Heat and mass transfer in falling film evaporation of viscous liquids

Heat transfer in single component and multicomponent falling liquid films undergoing surface evaporation was experimentally and theoretically studied with emphasis on extending the data base and theoretical analysis to Prandtl and Schmidt numbers larger than those in existing studies.;For single component films, evaporative heat transfer coefficients were measured for two test fluids: water and propylene glycol. The maximum Prandtl number reached in our single component experiments was 46.6, which is about an order magnitude larger than that reached in Chun and Seban's (1971) well known data base. Comparison with Chun and Seban correlation indicated that the correlation was not reliable at Prandtl numbers larger than 10 because it underpredicted our propylene glycol data by up to 50% in the wavy laminar flow regime and overpredicted by up to 70% in the fully turbulent flow regime. In the wavy laminar flow regime, the dimensionless heat transfer coefficient was successfully correlated with Reynolds and Kapitza numbers. In the fully turbulent flow regime, a turbulence model capable of representing data quite well at all Prandtl numbers was developed and validated. A closed form approximate expression for the dimensionless evaporative heat transfer coefficient in turbulent films was obtained by applying asymptotic analysis as Prandtl number approaches infinity. The approximate expression was also capable of predicting the heat transfer coefficient of sensible heating and the mass transfer coefficient of absorption.;For multicomponent films, evaporative heat transfer coefficients were measured for three binary mixtures of glycerol and water. The maximum Prandtl and Schmidt numbers reached in our binary mixtures experiments were 220 and 95,000, respectively. The new data considerably extended the previously existing data base by a factor of 17 in Prandtl number and by a factor of 237 in Schmidt number. The measured heat transfer coefficients were found to be smaller than the expected value for single component films by up to 60% indicating strong mass transfer resistances. Comparison with the mass transfer model proposed by Palen, Wang, and Chen (1994) indicated that the model was satisfactory up to a Schmidt number of 7,000. For Schmidt numbers larger than 7000, Palen, Wang, and Chen model underpredicted the evaporative heat transfer coefficient data by up to 100% with the difference increasing with Schmidt number. An improved mass transfer model was developed and validated over the entire Schmidt number range covered in this study.