凸緣對側風中氣罩有效捕集範圍的影響

The Effect of Flange to the Capture Performance of Hood

本研究使用風洞實驗配合計算流體力學計算探討凸緣對側風中氣罩捕集區的影響。以往普遍認為氣罩設置凸緣可以減小抽氣範圍，在相同風量下提高氣罩前方的抽氣風速，但這並不能完全反映氣罩的捕集能力。因此本研究以氣罩在側風中的有效捕集範圍，也就是捕集區做為評估依據，並藉此探討凸緣對氣罩捕集功能的潛在原因。在風洞實驗中，氣罩安裝於風洞內，風洞產生均勻側風，使用二維雷射都卜勒測速儀量測風洞中央對稱面上的風速向量，並根據分隔流線描繪出捕集區範圍。流體力學計算則使用FLUENT 3D 5.5熱流計算套件，在模擬實驗狀況下以 κ-ε 紊流模式進行運算，並輔以勢流模式計算。研究結果顯示，在相同氣罩風量或開口風速下凸緣尺寸在側風中對氣罩捕集範圍並無顯著增加效果。而凸緣的主要功能在於降低氣罩壓力損失，在相同排氣機功率下，可以獲致較大的氣罩風量或開口風速，以達到較大的抽氣風速。因此凸緣的設置對氣罩捕集能力仍有其貢獻，但主要在於能源的有效運用，而一般所認知縮小抽氣範圍以提高抽氣風速則是較次要的貢獻。

This study employed wind tunnel tests and computational fluid dynamics to examine how a flange affects the capture performance of a hood. It is acknowledged commonly that a flange will decrease the space covered by the exhaust air flow, and consequently increase the exhaust speed in front of the hood opening under the same exhaust flow rate. However, the exhaust speed alone cannot fully reflect the capture performance of hood. Instead, this study used capture zone, within which a hood can undergo an effective capture under a cross-draft, as a criterion of evaluation. This study also investigated the implicit reason how a flange enhance the capture capability of a hood. A hood was installed inside a low-speed wind-tunnel. The wind-tunnel generated a uniform crossdraft. The air velocity vectors on the central plane of the wind-tunnel were measured by a 2D laser Doppler velocimetry. The measurement revealed the capture zone by tracing along a dividing streamline. The computation was carried out by using FLUENT 3D 5.5 package to simulate the experimental flow field based on κ-ε turbulence model. A potential flow model computation was also performed for comparison. The results showed that the flange size did not expand the capture zone significantly under the same exhaiust flow rate. However, the flange did increase the exhaust flow rate or opening flow rate at a given fan power due to less pressure loss, and consequently the exhaust speed in front of a hood. Therefore, a flange does enhance the capture capability of a hood. However, this enhancement results mainly from more efficient power usage, the common acknowledged flow distribution is a minor reason for the benefit of a flange.