利用可撓性細銅線、珠子和液體過熱於加熱表面上來增強流動沸騰熱傳

Abstract

本篇論文主要分為三個主題，分別是提出兩種具有主動特性之被動式熱傳增強的方法，以及一種測試段入口液體過熱的方式來增強介電液FC-72流經至一加熱水平銅板 ( 10 x 10 mm2 ) 之流動沸騰熱傳。其中，具有主動特性之被動式熱傳增強的兩種方法分別為：可撓性細銅線及輕巧的塑膠珠子。首先，第一部分的研究主題是於加熱表面上放置可撓性細銅線、兩端固定於加熱面邊緣且擺放的位置與流動方向互相垂直，藉由FC-72液體衝擊可撓性細銅線來增強FC-72飽和態流動沸騰熱傳。其中，實驗參數分別為：固定介電液FC-72質量流率為300 kg/m2s及細銅線之間的間距為0.1 cm、加熱量變化為0.1至11 W/cm2、細銅線線徑大小為79至254 μm、細銅線與加熱表面之間的高度變化為0至1.0 mm及細銅線長度變化為10至12 mm。實驗結果得知，施加最粗的細銅線緊貼於加熱表面上可以有效地增強30%飽和態流動沸騰熱傳。另外，從氣泡特徵發現，主要造成熱傳增強的原因是靠近細銅線附近的氣泡脫離直徑變大及脫離頻率變慢。 接著，第二部分的研究內容是探討當介電液FC-72流經塑膠珠子時，因珠子上表面受到流體的慣性力衝擊、下表面受到流體對加熱面造成的剪應力以及加熱面上的熱浮力作用，造成珠子受力不平衡，使得珠子於加熱表面上呈現不規則旋轉，進而增強飽和態及次冷態流動沸騰熱傳。其中，輕巧的塑膠珠子本身內部中空，類似圓環，且串於細銅線上，其旋轉方向與FC-72流動方向互相垂直。此外，在飽和態及次冷態流動沸騰熱傳中所探討之實驗參數分別為：改變介電液FC-72質量流率、次冷度的變化、加熱量的多寡、加熱表面上細銅線的數目、每條細銅線上珠子的顆數、珠子與加熱面之間的高度，以及細銅線與珠子的大小。實驗結果得知，利用不規則旋轉珠子於加熱表面上來增強流動沸騰熱傳，在飽和態流動沸騰中最高有55%的增加，而在次冷態流動沸騰最高也有50%的增幅。主要的原因是在加熱表面上的脫離氣泡受到不規則旋轉珠子的影響，可以快速地沿著珠子外表面迅速脫離，同時，靠近珠子周圍的新鮮FC-72主流液體，因流體黏滯效應的關係而快速衝向於加熱表面，進而增強飽和態及次冷態流動沸騰熱傳。另外，不規則旋轉的珠子也會造成初始沸騰的壁過熱度大幅度的降低，此效應對於應用在電子冷卻技術上有莫大的幫助。 最後一個部分的研究內容是直接將測試段入口處的FC-72液體溫度加熱至過熱態，但此時入口壓力還是維持在飽和壓力下來探討液體過熱對於流動沸騰熱傳之影響。其中，在測試段上游處包覆著輔助加熱器來控制FC-72液體流經測試段入口時的液體過熱溫度。實驗結果得知，入口液體溫度輕微地提升對於流動沸騰熱傳有明顯的增強效果。而且，當液體過熱度增加至1.2℃和1.5℃時，流動沸騰熱傳增強的幅度約有100%之成效。主要的原因是入口FC-72液體溫度過熱，造成加熱面上的熱邊界層內液體溫度更熱，使得氣泡脫離時直徑變大，脫離頻率變快，進而增強流動沸騰熱傳。最後，我們也分別將可撓性細銅線和不規則旋轉珠子與入口液體過熱作結合，此結合顯示可進一步提升流動沸騰熱傳之效果。

An experimental study is carried out here to explore possible enhancement of FC-72 flow boiling heat transfer over a small horizontal heated copper plate by two different active-like passive augmentation methods and by slight inlet liquid superheating. In the first part of the study, movable fine copper strings are installed above the plate. Specifically, parallel strings of uniform size and pitch with their ends only fixed at the plate edges are placed normal to the upstream flow direction. In this part of the experiment, the imposed heat flux is varied from 0.1 to 11 W/cm2, the diameter of strings from 79 to 254 μm, string-heated surface separation distance from 0 to 1.0 mm, and the length of the strings from 10 to 12 mm with the pitch of the strings fixed at 1.0 mm for the FC-72 mass flux maintained at 300 kg/m2s. In the second part of the study, small plastic beads like thick circular rings are mounted additionally on the fine copper strings, in addition. The beads can be irregularly rotated by the shear force from the boiling flow and by the buoyancy of the bubbles. In the test, the chosen reference beads have average outer and inner diameters of 1.45 and 0.65 mm, respectively, and thickness of 1.0 mm. The average weight of a bead is 0.0038 g. Straight parallel strings of the beads at selected pitch and height are placed above the heated plate normal to the incoming upstream flow with their ends fixed at the rig installed near the plate edges. The effects of the relevant parameters on the saturated and subcooled FC-72 flow boiling heat transfer enhancement, including the imposed heat flux, string pitch, number of beads on each wire, and bead-plate separation distance, are examined in detail. In the third and final part of this study, the inlet liquid superheating is controlled by the auxiliary heater which is installed at the upstream diverging portion of the channel. Meanwhile, the pressure in test section is maintained at saturated state. Besides, combination of the installation of copper strings and beads with the inlet liquid superheating to enhance boiling heat transfer is also examined. The experimental data obtained from the first part of the study for the installation of the copper strings show that installing the fine copper strings above the heating surface can enhance the FC-72 flow boiling heat transfer coefficient up to about 30% over that for a bare surface for a well selected set of the experimental parameters. Besides, the string size and length exhibit nonmonotonic effects on enhancing the boiling heat transfer due to complex influences of the strings on the bubble dynamics near the heating surface. Moreover, the presence of the strings is found to increase the size of nucleation bubbles and active bubble nucleation site density but meanwhile impede the bubble departure from the boiling surface. In the second part of the study, the experimental results indicate that the bubble pumping away from the heated surface from the rotating beads can effectively enhance the boiling heat transfer in the saturated and subcooled flows. Besides, the enhancement in the boiling heat transfer is more pronounced when the beads are placed closer to the plate at the medium string pitch. Moreover, there exists an optimal number of beads threaded on each wire. The best enhancement in the saturated boiling heat transfer coefficient in this study can be as high as 55 % for a suitable selection of the experimental parameters. The corresponding best subcooled boiling heat transfer enhancement is 50%. Moreover, the rotating beads can substantially reduce the wall superheat required for incipient boiling in both saturated and subcooled flows. This is particularly beneficial for electronics cooling. Finally, it is noted from third part of the present study that a slight inlet liquid superheating can be very effective in enhancing the FC-72 boiling heat transfer. The significant enhancement is found to mainly result from the increasing the bubble departure size and frequency at increasing liquid superheating. The best boiling heat transfer enhancement can be around 100% for the inlet liquid superheating of 1.2℃ and 1.5℃.