Fundamental understanding of swirling flow pattern in hydrocyclones

This work is concerned with establishing and validating a physics-based model that describes the swirling flow inside hydrocyclones. The physics is embedded in a Computational Fluid Dynamics (CFD) simulation model whose key features are presented and justified in the paper. Some features are selected in such a way that the model can eventually be used to simulate dense flow inside hydrocyclones. Nevertheless, its underlying physics is here within validated against dilute flow conditions. The model applies a Eulerian multi-fluid modelling approach for fluid–particle turbulent flows, and is computed using the semi-industrial code NEPTUNE_CFD. Simulation results are successfully compared to water split, velocity profiles inside the hydrocyclone and partition function measurements, either produced using our own experimental setup or from the literature. The work finds velocity profiles to be the most discriminating parameter for validation of the physics that describes the swirling flow inside the hydrocyclone. Water split on the other hand shows no relation to the choice of turbulence model and hence cannot be used to validate a mechanistic model of the hydrocyclone. The physics-based model presented here is the first building block towards describing and understanding hydrocyclone flow under dense regime

Development and validation of models for bubble coalescence and breakup

A generalized model for bubble coalescence and breakup has been developed, which is based on a comprehensive survey of existing theories and models. One important feature of the model is that all important mechanisms leading to bubble coalescence and breakup in a turbulent gas-liquid flow are considered. The new model is tested extensively in a 1D Test Solver and a 3D CFD code ANSYS CFX for the case of vertical gas-liquid pipe flow under adiabatic conditions, respectively. Two kinds of extensions of the standard multi-fluid model, i.e. the discrete population model and the inhomogeneous MUSIG (multiple-size group) model, are available in the two solvers, respectively. These extensions with suitable closure models such as those for coalescence and breakup are able to predict the evolution of bubble size distribution in dispersed flows and to overcome the mono-dispersed flow limitation of the standard multi-fluid model. For the validation of the model the high quality database of the TOPFLOW L12 experiments for air-water flow in a vertical pipe was employed. A wide range of test points, which cover the bubbly flow, turbulent-churn flow as well as the transition regime, is involved in the simulations. The comparison between the simulated results such as bubble size distribution, gas velocity and volume fraction and the measured ones indicates a generally good agreement for all selected test points. As the superficial gas velocity increases, bubble size distribution evolves via coalescence dominant regimes first, then breakup-dominant regimes and finally turns into a bimodal distribution. The tendency of the evolution is well reproduced by the model. However, the tendency is almost always overestimated, i.e. too much coalescence in the coalescence dominant case while too much breakup in breakup dominant ones. The reason of this problem is discussed by studying the contribution of each coalescence and breakup mechanism at different test points. The redistribution of the gaseous phase from the injection position at the pipe wall to the whole cross section is overpredicted by the Test Solver especially for the test points with high superficial gas velocity. Besides the models for bubble forces, the simplification of the Test Solver to a 1D model has an influence on the redistribution process. Simulations performed using CFX show that a considerable improvement is achieved with comparison to the results delivered by the standard closure models. For the breakup-dominant cases, the breakup rate is again overestimated and the contribution of wake entrainment of large bubbles is underestimated. Furthermore, inlet conditions for the liquid phase, bubble forces as well as turbulence modeling are shown to have a noticeable influence, especially on the redistribution of the gaseous phase

A Generalized Compressible Cavitation Model

A new multi-phase model for low speed gas/liquid mixtures is presented; it does not require ad-hoc closure models for the variation of mixture density with pressure and yields thermodynamically correct acoustic propagation for multi-phase mixtures. The solution procedure has an interface-capturing scheme that incorporates an additional scalar transport equation for the gas void fraction. Cavitation is modeled via a finite rate source term that initiates phase change when liquid pressure drops below its saturation value. The numerical procedure has been implemented within a multi-element unstructured framework CRUNCH that permits the grid to be locally refined in the interface region. The solution technique incorporates a parallel, domain decomposition strategy for efficient 3D computations. Detailed results are presented for sheet cavitation over a cylindrical headform and a NACA 66 hydrofoil

Methodology for tidal turbine representation in ocean circulation model

The present method proposes the use and adaptation of ocean circulation models as an assessment tool framework for tidal current turbine (TCT) array layout optimization. By adapting both momentum and turbulence transport equations of an existing model, the present TCT representation method is proposed to extend the actuator disc concept to 3-D large-scale ocean circulation models. Through the reproduction of experimental flume tests and grid dependency tests, this method has shown its numerical coherence as well as its ability to simulate accurately both momentum and turbulent turbine-induced perturbations in both near and far wakes in a relatively short period of computation time. Consequently the present TCT representation method is a very promising basis for the development of a TCT array layout optimization tool

Segregation in dense gas-fluidised beds: validation of a multi-fluid continuum model with non-intrusive digital image analysis measurements.

A non-intrusive digital image analyses technique is applied to study size driven segregation of a binary mixture of coloured glass beads in a bubbling gas-fluidised bed. Segregation rates and patterns obtained from experiments are compared to numerical simulations performed with a two-dimensional multi-fluid Eulerian model, that uses closure laws according to the kinetic theory of granular flow. It is demonstrated that prediction of segregation is a rather severe test case for fundamental hydrodynamic models, since bubble dynamics and momentum transfer between particles of different classes have to be modelled correctly. At all gas velocities segregation rates predicted by the multi-fluid model were much higher than those observed in experiments. At gas velocities higher than the minimum fluidisation velocity of the largest particles the model still predicts segregation, when it does not occur in experiments. It is concluded that the predicted intensity of bubbling is too low, since energy dissipation by particleparticle\ud interaction is still underestimated in the applied kinetic theory closure model

Recent progress towards hydrodynamic modelling of dense gas-particle flows

In this paper a state-of-the-art review will be presented on hydrodynamic modeling of dense gas-particle flows as encountered in the fluid\ud bed family of gas-solid contactors. After a brief introduction the different classes of fundamental hydrodynamic models will be discussed together with their physical basis and mutual advantages and disadvantages. Thereafter some typical results will be presented on first principles modeling of dense\ud gas-fluidized beds. Finally the conclusions will be presented and areas which need substantial further attention will be indicated

Nonlinear evolution of the magnetized Kelvin-Helmholtz instability: from fluid to kinetic modeling

The nonlinear evolution of collisionless plasmas is typically a multi-scale process where the energy is injected at large, fluid scales and dissipated at small, kinetic scales. Accurately modelling the global evolution requires to take into account the main micro-scale physical processes of interest. This is why comparison of different plasma models is today an imperative task aiming at understanding cross-scale processes in plasmas. We report here the first comparative study of the evolution of a magnetized shear flow, through a variety of different plasma models by using magnetohydrodynamic, Hall-MHD, two-fluid, hybrid kinetic and full kinetic codes. Kinetic relaxation effects are discussed to emphasize the need for kinetic equilibriums to study the dynamics of collisionless plasmas in non trivial configurations. Discrepancies between models are studied both in the linear and in the nonlinear regime of the magnetized Kelvin-Helmholtz instability, to highlight the effects of small scale processes on the nonlinear evolution of collisionless plasmas. We illustrate how the evolution of a magnetized shear flow depends on the relative orientation of the fluid vorticity with respect to the magnetic field direction during the linear evolution when kinetic effects are taken into account. Even if we found that small scale processes differ between the different models, we show that the feedback from small, kinetic scales to large, fluid scales is negligable in the nonlinear regime. This study show that the kinetic modeling validates the use of a fluid approach at large scales, which encourages the development and use of fluid codes to study the nonlinear evolution of magnetized fluid flows, even in the colisionless regime