Numerical investigation on convective heat transfer of supercritical pressure CO2 in horizontal tubes under cooling condition is carried out by using SST k-ω turbulent model. The effects of heat flux, tube diameter and buoyancy on heat transfer characteristics are discussed. The results show that the temperature stratification and secondary flow are generated, and the radial velocity and turbulent kinetic energy profiles are asymmetric on the cross-section due to buoyant effect. The peak of heat transfer coeffcient near the pseudo-critical temperature appears earlier but with a smaller peak value on the bottom surface than on the top surface. The heat transfer discrepancy on the cross-section can be explained by fleld synergy principle well. The heat flux has no evident in fluence on the peak value of heat transfer coeffcient, but affects its position seriously. The larger the heat flux and the tube diameter are, the more significant the buoyant effect becomes.
Conclusions
The convective heat transfer of supercritical pressure CO2 in horizontal tubes under cooling condition is numerically investigated by using SST k-ω turbulent model in software ANSYS CFX. The numerical results with SST k-ω turbulent model are in good agreement with the experimental results. The main conclusions are obtained as follows:
(1) The wall temperature distribution is significantly non-uniform in circumferential direction. The wall temperature on the top surface is higher than that on the bottom surface due to the buoyant effect. The higher heat flux leads to the greater wall temperature difference between the top surface and the bottom surface.
(2) The radial velocity and turbulent kinetic energy profiles are asymmetric on the cross-section. The fluid with high velocity mainly concentrates on the upper part of the tube, and the turbulence kinetic energy of the fluid on the upper part is higher than that on the bottom.
(3) The heat transfer coefficient along the tube has a peak value in the vicinity of the pseudo-critical temperature. The peak of heat transfer coefficient appears earlier but with a smaller peak value on the bottom surface than on the top surface, which can be explained by field synergy principle well. The heat flux has no evident influence on the peak value of heat transfer coefficient, but affects its position seriously. The larger the heat flux and the tube diameter are, the more significant the buoyant effect becomes. The buoyant effect has great influence on the heat transfer coefficient in the region of Tb < Tpc, but has little effect on the heat transfer coefficient in the region of Tb > Tpc.
(4) The temperature stratification appears on the tube cross-section due to the drastic changes of properties near the wall, and the stratification becomes more obvious as the heat flux increases. The secondary flow and vortexes are generated on the cross-section due to the buoyant effect, and the secondary flow intensity and the twist degree of vortex can be used to characterize the buoyant effect.
The results have been published on Journal of Supercritical Fluids 130 (2017) 389-398.
Fig.1. Physical model of horizontal tube
