Evaporation and condensation of steam-water in a vertical tube

Heat Transfer data have been obtained for water from single-phase flow to t wo-phase annular Row at 0.07-0.09 MPa in a 9.5 mm vertical bore tube under conditions of evaporation and condensation in the same test section. The ma in aim of the experiments was to elucidate the mechanism of heat transfer i n annular flow by distinguishing between the conventional explanation of a purely convective mechanism at high quality region and the alternative hypo thesis in which heat transfer is enhanced by secondary nucleation in the re gion. To avoid ambiguities in local hydrodynamic conditions the experiments were carried out under the same conditions (namely equilibrium annular flo w) for both evaporation and condensation in the same test section. The resu lts indicated a forced convective mechanism of the conventional type rather than the alternative thin film boiling mechanism (secondary nucleation) as suggested by Mesler (AIChE, 23 (1977) 448). The heat transfer coefficients in single-phase flow and annular flow regimes are compared with literature correlations. The results show that the present data are in reasonable agr eement with existing correlations. (C) 2001 Published by Elsevier Science B .V.