Study On Heat Transfer Characteristics Of Supercritical CO₂ In Helically Coiled Tube

As an environmentally friendly, nontoxic and nonflammable refrigerant,carbon dioxide?CO₂? offers attractive thermal characters such as small viscosity and large refrigerant capacity. Supercritical CO₂ has wide applications in air conditioners, nuclear reactors and aerospaceengineering. The heat transfer of supercritical CO₂ in straight tubes, enhanced tubes and shaped tubes appears in these field. Higher heat transfer rate and less volume occupation in helical tubes give rise to people's interest. The helical-coil-in-fluted-tube gas cooler is compact and avoid cross contamination. water and CO₂ flow in the helically coiled tube, so the heat transfer of the gas cooler is enhanced. The gas cooler are well suited for supercritical heat pump, however, the heat transfer mechanism of supercritical CO₂ in the helically coiled tube is still deficient. Experiment and numerical simulation are applied in this paper to study the heat transfer characteristic of supercritical CO₂ in the helically coiled tube.In this research, the heat transfer of supercritical CO₂ cooled in the helically coiled tube is investigated experimentally. The effects of mass flux, heat flux and pressure on the heat transfer of supercritical CO₂ in the cooled helically coiled tube are studied. While the effect of pressure is related to the corresponding variation of thermo-physical properties along flow direction, with increase of pressure, the maximum value of heat transfer coefficient decreases greatly. The effect of heat flux is mainly related to the distribution of properties in radial direction of tube transverse section, with increase of heat flux, heat transfer coefficient takes smaller value in the pseudo-critical temperature region, however, heat transfer coefficient almost invariably change in the liquid-like region.The heat transfer coefficients in the horizontal helically coiled tube are higher than those of vertical helically coiled tube, It is demonstrated that the effect of buoyancy cannot be ignored. Using three buoyancy parameters, Bup, Grth/Grq and Rito predict the effect of buoyancy on the heat transfer of supercritical CO₂ in the helically coiled tube. The three parameters overestimate the effect of buoyancy.Numerical simulation on heat transfer of supercritical CO₂ in heated and cooled helically coiled tubes is performed. The three models, SST, standardk-e, RNG k-e, used in the paper yield similar tendencies in heat transfer coefficient in the helically coiled tube qualitatively. But,the SST model gives the best prediction of the experimental data. The comparison and analysis of heating and cooling heat transfer coefficient in helically coiled tube is conducted by using SST model, and the heat transfer coefficients of cooled helically coiled tube are higher than those of heated helically coiled tube. The flow fields are analyzed and the effect of heat flux is mainly related to the distribution of cp in radial direction of tube transverse section.