EXPERIMENTAL STUDY ON THE HEAT TRANSFER CHARACTERISTICS OF SUPERCRITICAL CARBON DIOXIDE IN THE STRAIGHTLY-RIBBED TUBES

The supercritical CO₂ (S-CO₂) Brayton cycle shows great advantages in structural compactness and thermal efficiency. To ensure the safety of the heating surface under high load, the flow and heat transfer characteristics in straightly-ribbed tubes was examined to study the improvement on the convective heat transfer ability of S-CO₂ and inhibition on the deteriorated heat transfer. An experimental system of S-CO₂ was established, and the heat transfer characteristics of S-CO₂ in both vertically upward smooth and straightly-ribbed tubes were investigated within a wide operating parameter range (pressure P = 7.5–9 MPa, mass flow rate G = 400–1000 kg/sq m-s, heat flux q = 40–600 kW/sq m). The rib width and rib height of tubes are 1 mm, with an inner diameter of 6 mm. The effect of flow parameters on the heat transfer performance was examined. The results demonstrated the convective heat transfer coefficient of straightly-ribbed tubes is about 1.5–2.3 times that of smooth tubes at supercritical pressure. The rib structure could effectively prevent the occurrence of deteriorated heat transfer of SCO₂. A new heat transfer prediction model in the vertically straightly-ribbed tube was proposed based on the result of this experiment.