Comparative analysis of a concentric straight and a U-bend gas cooler configurations in CO2 refrigeration system

This paper compares the heat transfer performance of two gas cooler configurations using CO2 as refrigerant. Straight gas cooler vs U-bend gas cooler was analyzed using 3D numerical simulation employing ANSYS FLUENT commercial program. The importance of the structure of the mesh was discussed and the unstructured sweep method was selected. Numerical results were satisfactorily compared with those of eight different empirical correlations for the Nusselt number applicable to similar heat exchangers. Behaviors of the local CO2 thermophysical properties, turbulence parameters, and heat transfer parameters in both straight and U-bend gas cooler configurations were analyzed. The results of this analysis were used to predict the transient behavior of the heat transfer rate and the thermal effectiveness of the two configurations. An increase of 16% was observed in the thermal effectiveness of the gas cooler based on U-bend configuration compared to the straight gas cooler configuration. Finally, the transient behavior of the overall efficiency of the transcritical refrigeration facility in which the two gas cooler configurations are supposed to be installed was assessed. The installation based on U-bend gas cooler geometry has presented higher performance of about 20% of efficiency when compared with the straight geometry.