側風與擴散效應對捕集裝置的捕集效率影響

The Effects of Cross-draft and Diffusion to the Efficiency of a Collection Device

氣罩與採樣頭之類裝置均藉由吸取空氣而收集空氣中的物質。在側風與吸氣共同作用下，捕集裝置前方會形成一捕集區，捕集區內的流線均會被導至吸氣開口。因此位於捕集區內的物質較有可能隨著流線進入捕集裝置而被捕集。但空氣中物質會藉由擴散等散佈作用跨越捕集區邊界，於是捕集區邊界形成一帶狀的「過渡區」，在此區域內捕集裝置對物質的捕集效率隨物質發生源的位置而異，因此只要針對此區域深入探討，即可獲得捕集裝置捕集效率對物質發生源位置的關係。本研究以理論計算模擬方式探討此區域的大小，針對捕集裝置中軸位置的粒子發生源進行探討。本研究以位於無限平面上圓形開口所產生的流場模擬捕集裝置吸氣氣流，再加上橫向側風產生流場，計算流場中粒子的軌跡，並以亂數法模擬粒子擴散，再評估粒子可被圓形開口捕集的機率而得在流場中給定點的捕集效率。配合大量計算結果與因次分析獲得一可用以估算捕集效率的經驗公式，此公式可用以評估氣罩或採樣器的捕集效率。

A collection device, such as an exterior hood or a sampling probe, collects air-borne materials by extracting air from an open air. Under the combined effects of the cross-draft, the extraction and the diffusion, a “transition region” is formed in front of the extracting opening. The collection efficiency of the device to the air-borne materials varies in this region. This study evaluated the size of the transition region by using computational simulation. The present study concerns the particles released on the central axis of the collector. A circular opening on an infinite plane was modeled as a collector. A potential flow field was generated by the opening and superposed by a uniform cross-draft. The particle trajectories were calculated by tracing the streamline. The dispersion due to diffusion was simulated by random numbers. The collection efficiency then was determined by the probability of the particles being collected at a given releasing position in the flow field. An empirical formula was established by compiling a large amount of computational results. This formula can be used to evaluate the performance of an exterior hood or a sampling probe.