Numerical Investigation of Thermal Hydraulic Behavior of Supercritical Carbon Dioxide in Compact Heat Exchangers

by Roma Fatima1, Alan Kruizenga2, Mark Anderson2, & Devesh Ranjan1
1Texas A&M University
2University of Wisconsin, Madison

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Three dimensional Computational Fluid Dynamics (CFD) models using FLUENT were developed to investigate the flow and the heat transfer characteristics of supercritical carbon dioxide in three different geometries – a single semi-circular channel, a series of nine parallel semi-circular channels and a single zigzag channel of diameter 1.9mm and length 500mm. Grid refinement studies were carried out in order to assess numerical errors associated with these calculations. In order to fully resolve the boundary layer in these calculations, all the computational meshes developed for this study incorporated the first node cell within the viscous sub-layer i.e. y+<1. Since the flow is turbulent, an appropriate choice of turbulence model is highly desirable. Henceforth, k-ε realizable and k-ω SST turbulence models were used to assess its impact on the heat transfer solution in this study. All the simulations are carried out at pressures 7.5 and 8.1MPa with varying mass flux (326 kg/m2-s and 762 kg/ m2-s), wall temperature and fixed inlet temperature (14.99ºC). The simulations were carried out both for cooling and heating mode of heat transfer. The heat transfer enhancement was observed at critical conditions for all the operating conditions considered. The zigzag configuration shows values of heat transfer coefficient higher by a magnitude of three compared to the semi-circular case but the pressure drop is almost higher by a magnitude of six in case of zigzag channel.