Abstract
摘要:Leidenfrost效應是指在固體溫度高於Leidenfrost溫度(LFP)時在固液介面形成絕熱之蒸氣層之現象,此Leidenfrost效應會抑制液滴與固體表面間之質量和熱量傳遞,且可能會導致熱系統的災難性失敗,例如福島核電廠事故。因此,升高LFP(也 稱為LF抑制) 在許多應用中是有利的,我們之前的初始研究探討超親水矽奈米線陣列之LFP,由於超親水矽奈米線陣列可提供一很大的毛細作用力,因此矽奈米線陣列可提升LFP,在高度為93 µm的矽奈米線陣列表面所得到之LFP為652.0 ± 2.2 °C,此值為目前文獻上之最高LFP。 此外,液滴撞擊固體表面的接觸時間指的是液滴與固體表面接觸的時間,考慮到縮短接觸時間可減少能量消耗,並增加熱量和質量傳輸,故減少液滴接觸固體表面時間是重要的。我們之前的初始研究亦量測了在過熱矽奈米線陣列的液滴接觸時間 ,在過熱矽奈米線陣列表面觀察到劇烈的沸騰現象,此劇烈的沸騰現象造成液滴產生噴流(jet),也縮短液滴接觸表面時間,在93 µm的矽奈米線陣列所得到之無因次化接觸時間為0.2,此值為目前在文獻上所得之最低之液滴接觸表面時間。 此計畫將首先探討LFP及接觸時間之理論機制,並提出理論模型來預測LFP及接觸時間。此外,目前假設矽奈米線陣列因為有大的毛細力,故能提高LFP,因此在此計 畫中將採用雙層粗糙尺度的矽奈米線陣列來增加毛細限,以進一步提升LFP。另外 ,目前推測在過熱的矽奈米線陣列,因為表面激烈的沸騰情形,而縮短液滴接觸時間;根據此假設,則液滴在過熱表面的接觸時間會受表面沸騰現象的影響,因此在 此計畫中我們希望能系統性探討表面成核密度與接觸時間的關係。另外,在實際應用上矽基材受到相當的限制,故在此計畫中我們預計合成銅奈米線陣列,並以銅奈米線陣列來增加LFP,與減少液滴接觸時間,以更符合實際工業系統的需求。
Abstract: The Leidenfrost effect refers to the phenomenon of forming an adiabatic vapor layer at the solid-liquid interface at a solid temperature above the Leidenfrost temperature (LFP). The Leidenfrost effect suppresses the heat and mass transfer between the droplet and the solid surface. Besides, it may also lead to a catastrophic failure of a thermal system, such as Fukushima nuclear plant meltdown. Therefore, elevating LFP (also known as LF suppression) is beneficial in many applications. Our preliminary study explored the LFP on superhydrophilic Si nanowire arrays. Given that the superhydrophilic nanowire array can provide a large Capillary force, the LFP on the silicon nanowire array was substantially elevated. The LFP obtained on the silicon nanowire array with a height of 93 μm was 652.0 ± 2.2 °C, which was the highest reported LFP in the literature. In addition, the contact time of an impacting droplet to a solid surface is the time it takes for the droplet to come into contact with a solid surface. It is important to reduce the contact time because of shortening the contact time can reduce energy consumption and increase heat and mass transfer. Our preliminary study also examined the droplet contact time on superheated superhydrophilic silicon nanowire arrays. Vigorous boiling was observed on the superheated silicon nanowire array, which caused the formation of liquid jets on the liquid droplets and also shortened the contact time of the droplets. The obtained dimensionless contact time on the silicon nanowire array with a nanowire height of 93 μm was 0.2, which was the smallest reported value in the literature. In this project, we will first explore the mechanisms of LFP and contact time on the silicon nanowire arrays, and the theoretical models for predicting the LFP and the contact time on the surfaces will be established. In addition, it was assumed that the high LFP on the silicon nanowire array was associated with its large capillary force. Therefore, a bi-porous silicon nanowire array will be used in this project to increase the capillary limit and to further enhance the LFP. In addition, it was assumed that the contact time reduction on the silicon nanowire was due to the intense boiling happening on the surface. According to this assumption, the droplet contact time on the superheated surface will be affected by the surface boiling phenomenon. Thus, we will investigate the effect of nucleation site density on contact time. Furthermore, in practical applications, silicon substrates are subject to considerable limitations. Therefore, in this project, copper nanowire arrays will be synthesized. The copper nanowire arrays will be exploited in increasing the LFP and reducing the contact time for the application in industrial systems.
Keyword(s)
萊頓弗羅斯特現象
萊頓弗羅斯特溫度
超親水性
矽奈米線陣列
接觸時間
銅奈米線陣列
Leidenfrost effect
Leidenfrost temperature
Superhydrophilic
Silicon nanowire array
Contact time
Copper nanowire array