Study On Two Phase Flow Instabilities In A Natural Circulation System With Lower Pressure

Flow instabilities have a crucial impact on natural circulation system.Therefore,the prediction and analysis of influencing factors of flow instabilities can promote us a comprehensive understanding of natural circulation system with low pressure.The best estimate system analysis code RELAP5 was used to analyze the natural circulation systems and the corresponding computational model and the scheme of nodalization were built.By comparison with related experimental data,using RELAP5 was proved to be effective and feasible to simulate flow instabilities in natural circulation with low pressure.To analyze the different kinds of flow instabilities in different characteristics natural circulation systems with low pressure,the node dividing of three different characteristics natural circulation systems were studied firstly.The reasonable schemes for dividing the node were obtained.Then,by extensively computing the result showed that:under low pressure conditions,in the natural circulation system with nuclear feedback could be prone to induce the flow instabilities with a short period and iso-amplitude by subcooled boiling;in the natural circulation system with long adiabatic riser could be prone to induced flow instabilities by periodic,intermittent flashing and saturated boiling;in the large-diameter heating pipe natural circulation system,flow instabilities could be caused by many kinds of factors.And these specific parameters of flow instabilities were obtained.To study the influence of system parameters on the flow instabilities,the subcooling of heating inlet,heating power,resistance coefficient of heating inlet,location of cooling section and nuclear feedback effects were selected as the variable respectively.By respectively changing the above parameters,their effects on the flow instabilities were obtained.Under low pressure conditions,the natural circulation system with long adiabatic riser could be sensitive to the subcooling of working fluid at the heating inlet and there was a stable region between them.Moderately increasing the heating power could make flow instabilities induced by saturated boiling transition from periodic to chaotic oscillations and suppress flashing-induced flow instabilities until the system has operated in steady state.Moderately increasing resistance coefficient at the heating section inlet could make flow instabilities induced by saturated boiling transition from single-phase to two-way pulsating and make flow instabilities transition from the leading of gravitational pressure drop to frictional pressure drop.Moving the cooling section from downcomer to the high horizontal pipe could suppress flow instabilities which induced by saturated boiling;and increase the strength of flashing so that the system is more unstable.Finally,the boundaries of instabilities were carried on study and found that:at the pressure of 0.1~0.2MPa,the natural circulation system with large-diameter heating pipe had a strong characteristic of non-linear and flow instabilities caused by many kinds of factors could occur.At the pressure of 1.0~2.0MPa,instability boundary of the natural circulation system with long adiabatic riser gradually narrowed as the pressure increased;when the pressure was 2.0MPa,the left instability boundary coincided with_ex=0 and flashing-induced flow instabilities completely disappeared.