Theoretical and experimental study on liquid film thicknesses of unsteady slug flows in a capillary tube

Abstract Liquid film thickness is an important parameter for predicting evaporation and condensation heat transfer performance of two-phase slug flow in a capillary tube. Under flow boiling condition, the bubble of two-phase slug flow is elongated and accelerated. In the present study, the effect of acceleration on the liquid film thickness of unsteady two-phase slug flow is theoretically and experimentally investigated. It is first theoretically found that the accelerated slug flow is affected by the dimensionless numbers of capillary number (Ca), Weber number (We), and Bond number (Bo). The relation of the dimensionless liquid film thickness with the capillary number, Weber number, and Bond number for unsteady accelerated slug flows was obtained. The liquid film thicknesses of two-phase slug flows under acceleration condition were directly measured using laser focus displacement metering technique. Circular capillary tube with inner diameter of 1 mm was used for the test tube, and water, ethanol and FC-40 were used as working fluids. The novel prediction correlation of liquid film thickness for accelerated slug flows was proposed and it well predicts all the experimental data for all conditions. As the Bond number increases, the liquid film thicknesses are gradually shifted in the direction of decreasing film thickness due to acceleration effect. At a fixed capillary number, initial liquid film thickness becomes thinner with increasing Bond number.