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標題: | 非對稱反摺微溝槽表面之液滴自推性能 Droplet Self-propulsion on the Asymmetric Reentrant Microgroove Surface |
作者: | 許庭瑜 Ting-Yu Hsu |
指導教授: | 呂明璋 Ming-Chang Lu |
關鍵字: | 萊頓弗羅斯特現象,非對稱雙反摺結構,液滴自推,氣泡動量,過熱表面,疏水表面,半導體製程, Leidenfrost Effect,Single & Double Reentrant Structure,Droplet self-propulsion,Bubble momentum force,Superheated Surface,Hydrophobic Surface,Semi-conductor producing process, |
出版年 : | 2023 |
學位: | 碩士 |
摘要: | 液滴對加熱表面的衝擊普遍存在於各種工業應用上,例如噴霧冷卻、微流道、液體收集與抗垢表面等都會應用到液滴彈跳。在處理過熱表面的情況下,防止薄膜沸騰在表面上發生,同時保持液滴高運動性對於這些應用至關重要。儘管先前的文獻已經展示了利用微奈米結構表面來抑制萊頓弗羅斯特效應,但液滴在高溫下的運動性通常無法保持,事實上,沒有任何一種表面能夠在超過500 ℃的溫度下實現液滴的自推能力、其中液滴自推能力的定義為受到過熱表面結構的影響液滴朝單一方向前進的能力。目前在工業應用上,涉及液滴撞擊過熱表面的主要挑戰包括:(1) 減少液滴的接觸時間;(2) 實現液滴的方向性自推,以防止液體在過熱表面上積累;以及 (3) 在廣泛的溫度範圍內抑制萊頓弗羅斯特效應。
在本研究中,我們提出了非對稱反摺微溝槽表面來應對上述挑戰,而微溝槽頂部有懸掛結構,這些懸掛結構類似於順時針旋轉的字母“L”,下垂的一側是雙反摺 (Double Reentrant) 懸掛結構,而水平的一側則是單反摺 (Single Reentrant) 懸掛結構,因此,在本研究中此表面被命名為非對稱反摺微溝槽 (ARG)。我們針對液滴的接觸時間、質心速度和萊頓弗羅斯特溫度點 (LFP) 進行測量後,觀察到在ARG表面上會發生液滴的拉伸彈跳,並且在400 ℃至650 ℃的溫度範圍測得約13毫秒的接觸時間,低於理論的毛細限 (Inertia Capillary Limit),表明ARG表面在高溫下降低了接觸時間的能力,而接觸時間的降低是由於液滴反彈的過程中產生對稱性的破壞所導致。同時,ARG的非對稱特性還提供了對撞擊液滴的單向自推能力,使得液滴在韋伯數為9.09時自推速度最高為0.5 m/s。ARG表面上的懸掛結構還起到阻止蒸氣層在表面上形成的作用,而表面上的溝槽則提供了一條通道,將蒸氣引導向外。因此,ARG表面能夠將萊頓弗羅斯特溫度點 (LFP) 提高至775 ℃。 ARG表面能夠在300 ℃至750 ℃的過熱表面上降低接觸時間,同時使液滴能夠達到最大0.5 m/s的自主推進速度,並在750 ℃的溫度範圍內抑制萊頓弗羅斯特效應。根據ARG結構的獨特性,ARG表面在需要液滴推進、抗垢表面、噴霧冷卻或衝擊冷卻的高過熱系統應用中具有相當的潛力。 The impact of the droplet on heated surfaces is commonly seen in various industrial applications, such as spray cooling, microfluidics, fluid collection, and antifouling. When dealing with droplets impacting superheated surfaces, preventing film boiling from happening on the surface while maintaining high droplet mobility is crucial for these applications. Although previous literature has demonstrated Leidenfrost suppression using micro- and/or nano-structured surfaces, the droplet mobility at high temperatures is usually not retained. As a matter of fact, no surface is able to achieve high droplet mobility at a temperature beyond 500 ℃. At present, the main challenges regarding the droplet impacting a superheated surface in industrial applications include: (1) reducing the contact time of the droplet, (2) achieving directional bouncing of the droplets to prevent fluid accumulation on the surface, and (3) suppressing the Leidenfrost effect over a wide temperature range. This work proposes the asymmetric reentrant microgroove surface to tackle these challenges. The microgroove surface consists of overhanging structures on the top of the microgroove walls. The overhanging structures resemble a clockwise rotated letter "L". The side that hangs down is the double reentrant overhanging structure, and the other side with the horizontal overhanging is the single reentrant overhanging structure. Consequently, the surface is denoted as the asymmetric reentrant groove (ARG) surface. The droplet's contact time and centroid velocity, and the Leidenfrost point (LFP) on the surface were examined. The elongated bouncing observed on the ARG surface, and a contact time of ~ 13 ms was obtained at temperatures between 400 ℃ ~ 650 ℃. This contact time was lower than the theoretical inertia-capillary limit, suggesting the ability of the ARG surface to reduce contact time at high temperatures. The contact time reduction is attributed to the breaking of symmetry in the droplet-bouncing dynamics. The asymmetric structure of the ARG also provided a force for the unidirectional self-propelling of the impacting droplet before the Leidenfrost point (LFP). The droplet's self-propelled velocity of 0.5 m/s was obtained at a Weber number of 9.09. The overhanging structure on the ARG surface also acts as a barrier preventing the vapor layer formation on the surface, while the grooves on the surface provide a pathway to channel the vapor outwards. Thus, the ARG surface could elevate the Leidenfrost point (LFP) to 775 ℃. The ARG surface could lower the contact time on a superheated surface between 300 to 750 ℃ while enabling droplets to achieve a large self-propelling speed up to 0.5 m/s and suppressing the Leidenfrost effect up to 750 ℃. Based on the unique characteristics of the ARG structure, the ARG surface is promising in applications in high-temperature thermal systems requiring droplet propulsion, antifouling, spray cooling, or impingement cooling. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/90646 |
DOI: | 10.6342/NTU202303808 |
全文授權: | 同意授權(全球公開) |
顯示於系所單位: | 機械工程學系 |
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