氣體於微流道內之熱流特性研究及其應用

Heat Transfer and Friction Characteristics of Gaseous Flow in Microtubes

陳加偉(Chia-wei Chen)

指導教授 ： 楊建裕

國立中央大學/工學院/機械工程學系/博士(2011年)

摘要

本研究以實驗方法探討氣體於微流道中之熱流特性，測試段分為平滑管與粗糙管兩類。平滑管管徑分別為86、308以及920 ?m三種，工作流體為空氣氣體，探討微流道熱流實驗尺寸效應所產生之影響，依不同管徑各實驗條件下最大Kn數範圍為2×10-4~7.2×10-5，故本研究微流道內流體流動型式皆仍於連續流體流動狀態內。實驗結果證實傳統大管徑壓降與熱傳關係式依然適用於本研究所使用管徑範圍之微流道，並無明顯尺寸效應。
粗糙管研究則藉由特殊製作出之內管徑約為1 mm之粗糙流道來探討表面粗糙度於微流道中對流體流動特性之影響，粗糙結構依類型可分為結構性與非結構性粗糙管。研究結果發現於粗糙管實驗中以非結構性粗糙表面有較佳之熱傳增強效果，但無論是哪種粗糙表面皆會使管內摩擦係數明顯增加；另於粗糙管研究中，同時使用空氣與二氧化碳兩種氣體為工作流體進行比較，實驗結果發現不同工作流體所得到之實驗結果相同，沒有明顯差異。
於本研究中，因氣體具有可壓縮性，故於實驗數據分析時將使用壓縮流理論所推導出之關係式進行數據換算，以探討壓縮性對於微流道內熱流特性影響，研究結果發現考慮到氣體壓縮性後，管內摩擦係數和傳統關係式相符。於熱傳實驗時考慮到氣體熱傳量小，須注意熱分流(Thermal Shunt)問題，管壁溫度量測方式使用Yang and Lin [2007]所使用之液晶熱像法(Liquid Crystal Thermography)進行溫度量測，同時為了驗證熱分流對實驗所產生之影響，另以傳統黏貼熱電偶方式進行溫度量測，實驗結果證實以液晶熱像法可成功避免熱分流問題而得到正確之管壁表面溫度，而使用熱電偶量測溫度時，則可發現當管徑與熱傳量逐漸變小時，因熱分流使得所量測到管壁溫度失真問題越顯嚴重，進而影響實驗結果，於微流道氣體熱傳實驗，熱分流問題已不再可以忽略，需加以避免。
本研究除了針對微管內熱流特性進行基礎研究外，並應用其相關研究結果設計製作一扁平管微流道熱交換器進行測試，除了熱交換器單體性能測試以驗證微流道內熱流特性研究結果外，並加裝一噴霧裝置以進行熱交換器熱傳增強研究，研究結果顯示微流道熱交換器設計時依舊可使用傳統關係式進行計算設計，和前述微管內熱流特性研究結果相符，且適當的噴霧量可大幅增加熱交換器熱傳性能。

關鍵字

表面粗糙度 ； 液晶熱像法 ； 熱分流 ； 微流道熱交換器 ； 噴霧冷卻 ； 尺寸效應 ； 微流道 ； 氣體

並列摘要

Experiments were conducted in this research to investigate size and roughness effect on flow characteristics and heat transfer coefficient of air and CO2 flow in circular micro-tubes.
For smooth microtubes with inside diameter of 86, 308 and 920 ?m, the Liquid Crystal Thermography method was used to measure the tube surface temperature for avoiding the thermocouple wire thermal shunt effect. The experimental results show that the friction coefficient of gas flow in micro tube is the same as that in the conventional larger tubes if the effect of compressibility was well taken into consideration. The conventional heat transfer correlation for laminar and turbulent flow can be well applied for predicting the fully developed gaseous flow heat transfer performance in microtubes. There is no significant size effect for air flow in tubes within this diameter range.
For rough tubes, the internal surfaces are structure helical fin and random roughness. The rough circular tubes were lab made Nickel tube with diameters ranging from 901 to 977 ?m and roughness elements from 5.3 to 44.6 ?m in height. The experimental results indicated that the friction factor was significantly higher than the prediction of conventional correlations for smooth tube both in laminar and turbulent flow. Heat transfer enhancement in laminar flow is slight, but in turbulent flow the heat transfer enhancement was significant and the enhancement increases with the increasing of Re for the random rough tubes.
In order to verify the conclusions of the above experiments, a micro-channel heat exchanger was also designed and tested in this study. It provides an experimental analysis on the heat transfer performance of a flat aluminum tube micro-channel heat exchanger with/without spray cooling. The effects of water spraying rate, air flow rate and relative humidity were investigated. The test results show that the analysis methods for conventional size heat exchanger are still well applied in micro-channel heat exchanger; the spray cooling can increase the heat transfer performance with increasing spraying rate but without penalty of increased flow resistance at low spray conditions.

並列關鍵字

micro-channel heat exchanger ； thermal shunt ； liquid crystal thermography ； roughness ； size effect ； micro tube ； gaseous flow ； spray cooling

參考文獻

王啟川，2001，熱交換器設計，五南圖書出版公司，台北。

Adams, T. M., Abdel-Khalik, S. I., Jeter, S. M., and Qureshi, Z., H., 1998, “An Experimental Investigation of Single-phase Forced Convection in Microchannels,” International Journal of Heat and Mass Transfer, Vol. 41, No. 6-7, pp. 851-857.

Adams, T. M., Dowling, M. F., Abdel-Khalik, S. I., and Jeter, S. M., 1999, “Applicability of Traditional Turbulent Single-phase Forced Convection to Non-Circular Microchannels,” International Journal of Heat and Mass Transfer, Vol. 42, pp. 4411-4415.

Araki, T., Kim, M. S., Iwai, H., and Suzuki, K., 2002, “An Experimental Investigation of Gaseous Flow Characteristics in Microchannels,” Microscale Thermalphysics Engineering, Vol. 6, pp. 117-130.

Arici, M. E., and Aydin, O., 2009, “Conjugate Heat Transfer in Thermally Developing Laminar Flow with Viscous Dissipation Effects,” International Journal of Heat and Mass Transfer, Vol. 45, pp. 1199-1203.