Supercritical and ultra-supercritical pressure boilers have been widely used in China because of its advantages of high capability, high thermal efficiency and low pollutants emission. In view of the high working parameters and complicated working conditions of such boilers, safe and stable operations of these boilers have become the focus of attention of related researchers and engineers for many years. As one of the most important phenomena that may occur in thermal power plants, flow instabilities of water in boilers’ water-cooled wall may result in heat transfer deterioration, thermal fatigue of pipes and even breakdown of the supercritical units, and have thus attracted much attention of developers and researchers of the boilers.

However, due to difficulties in carrying out experiments on the density wave oscillation of water under supercritical pressures, the related experimental data is very rare, and moreover, the characteristics of density wave oscillations of water under supercritical pressures has not been well understood.

A series of experiments have been conducted on the density wave oscillation of water flowing at supercritical pressures in a 6-m long vertical upward tube with 20.0-mm internal diameter. The experimental parameters cover the pressures from 23 to 27 MPa, the mass fluxes from 300 to 600 kg·m−2·s−1, and the heat fluxes from 225 to 500 kW·m−2. Three types of density wave oscillation were found in the present study: 1) stable periodic density wave oscillation, 2) attenuated density wave oscillation, 3) divergent density wave oscillation. Effects of pressures, mass fluxes and heat fluxes on the density wave oscillation were analyzed. With the increase in pressures and mass fluxes, the density wave oscillation of supercritical pressure water can be postponed, and is difficult to occur. And the density wave oscillation will be triggered and enhanced by increasing the heating flux. The mechanism of density wave oscillation of supercritical pressure water in tubes was also analyzed, and a dataset was established for the verification of related numerical calculations and modeling.

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