Flow boiling heat transfer of binary mixtures of R1234yf/R32 and R1234ze (E)/R32 in a horizontal minichannel

This study experimentally investigates the flow patterns and boiling heat transfer of binary mixtures R1234yf/ R32 and R1234ze(E)/R32 in a horizontal circular minichannel with an inner diameter of 2.0 mm. The effects of mass flux, heat flux, vapor quality, and mass composition are evaluated. In the adiabatic two-phase flow, the observed flow patterns included slug/plug (intermittent), wavy, churn, and annular flows. A simple flow pattern transition boundary is proposed to distinguish between two types of flow regimes: one controlled by shear stress and one controlled by surface tension and gravity. The results showed that heat flux significantly affects slug/ plug flows, but it has minimal impact on other flow patterns. The heat transfer coefficient increased as the mass flux increased, particularly at higher qualities. At higher mass fluxes, the heat transfer coefficient increased as the transition to annular flow occurred with increasing quality. Conversely, at lower mass fluxes, the heat transfer coefficient either slightly decreased or remained almost constant as the quality increased. Heat transfer degradation decreased as the mass flux increased, and the effect of degradation in annular flows was minimal. R1234ze (E)/R32 (80/20 mass%), which had the largest temperature glide, exhibited the lowest heat transfer coefficients; however, this degradation decreased as the mass flux increased. The heat transfer coefficients for both the pure and mixed refrigerants increased in proportion to the heat flux raised to the power of 0.7. The previous correlation predicted the heat transfer coefficients of the pure refrigerants with a mean absolute percentage error (MAPE) of 12.6 %; however, it overestimated for mixtures. The proposed correlation predicted the heat transfer coefficient for mixtures with an MAPE of 13.4 %.