筆電散熱風扇氣動噪音之探討

Abstract

噪音是筆記型電腦使用者面臨的一大困擾，為了減少因空氣流動造成的氣動噪音，需先了解其成因及影響因素。本文主要以氣動聲學理論，使用計算流體力學進行數值模擬，了解改變風扇本身的幾何構造與額外增加結構對於風扇噪音頻譜與音量大小造成的改變。使用暫態大尺度渦流模擬法（LES）算出風扇流場，以此流場估算Ffowcs Williams and Hawkings（FW-H）聲學類比的聲音訊號。模擬結果顯示，縮減舌部間隙和優化流道結構皆能讓質量流率上升，提升風扇散熱效率，同時僅引起輕微的噪音增加。其中，縮減舌部間隙使質量流率上升4.95%，入口與出口聲音大小分別增加1.0 dB及1.5 dB；增加20毫米的流道結構可以提升5.01%的質量流率，入口聲音大小上升4.9 dB、出口降低聲音大小0.7 dB。本研究預期能為未來風扇降噪技術的開發提供指引，有助於減少筆記型電腦使用者所受的噪音困擾。

Fan noise is an inevitable annoyance for laptop users. Further investigation of causes and influencing factors is needed to reduce aeroacoustic noise from fans. Based on aeroacoustics and computational fluid dynamics (CFD), the study aims to examine resultant noise in different fan cases. This includes analyzing changes, in terms of its spectrum and the total sound pressure level, due to modifications in the fan’s structure and extra structures. Using the transient large eddy simulation (LES), we first obtain the detailed flow field. Next, the fan noise is computed from the flow field with the help of Ffowcs Williams and Hawkings (FW-H) acoustic analogy. Here we show that reducing the cut-off clearance and extending the flow discharge can increase the mass flow rate, enhancing the fan's cooling efficiency while only slightly increasing noise levels. Specifically, reducing the cut-off clearance increases the mass flow rate by 4.95%, with sound pressure levels at the inlet and outlet increasing by 1.0 dB and 1.5 dB, respectively. The 20 mm flow discharge extension increases the flow rate by 5.01%, with the inlet sound pressure level rising by 4.9 dB and the outlet sound pressure level reducing by 0.7 dB. We anticipate that our findings will serve as a guide for future research on fan noise reduction, potentially alleviating the noise-related discomfort experienced by laptop users.