- 學位論文
冷媒基質之奈米流體於池沸騰熱傳增強研究
Enhancement of pool boiling heat transfer with refrigerant based nanofluids
摘要
奈米流體近年來被視為極具發展潛力之熱傳增強技術之一，但目前文獻多集中於單相水基之熱傳研究，關於其他工質或沸騰熱傳領域則鮮少有報導，然而又因粒子分散情況之不同，將導致熱傳性能表現不一。因此，本研究特別以冷媒為基質之二氧化鈦奈米流體進行池沸騰熱傳增強研究，主要利用有機改質方法使粒子形成具長碳鏈之立體結構，並搭配超音波震盪將奈米粒子分散於冷媒流體中。經由分散性檢驗後進行熱性能測試，最後探討分散效應與濃度效應對熱傳性能之影響。 分散性檢驗結果顯示，經改質之奈米粒子能均勻且穩定分散於冷媒流體中。在熱性能測試方面，改質後奈米流體與未改質相比，其對流熱傳係數增強倍數由30% 提升至65%，性能提升之可能解釋為粒子分散性變好，因布朗運動導致微對流機制而加速熱能傳遞，使壁面過熱度下降，增強熱傳性能，但臨界熱通量則無顯著提升。經改質之奈米流體於濃度0.001wt%、0.01wt%、0.1wt% 測試範圍中，發現熱傳性能隨濃度增加呈現先升後降之趨勢，其中以濃度0.01wt%熱傳性能表現最佳，與純質R-123相比對流熱傳係數可提升65%，可能原因為隨著濃度增加，均勻分散之粒子增多有利於微對流機制，但過多的粒子會沉積於加熱表面而形成額外熱阻，使熱傳性能惡化。進一步將具奈米粒子之流體物性代入Rohsenow熱傳預測式後發現，熱傳增強之表現並不顯著，因此間接證明流體物性改變似乎並非主要影響因素。奈米流體熱傳機制較為複雜，有待進一步分析探討。
並列摘要
Nanofluids is expected to be one of the most potential techniques for heat transfer enhancement in recent years. However, to date, there has been relatively little research conducted on boiling heat transfer regime, with refrigerants based are even bare reported. In addition, the varied characterization of suspension conditions in pool boiling causes the inconsistency between experimental results. Therefore, the aim of this article is to investigate the pool boiling heat transfer enhancement with refrigerant based Titania nanofluids. Using organic surface modification technique to produce nanoparticles with long-chain three-dimensional structure, cooperate with ultrasonic vibration to disperse nanoparticles effectively into refrigerants. Boiling experiments were conducted after suspension ability tests. Finally, the effect of suspension and concentration to boiling heat transfer of nanofluids are discussed. The result of suspension tests show, modified nanoparticles could be uniformly and steadily suspended in refrigerants. Boiling experiments reveals that 30% to 65% enhancement of convective heat transfer coefficient compare with unmodified nanofluids. One possible explanation for this is that the better suspension condition increases Brownian-motion-based mirco-convection heat transfer efficiency, which decreases the wall superheat, hence, enhances heat transfer. Boiling experiments of modified nanofluids under concentrations 0.001wt%、0.01wt%、0.1wt% found that, the heat transfer efficiency firstly increase and then decrease with increasing concentration. The highest enhancement of convective heat transfer coefficient is 65% compare with pure R-123 in concentration 0.01wt%. The possible reason is that, along with increasing concentration, the increasement of well-dispersed nanoparticles are favor to micro-convective mechanism, but too many nanoparticles would cause sedimentation on heating surface, which adds extra resistance of heat and decreases the boiling heat transfer. In addition, comparisons between the heat transfer data and the Rohsenow correlation indirectly affirm that the changed physical properties of the base liquid is not the main factor to performance, other mechanisms has to be considered. Further, correlations need to be proved by detail experiments and precisely theories.