A study of pool boiling and falling-film vaporization with R-245fa/oil mixtures on horizontal tubes

This paper presents the effect of oil concentration on the pool boiling and falling-film vaporization of oil/R-245fa mixtures with 1%, 2%, and 5% lubricant mass fraction. The saturation temperature ranges from 5 to 20 °C, and the heat flux varies between 6 kW/m² and 55 kW/m². Two types of tubes with a 19-mm outer diameter are tested: a smooth tube and a fin tube with 0.4-mm fin height, 60 FPI (fins per inch), and a 0.233-mm fin gap. For both the pool boiling and falling-film vaporization tests, the local oil concentration measured by the refractive index sensors is in good agreement with the average oil concentration obtained from the weight measurement. In case of pool boiling, the heat transfer coefficient for the smooth tube decreases with increasing oil concentration. In contrast, it increases with increasing oil concentration for the fin tube because of a more significant bubbly foam, which facilitates bubble nucleation. For the falling-film evaporation, the heat transfer coefficient increases with increasing oil concentration for both the smooth and fin tubes because the bubble nucleation is enhanced by the foam at a high oil concentration. However, the heat transfer coefficient for the second fin tube decreases with the oil concentration at a high heat flux (>20 kW/m²) because of the partial dry out caused by the liquid held on the top tube.