具高微粒負載特性之正壓式虛擬旋風分徑器效能提升研究

Abstract

自1995年3月東京地下鐵的沙林事件以及一連串恐怖攻擊事件發生，引起了一連串的恐慌，恐怖組織使用核生化（chemical, biological, radiological, or nuclear, CBRN）武器引發之戰爭，造成嚴重的傷亡。如今各國皆積極研發新的方法與技術以偵測、控制、並移除有關化學性、生物性、放射性、微粒態的污染。 一個標準裝甲車的空氣清淨系統應包含供氣之動力風扇、一旋風式分徑器以去除大粒徑微粒、折疊式濾材去除通過旋風式分徑器的剩餘微粒、活性碳層吸附空氣中的有機蒸氣，每單元皆有其作用，缺一不可。虛擬旋風分徑器由於底部有一小孔，可將蒐集之微粒由底部開口排出減少微粒負載效應，因此格外重要。為增進虛擬旋風分徑器之效能，本研究分三個階段，其概述分別如下: 第一階段，旋風式分徑器品質提升研究: 採用過濾品質(filtration quality factor)的觀點，於相同的流量下其單位阻抗所能提供的微粒收集效率高低，全面性的測試旋風式分徑器的各構型參數，依過濾品質的觀點檢視Stairmand構型發現該構型於分徑器高度(H)與出口管長(S)的建議已是最佳化的設計，若能改變其他的構型規格如使入口更為狹長 (a/b由13/5調整至20/3.2)、盡可能增加錐狀底高的比例( hc/H由62/100調整至70/100)與減少錐狀底底部直徑(B由9 mm減至4 mm)、減少出口管直徑(De由13 mm調整至8 mm)，將可使旋風式分徑器之過濾品質有效的提升；實驗的結果顯示若能有效調整構型參數於設定流量(30 L/min)下其分徑品質比起Stairmand構型將可提升5倍，不同流量下其差異不同，於較小流量下(15 L/min)其品質差異可達12倍。 第二階段，正壓式虛擬旋風分徑器效能提升研究: 本研究引用並調整市售空氣清淨機常用的效能指標「CADR」，探討不同入口流量、底部開口大小位置、不同錐狀底、不同背壓的改變對虛擬旋風分徑器效能影響，結果可知若能提升風扇所提供之風量可有效的提升CADR值。底部開口越小對大粒徑微粒而言其CADR值越高，但須注意阻塞的問題。底部開口位置不同其CADR值差異不大，唯底部開口置於中心將導致氣體無法由底部開口排出，反而變成吸入，且開口於最邊緣其副排出氣流有最佳之排出效率，因此開口置而離中心最遠的切邊處是最理想選擇。錐狀底越小將使CADR值越高。折疊式濾材與活性碳層所形成的背壓越大則分徑器的CADR值越低。使用離心風扇供氣後，會造成Qmajor減少亦會造成總流量減少，連帶影響微粒的收集效率，其複雜的關係需要進行更多的研究方能進一步的了解其影響。 第三階段，正壓式虛擬旋風式微粒排出特性研究: 虛擬旋風分徑器的微粒排出特性具有濃縮效果，因此在探討微粒排出的同時，亦可以同時將其視為一個微粒濃縮器，一個理想的微粒濃縮器應能控制微粒濃度、粒徑與分佈。在本實驗中證明，透過改變正壓式虛擬旋風分徑器之參數，的確對微粒濃度、粒徑與分佈造成影響。實驗結果顯示副排出氣流之微粒濃縮比會受入口氣流流量大小、底部開口大小、與錐狀底部直徑(B)的影響，入口流量越大其濃縮比越低，底部開口越大其濃縮比越低，錐狀底直徑(B)越小其濃縮比越高，底部開口位置雖對於微粒濃縮比影響不顯著，但對於較大粒徑微粒亦會有少量的影響。以排出微粒量而言，副排出氣流之微粒粒數排出比之大小受錐狀底部開口大小與開孔位置影響，底部開口越大其微粒粒數排出比越高，底部開口位置離中心越遠其微粒數排出比亦越高。副排出氣流所排出之微粒粒數比分佈曲線之分佈範圍(GSD)，將因入口流量增大、底部開口變小與錐狀底變小而使GSD變小，其中改變底部開口大小對其GSD影響最為顯著。

