Flow Instability At Transcritical Natural Circulation

After the Fukushima accident, nuclear power has entered a new stage of slow recovery and development. Supercritical water-cooled reactor, as the only kind of reactor which is cooled by water of the 6 kinds of reactors in Generation IV Nuclear Energy Systems, has many advantages in thermal efficiency, economy, conciseness, continuity and sustainability, comparing with pressurized water reactor. As the basic research of SCWR, study on transcritical natural circulation flow and heat transfer is meaningful to the whole nuclear power, which also has the possibility of applied in advanced nuclear power system, inherent safety of nuclear reactors can also be improved. Heat transfer characteristic may be influenced by flow instability at transcritical natural circulation, which has an important effect on nuclear reactor inherent safety. So study on transcritical natural circulation flow instability has important engineering significance and academic value.Experiments were conducted on transcritical natural circulation device, which was designed by North China Electric Power University, the influence of system parameters on natural circulation flow instability were studied. It turned out that fluid temperature tends to be stable in transcritical section, flow excursion and flow oscillation also happen tempestuously in this section. The period of flow oscillation decreases with heating power increases. Mass flow rate increases first and decreases with pressure. The period of flow oscillation also increases with pressure increasing. The change of pressure difference results to the switch of high fluid density section and low fluid density section, and it is opposite to flow rate in phase position.Based on ANSYS software and Fortran language to write program, heating power, pressure and inlet temperature are analyzed via numerical simulation and calculation. The result showed that fluid temperature tends to be stable in transcritical section and the value is critical temperature. Flow rate also changes in this section, it increases in subcritical section and decreases in supercritical section. The heat transfer characteristic of circle channel is better than that of rectangular channel in subcritical condition, but rectangular channel is better in transcritical condition.By comparing mechanism of flow instability and heat transfer deterioration, fluid density, viscosity, expansion coefficient changes result to transfercriticalflow instability. Decreasing of constant pressure volume is the main reason of heat transfer deterioration. At the same time, mass velocity, channel shape are the cofactor of heat transfer deterioration. Bubbles generation, growth and disappearance are the reasons of flow instability. Bubbles generated on heating wall make heat transfer deterioration. Heat transfer deterioration appears both in subcritical and transcritical condition, it results from physics property changes. Flow excursion happens more quickly in subcritical condition than that in transcritical condition, but period of flow oscillation is longer in transcritical condition. As for subcritical condition, flow excursion and flow oscillation phenomenon both appear in narrow rectangular channel, and rectangular channel contributes to system stability.