HFO1234yf has been proposed for mobile air-conditioners due to its low global warming potential (GWP) and performance comparable to that of R134a. However, its performance is inferior to that of R410A. This makes it difficult to be applied to residential air-conditioners. In order to apply the low-GWP refrigerant to residential air-conditioners, refrigerant mixtures of HFO1234yf and R32 are proposed, and their flow boiling heat transfer performances were investigated at two mass fractions (80/20 and 50/50 by mass%) in a smooth horizontal tube with an inner diameter of 2 mm. The experiments were conducted under heat fluxes ranging from 6 to 24 kW/m(2) and mass fluxes ranging from 100 to 400 kg/m(2) s at the evaporation temperature of 15 degrees C. The measured heat transfer coefficients were compared with those of pure HFO1234yf and R32. The results showed that the heat transfer coefficients of the mixture with an R32 mass fraction of 20% were 10-30% less than those of pure HFO1234yf for various mass and heat fluxes. When the mass fraction of R32 increased to 50%, the heat transfer coefficients of the mixture were 10-20% greater than those of pure HFO1234yf under conditions of large mass and heat fluxes. Moreover, the heat transfer coefficients of the mixtures were about 20-50% less than that of pure R32. The performances of the mixtures were examined at different boiling numbers. For refrigerant mixture HFO1234yf and R32 (80/20 by mass%), the nucleate boiling heat transfer was noticeably suppressed at low vapor quality for small boiling numbers, whereas the forced convective heat transfer was significantly suppressed at high vapor quality for large boiling numbers. This indicates that the heat transfer is greatly influenced by the mass diffusion resistance and temperature glide of the mixture. (C) 2012 Elsevier Ltd. All rights reserved.