Modelagem computacional da circulação natural do CO2 supercrítico

Abstract

Passive safety systems for residual heat removal through natural circulation are indispensable for the future of nuclear energy. In an attempt to optimize such systems, the use of supercritical CO2 is an alternative considered by many researchers due to the high rates of heat transfer and low pressure drop. This work aims to investigate one of the main intrinsic limitations of this system type, the phenomenon of heat transfer deterioration (HTD). The four most common configurations of a rectangular natural circulation loop were analyzed, differed by the orientation of the heater and the cooler. The simulations were performed in steady state, using a three-dimensional model. The turbulence model chosen was RNG k-ε. The code used was ANSYS Fluent v. 18.2. The natural circulation mass flowrate and the average temperature were analyzed as a function of the heating power for the four loop configurations and for three pressures (8.5 MPa, 9.0 MPa and 9.5 MPa). The results were compared with the empirical correlations of SWAPNALEE et al. [1] and YADAV et al. [2] in order to verify the simulation. The HTD regime was observed for all the cases studied. It was observed that the increase in pressure delays the beginning of HTD regime and also induces a significant increase in the natural circulation flowrate after the emergence of HTD for all studied cases. In order to investigate the heat transfer of the system, the Nusselt number was analyzed as a function of the Rayleigh number and it was observed that the correlation described by YADAV et al. [2] presented good agreement with the results obtained from the simulation. Finally, correlations were proposed for the heat transfer and the Reynolds number, individually for each configuration, and a unified correlation was proposed for the friction factor, for all the cases studied.