low GWP, refrigerant mixtures, evaporation, heat transfer
Refrigerant mixtures R32/R1234yf and R32/R1234ze(E) are considered to be the low GWP alternatives of R32 and R410A for air conditioners. However, according to the recent reports, the severe heat transfer degradation is encountered during the evaporation process. This implies that much larger heat exchangers are required to maintain the COP and cooling/heating capacity with R32/R1234yf and R32/R1234ze(E). Therefore, the effects of mixture component and composition on the heat transfer degradation is experimentally investigated in this paper. Heat transfer coefficient of those two mixtures and their components (i.e., R32, R1234yf and R1234ze(E) alone) are experimentally quantified with horizontally set copper microfin tubes of 6.00 mm in outer diameter having 48, 58, and 64 fins of 0.26 mm in height and 19 Â° in helical angle. The evaporation test was conducted at an average saturation temperature of 10 Â°C, a heat flux of 10 kW m-2, and mass fluxes from 150 to 400 kg m-2s-1. The heat transfer is degraded most at the composition where the temperature glide and concentration difference between vapor and liquid phases are maximized. This suggest that the relevance of the heat transfer degradation and mass transfer resistance caused by the concentration boundary layer and the additional sensible heat transfer. Although the heat transfer coefficients of R1234yf and R1234ze(E) alone are comparable, the magnitude of heat transfer degradation is obviously severer for R32/R1234ze(E) than that for R R32/R1234yf. This can be explained with the larger concentration difference and the temperature glide of R32/R1234ze(E) that those of R32/R1234yf. With increasing mass flux and number of fins, the heat transfer degradation is mitigated somewhat. It appears to be that mixing of concentration boundary layer contributes to recover the evaporation heat transfer. For the real possibility to use R32/R1234yf and R32/R1234ze(E) in air conditioners, the heat transfer recovery or enhancement could be the key technology.