以多孔金屬片作為慣性衝擊器收集板的微粒收集效率

Abstract

本研究探討多孔金屬片作為單階圓孔慣性衝擊器的收集板時的微粒收集效率，並比較與平板收集板的收集效率之異同。多孔金屬片的孔隙直徑分別為100、40、20、5 micro meter，衝擊器的噴嘴大小為2.6、3.6 mm，測試流量範圍為1.5∼3 LPM。本研究利用超音波霧化器產生多徑微粒，以氣動直徑偵測儀 (APS) 量測衝擊器上下游的微粒濃度差，探討影響微粒收集效率的因子。實驗結果顯示，對於液體油酸微粒而言，當噴嘴雷諾數(Re)越大且多孔金屬片阻力係數 (K) 越小的情況下，孔隙直徑100、40 micro meter的多孔金屬片比平板的收集效率曲線平緩，截取氣動直徑亦較小。此原因在於部份氣流可以貫入多孔金屬片內部，增加微粒的收集效率。相較之下，孔隙直徑20、5 micro meter的多孔金屬片孔隙小，氣流貫入多孔金屬片內部的量不多，本研究發現其與鋁箔在微粒的收集效率上沒有多大的差異。 對於固體氯化鉀微粒而言，在sqrt(stokes)大於sqrt50且收集板沒有塗敷矽油(Silicone oil)的情況下，孔隙直徑越大(100、40 micro meter)的多孔金屬片相較於鋁箔有著越高的收集效率，sqrt大於0.8時的收集效率可達70％∼85％，比鋁箔的收集效率45％高很多，具有改善微粒彈跳的效果。對孔隙直徑為100、40 micro meter且塗敷矽油的多孔金屬片而言，在sqrt大於1時的微粒收集效率可以達到90％∼95％，沒有塗敷矽油的對照組其收集效率則低很多。收集板在微粒負載0.24 mg且沒有塗敷矽油的情況下，不管是多孔金屬片或是鋁箔，微粒的收集效率皆不好；塗敷矽油的多孔金屬片(孔隙直徑為100、40 micro meter)在大於截取氣動直徑的微粒收集效率可以達到95％，理由是因為孔洞結構的毛細作用使得收集板上累積的微粒層因而會含浸有矽油，可防止固體微粒的彈跳並增加微粒的收集效率；反之，使用鋁箔收集板時，因為塗敷的矽油被微粒層覆蓋住了，後繼的固體微粒會自微粒層上彈跳，造成收集效率降低，使得大於截取氣動直徑的微粒收集效率僅為45％左右。

This study has investigated particle collection efficiency experimentally of a single round nozzle inertia impactor using porous metal substrate. The collection efficiency was also compared to that of a impactor that uses flat plate substrate. The diameter of the porosity of the porous disc is 100, 40, 20 and 5 micro meter, and the nozzle diameter is 2.6 and 3.6mm, with the tested flow rate from 1.5 to 3 LPM. The range of nozzle Reynold number is from 753 to 1563 and the resistance factor of the porous disc is from 4.94×10^9 to 1.02×10^11 (1/m2). This study used an ultrasonic aerosol generator to create polydisperse particles and used an aerodynamic particle sizer to measure particle concentrations at the upstream and downstream of the impactor and determine the collection efficiency. Test results is show that for liquid oleic acid particles, for larger Reynold number and smaller resistence factor of the porous disc, porous disc with pore sizes of 100 and 40 micro meter not only produces smoother collection efficiency curves than that of flat plate substrate, but also result in smaller cut-off aerodynamic diameters. The reason is that partial airflow penetrates into the porous disc to increase the collection efficiency. Owing to its large resistence factor, porous disc with smaller pore sizes (20 and 5 micro meter) does not have too much airflow penetration into the disc and our experimental results show its collection efficiency is not too much from that of the flate plate. For solid KCl particles, when sqrt(stokes) is greater than sqrt50, the uncoated porous disc with larger pore sizes (100 and 40 micro meter) has higher collection efficiency. For example, when sqrt is greater than 0.8, the efficiency of the porous disc(100 and 40 micro meter) can be as much as 70% to 85%, which is much higher than 45% of the aluminum foil. That is, solid particle bounce is reduced when using porous disc. Using the porous disc with pore size of 100 and 40 micro meter and coated with silicone oil of about 70 mg in mass, the particle collection efficiency can reach 90% to 95% when sqrt is greater than 1, which is a substantial improvement in the collection efficiency of the uncoated disc. This is the case for clean disc. When the loaded particle mass of the coated porous disc is 0.24 mg, the efficiency still remain as high as 90％∼95％ for sqrt＞0.8. In comparison, the collection efficiency drops sharply when sqrt＞0.8 for uncoated porous disc or aluminum foil at the loaded mass of 0.24 mg. The reason for such high efficiency when using coated porous disc with large pore sizes (100 and 40 micro meter) is because the capillarity effect of the pore structure. Due to the capillarity effect, silicone oil is absorbed into the deposited particle layer, which can prevent solid particles bounce and increase the particle collection efficiency. In contrast, silicone oil coated on aluminum foil, silicone oil would be covered by the deposited particle layer quickly, therefore, solid particles would bounce from the deposited particle layer and decrease the collection efficiency. This results in particle collection efficiency as low as 45% for sqrt＞1.0.