| Summary: | 碩士 === 國立清華大學 === 動力機械工程學系 === 103 === This study aims to investigate the characteristics of heat transfer in a 90-degree bend PDMS microchannel under one side constant wall heat flux thermal boundary condition with acquired velocity, fluid temperature and wall temperature distributions. The velocity profiles in the microchannel were obtained by Micro Particle Image Velocimetry (Micro-PIV), and the temperature distributions were measured by Temperature Sensitive Paint (TSP). The experimental results are compared with simulation data to verify the accuracy of measurement technique. In order to discuss the secondary flow in the 90-degree bend microchannel, two kinds of turn, sharp turn and round turn, are designed in this study.
Due to the concern that electrical double layer effect may affect the flow field and heat transfer in microchannel flow, the current monitoring technique is used to measure the zeta-potential of common TSP solutions, including Rubpy, Rhodamine B, Pyronin B and Pyronin Y. The thicknesses of electrical double layer are also calculated to find the ratio between hydraulic diameter and it. After carefully considering this ratio, the extent of electrical double layer effect could be determined.
In the flow field analysis using micro-PIV technique, the depth of correlation will change the acquired velocity profiles if using different depth range during velocity. Therefore, the depth of correlation based on the fluorescent particles and microscope system used in current experiment is first calculated to determine the influence to the results of velocity measurement. The median band-pass filter is performed in the measurement of turning region velocity during image process. Therefore, the secondary flow is observed by measuring velocity distribution at different depth in the microchannel flow.
In the heat transfer analysis, a micro-heater is fabricated as the heating device for the experiment to reduce the heat loss and avoid the axial heat conduction effect. The image process of 5×5 median filter is performed to reduce the noise in the fluid and wall temperature using TSP technique. From Nusselt number distribution calculated by fluid and wall temperature, it can be noticed that the mean Nusselt numbers along the microchannels with two different 90-degree bend designs, a sharp turn and a round turn, are increasing with increasing Reynolds number, and the Nusselt number in the sharp turn are larger than the round turn.
The pumping powers to drive the flow in the microchnanel device are calculated by using the pressure difference between inlet and outlet from simulation results, and they increase with increasing of Reynolds number. The sharp turn requires higher pumping power than the round turn. After calculating the ratio between mean Nusselt number and pumping power and considering the relative magnitude of ratio, the overall performance of heat transfer effect between sharp turn and round turn can be concluded. The sharp turn design shows higher heat transfer enhancement, but the round turn design has better performance if considering the pumping power.
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