凹槽慣性衝擊器的理論研究

Abstract

慣性衝擊器廣泛應用於大氣中粒徑分布的採樣.針對液體微粒來說,實驗與理論有相當好的一致性,且收集效率曲線都是S形狀.但是,對於固體微粒來說,微粒反彈的現象一直是大家頭痛的問題並且嚴重影響採樣結果.為了克服固體微粒的反彈作用,許多新型的採樣器陸續被開發出來.其中一個便是將傳統慣性衝擊器的平板改為凹槽的微粒補捉器(particle trap impactor). 在本研究中,討論影響微粒捕捉器收集效率的因子.首先,模擬在不同D/W及H/W(D:凹槽的開口直徑.W:圓形噴嘴的直徑.H:凹槽的深度)情況下,微粒補中器的流場.在流場計算完成後,計算在此流場中,固體微粒以及液體微粒的運動軌跡,進一步計算出效率曲線.同時利用固體微粒的反彈模式模擬固體微粒打到衝擊板表面時的反彈情形.研究也同時發現,理論上計算所得的反彈關鍵速度(微粒反彈所需要的最小速度),必須要乘以一個修正參數,才能夠與實驗結果相吻合. 研究結果顯示,微粒捕捉器的凹槽開口大小對收集效率有重要的影響.對於液體微粒而言,當開口越大時,能有越好的收集效率.這主要是因為收集濾紙上的正向速度,隨著開口的增加而增加.當凹槽深度增加時,也會有較佳的收集效率.但是對於全開的微粒捕捉器,凹槽深度的增加並不會對收集效率有所幫助.針對固體微粒方面,也能有效的防止微粒反彈而無法收集.

Inertial impactors are widely used in aerosol sampling devices to determine the size distribution of particles. For liquid particles, the experimental collection efficiency is S-shaped, similar to the ideal impactor efficiency and fits well with the theory critical curve. However, for solid particles, particle bounce occurs and will severely affect the measured size distribution. In order to overcome the problems of sampling solid particles, many new impactors have been designed. One of them is the particle trap impactor, where a cavity replaces the flat-plate of the traditional impactor to prevent particles from bouncing and being lost. In this study, factors influencing the collection efficiency of the particle trap impactor have been investigated. Two-dimensional flow fields of trap impactor for different D/W and H/W (D: the diameter of the trap opening, W: the diameter of round nozzle, H: the trap depth) were first simulated. Particle trajectories for both liquid and solid particles were then calculated to obtain the particle collection efficiency. A bounce model was applied to calculate the solid particles bounce upon impaction. The study found that the critical velocity, which is the minimum normal particle velocity for particle to bounce, must be multipied by a correction factor in order to obtain good agreement between experimental researches and theory. This study shows that the trap opening influences the collection efficiency significantly. For liquid particles, when the diameter of trap opening increases, the collection efficiency is better because of the normal velocity above the filter surface increasing and helping the particle impacting. When the trap depth increases, the collection efficiency is improved, too. But for the open trap impactor, the collection efficiency is almost not affected by the depth of the trap. For solid particles, the cover of the trap impactor successfully prevents the particles from rebounding from impactor.