Flow boiling heat transfer of new refrigerant blends: Experimental data in a microchannel and modelling

Mixtures of hydrofluorocarbons (HFCs) and hydrofluoroolefins (HFOs) are suitable as drop-in substitutes in refrigeration and air conditioning, due to the low global warming potential (GWP) and desired thermodynamic properties. In the present work, the flow boiling heat transfer of four HFOs/HFCs mixtures has been studied inside a 0.96 mm diameter channel. Three of those mixtures, R513A (R1234yf/R134a, 56/44 % by mass, GWP_100-y = 629), R516A (R1234yf/R152a/R134a 77.5/14/8.5 % by mass, GWP_100-y = 131) and R515B (R1234ze(E)/R227ea, 91/9 % by mass, GWP_100-y = 299), are azeotropic mixtures, while the fourth is quasi-azeotropic mixture R450A (R1234ze(E)/R134a, 56/44 % by mass, GWP_100-y = 547, ΔT_GL = 0.63 K at 30 °C). The experimental campaign was conducted using a test section where the flow boiling is promoted by a secondary fluid, at 30 °C mean saturation temperature and mass flux between 300 kg m⁻² s⁻¹ and 600 kg m⁻² s⁻¹. The present data have been compared with the heat transfer coefficient of R134a, in order to assess the suitability of its drop-in substitutes. From the comparison between experimental data and the predictions from some semi-empirical models, a modified method is presented. The new flow boiling heat transfer correlation has been successfully tested with data of propane, propylene, R32 and R1234yf.