單反摺微米柱陣列之沸騰熱傳

Abstract

隨著時代進步，工業技術不斷發展，電子產品的效能不斷提高，這也意味著它們產生更多的熱量。熱管理也因此變得非常重要，以預防高功率的電子設備損壞。在眾多的散熱方法中，池沸騰熱傳被證明是一種非常有效的方式。它的原理是冷卻液在散熱過程中相變，相變過程帶走了大量的潛熱，實現快速的散熱效果。 本研究是基於單反摺微米柱陣列複合結構表面的池沸騰傳熱性能進行實驗，其表面具有單反摺微米柱陣列區域和光滑區域，單反摺微米柱陣列頂部有疏水塗層以利於成核，光滑區域則是降低水流入的阻力幫助補水。此微米柱陣列複合結構表面預計能將液氣分流，避免產生流體不穩定性，以突破熱通量的限制。 實驗中使用高速攝像機和紅外線熱樣儀來捕捉不同熱通量下的沸騰現象並進行研究。高速攝影機用於拍攝樣品的側視圖，以確認氣泡的成核位置及情況。紅外線熱像儀用於觀測樣品表面的溫度分布，並能夠分析氣泡的成核密度以及液體和氣體在表面上的比率。 在本研究中，我們進行了一系列實驗，以評估不同樣品表面結構對熱傳導性能的影響。這些樣品包括二氧化矽平表面（SiO2）、微米柱陳列樣品（Pillar）、單反摺微米柱陣列樣品（SR）以及單反摺微米柱陣列並在柱頂蒸鍍鐵氟龍的樣品（SRT）。結果表明，結構化樣品的性能在多個方面顯著優於平面樣品，這為熱管理領域提供了重要的見解。首先，透過對熱傳係數（HTC）的分析，我們觀察到具有結構的樣品(Pillar、SR 和SRT)具有更高的HTC，像是分別Pillar 的HTC 為5.34 W/cm2-K，SR 的 HTC 為 5.92 W/cm2-K 和 SRT 的 HTC 為 6.37 W/cm2-K，都遠高 於 SiO2 平表面的HTC 為 2.00 W/cm2-K，其中具有鐵氟龍來料的 SRT 又是其中之最。這反映了具有結構的樣品對於熱量傳遞的顯著增強，這種增強可以歸因於結構化表面會增強成核效應以及疏水性材料的存在，從而減少了表面過熱度。 其次，我們研究臨界熱通量（CHF），即表面所能承受的最大熱通量。結果顯示，具有結構的樣品其 CHF 明顯高於 SiO2 平表面的樣品。其中，Pillar 的 CHF為119.62 ± 1.40 W/cm2、SR 的CHF 為143.79 ± 5.12 W/cm2 和SRT 的CHF 為141.71 ± 3.16 W/cm2，而SiO2 平面樣品的 CHF 僅為 87.89 ± 3.4 W/cm2。 這種性能提升可歸因於結構化表面的平滑水道，有助於限制氣泡的脫離直徑，從而延遲了氣泡覆蓋整個表面的時間，使其能夠承受更多的熱通量，而 SR 與 SRT 樣品的 CHF 高於Pillar 樣品，推測其原因為單反摺結構內能夠在高溫下儲存更多的水，使液膜增厚，進而延後樣品表面燒毀。此外，本研究採用了基於流體不穩定性的臨界熱通量模型，用於預測CHF，並發現實際觀察到的CHF 與理論值相符，這進一步驗證該模型的準確性。 總之，我們的研究結果強調了表面結構和疏水性材料在熱傳導和熱管理中的潛在重要性。這些發現為設計和開發高效能熱管理系統提供了有力的指導，並為未來的熱管理研究提供了新的方向。

As time progresses and industrial technology advances, electronics are becoming more compact, which means that the power density is getting higher. Thermal management has, therefore, become very important to prevent damage to high power electronic devices. Among the many heat dissipation methods, pool boiling heat transfer has proven to be very effective. It is based on the principle that the cooling liquid changes phase, carrying away the latent heat and realizing rapid heat dissipation. The present work studies the pool boiling heat transfer performance of the surface consisting of a single-reentrant micropillar array, with the top of the single-reentrant micropillar array having a hydrophobic coating to facilitate nucleation and the smooth region to reduce the resistance of the water inflow to help recharge the water. A high-speed camera and an infrared camera were used to capture the boiling phenomenon at different heat fluxes. The high-speed camera was used to capture boiling dynamics. The infrared camera was used to measure the temperature distribution on the sample surfaces. The images were then used to analyze the nucleation density and the vapor-to-liquid ratio on the surface. Three samples included a flat silicon dioxide surface (SiO2), a micrometer column array sample (Pillar), a single-reentrant micrometer column array sample (SR), and a single-reentrant micrometer column array sample with Teflon coating on the top of the column (SRT) were studied in this work. The results show that the structured samples significantly outperform the flat samples in a number of ways, hich provides important insights into the field of thermal management. First, by analyzing the heat transfer coefficients (HTC), we observed that the structured samples (Pillar, SR, and SRT) had higher HTCs, such as 5.34 W/cm2-K for Pillar, 5.92 W/cm2-K for SR, and 6.37 W/cm2-K for SRT, respectively, which were much higher than 2.00 W/ cm2-K on the flat SiO2 samples. The SRT with the Teflon coating gave the highest HTC among them. This reflects the significant enhancement of heat transfer for structured samples, which can be attributed to the enhanced nucleation effect of structured surfaces as well as the presence of hydrophobic materials, which reduces surface overheating. Secondly, we investigated the critical heat flux (CHF) of the surfaces, which is the maximum heat flux that a surface can withstand without experiencing burnout. The results show that the CHF of the samples with structures is significantly higher than that of the SiO2 surface. The CHF of the Pillar was 119.62 ± 1.40 W/cm2, the CHF of the SR was 143.79 ± 5.12 W/cm2, and the CHF of the SRT was 141.71 ± 3.16 W/cm2, while the CHF of the flat SiO2 samples was only 87.89 ± 3.4 W/cm2.This performance enhancement can be attributed to the smooth water channels on the structured surface. They help to limit the diameter of the air bubbles before they depart. The reduction in the departing bubble size prohibits the air bubbles from covering the whole surface at the same heat flux. This allows the structured surface to withstand even higher heat flux before the vapor bubbles can cover the whole surface. Thus delaying the CHF.The higher CHF of the SR and SRT samples compared to that of the Pillar samples can be attributed to the fact that the single-everse-folded structure is capable of storing more water at high temperatures, which leads to the thickening of the liquid film, and thus delaying the surface burnout of the samples. In addition, a CHF model based on fluid instability was adopted in this study for the prediction of CHF, and the actual observed CHF was found to be consistent with the theoretical value, which further validated the accuracy of the model. Overall, our results emphasize the potential importance of surface structures and hydrophobic materials in pool boiling. These findings provide a strong guideline for the design and development of high-performance thermal management systems and offer new directions for future thermal management research.