Liquid velocity distribution in slug flow in a microchannel

The liquid velocity distributions in two-phase slug flows in a nearly rectangular microchannel etched on a microfluidic chip were investigated using a three-dimensional tracking method for submicron fluorescent particles seeded in the working liquid (water). The Taylor bubbles generated from dissolved air in the water through heating the micro-fluidic chip to 35–55°C had low velocities, so they had the very small Capillary and Reynolds numbers. The change in the Taylor bubble shape with the flow conditions such as the bubble velocity and temperature was negligible in the present study. The shapes of the fully developed Taylor bubbles were determined by analyzing the interference fringes in in-line holographic images of the microchannel section containing the bubble to estimate the bubble cross-sectional area. The three-dimensional liquid velocity distribution for the two-phase slug flow was obtained using a least-squares fit of the measured tracer particle velocities to an analytic velocity distribution for Poiseuille flow in a rectangular channel to determine the mean liquid velocity and the liquid flow rate in the slug. The liquid velocity was normalized using the measured instantaneous bubble velocity to remove the influence of the slug and the bubble velocity fluctuations on the liquid velocity distribution. The results show that the mean liquid velocity through the microchannel corners is 2.3 times the bubble velocity, which is in agreement with previous observations of the maximum liquid velocity in the corners of 3.5–3.8 times the bubble velocity.