Abstract
The recently developed GENTOP (Generalized Two Phase Flow) concept, which is based on the multifield Euler‒Euler approach, was applied to model a free-surface vortex—a flow situation that is relevant for hydraulic intake. A new bubble entrainment model has been developed and implemented in the concept. In general, satisfactory agreement with the experimental data can be achieved. However, the gas entrainment can be significantly affected by several parameters or models used in the CFD (Computational Fluid Dynamics) simulation. The scale of curvature correction C s c a l e in the turbulence model, the coefficient in the entrainment model C e n t , and the assigned bubble size to be entrained have a significant influence on the gas entrainment rate. The gas entrainment increases with higher C s c a l e values, which can be attributed to the stronger rotation captured by the simulation. A smaller bubble size gives higher gas entrainment, while a larger bubble size leads to a smaller entrainment. The results also show that the gas entrainment can be controlled by adjusting the entrainment coefficient C e n t . Based on the modeling framework presented in this paper, further improvement of the physical modeling of the entrainment process should be done.
Highlights
A free-surface vortex may exist in a wide range of scales; it can be as small as a “bathtub vortex” [1,2,3] or can be as big as an ocean whirlpool [4]
In the proposed entrainment model described in Equation (37), the vorticity magnitude is used to take into account the rotation and to identify the location of the gas core tip, which is not included in the entrainment model of [65]
The results show that the entrainment rate that is controlled by the entrainment model is highly influenced by the entrained bubble size distribution
Summary
A free-surface vortex (see Figure 1) may exist in a wide range of scales; it can be as small as a “bathtub vortex” [1,2,3] or can be as big as an ocean whirlpool [4]. A free-surface vortex and its associated gas entrainment may lead to several operational and safety problems [5,6,7,8,9]. They may cause mechanical damage and loss of performance in fluid machinery such as turbines and pumps [6,7]. In the case of bubble entrainment driven by the free-surface vortex, the entrainment process needs to be resolved This requires a very fine mesh, especially at the tip of the gas core. A new bubble entrainment model considering the physics of the flow has been developed and implemented in the GENTOP concept in this study
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