- 學位論文
透過微奈米結構以延緩萊頓弗羅斯特現象和降低水滴與過熱表面接觸時間
Leidenfrost Suppression and Reduced Contact Time of a Drop Impacting on Micro- and Nano-Structured Hot Surfaces
摘要
由於在大多數的能源過程中都有固液接觸的部分，因此可以通過優化固液間的交互作用來最大程度地提高產生能量的效率。對於衝擊平面的液滴，一旦表面溫度高於萊頓弗羅斯特點（LFP），就會在固體表面上形成穩定的蒸氣膜，而由於隔熱蒸氣層的形成，質傳和熱傳的速率將被極大的抑制。因此，提高萊頓弗羅斯特點，或者說對萊頓弗羅斯特現象（LF）的抑制，在許多工業應用中都是有利的。由於擁有卓越的毛細力和強大的洩除蒸汽的能力，微米和納米尺度的表面結構普遍被認為可以抑制LFP。在本次抑制LF的研究中，使用了在矽表面上垂直排列的矽納米線（SiNW）陣列。我們發現：在過熱的超親水矽納米線陣列塗層的表面上，萊頓弗羅斯特現象被極大的抑制。在納米線高度為90μm的SiNW陣列塗層的表面上觀察到了655±14°C的LFP。且這是文獻中的最高值。理論模型表明，LFP的升高是由於納米線陣列卓越的毛細力和因納米線陣列增強的蒸氣洩除能力。 此外，在縱觀整體的熱傳性能時，液滴與加熱表面的接觸時間（t_c）是重要的因子。對於衝擊平面的液滴，t_c是指液滴與固體表面接觸的時間。較短的t_c能夠增強熱傳。因此，研究人員感興趣的是較短的t_c。在室溫下，超疏水表面已被設計成具有良好的洩除液體能力，但在加熱表面上洩除液體的概念則尚未被探索。受到使用微納米結構來減少液滴接觸時間的研究的啟發，我們猜想或許能利用過熱的超親水表面上強烈的固液交互作用來減少液滴的接觸時間。對於帶有v形矽（Si）微溝槽的氮化矽（SiNx）表面（下面稱為SNVM），在287℃時觀察到了液滴拉長的樣子。在該溫度下，在界面處同時發生了沸騰和萊頓弗羅斯特效應（即所謂的亞努斯熱狀態）。有趣的是，由於表面上的拉伸彈跳而觀測到了微小的t_c，10.36 ms。這是文獻中在亞努斯熱狀態下的最低值。t_c的降低和在表面上觀察到的液滴拉伸均是由表面上氣泡產生的不對稱慣性力所引起的。 除了LF的抑制，我們也研究了SiNW陣列上液滴的接觸時間。由於伴隨著快速彈跳的劇烈沸騰作用，形成了在液滴上表面的噴流。氣泡擴張和界面處的強剪切動量所引起的壓力則造成了接觸時間的減少。在SiNW高度為90μm時，接觸時間減少到1.3±0.1 ms，與理論極限值相比減少了93％。 這篇論文呈現了微納米結構表面減少接觸時間的效果，以及此表面對萊頓弗羅斯特顯著地抑制作用。而提高萊頓弗羅斯特溫度和縮短接觸時間的表面能為增進液滴的流動性及對加熱表面的熱管理鋪平道路。
並列摘要
By considering the fact that liquid-solid contacts are ubiquitous in most of the energy processes, the efficiency of energy production can be maximized with the optimized liquid-solid interaction. When a drop impacts to a surface and the surface temperature is higher than the Leidenfrost point (LFP), a stable vapor cushion forms on the solid surface. The mass and heat transfer rates would be greatly suppressed because of the formation of the thermally insulating vapor layer on the surface. Thus, elevating Leidenfrost point, also called Leidenfrost (LF) suppression, is advantageous in many industrial applications. Micro- and nanoscale surface modification is widely recognized to suppress the LFP because of superior capillary force and strong vapor dispersion ability. In the present work, vertically aligned silicon nanowires (SiNW) array on a silicon surface was used for LF suppression study. The present study revealed substantial Leidenfrost suppression on a superheated superhydrophilic silicon nanowire (SiNW) array-coated surface. An LFP of 655 ± 14 °C was obtained on the SiNW array-coated surface with a nanowire height of 90 m. This LFP was the highest reported value in the literature. A theoretical modeling revealed that the elevated LFP was caused by a superior capillary force and enhanced vapor dispersion into the nanowire array. In addition, the contact time (tc) of a drop on the hot surfaces is an important consideration while dealing with the overall heat transfer performance. The tc of an impacting drop is the time during which the drop comes in contact with the solid surface. A short tc enhances heat transfer. Thus, a short tc is of interest to researchers. Even though superhydrophobic surfaces are designed for better liquid shedding ability at room temperature, the concept of drop shedding on hot surfaces has not been explored. Inspired by studies that have used micro- and nano-structures to reduce drop contact times, we suspect that the strong solid-liquid interaction on a superheated superhydrophilic surface might be utilized to reduce the drop contact time. For the silicon nitride (SiNx) surfaces with v-shaped silicon (Si) microgrooves called SNVM hereafter, an elongated shape of the drop is observed at a temperature of 287 °C. At that temperature, concurrent boiling and Leidenfrost effect is observed at the interface (the so-called Janus thermal state). Interestingly, a small tc of 10.36 ms was obtained on the surface due to the elongated drop bouncing from the surface. The obtained contact time was the lowest value in the Janus thermal state in the literature. The tc reduction and the observed drop elongation on the surface both resulted from an asymmetric momentum force created by the vapor bubbles on the surface. In addition to LF suppression, the contact time of a drop on SiNW arrays was also studied. The drop experienced ejection of liquid-jet from the top surface of the drop due to vigorous boiling along with rapid bouncing. The pressure induced due to the vapor bubble explosion and strong shear momentum at the interface was considered for the contact time reduction. A reduction of contact time of 1.3 ± 0.1 ms is obtained for SiNW height of 90 μm, which is 93% reduction from the theoretical limit. This thesis has reported significant Leidenfrost suppression and reduced contact time on a micro- and nanostructured surfaces. Surfaces with elevated Leidenfrost temperature and reduced contact time paves a way for improved drop mobility, and thermal management on hot surfaces.