BT-14 Falling Film Evaporation Under Vacuum Conditions

J. C. Chen, A. Alhusseini, and K. Tuzla

The Phase II project was undertaken specifically to study falling film evaporation of wide boiling-range mixtures under vacuum operating conditions. The objectives were to

  • Obtain experimental data at subatmospheric pressures over an extended range of fluid viscosities, Prandtl numbers, and Schmidt numbers.
  • Assess the validity of existing models for predicting falling film heat transfer coefficients over this extended range of parameters.
  • Develop improved correlations for evaporative heat transfer that would be applicable for both atmospheric and vacuum conditions, if necessary.

A new experimental loop, funded by Lehigh University, was designed and fabricated to permit falling film experiments under vacuum conditions. The test section consisted of a 1.5 in. (38.1 mm) OD tube with a heated length of 9.75 ft (2.97 m). Liquid flowed down the outside of this vertical tube as a falling film, which was observable through a glass column that enclosed the entire test tube. Experiments to measure the heat transfer coefficients for evaporation of single component films (water and propylene glycol) and for films of binary mixtures (aqueous solutions of glycerol) were conducted. A total of 86 runs were obtained for the single component films and 117 runs for the mixture films. These new data considerably extended the previously existing database, increasing the range of Prandtl numbers by a factor of 17 and Schmidt numbers by a factor of 237.

Using these new data to assess the validity of existing models and correlations, we conclude that

  • The Chun-Seban model for heat transfer coefficients of single-component films is inadequate for fluids with Prandtl numbers greater than 10.
  • The Palen-Wang-Chen model for mass transfer coefficients in falling film evaporation is satisfactory up to a Schmidt number of 7000. This is surprisingly good since the original database for Palen et al. was limited to Schmidt numbers less than 500.
  • For mixture films of Schmidt numbers greater than 7000, the Palen-Wang-Chen model underpredicts the mass transfer coefficient; the difference increases with the Schmidt number.

Using the new database, improved models were developed for

  • Single-component evaporative heat transfer coefficients in falling films for laminar, transition, or turbulent regimes
  • Mass transfer coefficients for falling film evaporation of mixtures, applicable in laminar, transition, or turbulent regimes

These improved models satisfactorily correlated all the new data over the entire extended parameter range.