Pressure Drop in Condensing Superheated Zone

Traditionally, condensation is characterized by three zones: desuperheating, condensation and subcooling. It was shown that condensation starts even in the presence of superheated vapor, when the wall temperature reaches saturation. Heat transfer effects of that process were described earlier but not pressure drop that was found to indicate the presence of condensate, the same as shown by heat transfer results. Pressure drop in condensers has traditionally been modeled separately for single-phase and two-phase regions. When plotted as a function of enthalpy, the correlations show a discontinuity between the single-phase and two-phase zones because the models typically assume thermodynamic equilibrium during condensation. This assumption indicates that the first drop of condensate would form when the bulk refrigerant enthalpy reached the saturated vapor enthalpy. The zone beginning when the wall temperature reaches the saturation temperature and ending when the bulk enthalpy reaches saturated vapor is classified as the condensing superheated zone. In this condensing superheated zone, the interaction between the condensate film and vapor increases the pressure drop compared to vapor alone. This paper presents experimental results for pressure drop in that zone of the condenser for R32, R134a and R1234ze(E). The pressure drop was found to be higher than predicted by single-phase correlations such as Colburn (1933)