Correlating liquid film thickness in microchannel slug flow to bubble length and velocity

Two-phase gas-liquid flows are commonly encountered in various industrial processes. The slug flow regime, characterized by alternating streamwise segments of gas bubbles and liquid slugs, is frequently observed in microscale channels where capillary forces play an important role. Accurate knowledge of the liquid film thickness surrounding bubbles during slug flow is crucial for predicting various hydrodynamic, interfacial, and thermal transport characteristics of interest, such as heat transfer during microchannel flow boiling. In this study, we experimentally and numerically investigate the effect of bubble length and velocity on liquid film thickness in an air-water slug flow in a single microchannel of circular cross-section. In the experiments, an open-loop flow facility is used to generate air-water slug flow in a circular microchannel with independently varying bubble lengths and velocities. A laser confocal displacement meter is used to measure the liquid film thickness, while the bubble length and velocity are extracted from high-speed visualizations. The liquid film thickness is observed to increase with increasing bubble velocity and length; while this effect of velocity has been reported in the literature, this is the first reporting of the influence of bubble length on liquid film thickness. Additionally, numerical simulations that replicate the experimental boundary conditions are performed using a two-phase volume-of-fluid approach to corroborate this trend. Furthermore, as the bubble length increases for a given bubble velocity, the film thickness asymptotically approaches a maximum value that agrees with the semi-infinite bubble approximation previously reported in the literature. A new empirical correlation is developed that is valid for all bubble lengths and offers accurate predictions [9 % mean absolute error (MAE)] of the liquid film thickness as a function of the dimensionless bubble length and dimensionless capillary number.