Forced Convection Boiling in Tubes

Local heat transfer coefficients and pressure gradients have been measured for bulk boiling of water in forced flow (vertically upward) in round tubes with internal diameters ranging from 0.1162 to 0.4317 in., lengths of 15 to 40 in., heat fluxes of 6 × 10⁴ to 1.45 × 10⁶ Btu/hr ft², mass fluxes of 49 to 911 lb/sec ft², exit qualities of 5 to 57%, and at pressures ranging from 42 to 505 psia. The local heat transfer coefficients expressed in the form of the Nusselt number have been correlated in terms of Reynolds and Prandtl numbers and two dimensionless groups characterizing forced convection vaporization, the Lockhart-Martinelli parameter X_tt, and the “boiling number,” Bo = (q/A)/Gh_fg. For large values of the latter, nucleate boiling predominates and the dependence of the heat transfer on X_tt is small. Conversely, at the small values of Bo, convective flow effects are dominant, and there is a strong dependence on X_tt.