Abstract
The aim of this study is to investigate the mixing effects of oxygen and nitrogen gases in micromixers with different designs of boundary obstructions. Numerical simulation and experimental measurement are used to verify and compare at various Reynolds numbers. At first, Numerical software ANSYS CFX is applied to simulate the flow field inside the T-type micromixers. The inlet of two gases and main (mixing) channels are 10 mm long, 1 mm wide, and 125 μm deep. After two gas flow intersecting at the T-junction, rib structures with different designs of symmetric, staggered or triangular are positioned in the main channel to form the micromixers with boundary obstructions. The flow field as well as the mixing efficiency and pressure drop due to these obstructions are investigated and discussed in the Reynolds numbers (Re) ranging from 1 to 150. In addition, different geometrical parameters of aspect ratio of length/width in the obstruction are discussed with simulation and the optimal parameters of obstructions of 400 μm long and 400 μm wide is found to be used in the following experiments. For the experiment of oxygen and nitrogen gases mixing, Pressure-Sensitive Paint (PSP) and the fluorescence microscope system are utilized to acquire the image of global oxygen concentration inside the four micromixers. The experimental results indicate that the mixing effect in T-type micromixers is merely dominated by diffusion in the range of inlet Re number from 10 to 150. The increase of flow velocity will shorten the residence time of fluid therefore decrease the mixing efficiency at the outlet from 74.63% to 24.41%. For the micromixers with symmetric obstructions, the gas flow passes through the obstructions with accelerating/decelerating while the flow contracts/expands. The distance for diffusion between two gas flows is reduced and mixing efficiency is improved. However, the flow is also accelerated due to constricted structures and reduces the diffused time, and mixing efficiency becomes lower if Re number increases. The pressure drop in the micromixer with symmetric obstructions is higher than the other three designs. Staggered micromixers provide better mixing efficiency by introducing the secondary flow while the gas flow passing the obstructions and generating transverse velocity and momentum, which increase the interfacial area between gases. For the Re number less than 50 (Pe number less than 38.19), mixing mechanism is dominated by diffusion. When Re number equals to 50 (Pe number equals to 38.19), the mixing efficiency reaches the lowest values among all Re number conditions. For the Re number greater than 50 (Pe number greater than 38.19), convection mass transfer dominates the mixing effect with the help of vortices formation behind the obstructions. The mixing efficiency is lower at Re number of 50 (Pe number of 38.19) and then increases with increase in Re number. Staggered obstruction can improve the mixing efficiency effectively at higher velocity and provides the best mixing efficiency among these four designs. Triangular micromixers have the similar mixing efficiency as the staggered one. The triangular obstructions will introduce agitation to the gas flow and generate vortices behind the obstructions at high flow velocity. The mixing efficiency in triangular obstruction has the same trend of staggered one and the minimum mixing efficiency is observed at Re number of 75 (Pe number of 57.28). However, the results might be deviated due to the imperfections of the sharp corners in the triangular obstruction due to manufacturing processes. The mixing efficiency reduces 21.75% compare to that in the staggered micromixers when Re number of 150, but the pressure drop is reduced by 2.69 kPa. In this study, global oxygen concentrations in micromixers with four different obstruction designs are successfully measured by PSP technique. The visualization and quantitative results are compared to simulation results with ANSYS CFX. This study provides detailed investigation of the gas mixing effects in microscale.