一般大氣或工作場所中之PM10及PM2.5微粒可能會對人體呼吸系統造成危害。計算流體力學(CFD)是利用數值計算方法來完成相關流體力學的研究目前已被廣泛應用於許多衝擊器效率之模擬計算。本研究之目的為發展一種新型態之主動式虛擬衝擊器來有效分離微粒，先藉由CFD模擬於不同樣品入口、邊鞘流及側邊流流速下，對不同粒徑微粒之分離效率，再根據模擬結果進行衝擊器實體設計與實驗驗證。
本研究選用CFD-ACE+做為模擬虛擬衝擊器分離微粒之計算流體力學工具。模擬前先找出最適模擬計算之結構化網格數，之後再模擬兩種不同尺寸之主動式虛擬衝器模型在不同邊鞘流速/樣品流速比值(Ratio等於0~20)之流場變化以及不同側邊流速(0~3 m/s)可分離之微粒粒徑(1.5 μm ~20 μm)。最後依照模擬結果製作虛擬衝擊器實體進行後續實驗比對。本研究使用流體化床氣膠產生器(Model 3400a, TSI)產生1.5 μm單粒徑微粒，來探討虛擬衝擊器在不同邊樣品流速/鞘流速比值(Ratio等於0~4)下之分離效率。微粒分離效率係以粉塵粒徑分析儀(Grimm 1.109)量測側邊流開啟前後衝擊器下方出口粒數濃度計算而得。
研究結果顯示Model-1(研究模型)及Model-2(實作模型)之最適模擬網格數分別為15000及10000在無邊鞘流對樣品流夾擊下(Ratio等於0)，Model-1及 Model-2中央內部管道皆有流場擴散導致微粒回流及管壁損失的現象，而當Raito大於等於1，該現象則消失。模擬結果也顯示在每一特定Ratio下，衝擊器分離微粒粒徑大小隨側邊流速增加而增加。Ratio值愈高，分離特定粒徑所需之側邊流速也隨之增加，實驗亦得到類似趨勢的結果。實驗與CFD模擬預測分離相同粒徑微粒所需之側邊流速相當接近，且兩者具有高度相關。
綜合結果顯示，本研究探討之兩種主動式虛擬衝擊器尺寸雖有差異，但微粒分離曲線皆具有相同的趨勢，且採用邊鞘流對樣品流進行夾擊之設計，能夠有效防止流場擴散、微粒回流及管壁沈積的損失；雖然使用CFD模擬預測衝擊器之微粒分離效率與實驗結果有所差異，但經校正修正後即可準確預測。此主動式虛擬衝擊器不但改善了傳統虛擬衝擊器流量不易控制以及分離效率較低和慣性衝擊器微粒損失之問題，是一有潛力運用於一般室內或是工作場所之空氣品質控制或微粒採樣的技術。 In the environment or the workplace, the PM10 and PM2.5 are the most harmfully particles to human respiratory system. Computational Fluid Dynamics (CFD) was used to study the particles mechanics. The numerical simulation has been widely researched in many field regarding to the collection efficiency of particles impactor. In this study, we developed an innovated type of active virtual impactor(AVI) to separate particles effectively. With different composition flow rate of sample flow, side flow and sheath flow, the operation parameters were established with CFD simulation results. The simulation results were putting in experimental and were verified to agreement with the experiment.
The optimal numbers of structured grids for numerical simulation were between 10000 with 150000. The sheath flow respected to sample flow ratio is from 0 to 20. The particle size was choosing from 1.5 µm to 20 µm. The different side flow was choosing from 0 to 3m/s. In the experiment, a single size of 1.5 μm particles was generated by Model 3400a. The sheath flow were respected to sample flow ratio can reach only from 0 to 4.
The simulation results showed that the particles were separated increasing when the side flow rate increased. When ratio value was increased the separation requirement for the specific particle size of the side flow rate increased. The experimental results agreed with the simulation results.
The sheath flow design was able to keep the particles flow in the middle of the flow channel and won’t loss on the wall. The applying of CFD simulations tool can predict the particle separation efficiency of the impactor and the operation parameters. This AVI design can improve the traditional virtual impactors which are not easing flow control and less separation efficiency.
