Experiment Study On Boiling Heat Transfer Characteristics And Pressure Drop Of CO2 In A Small Diameter Tube

As natural working fluid, carbon dioxide has no effect on ozone depletion and negligible effect on global warming comparing with traditional refrigerant, besides, carbon dioxide have advantages of high volumetric heat capacity, low viscosity and low surface tension. Therefore, carbon dioxide boiling heat transfer is being hotspot for developing of natural refrigeration. In recent years, studies on carbon dioxide boiling heat transfer with low mass flux in small diameter tube are quite limited. This study is aiming at acquiring experimental data, supplying reference for designing of CO2 mobile air-conditioning evaporators and enriching data by which general correlation of CO2 boiling heat transfer characteristics in small diameter is predicted. The heat transfer coefficient and pressure drop for CO2 flow boiling in horizontal round tube with 4 mm ID are experimental investigated. The test section has a length of 0.5 m with 1 mm thickness. The carbon dioxide heat flux vary from 2 to 18 kW/m2, the evaporating temperature is from 1 to 15℃, the mass flux is from 100 to 300kg/m2s, and the vapor qualities vary from 0.1 to 0.9.The test results indicate that the variations of heat transfer coefficient with respect to vapor quality are quite different from that of traditional fluocarbon refrigerant. Since low viscosity and low surface tension characteristics of carbon dioxide, the partial dryout begins at middle even low vapor quality. Evaporating temperature, mass flux, heat flux and vapor quality influence boiling heat transfer characteristics of carbon dioxide strongly. In details, heat transfer coefficient rise with heat flux increasing in low vapor quality regime, and heat transfer coefficient has an obvious trend of single-decrease in high heat flux. Heat transfer coefficient rise with evaporating temperature increasing. The onset of dryout is mainly influenced by mass flux, with the rise of mass flux the critical vapor quality increase. After dryout quality above four parameters have little effect on heat transfer coefficient. The correlation of boiling heat transfer coefficient based on traditional refrigerant is not fit for predicting for carbon dioxide.The pressure drop of carbon dioxide rise with heat flux and mass flux increasing, fall with evaporation temperature increasing. The pressure drop of carbon dioxide less than that of fluocarbon refrigerant because of lower viscosity.The carbon dioxide evaporator preferably should be designed with low mass flux, because the experiment results indicate that increased mass flux does not improve heat transfer largely while add pressure drop acts negatively.The carbon dioxide evaporator should select micro or small tubes, because that carbon dioxide has advantages of higher heat transfer coefficient and lower pressure drop than fluocarbon refrigerant. After optimize designing, micro or small tubes evaporator can realize more compact system. At the same time, since carbon dioxide evaporator has high working pressure, macro or small tubes will be more safety.