Abstract
Gas entrainment phenomena caused by a free surface vortex is a very important topic in design studies on various fluid machineries. For example, in sodium-cooled fast reactors, a free surface vortex of the sodium coolant can be generated in the reactor vessel due to the complex velocity profile/fluctuations in the coolant. With the increase in the free surface vortex intensity, the static pressure at the vortex center region decreases, and the free surface dent, i.e. gas core, develops towards a suction mouth. Finally, when the gas core is connected to the suction mouth, the gas (bubble) is entrained into the primary sodium coolant circuit. Such a gas entrainment can make the void fraction higher in the coolant, which may pose a threat to the stable and safe operations of sodium-cooled fast reactors because the high void fraction in the reactor core may cause the disturbance in core power. For this reason, the entrained gas flow rate by the gas entrainment should be controlled within an allowance level for the practical safety design of sodium-cooled fast reactors. However, the accurate evaluation method for the entrained gas flow rate has not been established successfully yet due to the non-linear characteristics of the gas entrainment phenomena, that is, detailed information of, e.g. vortical velocity profile and/or void fraction distribution is necessary to evaluate the entrained gas flow rate. Such data have not been obtained enough despite a lot of previous experimental studies by many researchers due to the difficulty in accurately measuring the physical values near the vortex center. Therefore, further researches with the help of CFD are required to deepen the understanding of the gas entrainment phenomena. The purpose of our study is the development of a transient CFD model which can well reproduce the gas entrainment phenomena. In this study, as a first step, the applicability of the CFD model is validated by the comparison between the CFD results and the experimental data of the gas entrainment in a cylindrical vessel with a suction pipe. As a result, the CFD and experiments show similar two-phase flow pattern inside the suction pipe, i.e., rotating annular flow, the shape of the gas core at the free surface is also very similar. Therefore, it is confirmed that the CFD can predict the gas entrainment phenomena triggered by a free surface vortex properly and accurately within the acceptable error range.
