Boiling heat transfer and dryout phenomenon of CO2 in a horizontal smooth tube

Evaporation heat transfer characteristics of carbon dioxide (CO2) in a horizontal tube are experimentally investigated. The test tube has an inner diameter of 6.0 mm, a wall thickness of 1.0 mm, and a length of 1.4 m. Experiments are conducted at saturation temperatures of 5 and 10 °C, mass fluxes from 170 to 320 kg/m2 s and heat fluxes from 10 to 20 kW/m2. Partial dryout of CO2 occurs at a lower quality as compared to the conventional refrigerants due to a higher bubble growth within the liquid film and a higher liquid droplet entrainment, resulting a rapid decrease of heat transfer coefficients. The effects of mass flux, heat flux, and evaporating temperature are explained by introducing unique properties of CO2, flow patterns, and dryout phenomenon. In addition, the heat transfer coefficient of CO2 is on average 47% higher than that of R134a at the same operating conditions. The Gungor and Winterton correlation shows poor prediction of the boiling heat transfer coefficient of CO2 at low mass flux, while it yields good estimation at high mass flux.