Condensation and vaporization heat transfer of low-GWP mixtures

The present work investigates the heat transfer performance of non-azeotropic ternary low-GWP (Global Warming Potential) mixtures during condensation and vaporization inside a 8.0 mm inner diameter channel. The tested mixtures are R455A, which is a ternary blend of R32, R1234yf and R744 (21.5/75.5/3.0% by mass composition), and R452B, which is a ternary blend of R32, R1234yf and R125 (67.0/26.0/7.0% by mass composition). R455A presents a GWP100-years value equal to 146 and a temperature glide of 10 K at pressure p=17.6 bar, while R452B is characterized by higher GWP, equal to 676, but lower temperature glide, about 1 K at p=22.9 bar. Condensation tests have been run with the two mixtures R455A and R452B at a mean saturation temperature (average value between dew point and bubble point temperatures at constant pressure and given composition) equal to 40 °C and mass velocity ranging between 100 and 600 kg m−2 s−1. Flow boiling tests have been performed at around 18 °C mean saturation temperature and mass velocity ranging between 200 and 400 kg m−2 s−1. The effects of vapor quality, mass velocity and heat flux on the heat transfer coefficient are investigated. Moreover, the adoption of zeotropic mixtures poses the problem of how to extend the correlations developed for pure fluids. In the present paper, the contribution of the mass transfer resistance is assessed and the corrections needed in the models are discussed.