The Study of Fluid Flow and Heat Transfer Inside Rectangular PDMS microchannels

Abstract

摘要 本論文主旨在探討微流道內之流體行為及熱傳現象，並與巨觀尺寸下之結果相比較，主要量測之物理量為流體的速度及溫度。分別利用微粒子影像測速法及溫度螢光分子感測法進行實驗，其中以微粒子影像測速法量測微直管與90。彎管流道內的速度場，並以溫度螢光分子感測法量測微流道內流場深度方向整 體之平均溫度分佈及微流道壁面溫度，此兩法皆具有非侵入式及全域性量測之特性，提供一個不干擾流場情形之量測結果。 以往溫度螢光分子感測法大都運用於航太工程相關領域中，偏向巨觀尺寸範圍之量測，直到近幾年來逐漸有學者將此技術轉移至微流場量測上。以往微機電系統溫度量測裝置都需要透過繁瑣的製程步驟來完成，而有別於傳統微機電系統溫度量測方式，溫度螢光分子製作方法簡易，且可提供高解析度之流場中全域性溫度分佈。本研究考慮不同螢光感測分子搭配適合之溶劑及黏著劑，製做出螢光溫度感測溶液及塗料，利用螢光溫度感測分子對溫度之光化學特性，即可開發出應用於微流道溫度量測之工具，且同時利用逐點校正法彌補傳統單點校正之不足，增加量測準確性及量測範圍，建立更完整的測量方法。 本論文以PDMS微矩形直管流道及微矩形90。彎管流道量測其內部流場速度向量變化並加以探討，在微直管部分以去離子水為工作流體，在雷諾數為0.37時，量測出在深寬比0.67及0.33之流道內部由發展區至完全發展區之速度分佈，空間解析度可至近壁2 μm之位置。且觀察到PDMS表面疏水性使邊界出現滑移現象，而滑移長度介於1.9~3.3 μm間，由中心位置往側壁推算80%距離內之範圍與解析解相當穩合。而由工作流體溫度變化發現在深寬比0.12之直管微流道中，熱發展長度為Gz-1/2~0.384，且在熱完全發展區中Nu為2.20；而在微彎管部分則是在雷諾數27時以分層方式量測不同深度平面速度分佈，並成功建構出其二次流情形，且由軸向及橫向溫度變化也可看出二次流使流體產生混合導致轉角後溫度驟升的效果。本研究得到二次流對微流道熱傳之影響，並可應用於往後微熱交換器之設計上。 ABSTRACT This study aims to examine the velocity and temperature fields in various microchannels and compares with theory. With novel molecule based temperature sensors, both surface temperature and fluid temeperature profiles inside the microchannels were acquired. 2-D velocity profiles were analyzed through μ-PIV techniques. These two techniques both provide non-intrusive and global measurements. The molecule based temperature sensor was widely used in aerodynamic engineering in the past decades, but applications in micro fluidic system measurements were just began in recent years. Differentiates from traditional micro temperature sensors prepared by complicated MEMS fabrication procedure, the molecule based temperature sensors simplifies the processes of sensor preparation as well as the installation. By selecting different kinds of luminescent molecule along with various solvent and binder, the molecular based temperature sensors have been investigated through theirs photochemistry properties and they have been applied to the measurement of micro fluidic system. At the same time, pixel-by-pixel calibration method was utlized to increase the accuracy and extend the range of temperature measurement. This study also performed on velocity measurement in rectangular microchannel with straight and 90。 sharp bend structures and DI water was used as working fluid. The μ-PIV system was demonstrated to be capable of acquiring velocity profiles at the distance 2 μm away from the side wall from the developing region to fully developed region in straight microchannels with aspect ratios of 0.67 and 0.33 at Reynolds number of 0.37. The slip length was calculated in the range of 1.9~3.3μm at fully developed region due to the hydrophobicity of PDMS material. The velocity profile calculated by Navior-Stokes equations with non-slip boundary condition agreed with the experimental results from center to 80% of the channel. The temperature distribution was measured in straight microchannels with aspect ratio of 0.12 and bottom side was heated as uniform wall temperature thermal boundary condition. Nusselt number variation in the channel was analyzed from thermal developing region to thermal fully developed region. Results showed thermal entrance length was Gz-1/2~0.384 and Nu reached 2.20 in the fully developed region. The velocity profiles of flow passing through a 90。 sharp bend were also measured at Reynolds number of 27.66 and 46.62. Secondary flow structure has been observed with multiple layers measurements around the corner along the depth of the microchannels. The temperature distributions of axial(x) and crosswise(y) directions before and after the corner show fluid mixing due to secondary flow effect. This study not only measured and analyzed the flow and thermal fields in microchannels but also provided essential information for future applications of micro-heat-exchanger.