The carbon dioxide(CO_2) Brayton cycle system is compact

efficient and flexible

and has a good application prospect in the third generation photothermal system and the fourth generation nuclear power system.The deterioration of CO_2 heat transfer affects the safe operation of the unit. In order to study the deterioration of CO_2 heat transfer in the vertical riser

a CO_2 heat transfer characteristic system is established for experimental research

and the CO_2 heat transfer characteristics under subcritical and supercritical conditions are compared. The influence of thermal parameters on the deterioration of CO_2 heat transfer is obtained

and the prediction correlation of CO_2 critical heat flux is established. The predicted value is in good agreement with the experimental value(error ±30%). It is found that the peak value of wall temperature is higher when CO_2 heat transfer deteriorates at subcritical pressure. Far away from the critical pressure and increasing the mass flow rate are conducive to restraining the occurrence of heat transfer deterioration.