Falling film evaporation of binary mixtures

The interactive effects of heat and mass transfer in evaporation of binary mixtures flowing as falling films on vertical surfaces was investigated. Evaporative heat‐transfer coefficients were measured for aqueous mixtures of ethylene and propylene glycol, with boiling ranges up to 55°C. Tests were carried out at atmospheric pressure with heat fluxes ranging from 3,000 to 25,000 W/m2, and film Reynolds numbers ranging from 300 to 3,000. Results indicated that the heat‐transfer coefficient for mixtures depends weakly on wall superheat and film Reynolds number, but strongly depends on mixture composition. Analysis of the results indicates that mass‐transfer resistance in the liquid film causes significant elevation of the interface temperature, causing a reduction of the effective temperature driving force. A semiempirical model for correlation of the interactive heat‐and mass‐transfer phenomena is proposed.