Flow boiling heat transfer of CO₂ at low temperatures in a horizontal smooth tube

Flow boiling heat transfer coefficients of CO₂ are measured in a horizontal smooth tube with inner diameter 6.1 mm. The test tube is heated by a secondary fluid maintaining constant wall temperature conditions. Heat transfer coefficients are measured at evaporation temperatures of -15 and -30°C, mass flux from 100 to 400 kg/m² s, and heat flux from 5 to 15 kW/m² for qualities (vapor mass fractions) ranging from 0.1 to 0.8. The characteristics Of CO₂ flow boiling are explained by CO₂ properties and flow patterns. The measured CO₂ heat transfer coefficients are compared to other published data. Experiments with R22 were also conducted in the same system and the results show that the heat transfer coefficients for CO₂ are 40 to 150% higher than for R22 at -15°C and low mass flux of 200 kg/m² s mostly due to the characteristics of CO₂ nucleate boiling. The presented CO₂ heat transfer coefficients indicate the reduction of heat transfer coefficient as mass flux increases at low quality regions and also show that dryout does not occur until the high quality region of 0.8, for mass fluxes of 200 and 400 kg/m² s. The Gungor and Winterton correlation gives a relatively good agreement with measured data; however it deviates more at lower evaporation temperature and high mass flux conditions.