應用自再濕潤流體之間歇性噴霧冷卻系統之效益分析

On the performance analysis of using self-rewetting fluid in intermittent spray cooling system

林家璿(Chia-Hsuan, Lin)

指導教授 ： 孫珍理

國立臺灣大學/工學院/機械工程學研究所/碩士(2018年)

https://doi.org/10.6342/NTU201801297

摘要

本研究將自再濕潤流體應用在間歇性噴霧上，藉由自再濕潤流體會往熱區回拉液體的性質，延長每次噴霧的散熱能力，並搭配間歇性噴霧較節省工作流體的特性，在固定加熱塊輸入功率下，探討改變噴霧時間及間隔時間，自再濕潤流體在不同工作週期 (duty cycle) 情況下之時間平均熱傳率，並與水的情況進行比較。我們亦運用高速攝影機與紅外線熱像儀記錄冷卻過程中的沸騰現象與液膜表面溫度變化，釐清不同熱傳機制對逐熱噴霧冷卻的影響，藉以得到最佳操作參數，達到最大冷卻效果並節省工作流體的消耗量。
由結果我們可歸納出四點：(1)逆Marangoni效應可以有效延長單一週期噴霧的冷卻時間，跟使用純水的情況比較，在較低的噴霧頻率即可達到較佳的冷卻效果，節省工作流體；(2)針對單一周期，相同工作週期條件下，噴霧時間越長，即噴霧頻率越低，使用自再濕潤流體的噴霧冷卻的瞬時熱傳率越高，意即其單一周期的時間平均熱傳率越高，這是因為噴霧時間越長，工作流體留在加熱表面的量越多，可維持較久的逆Marangoni對流所致；(3)針對單一周期中，相同噴霧時間條件下，工作週期越小，即噴霧頻率越低，瞬時熱傳率越高，所得之時間平均熱傳率越高，這是因為噴霧頻率越低會導致較高的表面溫度，使得逆Marangoni 對流越強；(4)雖然噴霧時間短時在單次週期內的冷卻能力不強，但在固定一段時間內卻可藉由增加噴霧次數來提高總冷卻量。而在相同的噴霧時間下，工作週期太低會造成加熱表面完全燒乾，而工作週期太高則會因為過多的噴霧量造成液膜累積並降低加熱面之溫度，進而減弱逆Marangoni對流，皆不利於冷卻效果。故由我們的實驗結果可得在噴霧時間為1.5 s工作週期為7.5%條件下達最高之冷卻熱傳量且擁有最高的冷卻效率與最低的耗水量。

關鍵字

逆Marangoni現象 ； 逐熱 ； 自再潤濕流體 ； 間歇性噴霧冷卻 ； 工作週期

並列摘要

In this study, we utilize the self-rewetting fluid in an intermittent spray system and study the effects of pulsed duration and duty cycle on the heat transfer mechanism and the cooling capacity.
Compared to water, using the self-rewetting fluid can successfully induced inverse Marangoni convection and proven to prolong the cooling time within a cycle. This leads to a lower fluid consumption because comparable heat transfer rate can be delivered by spraying the self-rewetting fluid at much lower frequency. For a given duty cycle, increasing the pulsed duration leads to a higher heat transfer rate per period. The working fluid left on the heated surface is so abundant that the inverse Marangoni convection is sustained. However, longer spray pulse decreases the spray frequency and is unfavorable when a given time span is consider. Although a shorter pulsed duration only shows moderate cooling capacity per cycle, the time-averaged heat transfer rate is improved by the increase in spray frequency. On the other hand, reducing the duty cycle also augments the heat transfer rate per period when the pulsed duration is fixed. As the spray frequency decreases, the surface temperature increases, so that a stronger inverse Marangoni convection is induced. Nevertheless, dry-out may occur if the duty cycle becomes too small. As a result, we find that a pulsed duration of 1.5 s with a 7.5% duty cycle produces the best cooling effect and with the highest spray cooling efficiency and the lowest fluid consumption.

並列關鍵字

inverse Marangoni effect ； heat chasing ； Self-rewetting fluid ； pulsed spray cooling ； duty cycle

參考文獻

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