In the wake of the March 1995 sarin attack in the Tokyo subway, as well as other recent terrorist incidents, governments and publics are viewing with the growing concern about the potential threat posed by chemical, biological, radiological, or nuclear (CBRN) weapons. The risk for terrorist events has led to the development of new approaches to sampling, testing, and controlling for both indoors and outdoors ambient air. A standard military armored vehicle is composed of a blower, a virtual cyclone, a pleated HEPA filter unit, and a charcoal pack. A positive pressure type virtual cyclone has been used for high aerosol loading and low loading influence. This study used three steps to improve the efficiency of the cycole. The first step is to expert on Performance Improvement of the Stairmand Cyclone Design. This work adopts the cyclone quality factor as the performance index, and the cyclone designed by Stairmand is adopted as the basic configuration in the present study. With the cyclone body diameter (D) fixed, we vary the cyclone length (H), inlet height (a) and width (b), cyclone length without the cone (hb), cone height (hc), cone bottom diameter (B), and vortex finder length (S). The results show that each of these design parameter has a different impact on the cyclone performance. Based on the cyclone quality factor, it is found that the H and S of the Stairmand cyclone are already the optimal conformations. The cyclone performance could be further improved, if further changes could be made to the other conformations, for example, narrowing down the inlet (adjusting a/b from 13/5 to 20/3.2), reducing the cone bottom diameter (adjusting B from 9 mm to 4 mm) and decreasing the vortex finder diameter (adjusting De from 13 mm to 8 mm), The cyclone quality factor appears to be a function of the volumetric flow rate. Results demonstrate that while the air flow is at 15 L/min, the newly improved cyclone could perform 12 times better than the Stairmand type cyclone, according to the cyclone quality factor. The second step is to improve the Performance characterization of a positive pressure virtual cyclone working at high aerosol loading. This step work adopts the clean air delivery rate (CADR), which is commonly used in air-cleaning devices as an indicator to understand the cyclone loading effect of different parameters such as the inflow rate, the position of bottom hole, the cone shape, and the back pressure.The results demonstrate that while increasing the air flow, the CADR would be well improved. As the bottom hole becomes smaller, the CADR of big particles will increase. There is no obvious differences between hole positions. However, when the bottom hole locates in the center, the air flow will turn to draw air instead of discharged. Therefore, the best position of the bottom hole is the margin of the circle and a small cone will produce a better CADR. When the back pressure caused by pleated HEPA filter unit and charcoal pack is bigger, the CADR is lower. If we replace the high pressure inflow air with general centrifugal fan, the Qminor will decrease as well as the total flow, thereby lowering the collection efficiency. The third step is to expert study of the positive pressure virtual cyclones as particle concentrators. There are lots of studies evaluating the fisibility of positive pressure virtual cyclones used as a particle concentrator. However, only a few studies diccussed the characteristics of parameters of the positive pressure virtual cyclone. An ideal particle concentrator is capable of controlling the particle concentration rate, particle size and its distribution. In this study, the particle concentration rate was proved to be effected by the above factors and the minor flow was effected by inlet flow, hole diameter at the cyclone bottom and the bottom diameter. When inlet flow rate was higher or the hole diameter was bigger, the particle concentration rate was lower. On the contrary, as the bottom diameter (B) was smaller, the concentration rate was higher. The position of the hole had no significant effect on particle concentration rate except particle diameter at 3-6 m. The particle number fraction in the minor flow was increased with increasing hole diameter and the distance between the hole to the center of bottom. When the inlet flow rate was higher, the hole diameter and the cone diameter were smaller, the GSD (Grain Size Distribution Curve) was narrower.