Experimental and theoretical investigation of in-tube condensation of steam-air mixtures

Experiments on steam condensation inside inclined tubes were varied out at the University of Rome "La Sapienza" - DINCE in the COTINCO test series, with the following aims: a) to investigate the physical phenomena involved in condensation of steam within tubes; b) to study the influence of the geometry (namely, tube inclination) on the heat transfer rate, also in presence of high concentration of non-condensables; c) to develop models and heat transfer correlations for the given conditions; d) to produce a database for modeling in-tube condensation with high percentage of non-condensables. A correlation fitting the experimental data in dimensionless form has been proposed, referring to the local values of main experimental parameters. The mixture and liquid water temperature profiles have been obtained by fitting the thermocouple measurements and it was possible to reproduce the heat flux distribution along the tube and the heat and mass balances locally. The profile of air concentration and the interface temperature along the tube were calculated as well. After evaluating the steam-air mixture properties (density, specific heat, viscosity, thermal conductivity and the diffusion coefficient of steam in air), it was possible to correlate the mixture Sherwood number to the Nusselt number (accounting for condensation and sensible heat transfer) inside the tube, locally along the tube but averaged on the internal circumference.