比較不同衝擊器收集板設計之固體微粒的收集效率曲線

Abstract

固體微粒於衝擊器內部的反彈現象往往造成微粒的損失，並且影響整個微粒粒徑分布的量測。本研究先利用油酸液滴探討影響慣性衝擊器收集效率的因素，包括S/W值(S：圓形噴嘴至收集板的距離；W：圓形噴嘴直徑)與用於當收集表面的不同的濾紙材質。本實驗所設計的單噴嘴慣性衝擊器的總抽氣流量為5.0 lpm，截取氣動直徑為2.0 mm。為了瞭解固體微粒的反彈現象，本研究設計了三種不同型式的微粒收集板，利用多孔金屬片作為濾紙的支撐物，且在多孔金屬片下方抽氣, 抽氣流量（在此稱為次要流量）控制在總流量的10 %以內，以防止細微粒的污染。為了進一步研究流經濾紙的抽氣流速及抽氣範圍對於固體微粒收集效率的影響, 本研究在濾紙與多孔金屬片間放置了不同圓孔直徑的投影片。 收集板設計一為深度2 mm，直徑16 mm的開放式圓形凹槽；油酸液滴的實驗結果顯示, S/W值增加對於收集效率曲線的影響並不顯著，且收集效率為50%時的Stk^0.5 值亦無明顯的偏移；但玻璃纖維濾紙的收集效率曲線較PC濾紙平緩，且收集效率為50%時的Stk^0.5 值由PC濾紙的0.47偏移為0.40，故濾紙材質的選擇是影響採樣結果的重要因素之一。 固體微粒的實驗結果顯示, 次要流量的抽氣的確可以降低固體微粒的反彈現象，並在次要抽氣流量為0.5 lpm時(抽氣面直徑為5 mm)可以達到最好的改善效果。對於收集板設計一而言，次要抽氣流量為0.5 lpm時（抽氣面直徑為5 mm）， 高達0.70時依舊可以維持80%左右的收集效率，顯示微粒反彈現象獲得了有效的控制。收集板設計二為開孔直徑5 mm，深度3.6 mm，槽寬18 mm的封閉式凹槽，與設計一同樣的次要流量抽氣條件下，設計二的收集效率偏低，維持在65 %左右。兩個設計均發現，當Stk^0.5 值高達0.70時，S/W=4的收集效率會維持在一個定值，但S/W=1則有收集效率持續下降的趨勢。 對於截取氣動直徑附近的固體微粒而言，內壁損失主要存在於衝擊器噴嘴下方，隨著粒徑的增加內壁損失提高。對設計一而言，內壁損失並不嚴重， 小於0.70的損失率均在2 %以下，唯 提昇至1.0時，損失率才提高至8 %，此時固體微粒的主要內壁損失發生於收集腔(Chamber)底部。設計二的內壁損失量較設計一嚴重，隨著Stk^0.5 的增加而提高，當 高達1.0時，內壁損失量在12%左右；當S/W值由1提升至4可有效降低微粒在噴嘴外側的附著量進而降低內壁損失，當Stk^0.5 在0.9以下，S/W=4的內壁損失率約為S/W=1的一半。 收集板設計三是一個深度為噴嘴直徑5.8倍的凹槽，實驗結果顯示，設計三的收集效率較設計一及設計二低，當 Stk^0.5 在0.55以上，效率維持在25-30 %之間，且內壁損失情形也較前二種設計嚴重。因此，從實驗結果看來，收集板設計一是作進一步應用與研究的較佳設計。

Inertial impactors are widely used in aerosol sampling devices to determine the size distribution of particles. However, for solid particles collected by the inertial impactor, particle bounce occurs and will severely affect the measured size distribution, especially when the particle concentration is high. In this study, liquid oleic acid particles were used to investigate the influence of the parameters, such as different S/W ratios and collection surfaces (S: distance from the circular nozzle to the impaction plate; W: the diameter of circular nozzle), on the collection efficiency of the inertial impactor. The impactor operates at a flow rate of 5.0 lpm and the designed cutpoint is 2.0 mm. To study the bounce of solid particles, different types of collection plates were designed in this work study. A porous metal disc was used as the support for the filter substrate. A minor flow drawn under the porous metal disc was fixed at the 10 % of the total flow rate to improve the collection efficiency while minimizing the contamination of fine particles. One of the designed collection plates (Design No. 1) is an open cylindrical cavity of 2 mm in depth and 16 mm in diameter. The experiment data of liquid droplets show that the collection efficiency and 50 % cutpoint are not significantly changed by different S/W ratios. ThesStk50^0.5 for the PC filter is 0.47 but is 0.40 for the glass fiber filter. Therefore, filter substrate is an important factor, which affects the sampling results. The experiment data of solid particles show that the minor flow can reduce the bounce of particles effectively. The proper minor flow rate is 0.5 lpm and the diameter of the suction area is 5 mm . For the Design No. 1, the collection efficiency reaches a value of 80 % at theStk^0.5 of 0.70 while S/W equals 1. But the collection efficiency decreases gradually when the Stk^0.5 is above 0.70 and S/W equals 1. Another design of collection plates (Design No. 2) has an enclosed cylindrical cavity of 3.6 mm in depth , 18 mm in diameter, and has an orifice of 5 mm in diameter at the top. Under the same minor-flow suction condition as the Design No. 1, the collection efficiency of the Design No. 2 is 65 % at the of 0.70 while S/W equals 1. When the Stk^0.5 is above 0.70, the collection efficiency of the Design No. 1 and 2 both maintain at 80 % and 70 % while the S/W ratio equals 4, but both decrease while the S/W ratio equals 1. The wall loss is mainly found on the outer surface of the nozzle when the Stk^0.5 is less than 0.70 and gradually increase as the increases. For the Design No. 1, the loss is under 2 % as the Stk^0.5 is less than 0.70 and reaches a value of 8 % as the is 1.0, regardless of the S/W ratio. The wall loss of the Design No. 2 is higher than that of the Design No. 1 and reaches a value of 12 % as the Stk^0.5 is 1.0. The increase of the S/W ratio from 1 to 4 can significantly reduce the wall loss for the Design No. 2. When the Stk^0.5 is less than 0.90, the percentage of the wall loss for S/W=1 is twice as much as S/W=4. The Design No. 3 is an enclosed cylindrical cavity of 14 mm in depth, 18 mm in diameter, and has an orifice of 5 mm in diameter at the top. The experiment data show that the collection efficiency for the Design No. 3 is lower than the Design No. 1 and 2 and maintain at 25-30 % as the Stk^0.5 is above 0.55. The wall loss for the Design No. 3 is severer than that of the Design No. 1 and 2. Therefore, the Design No. 1 is the better design for further study and application.