The abnormal heat transfer behavior of supercritical carbon dioxide(S-CO_2) with low mass fluxes in a horizontal tube was studied

the S-CO_2 heat transfer process in the horizontal tube under the condition of low mass fluxes was simulated with Fluent software

and the abnormal heat transfer behavior of heating and cooling conditions and the influence of heat flux on heat transfer were analyzed. The results show that when the thermal boundary conditions are P=8 MPa

G=200 kg/(m~2·s) and q/G=0.2 kJ/kg

the temperature of top and bottom walls in the S-CO_2 tube decreases along the way during the flow cooling process. When the mainstream temperature of SCO_2 reaches the pseudo critical temperature

the heat transfer coefficient of the top wall at 551.0 mm from the inlet has a sudden peak value

heat transfer enhancement occurs here. Under heating conditions

the temperature of the top wall first rises along the tube path

then drops to 395 K and then rises slowly. The temperature of the bottom wall drops briefly and then rises slowly. At the top wall 69.5 mm away from the inlet

the heat transfer coefficient has a valley value

and the heat transfer at this point deteriorates. The increase of heat flow density aggravates the deterioration of heat transfer under heating conditions

but has no obvious effect on cooling heat transfer. It can be seen that the thermal physical property distribution of the characteristic section is the main reason for the different heat transfer behaviors. Based on the low mass fluxes conditions

thermo-physical properties and buoyancy effects

a correlation equation for predicting supercritical heat transfer enhancement is constructed

which provides theoretical guidance for the design and operation optimization of supercritical fluid heat exchanger.