Experimental investigation of two phases evaporative heat transfer coefficient of carbon dioxide as a pure refrigerant and oil contaminated under forced flow conditions in small and large tube

• Two phase heat transfer coefficient of CO2/oil inside a horizontal tube is studied.
• The heat transfer coefficient decreases with the oil concentration and quality.
• The correlations of pure refrigerants can be used for the oil/CO2 refrigerant.
• The effect of the vapor pressure is only noticeable at higher oil concentration.

The evaporative two-phase heat transfer coefficient of CO2/oil contaminated as a refrigerant under forced flow conditions through a smooth horizontal tube was experimentally investigated. The experiments were carried out for two test sections of evaporators. The test sections were made of seamless precision steel tubes with a length of 1.12 m and two inner diameters of 4 and 10 mm to fulfill the influence of the evaporator geometry. Experimental parameters include mass fluxes varied from 90 to 750 (kg m−2 s), heat flux ranged from 5 to 40 (kW m−2), evaporation temperatures changed from −10 to −35 °C, and the oil concentration is varied from 0.2 to 7 %. The results from the experiment are compared with those calculated from correlations reported in the literature. The results of this study are of technological importance for the efficient design of evaporators when systems are assigned to utilize CO2 as a refrigerant.