Numerical study on the local heat transfer characteristic of supercritical CO2 in semicircular/circular channel under cooling condition

A mathematic-physical model was established to explore the local heat transfer characteristics of supercritical CO2 in semicircular and circular channels under cooling condition, which was applied in the field of supercritical CO2 Brayton cycle. The numerical method was validated by the experimental data with the maximum error of 12.44%, meeting the accuracy requirement. And the operation pressure, cooling heat flux, inlet velocity and hydraulic diameter were 8∼9 MPa, 10∼14 kW⋅m−2, 1.2∼5 m⋅s−1 and 1∼2 mm, respectively. The results showed that the local heat transfer coefficient of SCO2 would increase at first and then decrease along the flow direction regardless of the semicircular channel and circular channel, and the peak value of the local heat transfer coefficient gradually decreased with increasing operation pressure and disappeared when the inlet velocity exceeded 3 m·s−1. Besides, the heat transfer performance of the horizontal semicircular channel was weaker than that of the circular channel due to the influence of the buoyancy effect. For the horizontal flow, the buoyancy effect was increased, while the heat transfer was decreased with increasing hydraulic diameter of the semicircular channel. Finally, the heat transfer correlation of the supercritical CO2 cooling process in the horizontal semicircular channel was modified by considering the influence of the dimensionless buoyancy effect and hydraulic diameter, and the prediction errors of 93.3% correlation calculation results were within ±13.5% after the modification. These results will provide some theoretical guidance on the design and optimization of the cooler for the supercritical CO2 Brayton cycle system.