Experimental study on flow instability and oscillatory heat transfer characteristics of ultra-supercritical water in parallel channels

The flow instability and oscillatory heat transfer characteristics of ultra-supercritical water in vertical upward parallel channels were experimentally investigated under the condition range of pressure within 26-30 MPa, inlet pressure drop coefficient from 0 to 5, time-averaged mass flux of 420 kg.m(-2).s(-1), and heat flux in the range of 0-130 kW.m(-2). Two types of oscillations, called low-frequency oscillation and high-frequency oscillation, were observed during the experiment. The effects of mass flux, pressure, and inlet pressure drop coefficient on flow instability were discussed. The influences of mass flux oscillation on wall temperature and oscillatory amplitude ratio and oscillation frequency number on time-averaged Nusselt (Nu) number were analyzed. The time-averaged Nu number of oscillatory flow was compared with the supercritical heat-transfer correlation equations. Results showed that the increase in pressure, inlet pressure drop coefficient, and inlet mass flux were beneficial to system stability when low-frequency oscillation occurred. Pressure and inlet pressure drop coefficient had a slight effect on flow stability, but inlet mass flux could still improve flow stability for the high-frequency oscillation. The heat transfer was restrained in the quasi-critical enthalpy region when the oscillation occurred. The wall temperature fluctuated with mass flux oscillation with the same period and opposite phase, and the larger amplitude of mass flux oscillation led to the stronger fluctuation of the wall temperature. Upon the occurrence of low-frequency oscillation, the time-averaged Nu number decreased with the increase of oscillatory amplitude ratio, and initially decreased with the increase of oscillatory frequency number and then remained constant. Upon the occurrence of high-frequency oscillation, the time-averaged Nu number increased slowly with the increase of oscillatory amplitude ratio and decreased with the increase of oscillatory frequency number. The values of Nu number predicted by correlations were generally higher than those obtained by experiments. (C) 2020 Elsevier Ltd. All rights reserved.