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.

Moisture separator reheater for nuclear steam turbine system of light water reactor, a combined equipment of mist separator and in-tube side condensing multi-tube type heat exchanger, has an inherent issue of flow instability inside the tubes. The issue causes the temperature oscillation and possible thermal fatigue at tube to tube-sheet welding of the tube outlet end. Construction of its heat transfer is that heating main steam supplied by NSSS inside the tubes transfers its latent heat by condensing to the colder high pressure turbine exhaust steam cooling outside of the tubes. The condensed water is exhausted by friction of venting excess steam through the tubes which makes the flow in tubes gas-liquid two phase regime and results the steady drainage without sub-cooling and associated temperature oscillation at tube end. A measuring was conducted on MSR of an existing 600 MW class unit to clarify the unstable behaviors without excess steam, which showed the temperature oscillation at tube end with synchronizing to oscillation of both pressure difference between tube inlet and outlet chambers and tube surface temperature along the tube. The cycle and the amplitude of the temperature oscillation were 55 second and 40K. Only the effects of venting steam for the actual unit and presumption of the mechanism of subcool were reported in the original paper, Journal of Engineering for Gas Turbines and Power 2010 Vol. 132/ 102905, here is added qualitative and quantitative study for both dynamics of the condensed water and behavior at tube outlet end to assess adequacy of the mechanism.

For the flow of the supercritical water (SCW), the fierce variation of density and specific volume possibly cause flow instability. Based on the structure of parallel channels, mathematical and physical models were established to simulate the flow and heat transfer characteristics of the supercritical water in the parallel channels with semi-implicit scheme and staggered mesh scheme. Flow instability of super-critical water was obtained by using the little perturbation method. Pseudo-subcooling number (N SUB ) and pseudo-phase change number (N PCH ) are defined based on the property of SCW. The marginal stability boundary (MSB) is obtained with using the N SUB and N PCH . The effects of mass flow rate, inlet temperature and system pressure on the flow instability boundary were also investigated. When increasing the mass flows and system pressure, decreasing the heat flux, the stability in the parallel channels increases. The effect of inlet temperature in the low pseudo-subcooling number region is different from that in high pseudo-subcooling number region.

For efficient cooling applications in power plants, the use of small two-phase natural circulation loops becomes attractive. An experimental study was carried out to examine how thermal hydraulic stability and operation conditions of these devices are affected by nucleation sites. A very smooth glass tube with artificial nucleation sites have been employed as boiling channel. The mass flow rate has been determined as function of heat flux and nucleation site location. Particular for low heat flux levels, the nucleation sites have a strong impact to the stability behavior. The observed flow instabilities have been analyzed with regard to non-linear effects and chaotic behavior.