Experimental investigation of the heat transfer behaviors of CO2, propane and their binary non-azeotropic mixtures above critical pressure in helically coiled tube

The non-azeotropic natural mixture, propane/CO2, in trans-critical Rankine cycle shows some special advantages in operation safe, thermodynamic performance and exergo-economic performance. The improvement of the system performance depends on further in-depth investigation on the heat transfer of propane/CO2 mixture above critical pressure which has not been performed yet. In this paper, the heat transfer behaviors of CO2, propane and their non-azeotropic mixtures in helically coiled tube are investigated and compared by experiments. The mass fractions of the mixtures are measured by Fourier Transform Infrared Spectrometer. The effects of operating parameters under various mass fractions on heat transfer are discussed. The general law of mass fraction on heat transfer is got under the same pressure ratio (P/Pcr) by experimental results. The peak of heat transfer coefficient increases as propane increases when the mass fraction of propane changes from 22.24% to 100%. The heat transfer coefficient of the mixtures has little variations with pressure because the specific heat of the mixtures has a slight decrease as pressure increases. The complicated heat transfer characteristics of the propane/CO2 mixtures are analyzed based on the buoyancy force and the thermal effect in heat surface. In the helically coiled tube, the heat transfer of the mixture is mainly dominated by curvature effect and the effect of the low-density in boundary layer becomes smaller and smaller as propane increases. The experimental data are predicted by a correlation. The results show that for various compositions, 80% of the data are predicted within ±20%, and 95% are within ±30%.