狹小空間內微多孔表面之蒸發熱傳性能研究

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Title:	狹小空間內微多孔表面之蒸發熱傳性能研究
Authors:	劉建富;Liu,Chien-Fu
Contributors:	機械工程學系
Keywords:	微多孔;狹小空間;熱傳增強;池沸騰;Micro porous coating, Boiling heat transfer, Confined space
Date:	2013-07-26
Issue Date:	2013-08-22 12:15:00 (UTC+8)
Publisher:	國立中央大學
Abstract:	由於沸騰蒸發可以有較高的熱傳係數，以蒸發熱傳設計的微熱交換器，可能可以解決高熱通量電子設備的散熱需求，然而，微熱交換器的沸騰熱傳通常都被限制在很小的空間內，汽泡生長在狹小空間內會與開放空間差異很大，此研究以微多孔表面提高沸騰熱傳性能並且分別在開放空間與狹小空間內進行測試，探討微多孔表面應用在狹小空間內的可行性。使用甲醇為工作流體在垂直方向的加熱面上，分別進行平滑板與不同微多孔層厚度(81、109、150、182及225 m)以及開放空間與不同狹小空間(1、2和3 mm)，在1大氣壓時的池沸騰熱傳性能實驗與流譜觀察。探討微多孔表面的厚度與狹小空間的間距，對不同熱通量時的熱傳係數與臨界熱通量變化，並且以每秒拍攝1000張的高速攝影機，觀察汽泡生長與週期性的流譜變化，由汽泡成長過程的整體變化來驗證熱傳實驗結果。實驗結果顯示出微多孔表面具有大量的有效成核孔洞，增加核沸騰汽泡產生的數量，與平滑板相比可以增加熱傳係數4.5倍，但微多孔表面也會有較高的汽泡脫離阻礙與多孔層本身的熱阻，這三者相互作用下造成在開放空間與狹小空間都會有一個最佳厚度有著最佳的熱傳性能，在此實驗中150 m厚度的微多孔表面有最高的熱傳係數。而在間距的影響探討顯示出，間距對平滑板沸騰熱傳性能的影響主要參數有四個，(a)薄液膜蒸發、(b)液體強制對流、(c)蒸汽脫離的阻礙和(d)部分乾涸的影響。但是在微多孔表面由於多孔的毛細吸力可以保持加熱表面的潤濕，所以部分乾涸的影響消失，而微多孔的結構也會減少變形汽泡造成的薄液膜蒸發。間距對微多孔表面熱傳性能影響的主要參數只有(a)液體強制對流和(c)蒸汽脫離的阻礙。在兩者的交互作用下，造成微多孔表面對平滑板的熱傳增強率，在低和中熱通量時，會隨著間距縮小而下降，但在高熱通量下間距的影響較不明顯。實驗結果也顯示出，平滑板與微多孔表面的臨界熱通量都會隨著間距縮小而下降，在間距小於1 mm時除了間距會影響臨界熱通量外，微多孔層厚度也會造成臨界熱通量的下降。微多孔表面在低和中熱通量下，是非常有效的池沸騰熱傳增強方式，但在非常狹小的空間或是非常高的熱通量時，熱傳增強率會減少。 Attributed to its high heat transfer coefficient, evaporating cooling involving the use of micro heat exchangers is considered a possible thermal management solution for cooling of high heat flux electronic devices. The boiling heat transfer in micro heat exchangers is generally confined in a very narrow space. The heat transfer characteristics are indeed different from those of conventional unconfined boiling. This work provides an experimental analysis of the boiling heat transfer of methanol on plain and micro porous coated surfaces inside confined space. Three space confinements with distance of 1.0, 2.0, 3.0 mm and unconfined spaces and five micro porous coating surfaces with thicknesses of 81, 109, 150, 182 and 225 m were tested. Effects of space confinement, surface treatment and heat flux on the heat transfer coefficient and critical heat flux were discussed. The micro porous coating layer provides large amount of active nucleation sites that significantly enhances heat transfer coefficients up to a factor of 4.5 in comparing to that on plain surface. But the porous coating layer also exerts higher bubble leaving resistance and coating layer thermal resistance. The combination of these three effects brought about an optimum coating layer thickness for micro porous surfaces in both confined and unconfined spaces. From the test results, we may deduct that for boiling on plain surface, the heat transfer performance in confined spaces was affected by four major effects, i.e. (a) thin film evaporation, (b) vapor blowing and liquid suction effect, (c) vapor leaving resistance and (d) partial dryout effect. But for boiling on micro porous coating surface, partial dryout was not found owing to the porous capillary force to keep the heating surface wet. The wetted rough porous surface also reduced the effect of thin film evaporation under the deformed bubbles. The heat transfer performance was affected majorly by vapor blowing and liquid suction effect and vapor leaving resistance only. The combination of these effects resulted in the micro porous to plain surfaces heat transfer enhancement ratio to decrease with decreasing confined space distance at low and moderate heat flux conditions but insensitive to confined space distance at high heat flux condition. Micro porous coating is a very effective boiling heat transfer enhancement treatment at low and moderate heat fluxes conditions. The enhancement ratio reduced in very narrow space confinement or at very high heat flux condition.
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