Liquid Film Thickness in Micro Tube Under Flow Boiling Condition

Slug flow is one of the representative flow regimes of flow boiling in micro tubes. It is well known that the thin liquid film formed between the tube wall and the vapor bubble plays an important role in micro scale heat transfer. In the previous study [1], liquid film thickness under adiabatic condition was investigated and an empirical correlation for the initial liquid film thickness based on capillary number, Reynolds number and Weber number was proposed. In the present study, the effects of wall superheat and bubble acceleration on the liquid film thickness are investigated. Under flow boiling condition, bubble velocity is not constant but accelerated, and it is necessary to consider this acceleration effect on the liquid film thickness, since it may affect the viscous, surface tension and inertia forces in the momentum equation. In addition, viscous boundary layer develops, and it may also affect the liquid film thickness. Besides, viscosity and surface tension coefficient are sensitive to temperature change. If wall superheat is high, it is crucial to consider the property change according to the temperature variation. In order to investigate these effects, laser focus displacement meter is used to measure the liquid film thickness. Ethanol, water and FC-40 are used as working fluids. Circular tubes with three different diameters, D = 0.5, 0.7 and 1.0 mm, are used. It is observed that when the wall superheat is larger than 5°C, liquid film thickness becomes thinner than the adiabatic case due to the decrease of viscosity near the wall. The increase of liquid film thickness with capillary number is restricted by bubble acceleration. Finally, an empirical correlation is proposed for accelerated flows in terms of capillary number and Bond number based on bubble acceleration.