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

葉長泓(Chang-Hung Yeh)

指導教授 ： 陳友剛

長榮大學/健康科學學院/職業安全與衛生研究所/碩士(2007年)

https://doi.org/10.6833/CJCU.2007.00089

摘要

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，捕集效率由0%（距捕集裝置較遠處），逐漸增加至100%（距捕集裝置較近處），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。主要方式係以位於無限平面上圓形開口所產生的理想流場（無黏性、無紊流）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，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收集效率，粒子隨時間散佈的程度隨給定的擴散係數 D 而定。
經整理得七千餘次不同參數組合下的捕集效率計算結果，經過三個層次的迴歸：(1) 由捕集效率與發生源位置關係得到過渡區厚度；(2) 發現捕集區厚度與擴散係數的平方根成正比；(3) 利用因次分析與多變量迴歸得到其他參數（如側風風速、吸氣開口風速比、捕集裝置開口直徑等）的效應，最後獲得一可用以估算捕集效率的經驗公式。此經驗公式的迴歸範圍涵蓋捕集開口直徑 d = 0.07~0.2、捕集開口風速 Vf = 0.2~15，側風風速 Vc = 0.25~1（Vf/Vc = 0.2 ~ 30）與 D = 10-5 ~10-3（使用一致的單位），迴歸相關係數 R2 均達到 0.999 以上。在迴歸過程中同時發現，達到 50% 捕集效率的發生源位置 z50% 不會隨擴散係數改變。經更進一步驗證，發現所得經驗公式在適當條件下（z50% 遠大於過渡區厚度）可使用於更大的範圍： d = 0.05 ~ 0.5，Vc = 0.1 ~ 1，Vf/Vc = 0.1 ~ 30，D = 10-2 ~ 10-6。

關鍵字

收集裝置 ； 捕集區 ； 捕集效率 ； 擴散

並列摘要

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. A circular opening on an infinite plane was modeled as a collector. A potential flow field (without viscosity and turbulence) was generated by the opening and superposed by a uniform cross-draft. The particle source is assumed to be located on the central axis of the opening. 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. The dispersion of the particle is determined by the given diffusion coefficient D.
An empirical formula was established by compiling a large amount of computational results. More than 7,000 collection efficiency computations were performed under different combination of parameters. The regression was performed in 3 levels. (1) the thickness of the transition region ?? is determined from the collection efficiency vs source location z; (2) was found to be portortional to the square root of diffusion coefficient; (3) the effect of the other parameters were determined by dimensional analysis and multiple-variable regression. The range of the regression is d (opening diameter of the collection device) = 0.07 ~ 2, Vf (flow velocity at the opening) = 0.2 ~ 15, Vf (velocity of the cross draft) = 0.25 ~ 1 (Vf/Vc = 0.2 ~ 30), and D = 10-5 ~ 10-3 (all the units are consistent). It was also found that the location where 50% collection efficiency is achieved, z50%, is not affected by the diffusion coefficient. In a further verification, the present study also found, under certain constraint, the empirical formula can be used in a more extensive range: d = 0.05 ~ 0.5, Vc = 0.1 ~ 1, Vf/Vc = 0.1 ~ 30, and D = 10-2 ~ 10-6.

並列關鍵字

Collection device ； Capture envelope ； Capture efficiency ； Diffusion

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