Near-zero shear stress experiments with heat flux effects on falling film evaporation inside a vertical tube

Local heat transfer coefficients for falling film evaporation of isopropanol were experimentally measured ((Uh, 95%/h)max = ±3.5%) inside a vertical brass tube (dinside = 28 mm) at near-zero shear stress. Zero shear stress experiments with falling film evaporation inside a tube are extremely difficult to realize - not to say they are impossible. In literature respective heat transfer investigations and visual flow observations were mostly done at outside tube surfaces surrounded by an extended volume. For the present investigations at near-zero shear stress a special vapour flow sensor was developed and installed in the experimental tube to locate the level of near-zero vapour velocity. The measurements included film Re numbers up to 100, inner wall heat flux up to 12,500 Wm−2 without bubble formation in the superheated liquid, and vapour temperatures ranging from 8.5 °C to 36 °C (Prliquid, freesurface = 14.5-20.8). The heat transfer measurements were focussed on Re number and heat flux effects on falling liquid film evaporation. The basic intension of this paper is a comparison of the evaporation heat transfer measurements with the characteristics in the various Re number ranges, and also a comparison with the authors' earlier condensation results as reported in Refs. [1,2]. Thereby influences of Re, Pr and Ka numbers on heat transfer are discussed, correlated (within deviations of ±2.7% from measured data) and compared with the literature.