應用向上平行噴流陣列與漸擴型鰭片陣列提升 HFE-7100 在飽和沸騰下之熱傳性能與氣泡可視化分析

Abstract

本研究旨在探討於飽和沸騰條件下，應用向上平行噴流陣列結合壁面結構設計，以提升 HFE-7100於垂直加熱壁面之熱傳性能。實驗中，平行噴流陣列設置於加熱壁面底部，噴流出口與壁面垂直的距離 5 mm處。實驗設計涵蓋兩種噴流速度，分別為vPJA = 1.04 m s-1 和 vPJA = 3.91 m s-1，以及五種壁面結構，分別為光滑壁面、一致型圓柱、一致型魚鱗、漸擴型圓柱與漸擴型魚鱗鰭片陣列。 由研究結果可將沸騰劃分為四個階段：第一階段為單相強制對流；第二階段成核點陸續啟動，逐漸轉為雙相強制對流；第三階段則為完全發展的核沸騰，熱傳係數隨壁面熱通量增加而迅速提升；在第四階段中，氣泡開始堆積並於加熱壁面上滑動，形成波浪狀液氣界面，導致沸騰熱傳係數隨壁面過熱度下降並最終達到臨界熱通量。本研究發現，成核點開始啟動所需之壁面過熱度不受噴流速度影響，但其所對應之壁面熱通量則隨噴流速度增加而上升。但當熱通量進一步提升至成核點完全啟動的第三階段時，高速噴流下的壁面過熱度較低速噴流高出約 0.7°C 至 4.2°C，顯示高速噴流對熱邊界層具有更強壓制效果，成核點需較高的壁面過熱度才能維持完全啟動。 在相同噴流速度下，使用鰭片陣列皆能明顯提升熱傳性能與臨界熱通量，特別是漸擴型魚鱗鰭片陣列的熱傳性能最佳。由於該結構具有順應氣泡排除方向的流線型設計，且鰭片間距逐層擴張的設計，能配合平行噴流陣列有效引導氣泡向上排除，使其最高沸騰熱傳係數較光滑表面提升約 58%。值得注意的是，即使於低速噴流條件下，搭配漸擴型鰭片設計之平行噴流陣列可達到 24 W cm-2 的臨界熱通量，優於傳統單噴流衝擊的22 W cm-2。

This study investigates the enhancement of saturated boiling heat transfer of HFE-7100 from a vertical surface by employing an upward parallel jet array and pin-fin arrays designs. Two different jet velocities are used: vPJA = 1.04 m s-1 and vPJA = 3.91 m s-1. The pin-fin array has five different designs: smooth surface (SMO), uniform cylinder (UC), uniform fish scale (UFS), diverging cylinder (DC) and diverging fish scale (DFS). From the results, boiling can be divided into four regimes. In regime I, heat transfer is dominated by single-phase forced convection. In regime II, nucleation sites progressively activate, transitioning to two-phase forced convection. In regime III, all nucleate boiling sites are activated, and the heat transfer coefficient rapidly increases. In regime IV, vapor bubbles start to accumulate and slide along the heated surface, forming wavy liquid-vapor interfaces that deteriorate heat transfer, eventually leading to critical heat flux (CHF). As jet velocity increases, the heat flux corresponds to onset of nucleate boiling (ONB). However, the wall superheat at the ONB is not affected by jet velocity. In regime III, faster jet results in higher wall superheat though, approximately 0.7°C to 4.2°C higher than that of slower jet. This indicates that the high shear produced by the faster jet strongly suppresses the thermal boundary layer and delays full activation of nucleation sites, leading to a higher required wall superheat. All pin-fin array surfaces can significantly enhance the boiling heat transfer and CHF. In particular, the design of diverging fish scale can facilitate bubble removal, attributed to its streamlined geometry and increasing fin spacing. The diverging pin-fin array can still achieve a CHF of 23~24 W cm-2 under slower jet.