應用噴流衝擊與漸擴型鰭片陣列改善介電溶液 HFE-7100於飽和狀態之池沸騰熱傳

Abstract

本研究旨在探討應用正向噴流及鰭片設計對於池沸騰熱傳性能之提升。實驗共使用五種的鰭片陣列表面: 光滑表面、一致型圓柱表面、一致型魚鱗表面、漸擴型圓柱表面以及漸擴型魚鱗表面。每種表面均為垂直擺放下，在距其1公分處對準表面中心給予3.59 m s-1及1.68 m s-1兩種速度的正向噴流。 實驗結果顯示，沸騰曲線可分為四個階段；第一階段的主要熱傳機制為單相強制對流，表面尚未有成核點啟動；第二階段中加熱表面上開始有成核點啟動，並且成核點數量隨熱通量增加而迅速提升。在沸騰的第一、二階段中，於所有表面應用噴流都能透過低溫流體的強制對流來增強沸騰熱傳性能。第三階段中加熱表面上的成核點已完全啟動，加熱表面上被顆粒氣泡所覆蓋，透過噴流能將表面中心的氣泡排除，並補充低溫流體至表面上，故沸騰熱傳係數於此階段會隨壁面過熱度的增加而提升。然而，此階段噴流會破壞表面上的熱邊界層，進而抑制氣泡成核，提高壁面過熱度。這個現象在五個表面中對光滑表面的影響最為明顯，這是因為光滑表面的流阻最低，噴流在表面上擴散流動的範圍最廣，達到最大的抑制區。在第四階段中，表面上的氣泡互相合併為大型氣團，妨礙低溫流體補充至表面上。此階段噴流衝擊逐漸無法穿透表面上的厚氣泡層並將低溫流體補充至表面，沸騰熱傳係數因此發生反轉隨壁面過熱度上升呈下降趨勢。 在相同表面下，比較噴流速度的差異可發現在第一階段中，較高的噴流速度能延後進入第二階段，這是因為其帶來更多的低溫流體提升強制對流，大幅降低壁面過熱度。而在第三階段中，鰭片陣列表面也會發生噴流破壞熱邊界層，進而抑制成核點的現象。此階段低噴流速度的沸騰曲線較高噴流速度的沸騰曲線稍微左移，這是因為高噴流速度射出的流體具有較高的動能，噴流在表面上擴散流動破壞熱邊界層的範圍較大，較大的氣泡抑制區造成較高的壁面溫度。進入第四階段後，此時表面的氣泡層過厚，而高噴流速度穿透的能力較佳，高噴流速度沸騰熱傳表現因此超越低噴流速度。 此外，在相同噴流速度下，與光滑表面相比，鰭片陣列表面皆能大幅提升熱傳性能。這是因為鰭片陣列表面的表面積及表面粗糙度大幅增加，能提供更多的氣泡成核點。圓柱及魚鱗兩種鰭片形狀對沸騰現象的影響也有所不同；將魚鱗鰭片採用漸擴型排列，其沸騰熱傳係數能較一致型排列提升3%~15%；若使用圓柱設計在沸騰曲線的第三階段中，一致型排列卻優於漸擴型排列，這是因為漸擴型圓柱表面流阻較低，使噴流在壁面擴散的流動範圍較一致型圓柱表面大，進而破壞熱邊界層，抑制較多成核點。進入第四階段後，由於表面被大量氣團覆蓋，此時漸擴型圓柱表面的低流阻反而有助於排除氣泡，熱傳性能於此時超越一致型圓柱表面。

This study aims to investigate the enhancement of heat transfer performance in pool boiling by the use of jet impingement and pin-fin designs. Five types of pin-fin array surfaces were utilized in the experiment: smooth surface, uniform cylindrical surface, uniform fish scale surface, diverging cylindrical surface, and diverging fish scale surface. The surface was placed vertically, with a normal jet injected from 1 cm away of the surface center with two velocities: 3.59 m s-1 and 1.68 m s-1. We found that the boiling curve could be divided into four regimes. In the first regime, heat transfer mechanism was still dominated by single-phase forced convection, with no active nucleation on the surface. In the second regime, active nucleation started to emerge on the heated surface, and their number rapidly increased with increasing heat flux. During these first two regimes, jet impingement was able to enhance boiling heat transfer on all surfaces through effectively delivering the cold fluid. In the third regime, nearly all nucleation sites were activated, and the heated surface was covered with discrete bubbles. Jet impingement played a crucial role by removing these bubbles from the center of the surface and replenishing with cold fluid. This process enhanced the boiling heat transfer coefficient as wall superheat increased. However, jet impingement also disrupted the thermal boundary layer, inhibiting bubble nucleation. This negative impact was particularly severe to the smooth surface. Due to its low flow resistance, the jet could spread more widely and lead to large bubble suppression region. In the fourth regime, bubbles coalesced into large vapor slugs, preventing cold fluid from reaching the surface. Jet impingement became less important and the heat transfer coefficient decreased with increasing wall superheat. Jet with a higher velocity was found to delay the transition to the second regime. This was because the forced convection was enhanced and wall superheat was reduced accordingly. In the third regime, the boiling curve of lower jet velocity was slightly shifted to the left for the pin-fin array surface. This was ascribed to a smaller bubble suppression region produced by the slower jet. On the other hand, higher jet velocity was more capable of penetrating the thicker bubble layer in the fourth regime, leading to better heat transfer performance. Comparing to the smooth surface, pin-fin array surfaces performed much better in boiling heat transfer. The extended surfaces and more nucleation sites both contributed to the improvement. Moreover the diverging arrangement of fish scale pin-fin surface could increase the heat transfer coefficient by 3% to 15%. The opposite trend was found for cylindrical pin-fin surface. In the third regime, the diverging arrangement allowed the jet to spread more widely over the cylindrical surface, disrupting the thermal boundary layer and inactivating many nucleation sites. This became favorable in the fourth regime, as the large bubble slugs could be more effectively removed over the diverging cylindrical surface.