| Summary: | 碩士 === 國立臺灣科技大學 === 機械工程系 === 106 === Acoustic streaming is a steady flow phenomenon induced by high-frequency oscillation of the flow or obstructions in the flow regime. The oscillation can be transported through different media, usually by gas bubble or certain solid obstruction geometries in the flow. In this study, acoustic streaming flows inside micro-channels induced by three different media, which including air bubble, oil droplet and solid obstruction, are compared and investigated by flow visualization and Particle Tracking velocimetry (PTV). The micro-channels are made by poly(dimethylsiloxane) (PDMS) using a standard fabrication process for a microfluidic device. The casts are made by micro-milling process to reduce the manufacturing cost and minimize the background reflection due to channel surface roughness. The characteristic dimensions of the media are 0.2 mm and 0.5 mm in diameter, and the oscillation generated by piezoelectric actuators has frequencies of 1 kHZ, 2 kHz and 12 kHz and input voltages of 20 V and 40 V, respectively. The particle trajectories are visualized by a microscope with 10X and 20X magnification and long exposure experiments using 5 μm polyimide particles. The velocity fields are measured by PTV using 2.6 μm polystyrene tracer particles. The experimental results show that the gas bubble creates the strongest vortical streaming flow under the same operating condition, but the size of the bubble is hard to maintain steadily. The solid obstruction induced vortical flow is the weakest of the three, but the streaming flow in the far field has a large 3-D circulation feature. The strength of the droplet-induced streaming flow is in between the other two types, and a similar far field circulation feature is observed. PTV results show that bubble-induced acoustic streaming has the largest peak absolute velocity at 21 mm/s, followed by droplet-induced flows at 12 mm/s, and the solid obstruction has the smallest velocity of 2 mm/s. On the other hand, the streaming flow induced by bubbles is more concentrated at the side, but on average the droplet-driven streaming flows have larger mean velocity. The second order curve-fitted velocity profiles show that at Y/D = 0.08 the bubble-driven flow has the largest absolute velocity of 20 mm/s, and at Y/D = 0.66 the droplet-driven flow has the largest absolute velocity at 7 mm/s.
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