Study of the influence of interfacial waves on heat transfer in turbulent falling films

Abstract

This study explores the influence of interfacial waves on mass, momentum and heat transfer in turbulent, free-falling water films that are subjected to sensible heating. Measured temporal records of film thickness and temperature profile across the film are used to examine the film’s thermal response to the passage of large waves. The temporal variations of liquid temperature and heat transfer coefficient are generally opposite to that of film thickness; the heat transfer coefficient is highest in the substrate regions upstream and downstream of large waves and lowest in the waves themselves. Increasing the film’s Reynolds number increases the mean thickness and wave amplitude, and decreases the wave period, but results in appreciable attenuation in the measured liquid temperature response to the large waves. Using FLUENT, a computational model of the falling film is constructed and its predictions compared to the data. The computed results show good agreement with the measured mean film thickness, wave form and period, and both wall and mean film temperatures. The model captures the measured increase in liquid temperature in the film substrate and decrease corresponding to the large waves, but the predicted temperature response is less attenuated for higher Reynolds numbers than the measured. Velocity predictions point to acceleration of high temperature liquid from the upstream substrate toward the cold region within the large wave before losing the excess heat due to mixing downstream from the wave crest. Overall, the present study demonstrates the effectiveness of computational tools at predicting the hydrodynamic and thermal characteristics of separated flows involving a wavy liquid-vapor interface.