Local evaporative heat transfer coefficient in turbulent free-falling liquid films

Abstract

Evaporative heating of a free-falling turbulent liquid film has been investigated experimentally and numerically. The film exhibited a long thermal development length persisting up to more than one half of the 781 mm long heated test section. The increased length of the development region is attributed to the formation of a boundary layer at the film interface. This boundary layer was predicted numerically and observed in temperature measurements within the film. The heat transfer coefficients are averaged over the lower section of the tube and correlated as a function of the Reynolds and Prandtl numbers. Numerical predictions are made for the development region and for fully developed heat transfer coefficients using three different eddy diffusivity models. Comparison with experimental data reveals that two of these models are fairly successful in predicting the extent of the thermal development region and the time-averaged evaporative heat transfer coefficient; yet the data indicate the need for development of a new model which accurately accounts for local and spatial wave-induced variations of film thickness.