本文以暫態數值方法計算水平底板鰭片陣列之自然對流散熱性能，計算結果顯示尺寸是影響其流場與自然熱對流係數相當重要的因素，文中所計算之鰭片陣列尺寸為L=56~500mm、H=6.4~38mm、S=6.4~20mm。固定間距S=6.4mm之鰭片陣列當長度由56mm逐漸增加時會使得鰭片間之流場由穩態的single chimney逐漸轉變為具動態特性之sliding chimney流場，這是由於鰭片長度增加使得散熱氣流流經鰭片之時間加長，因而氣流得以在鰭片之間充分受熱並往鰭片陣列內部聚集同時往上流動，而鰭片陣列內部也因這些充分受熱的氣流導致其局部熱對流係數遠低於鰭片末端之外側區域。此外這些因熱浮力往上流動的氣流也同時引入鰭片上方相對較冷之氣流進入鰭片間而形成具動態特性之sliding chimney流場，這使得鰭片陣列整體自然熱對流係數出現震盪的情況。本文發現使用穩態數值方法計算具sliding chimney流場之鰭片陣列會低估其熱對流係數且不易收斂，因此需使用暫態的方法以得到收斂的結果並採取其時間平均值來決定熱對流係數，以此方式進行之自然熱對流計算可符合文獻以實驗數據所歸納出之經驗式，另外在鰭片陣列尺寸對散熱性能影響之探討上也顯示計算結果之趨勢符合文獻試驗結果。本文由數值結果發現大尺寸之鰭片陣列常因內部散熱不佳而呈現低熱對流係數，為改善此一缺點，本文提出在底板位置開口引進鰭片下方冷氣流以改善散熱效能，由於縮短了散熱氣流路徑總長，可大幅度提高局部熱對流係數。計算結果顯示底板開口靠近鰭片陣列內部之改善效果較靠外側為佳，採用較小而均布之開口改善效果最好，所計算之自然熱對流係數可達未開口鰭片之2.7倍。

This work numerically analyzes the dynamic behavior of natural convection from horizontal rectangular fin arrays. A parametric study is made using a 3-D unsteady model for fin lengths of L = 56500 mm, fin heights of H = 6.438 mm, and fin spacing of S = 6.420 mm. With an increasing L, the flow pattern evolves from a steady single-chimney to an oscillating sliding-chimney flow in which cold air is partly drawn downward from the upper ambience. The average convection heat transfer coefficient decreases with increasing fin length. For an intense sliding-chimney flow pattern from long and low fin arrays, an unsteady simulation yields higher average convection heat transfer coefficients than those using a steady-state simulation. The h–S relation exhibits a steep drop when S is narrowed below a threshold, which is larger for lower and longer fins. The optimum fin spacing Sopt occurs near the threshold S, below which the benefit of increasing heat transfer area surrenders to the decrease of h caused by excessive viscous drag. The predicted dependence of Sopt on H and L agrees well with experimental results and is explained based on numerical results of flow and heat transfer characteristics. The present predictions of Nu agree well with the correlations in the literature which use L/2 or S as the characteristic lengths.
For horizontal rectangular fin arrays with length L  200 mm, the overall convection heat transfer coefficients are quite low because the inner surfaces of the long fin arrays are poorly ventilated with cold surrounding air. In this study, we introduce perforations through the fin base to draw cold air directly from below the fin base. The perforations, especially locating in the inner region, improve ventilation and heat transfer performance significantly. The conditions with more but shorter perforations exhibit the most significant improvement in heat transfer. The overall heat transfer coefficients with short and distributed perforations, whose total perforated length equals L/2, can be as large as 2.7 times that without perforations.

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