單一電極線電暈單極充電器的奈米微粒充電效率改善研究

Abstract

本研究的目的在於利用包覆空氣減少微粒損失進而提升電暈單極充電器的奈米微粒充電效率。首先設計一個具有軸向包覆空氣的充電器，其構造包含一個內徑30 mm的圓柱外殼，及使用一直徑為50 μm、長度為2 mm的黃金電極線作為放電電極，並在固定氣膠流量1 L/min的條件下測試充電器對單徑2.5 ~ 20 nm銀微粒之充電效率。本研究並建立了一個二維數值模式來推估單極微粒充電器的奈米微粒充電效率，充電器內部的流場利用SIMPLER方法而得，電位場及離子濃度場則利用Poisson方程式及對流擴散方程式來求解；之後再利用對流擴散方程式配合Fuchs擴散充電理論求取帶電微粒的濃度場及充電效率。研究結果顯示，模擬的外在充電效率與實驗值相符。模擬結果說明了使用軸向包覆空氣減少帶電微粒損失的優點，並指出主要帶電微粒損失的發生位置。但由於較小的微粒較難被充電，使得具有軸向包覆空氣的充電器對粒徑小於10 nm的微粒之充電效率不佳，仍有改進的空間。 為進一步有效地提升奈米微粒的充電效率，本研究另設計一個具有徑向包覆空氣以減少帶電微粒損失的單一電極線單極氣膠充電器。此充電器之構造包含一個用來引入徑向包覆空氣的6 mm長接地多孔金屬套管，嵌入於內徑6.35 mm的絕緣鐵氟龍管正中央，及一直徑為50 μm、有效長度為6 mm的放電黃金電極線。本研究利用已建立的數值模式評估及最佳化此充電器的充電效能。研究過程中發現徑向包覆空氣的開口位置對減少帶電微粒損失的影響是重要的，故本研究針對兩種不同的充電器設計進行探討，在設計1中，徑向包覆空氣6 mm寬的開口兩端對準6 mm長的放電電極兩端，然而在設計2中，徑向包覆空氣的開口往電極線前端的左方偏移2 mm。 與具有軸向包覆空氣的充電器比較，模擬結果顯示具有徑向包覆空氣的充電器之充電區並未觀察到迴流場的存在，在相同的操作條件下，因為帶電微粒沉降區的減少，使得設計2的靜電損失小於設計1，模擬結果說明了使用徑向包覆空氣並配合適當包覆空氣的開口位置具有減少帶電微粒損失的優點。與目前文獻上具有最高外在充電效率的兩組單極氣膠充電器相較 (Chen and Pui 1999; Kimoto et al. 2010)，模擬結果顯示，針對10 nm以下微粒，在施加電壓為+3.5 kV、氣膠流量為0.5 L/min及包覆空氣流量為0.7 L/min時，具有徑向包覆空氣的充電器之設計2有相似的充電效能，對2.5–10 nm微粒的外在充電效率達到15.2%–65.8%，而Chen and Pui (1999)的充電器對3–10 nm微粒的外在充電效率為22%–65%，Kimoto et al. (2010) 的充電器對5–10 nm微粒的外在充電效率為59%–64%。 預期本研究設計的具有徑向包覆空氣的充電器可作為高效率的奈米微粒充電器，可改善監測儀器對奈米微粒的偵測靈敏度，未來可進行實驗驗證理論值以加強實用性。

The objective of this study is to develop a corona unipolar charger with sheath air to minimize particle loss and enhance the nanoparticle charging efficiency. At first, a unipolar charger with axial sheath air was designed which consists of a cylindrical casing of 30 mm in inner diameter in which a gold wire of 50 μm in diameter and 2 mm in length is used as the discharge electrode. The experimental charging efficiency was obtained at a fixed aerosol flow rate of 1 L/min using monodisperse silver nanoparticles of 2.5 to 20 nm in diameter. A 2-D numerical model was also developed to predict nanoparticle charging efficiency in the unipolar charger. Laminar flow field was solved by using the Semi-Implicit Method for Pressure Linked Equations (SIMPLER Method), while electric potential and ion concentration fields were solved based on Poisson and convection-diffusion equations, respectively. The charged particle concentration fields and charging efficiency were then calculated based on the convection-diffusion equation incorporating the Fuchs diffusion charging theory (Fuchs 1963). Good agreement between simulated and experimental extrinsic charging efficiency was obtained. Numerical results show the advantage of using axial sheath air to minimize charged particle loss and indicate the location where major charged particle loss occurs. However, the extrinsic charging efficiency of the charger with axial sheath air is still low for particles smaller than 10 nm in diameter due to low intrinsic charging efficiency. In order to improve the design of the charger, a single-wire corona unipolar charger with radial sheath air to minimize charged particle loss was proposed to enhance the charging efficiency of nanoparticles. The charger consists of an insulated Teflon tube (inner diameter ID = 6.35 mm) with a 6 mm long grounded porous metal tube inserted at the center from which radial sheath air is introduced, and a discharge gold wire of 50 μm in ID and 6 mm in effective length. The performance of the charger was evaluated and optimized by the present numerical model. The effect of the position of the sheath air opening on reducing the loss of charged particles was found to be important and two different designs were studied. In design 1, both ends of 6 mm wide sheath air opening are aligned with the ends of 6 mm long discharge wire, while in design 2 the sheath air opening is shifted 2 mm toward the left of the leading edge of the wire. Compared to the charger with axial sheath air, numerical results show that no flow recirculation region is observed in the charging zone of the charger with radial sheath air. At the same operating condition, design 2 was found to have less electrostatic loss than design 1 because of its smaller deposition region of charged particles. Numerical results show the advantage of using radial sheath air with an appropriate position of the sheath air opening to minimize charged particle loss. Compared with two existing unipolar chargers with the highest extrinsic charging efficiency for particles smaller than 10 nm in diameter (Chen and Pui 1999; Kimoto et al. 2010), results show that design 2 operated at the applied voltage of +3.5 kV, aerosol flow rate of 0.5 L/min, and sheath air flow rate of 0.7 L/min was found to have extrinsic charging efficiency of 15.2%–65.8% for particles ranging from 2.5 to 10 nm in diameter, which is comparable to that of the charger of Chen and Pui (1999), 22%–65% for particles ranging from 3 to 10 nm in diameter, and that of the charger of Kimoto et al. (2010), 59%–64% for particles ranging from 5 to 10 nm in diameter. It is expected that the charger with radial sheath air designed in this study could be used as an efficient nanoparticle charger to improve the sensitivity of monitoring instruments for nanoparticles. In the future, the experiments will be conducted to validate the simulated results and to further enhance the feasibility of the charger.