Flow boiling heat transfer characteristics of zeotropic mixture CO2/R152a with large temperature glide in a 2 mm horizontal tube

Abstract

The flow boiling characteristics of CO2/R152a zeotropic mixtures with three mass fractions (22.1/77.9, 47.4/52.6 and 65.3/34.7 wt.%) and the pure components in a 2 mm horizontal tube are experimentally studied. The flow boiling characteristics are tested in the range of temperature glide of 30.9∼38.3 °C, mass flux of 200∼400 kg/m2s, heat flux of 12∼36 kW/m2, and saturation temperature of 5∼15 °C. Flow patterns are observed and the flow pattern map is also described, and the heat transfer and pressure drop characteristics of working fluid are also conducted. Furthermore, a new correlation to predict the heat transfer coefficient of large temperature glide working fluid is proposed. The results show 7 types of flow patterns are recorded, showing the flowing characteristic between the conventional and microscale channel. The heat transfer coefficient of CO2/R152a increases slowly with vapor quality, and decrease rapidly when vapor quality reaches to the initial dryout vapor quality, which is lower than that of pure CO2. The mass transfer resistance and sensible heat resistance shows a severe deterioration effect on the flow boiling heat transfer due to the high temperature glide as the sensible heat accounts for 13.12∼15.44 % for the mixture CO2/R152a. The frictional pressure drop gradient of CO2/R152a increases with mass flux, and decreases with the increase of saturation temperature and mass fraction of CO2. Considering the mass transfer resistance and effects of non-negligible sensible heat contribution, a new heat transfer correlation is proposed, with the mean absolute deviation of 16.9 %, which shows an acceptable prediction accuracy compared with the reported data in the literatures.