Characterization of gas mixing in T-type micromixers in laminar and engulfment flow regimes

Autor: Hu, Yu-Siang, 胡宇翔
Rok vydání: 2017
Druh dokumentu: 學位論文 ; thesis
Popis: 105
The purpose of this study is to investigate gases mixing inside T-type micromixers. Both numerical simulation using ANSYS CFX and experimental approach with PSP (Pressure Sensitive Paints) are applied to analyze the oxygen concentration and fluid field inside the micromixers. Due to the different flow velocity in the micromixers, the fluid field is distinguished to different flow regimes: laminar flow regime and engulfment flow regime. The effect of gases properties on mixing effect has been observed in specific flow regimes and discussed. The height of the micromixers is 125 μm and the width used in this study are 550 μm and 350 μm, therefore, their aspect ratio are 0.23 and 0.35, respectively. The length of two gas inlet channels are the same as 5 mm, while mixing channel is set as 10 mm. The structures of micromixers are fabricated by soft photolithography. The pressure (2 kg/cm^2) and temperature (105 ℃) are applied to make the structure on the PMMA sheets with hot embossing. The micromixer is constructed by combing two PMMA sheets with thermal fusion bonding. Three sets of gases mixing are investigated in this study, and they are Nitrogen-Oxygen, Argon-Oxygen, and Helium-Oxygen. The inlet velocities of two gas flow are the same and controlled from 3 m/s to 50 m/s (Re_(O_2 )=36.1~600.9, Pe=7.7~489.9). The detailed oxygen concentration inside the micromixers are measured by using PSP sensor with different gases mixing. The experiment results show that the mixing effect is dominated by the molecular diffusion when inlet velocity is less than 15 m/s (Re_(O_2 )=199.4, Pe=147.0). For inlet velocity is 3 m/s (Re_(O_2 )=39.9, Pe=27.8), the mixing efficiency of Helium-Oxygen at the exit of 550 μm wide micromixer is 96.44%, which is greater than 76.03% of Nitrogen-Oxygen. The mixing efficiency of Argon-Oxygen is 72.46%, and it is the lowest compared to others. The gases with higher diffusion coefficient with oxygen molecules will have better mixing efficiency. It is also observed that the mixing efficiency decreases with increase of inlet velocity due to the reduced diffusion time of gases. As 350 μm wide micromixers are used, namely with the higher aspect ratio, the diffusion distance becomes shorter and the residence time of gases become longer by stronger shear stress. Therefore 350 μm wide micromixers have better mixing performance compared to 550 m wide micromixers. For inlet velocity of 3 m/s (Re_(O_2 )=36.1, Pe=27.8), mixing efficiency of Nitrogen-Oxygen has improved to 98.40% in the 350 μm wide micromixer, greater than that (76.03%) in the 550 μm wide micromixer. If inlet velocity is increased to higher than 30 m/s (Re_(O_2 )=360.5, Pe=293.9), the fluid field of Argon-Oxygen mixing changes to engulfment regime. Gas mixing is enhanced as the interfacial area of gases increases due to twisting of two gases in engulfment regime. Mixing efficiency of Argon-Oxygen is 58.20%, higher than 53.78% of Nitrogen-Oxygen. When the inlet velocity further increases to 50 m/s (Re_(O_2 )=600.9, Pe=489.9), mixing efficiency of Argon-Oxygen enhances to 90.45%, much higher than 60.07% of Nitrogen-Oxygen. The Reynolds number triggering fluid flow into engulfment regime has inverse relation with dynamic viscosity of the fluid. It is observed that the fluid field of Argon-Oxygen changed into engulfment regimes earlier than Nitrogen-Oxygen due to the higher dynamic viscosity of Argon. In addition, the fluid field of Argon-Oxygen mixing in 550 μm wide micromixer does not change into engulfment regime until Re_(O_2 )=398.7 while the 350 mm wide micromixer triggered earlier at Re_(O_2 )=360.5. Because the wider micromixer has larger surface at the upper and bottom walls, it will slow the development of vortices and delay the development of the engulfment flow. Engulfment flow is not observed inside both 350 m and 550 m micromixers for Helium-Oxygen mixing in the inlet velocities varying from 3m/s to 50 m/s and only asymmetry vortices has been observed at higher velocity due to the great difference of density between Helium and Oxygen. Therefore, mixing efficiency of Helium-Oxygen will not increase as inlet velocity increases at higher inlet velocity conditions. Finally, the experiment data acquired by PSP technique and simulation calculated with ANSYS CFX have good agreement in the laminar flow regime. The profiles of oxygen concentration obtained from experimental method deviate from simulation data in the engulfment flow regime, and it is attribute to the surface condition which is assumed smooth in the simulation. However, the critical Reynolds number triggering the engulfment flow shows favorable agreement in the experiment and simulations.
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