Effects of Supercritical CO2 Fluid Properties on Heat Exchanger Design

Abstract

Abstract Supercritical CO2 (sCO2) in industrial applications is a promising solution for the reduction of equipment size and for the potential increasement of overall efficiency. The characteristics of CO2 near the critical point are advantageous for the heat transfer, with high specific heat capacity when compared to conventional liquid or gas coolants. Moreover, dopants can be mixed to CO2, in a blend, to further increase the heat transfer and elevate the critical point of the fluid. Under the framework of the Horizon 2020 DESOLINATION (DEmonstration of concentrated SOLar power coupled wIth advaNced desAlinaTion system in the gulf regION) project, a heat exchanger was designed using CO2 and blend as working fluids in an innovative power cycle bank of a concentrated solar power (CSP) plant coupled to a water desalination system. In this paper, the innovative heat exchanger is evaluated in terms of the geometry, focusing on heat transfer and turbulence, using computational fluid dynamic (CFD) simulations with SST k-ω turbulence model and real-gas models with REFPROP library for thermodynamic and transport properties. Within each fluid, the diameter of the channels varied from 1.8–2.2 mm and the results are compared to analytical models. The geometry with 1.8 showed a larger pressure drop, but also larger overall heat transfer coefficient. It is observed that both pressure drop and heat transfer decrease with the diameter. Blend as the hot fluid resulted in higher average temperature and heat transfer coefficient, but in lower pressure drop than pure CO2. Nusselt number correlations were compared, showing the decrease of its values with the diameter for pure CO2, while for the blend it was observed higher values for 2.0 mm of diameter, except for Fang and Xu correlation that showed the same trend as the CO2.