濾清器的效能也開始受到重視。空氣濾清器發展至今，在要求效能的同時，也因此衍生了惱人之氣動噪音。為了抑制此氣動噪音，本研究選用亥姆霍茲共振器來進行離心扇之窄頻帶噪音的降噪，利用不同設計參數分析探討相對應之效果。首先利用CFD模擬軟體進行整機的優化設計，藉由更動整體外形、葉片出入口角、葉片數及電漿片擺放位置，完成優化設計並將之作為基準；接著經由計算結果探討流場與噪音之連動關係，試圖找出風扇的最大噪音源，且將共振器裝於最大噪音源進行降噪。隨後針對此具有共振器之風扇進行流場與噪音的數值模擬，以確認共振器應用於風扇的降噪效果；由模擬結果得知，共振器對風扇之氣動性能影響不大，而對於降噪效果最高可到3dBA；綜合歸納上述結論，本文所探討的亥姆霍茲共振器應用於離心扇確實有效果，但對特徵頻的降噪效果還需經過不同設計參數和安裝位置作更多探討，才能確保減噪效果之實用性。

Recently, the air-quality control attracts many research interests and is treated as the evaluation index for meeting strict environmental requirements in human life. Also, WHO's air pollution report points out that the particulate matter concentration surpasses the standard frequently and air pollution spreading throughout the world are leading people to pay attention on air cleaners, masks, and other air-filter products. So, the high-performance air cleaner has become an essential appliance in every family. However, the performance characteristics of air cleaner are rarely investigated in a systematical manner. Therefore, this research intends to investigate the physical mechanism of flow pattern, enhance the aerodynamic performance, and reduce the acoustic noise with the aids of numerical technology. Firstly, a commercial air cleaner is chosen to calculate numerically its aerodynamic performances for serving as the comparing base. The results show that reference product generates 610 CFM of maximum flow rate and 31.2 mmAq of maximum static pressure at 1,250 rpm. In addition, this calculated flow information is utilized to raise new design alternatives for finding out the optimum combination of housing, inlet guiding duct, blade angle, blade number, central body and geometry of rotor, cut-off clearance, outlet adaptor, and filter arrangement, which are modified methodically with the aids of parametric study on each component considered here. Consequently, these appropriate components are employed to construct the new air cleaner, which is estimated numerically to produce the free-delivery flowrate at 735 CFM and the no-delivery pressure at 30.8 mmAq. These performance outcomes represent a 20.5% increase on maximum flowrate with a similar static pressure. Note that it is found that the flow rate delivered by unit torque is improved substantially by 24.8%, which increases from the original 480 CFM/N-m to 599 CFM/N-m. Besides, the velocity pattern at the discharge becomes more uniformly distributed for the new air cleaner.
Thereafter, the Helmholtz resonator is designed and imposed to this air cleaner for reducing it annoying harmonic noise. Subsequently, the acoustic field of new air cleaner is calculated and analyzed carefully via the transient CFD simulation. So, an in-depth understanding on the acoustic features is attained and utilized to design several Helmholtz resonators targeting the 1st and the 2nd harmonics, which are installed at the cut-off (noise source) and the highest-pressure fluctuating location on the housing as illustrated in the numerical outcomes. According to the CFD simulation, the maximum noise reductions is observed as high as 3.9 dB on the 1st harmonic frequency when 7 Helmholtz resonators are installed in parallel to the air stream on the housing. Clearly, the noise-reduction effect needs to be further optimized by executing a comprehensive parametric study on resonator geometry. In conclusion, this study establishes a systematic and reliable scheme for performance enhancement and acoustic analysis of the air cleaner within the framework of CFD technology.

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