Study of convection heat transfer of CO2 at supercritical pressures during cooling in fluted tube-in-tube heat exchangers

Abstract The heat transfer characteristics of CO 2 at supercritical pressures during cooling in two fluted tube-in-tube heat exchangers and a smooth tube-in-tube heat exchanger are investigated. Both the overall heat transfer and the local heat transfer coefficients are analyzed at various CO 2 mass flow rates, inlet pressures, and flute pitches. The results demonstrate that the smaller hydraulic diameter of a fluted tube has a higher heat transfer coefficient at any pressure. The fluid is more disturbed in the fluted tube with a smaller flute pitch where the temperature gradient of CO 2 near the wall is larger, resulting in greater Nusselt number. The overall heat transfer coefficients of the fluted tubes are 2–3 times that of the smooth tube. A new correlation is developed by considering the fluid properties and the fluted tube structure effects for the convective heat transfer calculation of supercritical CO 2 in the fluted tube during cooling.