Measurements and modelling of R455A and R452B flow boiling heat transfer inside channels

Heat transfer coefficients during flow boiling of non-azeotropic mixtures are measured inside two horizontal smooth tubes of 8.0 mm and 0.96 mm inner diameter. The experimental tests are performed with ternary mixtures R455A (R32/R1234yf/R744 at 21.5/75.5/3% by mass) and R452B (R32/R1234yf/R125 at 67/26/7% by mass), displaying respectively a temperature glide of about 11 K and 1 K in the present tests. The effects of vapour quality, saturation pressure, heat flux, mass velocity and channel diameter on the heat transfer coefficient of the mixtures are investigated. The experimental results are compared against selected flow boiling correlations. Because of the zeotropic characteristics of the mixtures, the models developed for pure fluids are modified following the Shah (2015) approach to account for the mass transfer resistance, affecting both the nucleate and the convective boiling mechanisms. Simple tools to estimate the mass transfer resistance in the prediction of the flow boiling heat transfer coefficients in the case of a high temperature-glide mixture are here presented and assessed.