Condensation in plain horizontal tubes: recent advances in modelling of heat transfer to pure fluids and mixtures

Recent work on improving general thermal design methods for condensation inside plain, horizontal tubes is presented, summarizing primarily the advances proposed at the Laboratory of Heat and Mass Transfer at the EPFL in collaboration with the University of Padova and the University of Pretoria. This work has focused on the development of a unified flow pattern, two-phase flow structure model for describing local heat transfer coefficients for pure fluids, azeotropic mixtures and zeotropic mixtures. Such methods promise to be much more accurate and reliable than the old-style statistically-derived empirical design methods that completely ignore flow regime effects or simply treated flows as stratified (gravity-controlled) or non-stratified (shear-controlled) flows. To achieve these goals, first a new two-phase flow pattern map for condensing conditions was proposed, which has been partially verified by flow pattern observations. Secondly, a new condensation heat transfer model for pure fluids and azeotropic mixtures has been developed including not only flow pattern effects but also interfacial roughness effects. Finally, the widely used Silver-Bell-Ghaly condensation model for miscible vapor mixtures has been improved by including the effects of interfacial flow structure and roughness on vapor phase heat transfer and a new non-equilibrium effect added.