Experimental validation of 3-D Lagrangian VOF model : bubble shape and rise velocity

A novel 3-D computational fluid dynamics model using an advanced Lagrangian interface tracking scheme was studied to find the time-dependent behavior of gas bubbles rising in an initially quiescent liq. A novel least-square approach is used to det. the normal behavior at the interface for an accurate reconstruction and advection of the interface based on mollification of the color function by convolution. The incompressible Navier-Stokes equations are solved using an accurate discretization scheme to obtain the flow field of the gas and liq. phase. Detailed expts. of single rising bubbles of different sizes were performed to compare the shape, rise velocity and pressure signal of the bubble with the performed simulations. The developed Lagrangian vol.-of-fluid model could accurately track the motion and shape of the gas - liq. interface embedded in a flow field with significant vorticity. [on SciFinder (R)

Eulerian simulations of bubbling behaviour in gas-solid fluidised beds

In literature little attempt has been made to verify experimentally Eulerian-Eulerian gas-solid model simulations of bubbling fluidised beds with existing correlations for bubble size or bubble velocity. In the present study, a CFD model for a free bubbling fluidised bed was implemented in the commercial code CFX of AEA Technology. This CFD model is based on a two fluid model including the kinetic theory of granular flow. Simulations of the bubble behaviour in fluidised beds at different superficial gas velocities and at different column diameters are compared to the Darton et al. (1977) equation for the bubble diameter versus the height in the column and to the Hilligardt and Werther (1986) equation, corrected for the two dimensional geometry using the bubble rise velocity correlation of Pyle and Harrison (1967). It is shown that the predicted bubble sizes are in agreement with the Darton et al. (1977) bubble size equation. Comparison of the predicted bubble velocity with the Hilligardt and Werther (1986) equation shows a deviation for the velocity of smaller bubbles. To explain this, the predicted bubbles are divided into two bubble classes : bubbles that have either coalesced, broken-up or have touched the wall, and bubbles without these occurrences. The bubbles of this second class are in agreement with the Hilligardt and Werther (1986) equation. Fit parameters of Hilligardt and Werther (1986) are compared to the fit parameters obtained in this work. It is shown that coalescence, break-up, and direct wall interactions are very important effects, often dominating the dynamic bubble behaviour, but these effects are not accounted for by the Hilligardt and Werther (1986) equation. © 1998 Elsevier Science Ltd. All rights reserved

Gas Holdup in a Slurry Bubble Column: Influence of Electrolyte and Carbon Particles

This study deals with the effects of electrolyte and particle concns. on the gas holdup in both the homogeneous and the heterogeneous flow regimes in a slurry bubble column. Gas holdup measurements and video recordings of the bubble behavior were carried out in a 2D slurry column (0.015 * 0.30 * 2.00 m) under ambient conditions. The addns. of electrolyte (sodium gluconate, 0.05-0.2 M) and of solid carbon particles (diam. 30 mm, 0.1-1.0 g L-1) both lead to a considerable increase in gas holdup. In both cases, crit. concns. exist above which no further increase in gas holdup is obsd. The transition from the homogeneous to the heterogeneous regime is not significantly affected by electrolyte but is increased by the presence of particles. Three mechanisms are proposed that might account for the gas holdup increase resulting from particle and electrolyte addn. It is suggested that a layer of carbon particles around the gas bubbles results in a lower av. bubble rise velocity. Both the addn. of carbon particles and the addn. of electrolyte lead to bubble stabilization, a decreased rate of coalescence, and thus a higher gas holdup. It is further suggested that the presence of electrolyte changes the surface tension, leading to smaller bubbles, a lower av. bubble rise velocity, and thus a higher gas holdup. The combined addn. of electrolyte and carbon particles confirms these hypotheses. [on SciFinder (R)

CFD modeling of gas-fluidized beds with a bimodal particle mixture

A computational fluid dynamics model was developed for gas-solid fluidized beds containing a mixture of two particle species. To calculate stresses of the solid phase, the kinetic theory of granular flow was extended to consider a binary mixture of smooth, nearly elastic, spheres. The developed model was simulated to demonstrate key features of binary mixture fluidization. Bed expansion with a binary mixture of different size particles, but with identical densities, was much higher than that of a system consisting of mono-sized particles of the same mean size as the bimodal mixture. Minimum fluidization velocity for the binary particle system was significantly lowered. The mixing behavior of the binary mixture of particles, characterized by the mixing index, increased with increasing superficial gas velocity. For a binary mixture of particles of larger size with lower density and smaller size with higher density, larger, lighter particles segregated to the top of the fluid bed, while smaller, heavier particles segregated to the bottom. With increasing fluidization velocity, this segregation pattern reversed and inversion occurred. The drag and gravity force difference between small, heavy particles and large, light particles was dominant at low gas velocities. With an increase in gas velocity, however, the gradients in granular temperature and pressure became dominant terms in the equations for the relative force and thus velocity between two different particle species

Simulation of fluidized beds with lattice gas cellular automata

This paper introduces an approach for the simulation of the hydrodynamic behaviour of gas–solid fluidized beds via the use of lattice gas cellular automata. This approach is based on a two-speed model, developed by U. Frisch, B. Hasslacher, and Y. Pomeau. Simulation runs for different configurations of the automaton produce results that can be compared to actual data. The simulations show when and how bubbling will occur. Values for the bubble diameters as a function of bed height, as well as bed porosities in two horizontal planes have been obtained from the simulations. From these results correlation diagrams and the Kolmogorov entropy are calculated. Generally, the results of the simulations are qualitatively in good agreement with experimental observations, showing that this new approach could provide a useful tool in predicting the fluid dynamic behaviour of fluidized beds

Experimental validation of Lagrangian-Eulerian simulations of fluidized beds

The present study aims to validate two-dimensional Lagrangian-Eulerian simulations of gas-solid fluidized beds by comparing these with dedicated exptl. data obtained with polystyrene Geldart type D particles of 1.545 mm size. Exptl. data on pressure, voidage, and bed height fluctuations, and the power spectral d. are compared with three different implementations of the Lagrangian-Eulerian model. Though qual. trends found in the expt. are correctly reproduced by the simulations, the simulations are particularly sensitive to porosity estn. procedures used in the three different simulation strategies employed. Furthermore, the phenomenon of particle clustering predicted by the model does not conform to exptl. observations; this is because the physics of the break-up of clusters is not properly captured in the Lagrangian-Eulerian model. [on SciFinder (R)

Optimal placement of probes for dynamic pressure measurements in large-scale fluidized beds

Pressure data sampled at sufficiently high frequency (typically 20 Hz or higher) can yield much information about the hydrodynamic state of a fluidized bed. Since part of the pressure waves traveling through large (industrial) fluidized beds is only detectable in a limited area of the bed, pressure measurements need to be performed at several positions to cover the whole bed. The local pressure waves (caused by, e.g., passing bubbles or coalescing bubbles) in a 0.80 m diam. bubbling fluidized bed of Geldart B particles are examd. Expts. and simulations are performed to det. the intensity decrease as local pressure waves propagate from their origin. A new spectral method is applied to det. the degree of coherence for pressure signals measured at two different positions in a fluidized bed. For a superficial gas velocity of 5umf, local pressure waves can be detected up to a radial distance of .apprx. 0.5 m from their origin; this distance is somewhat lower for lower gas velocities. This means that the radial spacing of pressure probes should not exceed 1 m. For large diam. beds with a bed height <1.5 m, a set of probes at a single level and at several radial positions is sufficient to observe or monitor the dynamic state of the complete bed; the probes should preferably be placed at a height of 30-40% of the total bed height