EXPERIMENTAL STUDY OF INITIAL LIQUID FILM THICKNESS IN SQUARE MICROCHANNEL TWO-PHASE FLOW

Liquid film plays an important role in two-phase flow and flow boiling heat transfer in microchannels. Compared to circular microchannels, square microchannels have higher heat transfer coefficients due to the thinner liquid film. In the present study, liquid film thicknesses in square microchannels were measured. The hydraulic diameters of the microchannels were 0.39, 0.6, and 0.7 mm, respectively. Two working fluids - water and ethanol - were used in twophase flow. Transient measurement of liquid film thickness was synchronized to high-speed flow visualization. The experiments showed that the liquid film remained stable after reaching the initial liquid film thickness at small capillary numbers (Ca). But the liquid film thickness continued to decrease at higher Ca. Comparisons of the two working fluids indicated that the liquid film thickness depended on the physical properties, including liquid viscosity and surface tension. The liquid film was thicker in microchannels of larger hydraulic diameters. The initial liquid film thicknesses at different Ca were analyzed. It was found that the initial liquid film thickness decreased with the increase of Ca and reached the minimum value at Ca ~ 0.03. After that, the initial liquid film thickness increased with the increase of Ca. A model of initial liquid film thickness was constructed and a new semi-empirical formula was derived to predict the initial liquid film thickness in square microchannels.