The influence of variable physical properties and buoyancy on heat exchanger design for near- and supercritical conditions

Computational fluid dynamics simulations were done on heated supercritical carbon dioxide flowing up or down in a vertical pipe. The impairment or enhancement of heat transfer caused by the temperature-induced variation of physical properties was investigated, as well as the effect of buoyancy. The simulations show, for non-buoyant flow, that for pressures above 120 bar, the effect of variation in physical properties is small and a constant-property Nusselt relation can be used for a heat exchanger design. For pressures below 120 bar, the variation in physical properties has to be taken into account for a correct heat exchanger design. For non-buoyancy conditions the Krasnoshchekov-Protopopov equation can be used to calculate heat transfer coefficients. It was observed that buoyancy can enhance heat transfer coefficients up to a factor 3. When buoyancy is active, the highest heat transfer coefficients are realized when the fluid flows downward. The Jackson and Hall correction factor for the calculation of heat transfer coefficients under buoyancy was confirmed by the simulations.