The effect of bubble deceleration on the liquid film thickness in microtubes

• Liquid film thicknesses in decelerated microtube slug flows are measured.
• LFDM technique is used to accurately measure the liquid film thickness.
• Deceleration makes the liquid film thicker than that in the steady flow.
• Empirical correlation is proposed for the initial liquid film thickness in decelerated flows.

Liquid film thickness is an important parameter for predicting boiling and condensation heat transfer coefficients in microtube 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 microtube 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 initial liquid film thickness in decelerated flows.