應用自再濕潤流體於高效噴霧冷卻系統之設計

Applying a self-rewetting fluid for the design of a high-efficiency spray cooling system

臧淞(Sung Tsang)

指導教授 ： 孫珍理

國立臺灣大學/工學院/機械工程學研究所/博士(2019年)

https://doi.org/10.6342/NTU201904349

摘要

在本研究中，我們成功應用逆Marangoni對流來建構高效噴霧冷卻系統。自再潤濕流體被使用於噴霧冷卻系統中，且被噴灑在超親水性加熱表面上，形成液體薄膜，並於其上引起逆Marangoni對流。藉著逆Marangoni對流，液體將能迅速且持續地朝目標發熱面移動，維持熱區的濕潤，延後或甚至避免燒乾現象發生，增強表面上之蒸發冷卻，達到高效冷卻設計之目的。於研究的前期，單一短脈衝噴霧被用來探討使用自再潤濕流體來增強噴霧冷卻熱傳之可能性。我們發現連續且不斷之液體薄膜為逆Marangoni對流能否被引起之關鍵，故有助於液體成膜的超親水性表面之使用為成功之必要條件。當逆Marangoni對流被成功引起，蒸發冷卻可於單一短脈衝噴霧結束後持續作用，並維持一段甚長之時間，也因此能將大量之發熱帶走，大大優化噴霧冷卻能力。在相比於以往之噴霧冷卻系統的條件下，以及在固定噴霧噴灑量及噴霧覆蓋面積Acover約為15 ~ 30 cm2的條件下，本研究所提出之高效噴霧冷卻系統將可使噴霧冷卻量Qspray提升3至7倍；在小噴霧噴灑量的條件下，本系統將可達到更大的溫降。並且，對於此工作流體因表面張力驅動而由冷區移往熱區表面之遷移現象，我們亦領先提出以「逐熱」一詞來作描述。而儘管逆Marangoni對流已被證實可增強噴霧冷卻能力，但仍有幾點操作事項需被留意：首先，對於不同之噴嘴規格，為了要能成功引起逆Marangoni對流，噴嘴之噴霧高度將伴隨著一定範圍之限制。再者，雖然使用長噴霧噴灑能得到較高之噴霧冷卻量Qspray，但當加熱表面溫度Ts較低時，短噴霧噴灑將仍是適用的。換言之，對於低加熱表面溫度Ts來說，較短之噴霧時間是適合的；而當加熱表面溫度Ts較高時，則必需使用較長之噴霧時間。
接著，為了要更加符合實務需求，於本研究之後期，大面積噴霧冷卻被引入。單一噴嘴的使用被延伸至多重噴嘴配置，以滿足大散熱面積 (20.25 cm2)被覆蓋的需求，並以間歇型方式來進行噴霧冷卻，以達到高效冷卻之目的。間歇型噴霧冷卻被認為是其中一種最有效之散熱方法，相比於不間斷之噴霧冷卻方式，間歇型噴霧冷卻將可同時兼顧噴霧冷卻效果及噴霧冷卻效率。在此階段之研究中，我們首先成功證明，只要噴霧液體薄膜能被完整形成在加熱表面上，自再潤濕流體所引起之逆Marangoni對流作用就能被應用於間歇型大面積噴霧冷卻系統之設計中。藉著逆Marangoni對流的作用，大面積加熱區域將可被保持潤濕，加熱表面上之燒乾現象可於一間歇噴霧冷卻工作周期中大幅縮短，甚至完全避免，進而使得間歇型大面積噴霧冷卻之熱傳更加地依賴於表面上之液體薄膜蒸發及沸騰；而非如以往噴霧冷卻系統中，噴霧冷卻之熱傳偏向於依賴噴霧所帶來之高流體衝擊動量特性。另外，不同噴嘴配置及噴霧參數之影響亦於本階段中被探討，且為了要能公平地比較噴霧冷卻能力，不同實驗測試條件下之工作流體消耗量皆將被固定。換言之，當使用較多噴嘴進行噴灑時，噴霧時間將被縮短；以及進行短噴霧模式噴灑時，噴霧頻率將被提高。在固定之工作流體消耗量條件下，與水之冷卻情況相比，使用自再潤濕流體來進行間歇型噴霧冷卻，將可獲得更大之噴霧冷卻熱傳率，使加熱表面溫度能持續地被維持低溫，並且同時伴隨較小之溫度跳動。另外，在高輸入電功率之條件下，對於多重噴嘴配置來說，積累於流場停滯區上之液塊，將有助於維持加熱表面上之噴霧液體薄膜的完整性，使得後續逆Marangoni對流能順利發生。最後，概括來說，以四噴嘴配置進行短噴霧模式噴灑，將能在伴隨較小之溫度跳動及較佳之溫度均勻性條件下，帶來較高之噴霧冷卻熱傳率。

