雷射表面改質對核沸騰熱傳之影響

Abstract

核沸騰區間作為最有效的熱傳方式之一，在溫度變化很小的情況下能傳遞大量的熱量，因此在工業上被廣泛地運用，也是日常生活中常見的現象。核沸騰的機制非常複雜，其熱傳效能可由三個參數來表徵：核沸騰起始點(Onset of Nucleate Boiling, ONB)的過熱度、熱傳遞係數(Heat Transfer Coefficient, HTC)、臨界熱通量(Critical Heat Flux, CHF)，本研究將著重前兩者，並以共三種方式對表面進行改質：三種熱壓銅網結構在純銅表面、一種旋轉塗佈奈米流體薄層結構和四種雷射表面紋理化(Laser Surface Texturing, LST)結構在鋁6061合金表面，以去離子水為工作流體，探討其在常壓飽和狀態下池沸騰熱傳的影響，最後以雷射表面紋理化結構透過實驗驗證Hsu所提出預測孔徑對ONB影響的模型。 實驗結果顯示，相較於平滑表面，三種熱壓銅網結構和一種旋轉塗佈奈米流體薄層結構皆延後ONB過熱度，並抑制了熱傳，下降HTC。四種雷射表面紋理化皆提前ONB過熱度且提升HTC，增強了熱傳。四種結構提前ONB過熱度2.3-6.2 ℃，並在熱通量為500 kW/m2時，四種結構HTC提升90-300 %。 雷射表面紋理化結構在圓心間距400 μm下，部分驗證了Hsu所提出的預測孔徑對ONB影響的模型，實驗所得成核範圍曲線整體趨勢與預測模型的趨勢相似，實驗所得臨界半徑與一些學者預測熱邊界層厚度所預測臨界半徑相近，而在圓心間距1000、2000下，並未發現類似Hsu預測模型的影子。

The nucleate boiling regime is widely used in industrial applications as one of the most efficient modes of heat transfer, in which large amounts of heat can be dissipated from a heated surface with small changes in temperature, and is a common phenomenon observed in our daily lives. The mechanism of nucleate boiling is very complex, and its heat transfer efficiency can be characterized by three parameters: Onset of Nucleate Boiling (ONB) superheat, Heat Transfer Coefficient (HTC), and Critical Heat Flux (CHF). In this study, we will focus on the first two, and modify the surfaces in three ways: three copper mesh structures by hot-pressing on pure copper surfaces, a thin film structure by spin-coating nanofluid and four Laser Surface Texturing (LST) structures on the surface of aluminum 6061 alloy, with deionized water as the working fluid, to investigate the effect of pool boiling heat transfer under the saturated state of atmospheric pressure. Finally, the semi-theoretical model proposed by Hsu in 1962 provides the size range of active nucleation cavities. This study will attempt to validate the model experimentally. The experimental results show that compared to the smooth surface, three copper mesh structures and a thin film structure delay the superheat at ONB and decrease the HTC. Four laser surface texturing structures advance the superheat at ONB by 2.3-6.2 °C, and increase the HTC by 90-300% at heat flux of 500 kW/m2. The model proposed by Hsu for predicting the size range of active nucleation cavities is partially validated at a cavity pitch of 400 μm. The overall curve trend of the experimentally obtained nucleation range curve is similar to that of the predicted model, and the experimentally obtained critical radius is similar to that one predicted by some scholars for predicting thermal boundary layer thickness. However, in the case of cavity pitches of 1000 and 2000 μm, there is no trace of the Hsu model.