Flow boiling heat transfer of carbon dioxide in horizontal mini tubes

Flow boiling heat transfer coefficients of carbon dioxide (CO₂) in mini tubes with inner diameters of 2.0 and 0.98 mm are measured and analyzed as a function of operating parameters. The heat transfer coefficients are measured at high mass fluxes from 500 to 3570 kg/m² s, heat fluxes from 7 to 48 kW/m², and saturation temperatures of 0, 5 and 10°C. The measured heat transfer coefficients before dryout are in the range of 10 000-20 000 W/m² K. The heat transfer coefficients before critical vapor quality are strongly affected by heat flux at all mass flux conditions. However, the effects of mass flux on heat transfer coefficients before critical vapor quality are only significant when the superficial liquid Weber number is less than 100. The heat transfer characteristics after critical vapor quality are explained by partially dried liquid film and rate of liquid droplets in the vapor core. Based on the database obtained from this study and the literature, a boiling heat transfer correlation before critical vapor quality is developed.