以數值模擬分析狹縫型虛擬衝擊器之效能;A CFD Study of Effects of Flow and Geometrical Parameters on Slit Virtual Impactor Performance

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Title:	以數值模擬分析狹縫型虛擬衝擊器之效能;A CFD Study of Effects of Flow and Geometrical Parameters on Slit Virtual Impactor Performance
Authors:	張博凱;Chang,Po-Kai
Contributors:	環境工程研究所
Keywords:	數值模擬;COMSOL;狹縫型虛擬衝擊器;史托克數;CFD;COMSOL;Slit virtual impactor;Stokes number
Date:	2015-10-22
Issue Date:	2015-11-04 17:52:12 (UTC+8)
Publisher:	國立中央大學
Abstract:	本研究利用數值模擬軟體COMSOL Multiphysics分析現有的虛擬衝擊器，在不同流量與不同構形下的濃縮效能。不同流量的影響分為進氣流量的影響與次要流比的影響，不同構形的影響分為漸縮區與收集口管壁影響。由模擬結果得知，進氣流量與截取粒徑呈反比，當進氣流量為13 LPM時，截取粒徑為1.4 μm；當進氣流量為400 LPM時，截取粒徑為0.2 μm，而無論進氣流量大小，Stk50^0.5皆為0.8，可達到的最高濃縮倍率也都為9.5倍。另外，由於漸縮區的關係，會使微粒具有橫向慣性，造成約在Stk50^0.5為1.9時，微粒會發生交錯現象，並沉積在收集口管壁，因此大粒徑的濃縮倍率會下降。在不同的進氣流量下，無論是截取粒徑，或最高濃縮倍率可濃縮的粒徑範圍皆可用史托克數預測。次要流比同樣也與截取粒徑呈反比，次要流比為0.05時，截取粒徑為0.63 μm；次要流比為0.4時，截取粒徑為0.21 μm。由於濃縮倍率與次要流比呈反比，因此在次要流比為0.05與0.4時，可達到的最高濃縮倍率分別為18.0與2.5倍。由於原始的史托克數公式，無法用來預測不同次要流比的截取粒徑，因此本研究提出新的史托克數公式，其在公式中，加入次要流比r的影響，藉此用來預測不同次要流比下的截取粒徑，並且也經由實驗數據證明的此公式的可行性。在構形的影響的部分，將漸縮區由斜面漸縮改為弧口漸縮後，其降低橫向的慣性，延緩微粒交錯現象，但截取粒徑與截取粒徑的損失卻會增加，而將收集口管壁改為漸擴構形時，可在截取粒徑不變的情況下，降低截取粒徑與大微粒的損失，提高濃縮倍率。藉由以上四種流量與構形的分析，可依據虛擬衝擊器使用功能的不同，分析最佳的流量與構形組合。;The virtual impactor was well known as it could avoid particle bounce and overloading typically encountered with inertial impactors. In this study, the flow field and particle trajectory in a slit type virtual impactor was simulated numerically by a commercial Computational Fluid Dynamics (CFD) software (COMSOL Multiphysics v.4.3b). Effects of flow and slit geometry, including the total flow (Qin), the ratio of minor to total flow (r), the taper slip nozzle (45⁰ chamfer lip and arc lip) and the collection probe configuration (straight and divergent), were investigated. The performance parameters for evaluating the performance were the collection efficiency (CE), the particle loss, the cutoff size (d50), and the concentrating factor (CF). The results show that the d50 is strongly depending on either Qin or r. For instance, the d50 could be reduced form 1.4 μm to 0.2 μm as Qin increased from 13 LPM to 400 LPM at r = 0.1. When Qin was fixed at 80 LPM, the d50 will decrease from 0.63 μm to 0.21 μm as minor flow ratio increasing from 0.05 to 0.4. In addition, a new modified Stokes number for virtual impactor to include the effect of r was proposed. It was further found the square root of this modified Stokes number was retained at about 0.9 under different Qin or r, which can be considered as the characteristic performance parameter for this slit type virtual impactor. On the other hand, the numerical simulation results show particle loss would increase when Stk^0.5 lager than 1.9 due to the particle crossing phenomenon. The particle crossing in acceleration nozzle caused particle deposit on collection probe and CF would decreased. In geometry analysis, the gradual curving lip could postpone particle crossing and decrease the internal loss of larger particles, but the particle loss near cutoff size was increased. Moreover, the divergent probe could decrease the particle loss near the d50 and collection probe. At last, the preliminary experimental tests were conducted to validate the numerical simulation results.
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