Experimental studies on the evaporative heat transfer and pressure drop of CO2 and CO2/propane mixtures flowing upward in smooth and micro-fin tubes with outer diameter of 5 mm for an inclination angle of 45°

Heat transfer characteristics show different tendency according to the tube orientations such as horizontal, vertical, and inclined positions. In this study, evaporative heat transfer characteristics and pressure drop of CO2 and CO2/propane mixtures flowing upward are investigated in inclined smooth and micro-fin tubes. Smooth and micro-fin tubes with outer diameter of 5 mm and length of 1.44 m with inclination angle of 45° were chosen as test tubes. Average inner diameters of test tubes are 4.0 mm (smooth tube) and 4.13 mm (micro-fin tube). The tests were conducted at mass fluxes from 212 to 656 kg/m2 s, saturation temperatures from −10 to 30 °C and heat fluxes from 15 to 60 kW/m2 for CO2. In addition, for CO2/propane mixtures, the test was carried out at inlet temperatures from −10 to 30 °C for several compositions (75/25, 50/50, 25/75 wt%) with the same mass fluxes, heat fluxes applied for CO2. Heat transfer coefficients in inclined tube are approximately 1.8–3 times higher than those in horizontal tube and the average pressure drop of inclined tube exists between that of horizontal and vertical tubes.