Ng Niels Deen

Large eddy simulation of the Gas–Liquid flow in a square cross-sectioned bubble column

In this work the use of large eddy simulations (LES) in numerical simulations of the gas–liquid flow in bubble columns is studied. The Euler–Euler approach is used to describe the equations of motion of the two-phase flow. It is found that, when the drag, lift and virtual mass forces are used, the transient behaviour that was observed in experiments can be captured. Good quantitative agreement with experimental data is obtained both for the mean velocities and the fluctuating velocities. The LES shows better agreement with the experimental data than simulations using the k–var epsilon model.

Ensemble correlation PIV applied to bubble plumes rising in a bubble column.

This paper discusses an ensemble correlation, double-exposure single-frame, particle image velocimetry (PIV) technique that can be applied to study dispersed gas?liquid two-phase flows. The essentials of this technique will be reviewed and several important issues concerning the implementation of the PIV technique will be discussed. The capabilities of the newly developed PIV technique will be demonstrated by examining the gas and liquid flow fields induced by a bubble plume rising in a rectangular bubble column.

Parallelization of an Euler-Lagrange model using mixed domain decomposition and a mirror domain technique: Application to dispersed gas-liquid two-phase flow

We report a parallel algorithm applicable to a Euler-Lagrange model embedding four-way coupling. The model describing the dispersed phase dynamics accounts for bubble-bubble collisions and is parallelized using a mirror domain technique while the pressure Poisson equation for the continuous phase is solved using a domain decomposition technique implemented in the PETSc library [S. Balay, K. Buschelman, W.D. Gropp, D. Kaushik, M.G. Knepley, L.C. McInnes, B.F. Smith, H. Zhang, PETSc Web page: http://www.mcs.anl.gov/petsc, 2001]. The parallel algorithm is verified and it is found that it gives the same results for both phases as compared to the serial algorithm. Furthermore the algorithm shows good scalability up to 32 processors. Using the proposed method, a homogeneous bubbly flow in a laboratory scale bubble column can be simulated at very high gas hold-up (37%) while consuming a reasonable amount of calculation wall time.

Two-Phase PIV in Bubbly Flows: Status and Trends

Particle Image Velocimetry (PIV) is a measurement technique that has received a lot of attention for this purpose in the last decade. PIV is an optical and thus non-intrusive measurement technique that gives instantaneous 2D velocity data for a whole plane in a 3D flow field. In this paper we will focus on recent developments in PIV for dispersed two-phase flows in particular for bubbly flows.

Multi-scale modeling of dispersed gas-liquid two-phase flows

Dispersed gas-liquid two-phase flows are encountered in a variety of industrial processes such as the large-scale production of synthetic fuels and base chemicals employing bubble column reactors. Despite their widespread industrial application the detailed understanding of the fluid mechanics prevailing in bubble column reactors is unfortunately lacking (Tomiyama, 1998), which can be related to the inherent complexity of the underlying physical phenomena in (dense) bubbly flows such as bubble-liquid interaction (including turbulence) and bubble-bubble interaction (including coalescence and break-up).

Large Eddy Simulation for Dispersed Bubbly Flows: A Review

Large eddy simulations (LES) of dispersed gas-liquid flows for the prediction of flow patterns and its applications have been reviewed. The published literature in the last ten years has been analysed on a coherent basis, and the present status has been brought out for the LES Euler-Euler and Euler-Lagrange approaches. Finally, recommendations for the use of LES in dispersed gas liquid flows have been made.

Particle image velocimetry measurements in an aerated stirred tank

In state-of-the-art research of multiphase flows, numerical simulations are performed time-dependently and in three dimensions. For the validation of such simulations the availability of quantitative measurement data is crucial. In this study angle resolved and angle-averaged flowfields of the liquid in the vicinity of a Rushton impeller in a stirred tank were measured with the use of particle image velocimetry (PIV). Single-phase liquid flowfields were compared with flowfields of the liquid in an aerated stirred tank. The characteristic trailing vortices in the single-phase flow were observed. When the tank was aerated, the trailing vortices disappeared. Furthermore, the flowfield turned out to be less periodic than without gas. The measured liquid velocities in the impeller-swept region were 50% lower in the case where gas was present. The absolute liquid velocity fluctuations in the presence of gas were of the same order of magnitude as in the case without gas, but when scaled with the maximum radial velocity the relative velocity fluctuations were significantly larger than in the single-phase flow.

Asymmetry-induced particle drift in a rotating flow

We report on an intriguing phenomenon taking place in a liquid rotating around a fixed horizontal axis. Under suitable conditions, bubbles and particles are observed to drift along the axis of rotation maintaining a constant distance from it and a constant angle of elevation above the horizontal. Absence of fore-aft symmetry of the bubble or particle shape is a prerequisite for this phenomenon. For bubbles, this requires a volume sufficiently large for surface-tension effects to be small and large deformations to be possible. Particle image velocimetry and flow visualization suggest that the wake does not play a role. The dependence on bubble radius, particle shape, liquid viscosity, and speed of rotation is investigated.

Large-Eddy simulation of a particle-laden turbulent channel flow

Large-eddy simulations of a vertical turbulent channel flow with 420,000 solid particles are performed in order to get insight into fundamental aspects of a riser flow The question is addressed whether collisions between particles are important for the ow statistics. The turbulent channel ow corresponds to a particle volume fraction of 0.013 and a mass load ratio of 18, values that are relatively high compared to recent literature on large-eddy simulation of two-phase ows. In order to simulate this ow, we present a formulation of the equations for compressible ow in a porous medium including particle forces. These equations are solved with LES using a Taylor approximation of the dynamic subgrid-model. The results show that due to particle-uid interactions the boundary layer becomes thinner, leading to a higher skin-friction coefcient. Important effects of the particle collisions are also observed, on the mean uid prole, but even more o on particle properties. The collisions cause a less uniform particle concentration and considerably atten the mean solids velocity prole.

Effect of viscosity on droplet-droplet collisional interaction

A complete knowledge of the effect of droplet viscosity on droplet-droplet collision outcomes is essential for industrial processes such as spray drying. When droplets with dispersed solids are dried, the apparent viscosity of the dispersed phase increases by many orders of magnitude, which drastically changes the outcome of a droplet-droplet collision. However, the effect of viscosity on the droplet collision regime boundaries demarcating coalescence and reflexive and stretching separation is still not entirely understood and a general model for collision outcome boundaries is not available. In this work, the effect of viscosity on the droplet-droplet collision outcome is studied using direct numerical simulations employing the volume of fluid method. The role of viscous energy dissipation is analysed in collisions of droplets with different sizes and different physical properties. From the simulations results, a general phenomenological model depending on the capillary number (Ca, accounting for viscosi...