Experimental and numerical study on the heat transfer characteristic of supercritical water with high mass velocity in a helically coiled tube

Abstract

To explore the flow and heat transfer characteristics of supercritical fluid with high mass velocity in a helically coiled tube, experiments and numerical simulations were conducted to evaluate the wall temperature distribution and heat transfer coefficient in a vertical helically coiled tube under a pressure of 24–28 MPa, with a mass velocity of 2500–4000 kg·m−2·s−1 and with a heat flux of 210–420 kW·m−2. Results show that the coil's inner wall temperature is the highest and the outer wall temperature is the lowest because the centrifugal effect is much stronger than the buoyancy effect at a high mass velocity in a helically coiled tube and thus the fluid with lower temperature and consequently higher density shifts to the outer side of the coil. To describe the wall temperature difference in the different circumferential points, a metric called the circumferential wall temperature inhomogeneity (CWTI) is proposed, and the influence of the parameters on the CWTI is then analysed. As bulk fluid temperature increases, the CWTI first decreases to a minimum at the pseudo-critical temperature and then increases thereafter. The increase of mass velocity reduces the CWTI, and the influence of mass velocity on the CWTI is the most significant in the high enthalpy region. In the high enthalpy region with the bulk fluid temperature of 410.1 °C, when the mass velocity increases from 2500 to 3250 kg·m−2·s−1, the CWTI decreases by 34.2%. The CWTI also increases almost linearly with the increase in heat flux. Finally, the factors influencing the average heat transfer coefficient in the helical tube are discussed. The coefficient first increases and then decreases with increase in the bulk fluid temperature, and the peak value of the coefficient appears at the pseudo-critical point corresponding to the supercritical pressure. Increasing the mass velocity improves the average heat transfer coefficient, and the improvement effect is more obvious after the pseudo-critical point. Increasing the pressure causes the bulk fluid temperature at which the peak average heat transfer coefficient is achieved to increase, and the peak value gradually decreases. When the pressure increases from 24.0 MPa to 26.0 MPa and to 28.0 MPa, the peak value of the average heat transfer coefficient decreases by 34.2% and 46.4%, respectively.