Abstract
The aerator can reduce erosion by mixing a large amount of air into the water in the solid wall area. The effectiveness of erosion reduction is mainly based on air concentration and its bubble size distribution. However, simultaneous simulation of the air concentration and its bubble size distribution in numerical simulations is still a hot and difficult area of research. Aiming at the downstream aerated flow of hydraulic aeration facilities, several numerical models, such as VOF, mixture, Euler, and Population Balance Model (PBM), are compared and verified by experiments. The results show that the CFD-PBM coupled model performs well compared to other conventional multiphase models. It can not only obtain the evolution law of the bubble distribution downstream of the aerator but also accurately simulate the recombination and evolution process of bubble aggregation and breakage. The Sauter mean diameter of the air bubbles in the aerated flow decreases along the way and eventually reaches a stable value. The bubble breakage is the main process in the development of the bubbles. It reveals the aeration law that the small air bubbles are closer to the bottom plate, while the large bubbles float up along the aerated flow, which provides a powerful support for the basic research on the mechanism of aeration and erosion reduction.
Highlights
E phenomenon of high-speed flow aeration in the discharge structure belongs to the problem of the gas-liquid two-phase flow. e main characteristics of the flow field are as follows: (1) one phase is continuous and the other is discrete; (2) there are interphase mixing, diffusion, deformation, and relative slip; and (3) there is the exchange of momentum, energy, and mass between phases. e flow aeration is affected by the boundary condition, so the influence factors are very complex [8]
Compared with the model experiments, numerical simulation has the characteristics of short operation periods and less resource consumption. e most important thing is that it can obtain a lot of comprehensive and detailed flow information of the gas-liquid two-phase flow which cannot be measured or difficult to measure in model experiments
Wu et al [10] investigated the particle distribution in the scrubbingcooling chamber of the entrained-flow coal gasifier using a three-dimensional Eulerian–Lagrangian model. e collisions between particles were taken into account by means of the Direct Simulation Monte Carlo (DSMC) method based on the hard-sphere model. e results indicate that the axial distribution of the particle number concentration becomes wave-shaped in the pool of the scrubbing-cooling chamber
Summary
Each phase in the model has a set of momentum equations and continuity equations. E mixture model solves the momentum equation of mixture and describes the discrete phase by relative velocity [23]. E momentum equation for the mixture can be obtained by summing the individual momentum equations for all phases E transport equation of the phase fraction of the discrete element is expressed as follows:. When the PBM is set up, the probability density of the bubble size of each component in the air phase at the inlet of velocity can be given by the lognormal distribution function. E lognormal distribution for the number density n, as a function of the bubble size L, can be written as follows: n(L) √1 e− (ln L− μ)2/2σ2 , Lσ 2π (22).
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