一個電子式微孔多階衝擊器的設計及測試

Abstract

傳統多階衝擊器已被廣泛地使用於氣膠質量分布量測，然而為了能讓衝擊器各階採 集到足夠的微粒供後續分析，需要較長的採樣時間(一般為24 小時)為其主要缺點。為了 能達到即時量測的目的，過去已有學者開發出電子式低壓衝擊器(electrical low-pressure impactor, ELPI)，且Dekati 公司也已將其作商業販售。目前ELPI 為一部熱門儀器，其獲 取數據的反應時間最短可小於5 秒內，故常被用於微粒排放量測及空氣品質方面的研 究。然而ELPI 目前仍有許多問題存在，如不旋轉的衝擊板易造成微粒過度負荷、內部 壓差過大以及微粒充電效率過低等。另外, ELPI 的價格高昂，代理商報價高達3 佰3 拾 萬台幣之多。由於本團隊過去已成功地開發出一部30 L/min 的微孔多階衝擊器 (micro-orifice cascade impactor, MCI)及一個具有軸向包覆氣流(0.5 to 1.0 L/min)的高效率 單極充電器(Chien and Tsai 2012)，故提出此三年計劃，結合上述兩種已發展的技術研發 出一部10 L/min 的電子式微孔多階衝擊器(electrical micro-orifice cascade impactor, EMCI) 以解決上述問題。第一年，本計劃將先以實驗數據來驗證低流量(0.5 -1.0 L/min)的效率 單極充電器的數值模擬結果，同時也會設計、製作並測試一部10 L/min 的MCI。為了 進一步降低充電微粒濃度的稀釋效應，本研究將會把避免帶電微粒在充電區內損失所使 用的包覆氣流，經由充電區後方的多孔介質內壁抽離。第二年，本計畫將會把高效率氣 膠充電器的尺寸放大成流量為10 L/min，並作微粒充電效率的校正，使其可用於EMCI 上。該年度也會著手進行10 L/min MCI 的微粒收集效率曲線校正。另外也將建立EMCI 的數據轉換模式，以將測得之氣膠電流數據轉換為氣膠數目及質量分布。第三年，本計 畫將進行EMCI 和其他儀器如SMPS (掃描式電移動度微粒粒徑分析儀，scanning mobility particle sizer)、ELPI 及MOUDI (微孔均勻沉積衝擊器，micro-orifice uniform cascade impactor)的現場比對，以驗證EMCI 的量測結果，並對其做進一步地修改。最後，為了 驗證EMCI 具有較高的微粒負荷能力，本計畫也會將其和ELPI 進行長時間的採樣比對。 預期在未來本計畫所開發之EMCI 可提供更準確的氣膠質量及數目分布量測結果。

Traditional cascade impactors are widely used to measure aerosol mass distributions but the major drawback is the required long sampling time (typically 24 hours) due to the necessity of collecting sensible particle mass on each stage. To achieve real-time measurements, the electrical low-pressure impactor (ELPI, 10 L/min, 3 nm to 10 m) was developed and commercialized by Dekati Ltd., Finland. Now the ELPI is a popular instrument for emission measurements and air quality studies with the minimum response time of less than 5 seconds. However, there are still many practical problems exist in the ELPI such as particle overloading on the nonrotating impaction plate, relative high pressure drop, and low aerosol charging efficiency. Besides, the ELPI is very expensive. The price quoted by the agent is as high as NT 3.3 million per unit. Since our research team has successfully developed a 30 L/min NCTU micro-orifice cascade impactor (NMCI) and a high efficiency unipolar charger with radial sheath flow (0.5 to 1.0 L/min), this 3-year project is proposed to combine these two devices to develop a 10 L/min electrical micro-orifice cascade impactor (EMCI) to resolve these problems. In the first year, the numerical results will be validated experimentally first for the high efficiency unipolar charger at the low flow rate of 0.5 to 1.0 L/min. Meanwhile a 10 L/min MCI will be designed, fabricated and tested. To further reduce the dilution effect on charged particle concentration, the sheath air flow used in the charging zone to prevent charged particle loss will be removed through the porous wall installed after the charging zone. In the second year, the high efficiency aerosol charger will be scaled up to the high flow rate of 10 L/min and calibrated for particle charging efficiency in order to use in the EMCI. The calibration of particle collection efficiency curves of the 10 L/min MCI will be also conducted in this year. Data reduction models will be established to convert the responses of the EMCI to aerosol number and mass distributions. In the third year, the inter-comparison of the EMCI system with the SMPS (scanning mobility particle sizer), the ELPI and the MOUDI (micro-orifice uniform deposit impactor) will be conducted for further validation and improvement. Finally, to ensure that the EMCI has a higher particle loading capacity, a long-term sampling comparison with the ELPI will also be conducted. It is expected that the EMCI developed in this project can provide more accurate measurements of aerosol mass and number distributions in the future.