三維親疏水混合微米銅柱陣列之池沸騰熱傳

Abstract

以往之池沸騰研究中，多受限於臨界熱通量，導致沸騰過程受到阻礙;抑或是高熱傳系數，但卻導致臨界熱通量的提早發生，本研究利用三維親疏水混合微米銅柱結構，研究為了解決臨界熱通量及熱傳係數無法同時提高的問題，本研究利用三維親疏水混合微米銅柱，研究出了高效的池沸騰性能。於文獻回顧中可以發現，能有效提升沸騰效能的方法有兩種，第一種為改變樣品表面的濕潤性，使樣品能夠有更強的吸水力或是改變氣泡動態機制;第二種為利用微結構，使樣品有更多的散熱面積，同時藉由微結構提升毛細力，本研究之三維親疏水混合微米銅柱結構就基於以上兩點的機制而設計，利用微米柱結構搭配親水改質，最大化的提升臨界熱通量，再於親水微米柱結構上局部塗覆疏水結構，製造出三維親疏水混合結構，從而有更好得氣泡成核、脫離特性，以提昇熱傳係數。本研究使用銅塊加熱樣品，再利用安裝於銅塊上的熱電偶，量測溫差計算熱通量以及表面溫度，同時使用高速攝影機拍攝氣泡動態，再利用影像分析成核點密度、氣泡脫離頻率及氣泡脫離直徑。經實驗結果顯示，微結構搭配親水層能大幅提升臨界熱通量。其中，臨界熱通量以HP-1.0mmPA的提升效果最大，其臨界熱通量為374.17 ± 3.68 W/cm²，HP-1.0mmPA相對於Plain Cu能夠提升250%。再來是複合親疏結構的影響，本研究將疏水材料局部塗覆於柱頂，由實驗結果表明，複合親疏結構與全親水結構的臨界熱通量保持一致，但熱傳係數卻有顯著的提升，實驗結果顯示，Hybrid-1.0mmPA之臨界熱通量及熱傳係數分別為368.19 ± 5.86 W/cm²及28.02 ± 3.24W/cm2-K，熱傳係數相對於HP-1.0mmPA提升了197%;相對於Plain Cu則能提升456%。其中熱傳係數又以Hybrid-1.5mmPA的提升效果最佳，其臨界熱通量及熱傳係數分別為343.95 ± 0.38 W/cm²及31.34 ± 0.76 W/cm2-K，熱傳係數相對於Plain Cu提升了522%。由實驗結果能夠發現，單純的親水表面改質，雖然能夠對臨界熱通量帶來不錯的提升效果，但於熱傳係數的表現中卻不佳，因此本研究的主要目標在於，利用兩種不同的改質方法於單一材料中，於結果表明，此方法能夠大幅提升臨界熱通量以及熱傳係數，其親水改質對於延緩臨界熱通量的主要機制在於，可以增強材料表面的濕潤性，使液體能夠在高熱通量時，吸入更多液體潤濕表面，然而，親水表面因有較大的表面能，這導致氣泡需要在更高的表面溫度下才能夠產生成核現象，這使親水結構於低熱通量時有較低的熱傳係數，而本研究利用複合親疏水結構克服這項問題，於柱頂的疏水結構因低的表面能，能夠於低表面溫度下就開始成核反應，使熱傳係數相對於親水結構有大幅得提升。由研究結果能夠得知微結構搭配親水改質能夠大幅提升臨界熱通量，於親水結構的基礎上再加入疏水塗覆能夠改善親水結構低熱傳係數的問題，並達到氣、液分流的效果。

In previous studies on pool boiling, critical heat flux (CHF) has often been a limiting factor, hindering the boiling process. Alternatively, achieving a high heat transfer coefficient (HTC) has often led to an early CHF. This study employs three-dimensional hybrid hydrophilic and hydrophobic copper micropillars to address the challenge of simultaneously enhancing CHF and HTC. The goal is to achieve efficient pool boiling performance. The literature review identifies two main methods for effectively enhancing boiling performance. The first method involves altering the wettability of the sample surface to increase its water absorption capability or improve bubble dynamics. The second method utilizes microstructures to provide a larger heat dissipation area and enhance capillary forces. The objective of this study is to combine these two approaches in pool boiling research. The three-dimensional mixed hydrophilic and hydrophobic copper micropillar structure is designed based on these two mechanisms. By combining micropillar structures with hydrophilic modifications, the study aims to maximize the critical heat flux. Additionally, by partially coating the hydrophilic micropillar structures with hydrophobic materials, a three-dimensional mixed hydrophilic-hydrophobic structure is created. This design enhances bubble nucleation and departure characteristics, thereby improving the heat transfer coefficient. Consequently, this approach is expected to enhance both the CHF and HTC. A copper block is used to heat the samples, with thermocouples mounted on the copper block to calculate heat flux and surface temperature. A high-speed camera captures bubble dynamics, and image analysis is employed to determine nucleation site density, bubble departure frequency, and bubble departure diameter. Experimental results show that microstructures combined with hydrophilic layers significantly enhance CHF. The HP-1.0mmPA sample achieved the highest CHF of 374.17 ± 3.68 W/cm², representing a 250% increase compared to plain copper (Plain Cu). For the impact of composite hydrophilic-hydrophobic structures, hydrophobic material was partially coated on the tops of the pillars. The results indicate that while the CHF of the composite structure is comparable to that of the fully hydrophilic structure, the HTC is significantly improved. Specifically, the Hybrid-1.0mmPA sample exhibited a CHF of 368.19 ± 5.86 W/cm² and a HTC of 28.02 ± 3.24 W/cm²-K, which is a 197% increase compared to HP-1.0mmPA and a 456% increase compared to Plain Cu. The Hybrid-1.5mmPA sample achieved the best enhancement in the HTC, with a CHF of 343.95 ± 0.38 W/cm² and a HTC of 31.34 ± 0.76 W/cm²-K, representing a 522% increase compared to Plain Cu. The results indicate that while purely hydrophilic surface modifications can significantly enhance CHF, they do not substantially improve the HTC. Therefore, the primary objective of this study is to use two different modification methods on a single material. The results show that this approach can significantly enhance both CHF and the HTC. The main mechanism by which hydrophilic modification delays CHF is by enhancing the wettability of the material surface, allowing more liquid to wet the surface at high heat flux. However, the high surface energy of hydrophilic surfaces requires higher surface temperatures for bubble nucleation, resulting in a lower HTC at low heat fluxes. This study overcomes this issue by using composite hydrophilic-hydrophobic structures. The low surface energy of the hydrophobic structures on the pillar tops allows nucleation at lower surface temperatures, significantly enhancing the HTC compared to purely hydrophilic structures. The study concludes that microstructures combined with hydrophilic modifications can significantly enhance CHF. Adding hydrophobic coatings to hydrophilic structures can address the low HTC issue and achieving liquid-vapor separation.