HEAT TRANSFER PERFORMANCES OF R449A DURING FLOW BOILING IN A HORIZONTAL SMOOTH TUBE

In the HVAC sector, because of the environmental concern, many commonly used refrigerants, characterized by high values of atmospheric lifetime and global warming potential, are undergoing a phase-out process. Therefore, it is a priority to find suitable alternatives capable of combining both good environmental features and at least equivalent heat transfer performances. This work reports on the experimental investigation of a low GWP refrigerant: R449A, a zeotropic mixture composed of R32 (24.3%), R125 (24.7%), R1234yf (25.3%), R134a (25.7%), which is proposed as a replacement to both R404A and R507A. The study focused on flow boiling (operating pressure: 7.16 bar, bubble temperature: 5°C, temperature glide: approximately 5 K) inside a horizontal smooth tube (outer diameter: 9.52 mm, thickness 0.3 mm, heat transfer length 1.1 m, pressure drop length 1.3 m). The test section consisted of a thermally insulated water-refrigerant coaxial counter-flow heat exchanger working at typical operating conditions for HVAC devices (mass flux ranging from 90 to 220 kg m-2 s-1). Specifically, the average heat transfer coefficient for complete evaporation was measured by the Wilson plot technique. The collected data, including both the pressure drop and the heat transfer coefficient, were compared with the predictions obtained from correlations reported in the open literature. It was found that the model from Bertsch et al. provides the best prediction for the heat transfer coefficient, while the correlation proposed by Zhang and Webb is the most suitable for the pressure gradient.