單孔圓形噴嘴慣性衝擊器的基本研究

Abstract

本研究探討單孔圓形噴嘴慣性衝擊器中微粒的重力效應、衝擊板直徑、微粒密度和以多孔金屬材質作為衝擊板基質時對微粒收集效率的影響，並且進行慣性衝擊器的設計參數研究。 在微粒重力效應方面，係利用數值模擬及實驗方法探討微粒重力對慣性衝擊器中微粒收集效率的影響。結果顯示，當噴嘴雷諾數小於 1500 ，考慮重力效應時，sqrt(50% Stokes)的值會隨噴嘴雷諾數的降低而降低；在噴嘴雷諾數小於 100 時，sqrt(50% Stokes)的值會隨噴嘴雷諾數的降低而劇烈下降。相反的，未考慮重力效應時，sqrt(50% Stokes)的值則隨噴嘴雷諾數的降低而增加；在噴嘴雷諾數小於 100 時，sqrt(50% Stokes)的值則劇烈上升。本研究也探討衝擊板直徑對微粒收集效率的影響，結果顯示，在噴嘴雷諾數大於 1500 和 W/Dc 小於 0.32 的情況下，sqrt(50% Stokes)幾乎和 W/Dc 的大小無關。當噴嘴雷諾數小於 1500 時，不同噴嘴直徑下衝擊板直徑對sqrt(50% Stokes)值的影響較為顯著。在微粒密度對收集效率的影響方面，本研究顯示，當微粒密度從 0.5 增加至 10 g/cm3 時，sqrt(50% Stokes)的值不會改變，且微粒密度對收集效率的影響可忽略。 在衝擊板基質方面，本研究顯示，以多孔金屬材質作為衝擊板時，由於氣流會貫穿進入多孔金屬材質內部，造成微粒收集效率曲線和傳統理論預測曲線不同。此時，以多孔金屬材質作為衝擊板的微粒收集效率高於平面型衝擊板，數值計算結果符合本研究所得的實驗值。 當氣流進入以多孔金屬材質作為慣性衝擊器的衝擊板內部時，需進一步考慮多孔材質對微粒的過濾效應。本研究顯示，計算所得的收集效率曲線會比假設 100 % 過濾效率 (理想過濾) 下的收集效率曲線更為陡峭；並且在噴嘴雷諾數等於 3000 及多孔金屬阻力因子等於 568000 cm-2 的情況下，當sqrt(Stokes) = 0.05 時，微粒收集效率有一極小值約為 2 %。對於超細微粒而言，當sqrt(Stokes)趨近0 時，由於微粒的擴散機制，將會使微粒的收集效率趨近理想過濾效率下的值。

This study investigates the effect of gravitational force, impactor plate diameter, particle density and porous metal substrate on particle collection efficiency of single round nozzle inertial impactors, and parametric study of impactor design. In the study of gravity effect, particle collection efficiency in inertial impactors has been investigated numerically and experimentally. The results show that when gravity is considered,sqrt(50% Stokes)decreases as Re is decreased when Re is smaller than 1500. sqrt(50% Stokes)decreases very drastically when Re is smaller than 100. To the contrary, without considering gravity, sqrt(50% Stokes)increases as Re is decreased when Re is smaller than 1500. sqrt(50% Stokes)increases very drastically with decreasing Re when Re is smaller than 100. This study also investigates the effect of impaction plate diameter on particle collection efficiency of inertial impactors. The results show that computed sqrt(50% Stokes)values are nearly independent of W/Dc for Re > 1500, and when W/Dc is less than 0.32. When the nozzle Reynolds number is smaller than 1500, the influence of W/Dc on sqrt(50% Stokes) is found to be much more pronounced. In the aspect of particle density effect, this study shows that sqrt(50% Stokes) is not affected by particle density ranging from 0.5 to 10 g/cm3, and the particle density effect on the collection efficiency is negligibly small. In the study of impactor plate material, this study shows that some air may penetrate into the porous metal substrate resulting in different particle collection efficiency than that predicted by the traditional theory. The particle collection efficiency for the impactor with the porous metal substrate is higher than that with the flat plate substrate, and numerical results are in good agreement with the experimental data. As the penetrating air carries the particles into the porous metal substrate, the effect of particle filtration by the porous metal substrate on the particle collection efficiency must be considered. The calculated collection efficiency curve will be sharper than that assuming 100 % filtration efficiency (ideal filtration) and there is a minimum value of about 2 % at sqrt(Stokes) = 0.05 when Re = 3000 and K = 568000 cm-2. For the ultrafine particles with sqrt(Stokes) approaching 0, the collection efficiency curve will approach to the curve assuming ideal filtration due to diffusion mechanism.