Condensation and flow boiling heat transfer of a HFO/HFC binary mixture inside a minichannel

Refrigerant blends obtained mixing hydrofluorocarbons (HFC) and hydrofluoroolefins (HFO) have recently been proposed as substitutes for high Global Warming Potential fluids employed in refrigeration and air-conditioning systems (e.g. R404A and R410A). Since these new mixtures have usually a non-azeotropic behavior, exhibiting a temperature variation during isobaric phase change, new heat transfer coefficients data are needed to assess available models or to develop new correlations. In the present paper, condensation and flow boiling heat transfer coefficients are measured inside a 0.96 mm diameter circular minichannel with a binary mixture of R32 and R1234ze(E) (0.75/0.25 by mass composition). Condensation heat transfer coefficients are measured at saturation pressure of 21.8 bar (corresponding to a dew point temperature of 41.5 °C) with mass flux ranging between 150 kg m−2 s−1 and 800 kg m−2 s−1. Flow boiling tests are run at 17 bar (corresponding to a bubble temperature of 28.7 °C), mass velocity between 300 and 600 kg m−2 s−1, and heat flux between 30 and 245 kW m−2. Mixture heat transfer coefficients are compared against data of pure fluids components and against data taken with R32/R1234ze(E) blends at varying compositions. The importance of the mass transfer resistance and the heat transfer penalization are discussed. Experimental data are used to assess correlations for the estimation of the condensation and flow boiling heat transfer coefficient.