Study on Turbulent Modeling in Gas Entrainment Evaluation Method

Abstract

Suppression of gas entrainment (GE) phenomena caused by free surface vortices are very important to establish an economically superior design of the sodium-cooled fast reactor in Japan (JSFR). However, due to the non-linearity and/or locality of the GE phenomena, it is not easy to evaluate the occurrences of the GE phenomena accurately. In other words, the onset condition of the GE phenomena in the JSFR is not predicted easily based on scaled-model and/or partial-model experiments. Therefore, the authors are developing a CFD-based evaluation method in which the non-linearity and locality of the GE phenomena can be considered. In the evaluation method, macroscopic vortex parameters, e.g. circulation, are determined by three-dimensional CFD and then, GE-related parameters, e.g. gas core (GC) length, are calculated by using the Burgers vortex model. This procedure is efficient to evaluate the GE phenomena in the JSFR. However, it is well known that the Burgers vortex model tends to overestimate the GC length due to the lack of considerations on some physical mechanisms. Therefore, in this study, the authors develop a turbulent vortex model to evaluate the GE phenomena more accurately. Then, the improved GE evaluation method with the turbulent viscosity model is validated by analyzing the GC lengths observed in a simple experiment. The evaluation results show that the GC lengths analyzed by the improved method are shorter in comparison to the original method, and give better agreement with the experimental data.

Modeling of the Free-Surface Vortex-Driven Bubble Entrainment into Water

“…The gas entrainment may cause safety and operational problems and, for this reason, becomes an essential issue in SFR safety analyses [13][14][15][16]. This issue has been intensively investigated by the Japan Atomic Energy Agency (JAEA) and its institutional partners [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30]. The problems associated with gas entrainments in SFRs are: changes in reactivity when the gas reaches the core [11,14,18,[31][32][33], burnout of the fuel pin due to the trap of large bubbles [31], thermal stresses in the reactor structure [32,33], pump cavitation and fluctuations in pump discharge [14,33], reduction in the heat transfer efficiency [11,33], disturbance in the prompt detection of fission products leakage from failed fuel pins [34], disturbance of electromagnetic sensors used for shutdown systems [34], and trouble with acoustic or ultrasonic instrumentation such as boiling noise detector [31,32,34].…”