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

Abstract

The supercritical carbon dioxide (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 is studied to explore the improvement on the convective heat transfer ability of S-CO₂ and inhibition on the deteriorated heat transfer. An experimental system of S-CO₂ is established, and the heat transfer characteristics of S-CO₂ in both vertically upward smooth and straightly-ribbed tubes are studied within a wide operating parameter range (pressure <i>P</i> &#61; 7.5-9 MPa, mass flow rate <i>G</i> &#61; 400-1000 kg&#183;m^-2&#183;s^-1, heat flux <i>q</i> &#61; 40-600 kW&#183;m^-2). 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 is analyzed. The results show 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 can effectively prevent the occurrence of deteriorated heat transfer of SCO₂. A new heat transfer prediction model in the vertically straightly-ribbed tube is proposed based on the result of this experiment.