Evaporative Heat Transfer of Annular Two-phase Flow of Air-water in a Small Vertical Tube

The evaporative heat transfer of the non-boiling annular two-phase flow of air-water in a small vertical tube with uniform wall heat flux was studied both theoretically and experimentally. A simplified two-phase flow boundary layer model was used to calculate the thickness of the water film attached to the wall, and from the liquid film thickness the evaporative heat transfer coefficients of the annular two-phase flow were obtained. Theoretical equations and semi-theoretical equations were proposed for predicting the evaporative heat transfer of the annular two-phase flow of air-water in a small vertical tube. The semi-theoretical prediction agrees well with the experimental data. The mechanism of the heat transfer enhancement is the evaporation of the thin liquid film attached on the wall.