Experimental Investigations On Flow And Heat Transfer Of CO₂in A Helically Coiled Tube

Helically coiled tubes are widely used in a variety of practical applications such as steam generators and coolers, power plant boilers, nuclear reactors, petrochemical industry as well as refrigeration and air conditioning systems, due to their practical importance of high efficiency in heat transfer, compactness in structure and easiness in manufacturing. The investigation on natural refrigerant of carbon dioxide as an alternative refrigerant for CFCs and HCFCs was turned out to be a hot research topic, transcritical carbon dioxide cycle has a promising prospect of application in the areas of refrigeration, heat pump and low-grade waste recovery. The research on flow and heat transfer characteristics for CO2flowing through helically coiled tube has important scientific and practical significance for the design and application of new efficient heat exchangers. In full consideration of physical properties for CO2at supercritical and subcritical pressures especially near the pseudo-critical temperature, an experimental setup for CO2flow and heat transfer investigation was built up and the reliability of the setup was verified by flow resistance, heat transfer coefficients and thermal balance calibrations in this paper, experiments can be widely conducted on flow and heat transfer characteristics for CO2in the helically coiled tube in this experimental setup.Experimental investigation was conducted on the heat transfer characteristics of CO2at supercritical pressures under the uniform heat flux boundary conditions, the influences of operating pressure, mass flux and heat flux on heat transfer characteristics were investigated experimentally. It was found that the heat transfer coefficients first increased and then decreased. The maximum values of heat transfer coefficients always occurred at the temperature range of Tb<Tpc<Twi. By theoretical research, the coupling effects of thermal boundary layer developing, thermo-physical property variation, buoyancy and centrifugal forces have been disclosed, the mechanism explanation of heat transfer coefficients and temperatures distribution along the flow direction was given at different experimental conditions. Bases on the2346data obtained in the present experimental study, two correlations of Nusselts number which are suitable for different temperature regions have been proposed.Research about the flow resistance for CO2heated at supercritical pressures in the helically coiled tube was conducted. Effects of mass flux and heat flux on the variations of frictional pressure drop, acceleration pressure drop, gravitational pressure drop and the ratio of the frictional pressure drop to the total pressure drop were identified and analyzed. The friction factor was found to decrease gradually and then increase with heat flux. The mechanism of buoyancy force effect on the flow resistance is analyzed. A new correlation of friction factor is presented based on the experimental data.As for flow boiling heat transfer of CO2through the helically coiled tube, experiments were conducted on inner wall temperature distributions along the flow direction and the circumference at different pressures and different heat fluxes. The effects of vapor quality, mass flux, heat flux and operating pressure on heat transfer coefficient distribution were discussed. The experimental results show that inner wall temperature distributions along the circumference were non-uniform, and it was pointed out that the relative significance of the buoyancy force and centrifugal is the main reason for this phenomenon. The local average heat transfer coefficient increases with increasing heat flux and inlet pressure, but the increment of mass flux has no effect on the heat transfer, suggesting that the nucleate boiling is the dominant mechanism while the forced convection effect is weak. The intensity of nucleate boiling changes with increasing heat flux and the variation of heat transfer coefficient with outlet vapor quality can be divided into three different trends, which is induced by the alternative of wall dry-out and rewetting. A new correlation of local average heat transfer coefficient has been proposed based on the data points.Research on the flow resistance for CO2flow boiling at the subcritical pressure in the helically coiled tube was studied. The results show that the frictional pressure drop for flow boiling increases with increasing heat flux, it shows different trends with the mass flux at different heating power, the results show totally increasing trend but decreasing at low heat flux or high mass flux. The frictional pressure drop decreased with increasing operating pressure because of the ratio of density between liquid and gas decreasing. A new correlation of two-phase frictional pressure drop is presented based on the experimental data.This research work can provide useful knowledge and a reference for the development of high efficiency heat exchanger and improvement of the equipment for refrigeration and air-conditioning applications.