平板鰭片在自然及強制對流下之熱傳分析

Abstract

本文應用兩套分析模式來預測平板鰭片熱沉於強制對流及自然對流下的熱傳性能表現，並針對這兩種不同應用情況進行系統最佳化分析。 強制對流時，本文針對熱沉兩端壓降為10 mmAq、熱沉高度24 mm的情形下，分別探討熱沉的鰭片間距、基板厚度、鰭片厚度、熱沉寬度、熱沉長度對熱阻的影響，並針對這些變數進行最佳化設計。自然對流時，在本研究所設定的尺寸範圍內，鰭片高度、基板長度、基板寬度及基板厚度分別存在一極限值，超過此值後對於熱傳性能增強無太大的助益，本文考量成本因素後提出最適切尺寸大小的建議。 在強制對流的情況下，當通過熱沉的空氣流量較小時，其熱傳性能就必須考量自然對流的效應。以固定尺寸的熱沉及輸入散熱功率30 W時，本研究找出當強制對流空氣流量小於0.68 cfm(流速0.2 m/s)時，其強制對流的散熱性能(熱阻值為4.62℃/W)即低於自然對流的散熱性能(熱阻值為4.38℃/W)。 熱沉加裝風扇產生強制對流時，須考量風扇的性能操作曲線，再進行熱沉尺寸的最佳化設計分析。利用本分析模式，搭配某固定風扇的熱沉，可以在風量、壓降及熱沉尺寸上找到其最佳操作點。在本文中的例子，最佳操作點落在風量7 cfm、壓降0.858 mmAq、流道長度60 mm、鰭片厚度0.4 mm、鰭片間距2.08 mm與基板厚度11 mm，此時熱沉總熱阻為0.43 ℃/W。

Two analytical models were used in this study to predict the performance of a plate-fin array heat sink in forced convection and natural convection cases respectively. The optimization of geometry variables for designing a plate-fin heat sink in both cases was also analyzed in this study. While the pressure drop of heat sink was fixed at 10 mmAq, and the height was fixed at 24 mm in forced convection situations, optimal values for fin spacing, base thickness, base width and base length were found. In natural convection situation, it was found that there are lower limits for the variation of fin height, base length, base width and base thickness of heat sink. Increase of these parameters has limited benefit of performance. Considering the limits of these parameters and the cost factors, the optimal values of these design parameters were suggested in this study. As the airflow rate through the heat sink becomes small in force convection, the effect of nature convection has to be considered. For the case of a fixed size of heat sink with a slow airflow velocity considered in this study, it’s found the overall performance of forced convection (with thermal resistance of 4.62 ℃/W) is worse than that of nature convection (with thermal resistance of 4.38 ℃/W) when airflow rate was under 0.68 cfm. In the case of forced convection using fan as driving force, the effect of fan performance curve has to be considered in the design optimization process of heat sink’s geometry variables. The optimal operating points of airflow rate, pressure drop and sizes of heat sink, can be found with this model for a single heat sink with a fixed fan. Example of the optimal operating point was illustrated in this study.