A computationally derived heat transfer correlation for in-tube cooling turbulent supercritical CO2

Abstract

This paper computationally investigates the turbulent heat transfer of sCO2 flows cooled in large horizontal tubes with diameter of 15.75 mm, 20 mm and 24.36 mm using RANS turbulence models. The numerical models were validated against experimental data published in literature to demonstrate the reliability of CFD simulations on the heat transfer coefficient prediction and buoyancy effect capture to turbulent sCO2. Based on the validated model, a number of computations, involving a wide range of operating conditions, have been carried out. The effect of mass flux (200–800 kg/m2s), pressure (8–10 MPa), heat flux (5–36 kW/m2) and tube diameter has been analysed. Results demonstrate that the AKN model shows the best consistencies with the experimental measurements and is also able to well reproduce the heat transfer characteristics under various conditions. As the mass flux increases, the heat transfer coefficients go up due to the enhanced turbulence diffusion. Pressure has a significant effect on the distribution of heat transfer coefficient, and its peak drops sharply with rising pressure. At Tb > Tpc, with the heat flux and tube diameter increasing, sCO2 heat transfer performance is improved; whereas at Tb < Tpc, the heat flux and tube diameter almost have no effects on the heat transfer performance. Considerable deviations with the existing heat transfer correlations necessitate the development of a new correlation to predict the heat transfer coefficients of cooling turbulent sCO2 in large horizontal pipes. Based on the reliable computational datasets, a Nusselt number equation based on the Gnielinski form with the ratio of density incorporated is formulated.