BT-16 Falling Film Evaporation of Ultra-Viscous Fluids
Phase III of Lehigh University’s falling film evaporation research focused on adding ultra-viscous film data to the existing database. The objective was to measure heat transfer coefficients for evaporating falling films with polymer solution viscosities up to 2419 lb/hr ft (1000 mPa s).
We performed experiments using aqueous mixtures of polyethylene glycol (PEG) with polymer weight fractions varying from 0.50 to 0.95. Two hundred thirty-two experimental runs were conducted, and fluid viscosities reached 2902 lb/ft hr (1200 mPa s). These new experiments extended the Prandtl number range by a factor of 57, the Kapitza number range by a factor of 10 to the 7th, and the Schmidt number range by a factor of 1700.
Using this extended database to assess existing correlations for falling film evaporation, we determined that the Alhusseini-Tuzla-Chen (ATC) correlation from Phase II works well within its original parameter range. However, the model often underpredicted heat and mass transfer coefficients for fluids with Prandtl numbers greater than 220 and Schmidt numbers greater than 10 to the 5th. To improve the model, we developed new correlations for the laminar contribution to both heat and mass transfer. The revised model, which satisfactorily represents all data through Phase III, is suitable for the design of falling film evaporation exchangers with Prandtl numbers up to 1.3 (10 to the 4th) and Schmidt numbers up to 1.8 (10 to the 7th).