MODELING OF THE FLOW AND HEAT TRANSFER OF SUPERCRITICAL CO2 FLOWING IN SERPENTINE TUBES

As a non-flammable, non-toxic refrigerant, supercritical CO2 (ScCO2) has been increasingly used for heat transfer applications. In this study, the ScCO2 flow and heat transfer in a set of full-size three-dimensional serpentine tubes were modeled with different inner diameters and tube pitches. The standard k-epsilon model was used for the turbulence modeling. The results show the effect of the different tube inner diameters and tube pitches on the flow and heat transfer of ScCO2 for a given flow flux or inlet Reynolds number. The heat transfer coefficient decreases as both the tube pitch and the inner diameter increase for a given mass flow rate. However, for a given inlet Reynolds number, the heat transfer coefficient first increases but then decreases with increasing tube inner diameter. The effect of the flow direction on the heat transfer performance was also studied for various inlet conditions. Downward flow results in a higher heat transfer coefficient than upward flow for inner diameters larger than 0.5 mm and the buoyancy effect can be ignored for Bo1×10-7 for the conditions studied here. These results can be used to optimize the tube shape and size in heat exchanger designs.