- This work investigates the in-tube condensation of low global warming potential (GWP) zeotropic refrigerant mixtures R448A, R450A, R452A, R454B and R454C. These refrigerants have been proposed for a wide range of commercial heating, ventilation, air-conditioning applications, but their heat and momentum transport phenomenon were not well understood. Thus, in this study, pressure drop and quasi-local heat transfer coefficients were measured during condensation in a 4.7 mm horizontal tube at mass fluxes ranging from 100 kg m−2 s−1 to 500 kg m−2 s−1 at three different saturation conditions (40, 50 and 50◦C). The corresponding temperature glides for these mixtures range from 0.7◦C to 6.3◦C.
Past research has shown that a strong coupling between the heat and mass transfer exists for zeotropic condensation. As a result, the correlations developed for pure fluids generally translate poorly to predictions for mixtures. Therefore to determine the best modeling method for low GWP mixtures, the measured data from this study were compared against predictions from various heat transfer models from the literature. The heat transfer coefficients for all five refrigerants are best predicted by the Cavallini et al. (2006) correlation (Mean absolute percent error, MAPE = 8%), with the mixture effects accounted for using the Silver, Bell and Ghaly correction. A comparison of the heat transfer and pressure drop of the new HFC/HFO mixtures against their HFC predecessors indicates that the low GWP mixtures may be considered as viable substitutes from a heat and momentum transport perspective. The two-phase frictional pressure drop for complete condensation of mixtures was also measured. The pressure drop for these low GWP refrigerants was predicted by the Cavallini et al. (2009) correlation, with the MAPE equal to 24%.
In addition to the investigation of saturated condensation, an emphasis was placed on the superheated and subcooled condensation. A review of the literature reveals limited investigations on this topic, with only a handful of predictions models available for pure fluids. These models were developed based on various simplifying assumptions, which may be why they exhibit a poor agreement with the measured data for refrigerant mixtures. Therefore, a new model is introduced to predict the heat transfer in the superheated region. The model predicts superheated and complete condensation data with good accuracy, with MAPEs equal to 8% and 12%, respectively. Implementing this model allows for a more accurate representation of the superheated condensation phenomenon and by doing so, permits for a more compact condenser geometry. More broadly, this work will aid in the adoption of low GWP refrigerant technology.