表面處理對露點及其熱傳之影響

Abstract

物質相變化會伴隨帶來大量的能量交換現象，沸騰、凝結、冰塊融化等都屬於物質相變化的範疇，水氣的結露現象也是一個相變化現象，因此預期會有較大的能量交換現象。 本研究將以Al-1050、SUS304、銅與鈦作為基板，以數種表面處理方式對其表面進行改質，來觀察表面改質對於表面結露現象的影響，改質後的表面藉由顯微鏡表面觀測、接觸角量測、表面熱傳量計算、露點溫度判斷來做討論。 將表面處理之樣品與原始表面之樣品進行梯次降溫之實驗，於梯次降溫的過程中，以資料擷取器紀錄每次降溫狀態達到穩態時之溫度數據，利用水套內部溫度梯度推算表面之熱傳量，並且於表面溫度降溫至接近露點溫度時，減少降溫幅度，以得到較為準確之露點溫度。數種樣品當中，以陽極處理之Al-1050樣品對於結露溫度之改變最為顯著，表面產生結露之溫度較理論露點溫度下降2.5oC，陽極處理之Al-1050樣品產生滴式結露，其他處理之樣品則產生膜式結露之現象。 Al-1050鋁合金行陽極處理的過程，以4 oC之0.3 M草酸作為電解質，30V外加電壓下進行第一步驟陽極處理3小時，再以50%之磷酸去除不規則氧化鋁，留下排序整齊之孔洞於表面上，再以25oC之0.3 M草酸作為電解質，30V外加電壓下進行第二步驟陽極處理1.5小時以增加氧化層厚度，可製作出孔徑30~50 nm結構較完整且均勻之陽極處理表面。 由於液滴凝結量較少，因而產生的潛熱量不大，於本量測系統中不能看到表面之熱傳量因液滴凝結而產生變化之現象

The phase-change process is going to absorb or release a large amount of energy. Boiling, condensing and melting are all phase-change process. Dewing of wet air is also a phase-change process, so is expected to transfer a large amount of energy. In this study, Al-1050, SUS304, Copper and Titanium are the substrates. Several surface treatments were conducted on the substrates. The modified surfaces by observing with microscope, measuring contact angle, calculating surface heat transfer rate and dew point, were discussed in this work. The modified surface and the original surface were conducted stepwise cooling experiments. A data logger recorded the steady state temperature of every cooling step during the stepwise cooling experiment, and the temperature gradients in the cooling system were to calculate the heat transfer rate of each surface. When the surface temperature was close dew point, the temperature difference of every cooing step was reduced to get an accurate dew point. The maximum difference between the surface dew point and theoretical dew point of surrounding, 2.5 oC was achieved by the anodic aluminum oxide on the Al-1050 surface. The AAO- Al-1050 surface showed the formation of DWC in the cooling experiment, but the other surfaces with different surface treatments showed the formation of FWC in the cooling experiment. To preparation of AAO Surfaces on the commercial aluminum alloy 1050, the first anodization step applied constant anodization voltage of 30 V in electrolytes of 0.3 M oxalic acid at 4 oC. After 3 hours of first anodization, the alumina layer was removed from the aluminum substrate using a mixture of 50 wt % phosphoric acid, leaving a highly ordered concave pattern on the surface of the samples. Following, a second anodization step applied constant anodization voltage of 30 V in electrolytes of 0.3 M oxalic acid at 25 oC. for 1.5 hours in order to increasing the film thickness. After the two-step anodization process, the AAO- Al-1050 surface with pore diameter about 30 nm to 50 nm was manufactured. There were very few droplets dewing on the surface, so the measured heat transfer rate didn’t increase when dewing occurred.