New correlation to predict the heat transfer coefficient in-tube cooling of supercritical CO2 in horizontal macro-tubes

The heat transfer coefficient during gas cooling process of supercritical CO2 without lubricating oil in horizontal macro-tubes has experimentally investigated. Investigation has done on two stainless steel circular tubes having inside-diameter of 4.55 mm and 7.75 mm and carried out for CO2 mass fluxes of 200–600 kg/m2 s, inlet fluid pressures of 7.5–10.0 MPa, and the inlet fluid temperatures of 90–100 °C.The experimental results indicate that the gas cooling pressure, the inner tube diameter, the mass flux and the temperature of CO2 have significant effects on the heat transfer coefficient, especially near pseudo-critical region. The heat transfer coefficient decreases as the cooling pressure increases otherwise increases as mass flux increases. At any temperatures, smaller inner tube diameter shows higher heat transfer coefficient.In comparison between present experimental data and existing correlations from references, most of supercritical heat transfer correlations show large deviations with this present experimental data. Therefore, based on experimental data obtained in this present work, a new correlation is proposed to predict more accurate heat transfer coefficient of supercritical CO2 in horizontal macro-tubes under cooling conditions. The majority of the experimental values are within 13% of the values predicted by the new correlation.