| Summary: | 碩士 === 國立清華大學 === 動力機械工程學系 === 103 === This study aims to investigate the mixing effects of oxygen and nitrogen gases in T-type micromixers. Commercial CFD software ANSYS CFX is used to simulate the flow fields inside the T-type micromixers with different aspect ratios of microchannels and Reynolds number effect, and the physic phenomena in flow field and mixing efficiency are analyzed quantitatively. For experimental approaches, PMMA sheets and double-sided tape made by PET film are used to fabricate T-type micromixers, which have rectangular cross-section and inlet and main (mixing) channels are in the same size. The microchannel is 10 mm long, 125 um deep and 550 um / 720 um wide. A non-intrusive experimental technique, pressure-sensitive paints (PSP), is applied to T-type micromixers for acquiring the global flow field with detailed oxygen concentration during the measurements. The spatial resolution and accuracy in the acquired data has been further improved by integrating a luminescence microscope with 2X objective lens and applying in-situ and pixel-by-pixel calibration during data processing.
From the experimental results, it can be clear seen that molecular diffusion is dominant during gaseous mixing at low Re numbers from 13.9 to 50.9. The mixing efficiency at micromixer outlet can reach to 95.92% while the width of microchannel is 550 um and Re is 13.9. If axial flow speed in the main (mixing) channel increases and inlet gas flow rate increases (Re number of 50.9), the flow quickly brings fluid to downstream and only partly fluid are mixed; therefore, the mixing efficiency decreases to 64.64%. Furthermore, the molecular path increases and shear stress on the upper/lower wall become relatively large if using microchannels with width of 720 um since aspect ratio decreases. The mixing efficiency at the outlet decrease to 8.7% compared to the microchannel with 550 um width.
If the inlet flow rate further increases to 149.7 ccm in the 550 um wide microchannel (Re=596.1), the symmetry in the flow field is no longer exist and engulfment flow regime is identified. In this regime, the interfacial area between nitrogen and oxygen gases increases due to the engulfment flow which shortens diffusion path and improve mixing performance. The asymmetrical flow rates at the inlets of T-type micromixers can also lead to the improvement of mixing. Engulfment flow cannot be seen in 720 um wide micromixer with the same flow rate due to the lower aspect ratio.
To sum up, the feasibility of PSP sensor (Ru(dpp) with silicone rubber) in T-type micromixers has been demonstrated to provide global oxygen concentration information which has good agreements with numerical results.
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