R744 Flow Boiling Heat Transfer With and Without Oil at Low Temperatures in 11.2 mm Horizontal Smooth Tube

摘要

Flow boiling heat transfer characteristics of R744 with and without oil were investigated experimentally in horizontal smooth tubes with an inner diameter of 11.2 mm. In order to investigate the effect of the miscible oil on the heat transfer of R744, POE (polyolester) RENSIO C85E oil is added to give an oil circulation rate (OCR) between 0.5% and 4%. Results are compared with those of pure R744. The experimental conditions include evaporation temperatures of-15 and -30 °C, mass fluxes from 40 to 200 kg/m2 s, heat fluxes from 2 to 10 kW/m2, and vapor qualities from 0.1 to 0.8. The heat transfer coefficients of pure R744 show strong dependence on heat flux due to the dominance of the nucleate boiling. However, heat transfer also depends on mass flux and evaporation temperature due to enhanced convective boiling contribution at higher mass flux, 200 kg/m2s, and change in thermo-physical properties. The pressure of oil generally deteriorates the heat transfer coefficient of pure R744. The reduction in heat transfer coefficient is most apparent at low qualities, 0.1 to 0.4, and at low mass fluxes, 40 kg/m2. It is caused by the suppression of nucleate boiling due to increased surface tension. At conditions where the convective boiling contribution is dominant, qualities above 0.5 and mass fluxes above 100 kg/m2s, oil increases heat transfer coefficients.