奈米流體之池沸騰熱傳

Abstract

本研究使用純鎳線於濃度0.01%wt.、0.1%wt.以及1%wt.之TiO2奈米流體沸騰製作奈米塗佈線，沸騰之熱通量參數為0 kW/m2、500 kW/m2以及1000 kW/m2。並以池沸騰實驗來探討不同參數製造的奈米塗佈線對沸騰熱傳的影響。同時以接觸角量測、SEM圖片、EDS分析和再現性實驗來探討奈米塗佈層的特性。 SEM和EDS的結果顯示純鎳線在奈米流體中沸騰將形成TiO2奈米塗佈層，此塗佈層隨流體濃度和熱通量參數變高而變厚。而沸騰曲線的結果顯示此奈米塗佈層於加熱表面形成熱阻使對流熱傳係數下降。然而，接觸角量測顯示奈米塗佈層為親水性介質，因此奈米塗佈層具有較高的臨界熱通量，最高約增強87%。為了測試奈米塗佈線的耐久度，本研究比較使用前和使用9次後之奈米塗佈線，結果顯示使用過後之奈米塗佈線其對流熱傳係數和臨界熱通量皆下降。表面型態的改變為對流熱傳係數下降的主要原因，而接觸角和臨界熱通量的反比關係不再，親水性指標不適用於使用過後之奈米塗佈線，因此臨界熱通量下降的原因仍未知。

The pool boiling behavior of water was experimently studied over a TiO2 nanoparticle-coated heater. The nanoparticle-coated wires were produced by boiling processes which submerge a pure nickel wire into nanofluid. Making nanoparticle-coated wires included two parameters: concentration of the nanofluids and heat flux.The concentrations of the nanofluids were 0.01%wt., 0.1%wt., and 1%wt., and the heat flux were 0 kW/m2, 500 kW/m2 and 1000 kW/m2. Furthermore, the contact angle measurement, SEM and EDS analysis were conducted to discuss the features of nanoparticle-coated wires. The SEM and EDS results showed that nanoparticles were deposited on the heating surface during boiling processes. Besides, the thickness of the nanoparticle-coated surface was enhanced as concentrations and heat flux increased. The boiling curves indicated that heat transfer coefficient of nanoparticle-coated wires decreased as a result of thermal resistance which was generated by nanoparticle-coated surface. However, the CHF was enhanced due to its hydrophilic surface which measured by contact angle experiments, and the maximum CHF enhancement rate was about 87%. It is believed that CHF enhancement is mainly caused by the nanoparticle coating on the heating surface. To test the reliability of nanoparticle-coated wire, boiling curve comparisons between nanoparticle-coated wire and used nanoparticle-coated wire were performed. The CHF and heat transfer coefficient decreased as using time increased. The modification of the heating surface was the main reason that heat transfer coefficient decreased. However, the relationship between contact angle and CHF disappear. Thus, the reason of CHF decrease is still unknown.