Abstract
The flashing phenomenon is relevant to nuclear safety analysis, for example by a loss of coolant accident and safety release scenarios. It has been studied intensively by means of experiments and simulations with system codes, but computational fluid dynamics (CFD) simulation is still at the embryonic stage. Rapid increasing computer speed makes it possible to apply the CFD technology in such complex flow situations. Nevertheless, a thorough evaluation on the limitations and restrictions is still missing, which is however indispensable for reliable application, as well as further development. In the present work, the commonly-used two-fluid model with different mono-disperse assumptions is used to simulate various flashing scenarios. With the help of available experimental data, the results are evaluated, and the limitations are discussed. A poly-disperse method is found necessary for a reliable prediction of mean bubble size and phase distribution. The first attempts to trace the evolution of the bubble size distribution by means of poly-disperse simulations are made.
Highlights
Flash boiling is a process of phase change from liquid to vapor
Flashing of liquid to vapor due to pressure drop represents highly complex two-phase situations
To its relevance to nuclear there safetyexists analysis, thetoflashing phenomenon has been studied forDue several decades
Summary
Flash boiling (flashing) is a process of phase change from liquid to vapor. It distinguishes itself from traditional boiling by the way the liquid gets superheated. Large break loss-of-coolant accidents (LOCA) of pressurized water nuclear reactors; Pressure release through blow-off valves at the pressurizer or steam generator; Two-phase critical flow problem through nozzles; Flashing-induced instability in natural circulation systems Under these circumstances, the properties of the flashing flow, such as the discharge flow rate, vapor generation rate, void hold-up, as well as two-phase morphology, are of key safety and economic importance. The flashing two-phase mixture has a strongly three-dimensional nature, which is accompanied with a large heterogeneous gaseous structure and high gas volume fraction All of these features along with the micro-scale bubble dynamic processes require a more sophisticated prediction tool with high time and space resolution, such as the CFD (computational fluid dynamics) technology. There exists a need to assess the restrictions and update the closures for flashing situations
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