Experimental and numerical studies of liquid-liquid two-phase flows in microchannel with sudden expansion/contraction cavities

The addition of micro cavities is a promising passive method to enhance mass transfer performance in micro-channels. Experimental and numerical methods are adopted to investigate the flow characteristics of velocity field, droplet shape and vorticity for the liquid-liquid two-phase flows in microchannels with micro cavities. The results indicate that the droplet expands in cavities for slug and transitional flow patterns with droplet length longer than the channel width, while the expansion is not obvious for droplet flow patterns. Dimensionless expansion width increases with decreasing Capillary number and with increases in droplet length. For long droplet with length more than 2.5 times of the channel width, the expansion width is nearly the same with increasing droplet length. Due to the sudden decrease and increase of cross-section areas in cavities, the droplet moving speed fluctuates in the cavities, leading to the pulsating flow in cavity-shaped microchannels. When the droplet passes through the cavity structure, the continuous phase near the wall enters the cavity and induces small vortexes in the cavities. The intensity of vortex is more pronounced with longer droplet due to the blocking effect of droplet expansion in cavities. The vorticity magnitude in cavities is always higher than the value in straight microchannel, indicating that micro cavities can enhance the fluid mixing in the microchannel.