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Liquid film thickness is an important parameter for predicting boiling and condensation heat transfer in micro tubes. In the present study, the effect of initial flow velocity on the liquid film thickness in accelerated flows under adiabatic condition is experimentally investigated. The laser focus displacement meter is used to measure the initial liquid film thickness. Circular tube with inner diameter of 1 mm was used for the test tube, and water, ethanol and FC-40 are used as working fluids. When the flow is accelerated from small initial velocities under small Bond number condition, initial liquid film thickness is identical to that of steady flow at small capillary numbers, and then deviates from the steady condition and eventually follows that of accelerated flow from zero initial velocity as capillary number is increased. When the flow is accelerated from large initial velocities, initial liquid film thickness deviates from the steady condition earlier and starts to follow that of accelerated flow from zero initial velocity

Liquid film thickness is an important parameter for predicting boiling and condensation heat transfer coefficients in micro tube slug flows. In the present study, the effect of bubble deceleration on the liquid film thickness is experimentally investigated under adiabatic condition. The laser focus displacement meter is used to measure the liquid film thickness. Circular tubes with three different inner diameters, D = 0.7, 1.0 and 1.3 mm, are used. Measurement is carried out using a micro tube with one open end and the other connected to an actuator motor. Water, ethanol and FC-40 are used as working fluids. It is found that deceleration makes the liquid film thicker than that in the steady flow, and it deviates from the steady case as the deceleration rate is increased. Liquid film thickness remains nearly unchanged just after the onset of deceleration, and then gradually decreases and eventually converges to the steady thickness as the velocity is further decreased. Finally, an empirical correlation is proposed to predict

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