CONDENSATION IN PRESENCE OF SUPERHEATED VAPOR FOR NEAR-CRITICAL CO2 AND R410A IN HORIZONTAL SMOOTH TUBES

Just below the critical pressure, superheat zone becomes longer and the enthalpy changes sometimes even greater than two-phase zone in condensers. Thus, the heat transfer mechanism in superheat zone becomes practically important. This paper provides experimentally determined heat transfer coefficient of CO2 and R410A heat rejection in a smooth tube of 6.1 mm ID at reduced pressures from 0.55 to 1.0, heat fluxes from 3 to 20 kWm(-2), and from superheat 20 K to vapor quality 0.02. The experimental coefficients are compared to correlations by Gnielinski and Cavallini et al, which valid for sufficiently superheated vapor and saturated condensation. When the tube wall reaches saturation temperature, the experimental coefficient starts gradually increasing from Gnielinski's correlation. This identifies the start of condensation in superheat zone. The semi-empirical correlation, which is here proposed as the combination of both correlations, satisfactorily predicts the heat transfer coefficient of the superheat zone condensation.