關鍵字

逆Marangoni對流 ； 逐熱 ； 自再潤濕流體 ； 間歇型噴霧冷卻 ； 大面積噴霧冷卻

並列摘要

In this study, we successfully utilize the self-rewetting fluid to enhance the evaporative cooling of a spray cooling system by inducing the inverse Marangoni convection. We find that the superhydrophilicity of the surface is essential to promote a continuous surface-tension-driven fluid flow, which helps to replenish the hot region with the working fluid and shorten the duration of dryout, or even fully prevent dryout from happening. As a result, cooling lasts much longer beyond the discharge and a significant amount of heat can be removed by a single shot of spray. We coin a term ‘heat chasing’ effect to describe the excellent cooling caused by the inverse Marangoni convection. Besides the surface superhydrophilicity, the spray height should be confined to a certain range so that a liquid film is formed. Once the inverse Marangoni convection commences, the spray cooling amount Qspray can be augmented three to seven times for a given spray amount supplied by a single pulsed spray as the spray cover area Acover is between 15 ~ 30 cm2. Although long spray always results in better heat transfer, enhancement with the inverse Marangoni convection is still present under the condition of the short spray for a surface temperature as high as 255°C.
Next, in order to satisfy the practical requirement, intermittent and large-area cooling are involved so that multiple nozzle arrrangements are applied to cover larger cooling area and spray periodically. Intermittent spray cooling is considered to be one of the most effective heat removal technologies with high spray cooling efficiency compared to the typical spray system. In this stage, we employ a self-rewetting fluid in intermittent spray over a larger cooling area of 20.25 cm2. The self-rewetting fluid is confirmed to be able to induce the inverse Marangoni convection on large-area cooling surface as long as a complete liquid film of the self-rewetting fluid is formed after the injection. We change the nozzle arrangements and spray parameters to study the effects of inverse Marangoni convection on the performance of the intermittent spray cooling. For comparison, the consumption rate of liquid is fixed so that increasing the nozzle number leads to a shorter pulse duration while the long-spray mode results in less frequent injections. By pulling the working fluid toward the heated region, the inverse Marangoni convection helps to extend the lifetime of liquid film and postpone dryout in the intermittent spray cooling process. Due to the stretching of the cooling duration, this significantly enhances the cooling performance and reduces the temperature fluctuation in time. For intermittent spray of the self-rewetting fluid, strong film evaporation and intensive nucleation play the major roles in cooling, not the impingement momentum. As a result, the configurations of multiple nozzles outperform single nozzle at high input power because liquid accumulation in the stagnation zone helps to preserve the liquid film on very hot surface. In general, using quadruple nozzles in the short-spray mode can lead to higher cooling rate with smaller temperature fluctuation and better thermal uniformity.

並列關鍵字

inverse Marangoni convection ； heat chasing ； self-rewetting fluid ； intermittent spray cooling ； large-area spray cooling

參考文獻

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