Experimental study on the evaporative heat transfer and pressure drop of CO2 flowing upward in vertical smooth and micro-fin tubes with the diameter of 5 mm

Because of the ozone layer depletion and global warming, new alternative refrigerants are being developed. In this study, evaporation heat transfer characteristic and pressure drop of carbon dioxide flowing upward in vertical smooth and micro-fin tubes were investigated by experiment with regard to evaporating temperature, mass flux and heat flux. The vertical smooth and micro-fin tubes with outer diameter (OD) of 5 mm and length of 1.44 m were selected as a test section to measure the evaporative heat transfer coefficient. The tests were conducted at mass fluxes from 212 to 530 kg/(m² s), saturation temperatures from −5 to 20 °C and heat fluxes from 15 to 45 kW/m², where the test section was heated by a direct heating method. The differences of heat transfer characteristics between the smooth and the micro-fin tubes were analyzed with respect to enhancement factor (EF) and penalty factor (PF). Average evaporation heat transfer coefficients for the micro-fin tube were approximately 111–207% higher than those for the smooth tube at the same test conditions, and PF was increased from 106 to 123%.