交錯潤濕性表面於紅銅圓管表面之池沸騰熱傳研究

Abstract

本研究在外直徑25mm的紅銅圓管上，以溶膠凝膠法搭配膠帶作為遮罩的方式製備出異質潤濕性表面，此異質潤濕性表面上共有兩種不同的潤濕性，一種為未經過改質的紅銅表面，其經過烘烤的步驟後，靜態接觸角為90°左右，若未經過烘烤則表面的靜態接觸角則為77°，而另一種為經過二氧化矽奈米粒子改質過後的超親水表面，其接觸角小於10°，在紅銅圓管上，以交錯式的方式，分別出現此兩種表面，稱為交錯式潤濕性表面，將完成製備的紅銅表面黏合於實驗設備後進行池沸騰的研究。 本研究主要有兩種變因來探討其個別對於池沸騰的影響，包括改變超親水表面於總體散熱面積的比例以及改變交錯式潤濕性界面數兩種，而在進行交錯潤濕性表面的研究前，首先以未改質的紅銅表面來進行研究，發現量測孔洞的位置會影響其溫度的量測，並由實驗結果可以得知，傾斜角越大的表面擁有較低的表面過熱度，並以此為基礎，若將實驗表面分為上、下兩個半圓進行探討，發現下半圓因受到表面阻擋的關係，其氣泡提前邁入聚合的階段，比起上表面擁有更佳的沸騰熱傳矽數，而在異質潤濕性的表面的研究中，首先，先固定交錯式潤濕性的界面數，探討超親水面積比例的影響，由實驗結果發現，其界面數在7條以下時，不同的超親水面積比例其沸騰熱傳曲線並沒有明顯的差異，而在界面數為15條的情況下，30%及50%的沸騰曲線相似，然而70%的沸騰曲線由於其散熱面上的氣泡除了可以與相同界面上氣泡進行聚合外，也可以與鄰近界面上的氣泡合併，因此沸騰曲線向左偏移並擁有較佳的沸騰熱傳表現，另外，交錯式潤濕性界面數的研究中，將固定超親水面積的比例，探討界面數的增加對於沸騰熱傳有何種程度的影響，由實驗結果可以發現，交錯潤濕性表面比起未改質表面皆擁有更佳的沸騰熱傳表現，且在相同的熱通量下，熱傳係數將隨著界面數的增加而提升。

This study used sol-gel method and tape as a mask to fabricate the heterogeneous wettability surface on a copper tube with 25mm in diameter. There are two wettabilities on the heterogeneous surface. One is plain surface with its contact angle equal to 90° after baking process. However, it is worth mentioning that the contact angle of the plain surface without baking is equal to 77°. The other one is the superhydrophilic surface modified with silica nanoparticles and the contact angle of the superhydrophilic surface is smaller than 10°. On the copper surface, the strip with one wettability will follow with the strip with the other wettability to form an “interlaced wettability” surface. This study performed pool boiling experiments with surfaces of interlaced wettability. There are two parameters in this study, including the ratio of superhydrophilic surface area and the number of the interlaced line. Before performing experiments with interlaced wettability, this study investigated the effect of measuring location on a copper cylinder. From present results, wall superheat increases with the decrease of inclined angle. On this basis, the heating surface was split into two surfaces. One is the upper surface facing up and the other one is the lower surface facing down. Experimental results showed that heat transfer coefficient of the upper surface is higher than it of the lower surface. Move on to experimental results of interlaced wettability. First, the number of the interlaced line was fixed to examine the effect of the ratio of superhydrophilic surface area. Experimental results showed that the ratio of superhydrophilic surface area won’t affect pool boiling curves when the number of interlaced line is lower than 7. However, at the condition of 15 interlaced lines, although the pool boiling curve of 30% case was similar to it of 50%. The pool boiling curve of 70% case had a shift to left because bubbles on the interlaced line could not only merge with bubbles beside them but also combine with bubbles in front of them. Due to this phenomenon, the surface of 70% case could have better heat transfer performance. Second, the ratio of superhydrophilic area was fixed to examine the effect of the number of interlaced line. From experimental results, wall superheat decreased with the increase of number of interlaced line among 30%、50% and 70% conditions.