單一氣泡破裂氣膠逸散之特性探討

Abstract

氣泡破裂為液體中非揮發性有害物質得以逸散至環境中重要的機制。雖然過去有許多相關研究，但都以連續且大量產生氣泡的方式進行，而本研究為了能夠清楚地界定氣泡直徑、表面張力等參數對微粒逸散的影響，因此，以單一氣泡為基礎，探討不同條件下，氣泡薄膜破裂後所產生的微粒分佈特性，以做為逸散控制方法設計的依據。 實驗中，主要探討之實驗變項包括氣泡直徑（0.50, 0.75, 1.00, 1.25, 1.50 cm）、氣泡維持時間（40, 45, 47, 48, 51 sec）系統內氣體流速的變化（16, 20, 25, 30, 35 cm/sec）、溶液表面張力（34, 36, 38, 41, 44, 46, 50 dyne/cm）與溶液體積濃度（0.1, 0.4, 0.8, 1.1, 1.6 %）。研究中使用自製的氣泡產生器，以每次產生一顆氣泡的方式並讓氣泡靜置於液面，待氣泡自然破裂後，引進乾淨氣流將液滴乾燥成微粒，並同時將微粒帶至系統下游進行微粒特性之分析。微粒特性之測量則利用氣動粒徑分析儀以及微粒凝結核計數器分別量測0.5~20 μm微粒之粒徑濃度分佈，以及小於1 μm 微粒之總微粒數，此外，也使用高速攝影機拍攝氣泡破裂的過程，並以膜厚儀量測氣泡膜壁破時厚度的變化。 研究結果顯示，單一氣泡直徑越大，逸散微粒粒徑分佈越大，且數量越多，而根據CPC與APS量測微粒總數的比值判斷，氣泡膜厚破裂所逸散的微粒分佈可能在微米與次微米各有一個峰值，分別為主粒徑 (Main droplets) 與伴隨粒徑 (Satellite droplets)所構成，而過去所謂的噴射液滴 (Jet droplets) 對於逸散微粒並無明顯貢獻；氣泡自然破裂時的厚度隨氣泡直徑以及表面張力的增加而增加，當任一尺寸氣泡的壽命超過一臨界值時，破裂時所產生的微粒總數會有顯著的增加。其次，系統內的氣體流率越大，則氣泡越早破裂，且當破裂的時間點小於前述的臨界值時，逸散微粒的數目便顯著下降：在直徑1 cm的氣泡隨流速從16至20 cm/sec，CMD與微粒總數皆約為2 μm與1000 #/bubble，但上升至35 cm/sec，CMD與微粒總數則下降至1.68 μm與313 #/bubble；此外，表面張力上升能有效降低微粒之生成，隨表面張力從34上升至41 dyne/cm，CMD從2.07上升至3.01 μm，但當表面張力從41上升至50 dyne/cm，CMD則隨之下降；最後，在相同表面張力不同體積濃度的溶液下，濃度越大所產生之微粒粒徑越大，但轉換為液滴粒徑後，液滴眾數皆維持約29 μm，表示改變溶液體積濃度對於氣泡破裂後液滴的生成並無變化。 依據本研究結果，若能控制氣泡破裂時的膜厚、釋放較小直徑的氣泡與提高溶液表面張力，便能控制微粒的產生；因此，從工程控制的角度，在相同體積的氣泡下，氣泡直徑越小，微粒逸散的總質量與總顆粒數便能有效降低；最後，提高溶液表面張力亦可有效降低微粒逸散至環境的能力。

At the surface of polluted river and reservoirs, the liquid droplets produced by air bubble bursting serve as vehicles fo r the transfer of non-volatile substances into the atmosphere. Thus, the purpose of this study aimed to experimentally characterize aerosol emission from a single bubble. In the present study, the major operating parameters included the bubble diameter (0.50, 0.75, 1.00, 1.25, 1.50 cm), bubble age (40, 45, 47, 48, 51 sec), conveying air velocity (16, 20, 25, 30, 35 cm/sec), surface tension (34, 36, 38, 41, 44, 46, 50 dyne/cm) and volume concentration (0.1, 0.4, 0.8, 1.1, 1.6 %). When the bubble bursting naturally, droplets would be dried and carried away from the chamber by an aerosol-free flow. Aerosol monitoring system was located at downstream the chamber and composed of an Aerodynamic Particle Sizer and a Condensation Particle Counter which were used to monitor the aerosol size distribution in the size range of 0.5 – 20 μm and total count of particle with diameter less than 1 μm , respectively. The high-speed micron camera was employed to record the bubble bursting process and film thickness detector was used to measure the bubble film thickness at bubble bursting. The results revealed that CMD and total counts increased with increasing bubble size. According to the ratio of particle count of CPC to APS, we concluded that the particle size distribution had two modes. One was the main droplets and the other was the satellite droplets, and no jet droplet was observed in this work. Moreover, the bubble film thickness increased with increasing bubble size and surface tension at the time of the bubble bursting naturally. This indicated that the particle emission could be controlled by controlling the film thickness of bubble bursting. The total counts and bubble lifetime decreased with increasing air velocity. The produced particle number counts decreased dramatically when the bubble lifetime shorter than the specific time. Finally, the total counts increased with decreasing surface tension significantly. CMD increased with increasing surface tension form 34 to 41 dyne/cm, but CMD decreased with increasing surface tension form 41 to 50 dyne/cm. Finally, the mode of particle increased with increasing volume concentration under same surface tension, but the mode of droplet size did not change. That means changing concentration did not effect on droplet emission from bubble bursting.