Search

Numerical simulations of contaminated spherical drops falling through a stagnant liquid at low Reynolds numbers are carried out using the finite difference method. The numerical results are used to describe the behavior of the surfactant concentrations and to understand the surfactant effects on the fluid motions in detail. The predicted interfacial surfactant concentration, Γ, is almost zero for angles, θ, below a certain value (the stagnant-cap angle, θcap ), whereas it steeply increases and reaches a large value for θ > θcap (the stagnant-cap region). The increase in the initial surfactant concentration, C0, in the drop enhances the adsorption from the drop to the interface, which results in the increase in Γ and the decrease in θcap . Peaks appear in the predicted Marangoni stresses around θcap, which causes similar peaks in the pressure distribution. The high-pressure spots prevent the fluid motion along the interface, which results in the formation of the stagnant-cap region and the attenuation of the tangential velocity in the continuous phase. The surfactant flux from the bulk to the interface decreases C in the vicinity of the interface for θ < θcap and the weak diffusion cannot compensate for the reduction in C by adsorption, which results in C at the interface smaller than C0 . The pattern of the low C region is determined by the advection and does not smear out because of a small diffusive flux.

Knowledge about collision dynamics of particles is fundamental for describing and modelling particulate processes. If liquids are additionally involved in the collisions, the rebound dynamics strongly depend on the liquid properties. However until now, wet particle collisions are still not fully understood. Hence, influence of liquid properties, particle attributes and impact conditions on rebound behavior of dry particles from wet walls is investigated experimentally and a force balance model is described, which can predict collision dynamics on a physical basis. The results of this proceeding were already published in [1]. .

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 flows. 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-fluid interactions the boundary layer becomes thinner, leading to a higher skin-friction coefficient. Important effects of the particle collisions are also observed, on the mean fluid prole, but even more o on particle properties. The collisions cause a less uniform particle concentration and considerably flatten the mean solids velocity prole.

The bubble characteristics in a 3-D cylindrical fluidized bed have been investigated both experimentally and numerically. Experiments were performed on a 0.1 m diameter fluidized bed, with alumina oxide particles (diameter ~1 mm) as a fluidizing material. Measurements were done at a spatial resolution of 1 mm and a temporal resolution of 1000 cross-sectional images per second, using an ultrafast electron beam X-ray computed tomography (XRT) setup (Fischer and Hampel 2010). A two-fluid model using kinetic theory of granular flow (Verma et al., 2013) was used to predict the bed dynamics numerically. The equivalent bubble diameter as a function of height is in close agreement with Darton et al. (1977) and Werther (1975) correlations. The bubble size distribution predicted from simulations is higher compared to experiments. Both the bubble rise velocity and the bubble size increase with increase in excess gas velocity. The experimental measurements and simulation predictions are in fair agreement with the Hilligardt and Werther (1986) correlation.

The present work focuses on the measurements of bubble properties in heterogeneous bubbly flows in a square bubble column. A four-point optical fibre probe was used for this purpose. The accuracy and intrusive effect of the optical probe was investigated first. The results show that the optical probe underestimates bubble properties, such as, bubble velocity, local void fraction. The presence of the probe in the bubble column influences the local flow conditions. Particularly, when the probe is placed close to the liquid surface, this influence is more pronounced. Furthermore, two methods determining interfacial area were compared. The results from both methods agree quite well at low superficial gas velocity, whereas large discrepancies are found when superficial gas velocity increases. Finally, effect of column height on bubble properties was studied. No significant difference was found for the three investigated columns.

A numerical simulation was performed on a turbulent gas-particle multi-phase flow in a circulating fluidized bed riser based on a hard-sphere discrete particle model (DPM) for the particle phase and the Navier-Stokes equations for the gas phase. The sub-grid scale stresses (SGS) were modeled with the SGS model proposed by Vreman (2004). The model enables the calculation of an arbitrary particle size distribution. In this work, binary mixtures of particles with different diameters are used in the simulation. From the numerical results it is found that the particles display a radial and axial diameter distribution. Small particles have a higher vertical particle velocity than the large particles. With increasing superficial gas velocity, the vertical particle velocity is increased. The average particle velocity and concentration vary both in the radial and axial directions. Finally, the numerical results are compared with the experimental and numerical results of Mathiesen et al (2000).

Hydrodynamics of gas-particle two-phase turbulent flow in a circulating fluidized bed riser is studied experimentally by Particle Image Velocimetry (PIV) and numerically with the use of a 3D discrete hard sphere particle model (DPM). Mean particle velocities and RMS velocities are obtained and the influence of turbulence on the flow is investigated. The experimental data are analyzed and compared with the numerical results showing a reasonable agreement.

Numerical simulations of the small bubble-big bubble-liquid three phase heterogeneous flow in a square cross-sectioned bubble column were carried out with the commercial CFD package CFX-4.4 to explore the effect of superficial velocity and inlet dispersed phase fractions on the flow patterns. The approach of Krishna et al. (2000) was adopted in the Euler-Euler framework to numerically simulate the gas-liquid heterogeneous flow in bubble columns. On basis of an earlier study (Zhang et al. 2005), the extended multiphase k - ε turbulence model (Pfleger and Becker, 2001) was chosen to model the turbulent viscosity in the liquid phase and implicitly account for the bubble-induced turbulence. The obtained results suggest that, first of all, the extended multiphase k - ε turbulence model of Pfleger and Becker (2001) is capable of capturing the dynamics of the heterogeneous flow. With increasing superficial velocity, the dynamics of the flow, as well as the total gas hold-up increases. It is observed that with increasing inlet phase fraction of the big bubbles, the total gas holdup decreases while the dynamic nature of the flow increases, which indicates that the small bubble phase mainly determines the total gas holdup while the big bubble phase predominantly agitates the liquid.

Direct numerical simulations can provide detailed understanding of micro-mechanics of individual particle collisions in the presence of liquid, which is crucial in wet granular flows in many industrial applications. In this work, a numerical model is developed for such detailed simulations of collision of a spherical particle on a wet surface. The model combines the immersed boundary method and the volume-of-fluid method supplemented with a contact model for the description of gas-liquid-solid contact line. Careful verifications of the model are conducted for droplet-wall as well as droplet-sphere contacts. Simulation results for normal collisions of a sphere on wet surfaces are reported, and compared with the corresponding experimental data.

In recent years, there has been growing interest to use computational fluid dynamics (CFD) for exploring dropletparticles interaction under the turbulent flow of air inside spray dryer systems. The final goal is to predict the dried solid particle size and mass flux distributions under the conditions of collision, coalescence and agglomeration of the droplets and particles. The coarse-grained simulation tool will be used to design more efficient spray dryers that can produce higher throughputs.