Expulsion of particles from a buoyant blob in a fluidized bed

1994 ◽  
Vol 278 ◽  
pp. 63-81 ◽  
Author(s):  
G. K. Batchelor ◽  
J. M. Nitsche
Keyword(s):  
Fluidized Bed ◽  
Fluidized Beds ◽  
Particle Concentration ◽  
Small Concentration ◽  
Numerical Calculations ◽  
Significant Feature ◽  
Average Particle ◽  
Remarkable Property ◽  
Gas Fluidized Beds ◽  
Secondary Instabilities

It is a significant feature of most gas-fluidized beds that they contain rising ‘bubbles’ of almost clear gas. The purpose of this paper is to account plausibly for this remarkable property first by supposing that primary and secondary instabilities of the fluidized bed generate compact regions of above-average or below-average particle concentration, and second by invoking a mechanism for the expulsion of particles from a buoyant compact blob of smaller particle concentration. We postulate that the rising of such an incipient bubble generates a toroidal circulation of the gas in the bubble, roughly like that in a drop of liquid rising through a second liquid of larger density, and that particles in the blob carried round by the fluid move on trajectories which ultimately cross the bubble boundary. Numerical calculations of particle trajectories for practical values of the relevant parameters show that a large percentage of particles, of such small concentration that they move independently, are expelled from a bubble in the time taken by it to rise through a distance of several bubble diameters.Similar calculations for a liquid-fluidized bed show that the expulsion mechanism is much weaker, as a consequence of the larger density and viscosity of a liquid, which is consistent with the absence of observations of relatively empty bubbles in liquid-fluidized beds.It is found to be possible, with the help of the Richardson-Zaki correlation, to adjust the results of these calculations so as to allow approximately for the effect of interaction of particles in a bubble in either a gas- or a liquid-fluidized bed. The interaction of particles at volume fractions of 20 or 30 % lengthens the expulsion times, although without changing the qualitative conclusions.

Prediction of the Flotsam Component in a Gas-Fluidized Bed of Two Dissimilar Solids

2012 ◽  
Vol 10 (1) ◽  
Author(s):  
Alberto Di Renzo ◽  
Francesco P. Di Maio ◽  
Vincenzino Vivacqua
Keyword(s):  
Fluid Dynamics ◽  
Experimental Data ◽  
Computational Fluid Dynamics ◽  
Simple Model ◽  
Discrete Element Method ◽  
Fluidized Bed ◽  
Fluidized Beds ◽  
Computational Simulations ◽  
The Difference ◽  
Gas Fluidized Beds

In the present paper the segregating behaviour of solids of different size and density in gas-fluidized beds is studied. In particular, the attention is focussed on pairs composed of a bigger/less dense species and a smaller/denser species. Typical industrial examples of such combinations are encountered in fluidized beds of biomass/sand mixtures. Their behaviour is not easily predictable, as the segregation tendency promoted by the difference in density is counteracted by the difference in size. While typically the denser component is expected to appear predominantly at the bottom of the fluidized bed, experiments on mixtures exhibiting the reverse behaviour have been reported (e.g. Chiba et al., 1980).A simple model to predict the segregation direction of the components, i.e. which species will segregate to the top of the bed (the flotsam), depending upon their difference in properties (size, density) and the mixture composition, is discussed. The predicted behaviour is compared with experimental data available in the literature and agreement is found for the majority of them. For one mixture, experiments are conducted as well as computational simulations based on the combined Discrete Element Method and Computational Fluid Dynamics (DEM-CFD) approach. This allows investigating how an initially mixed bed upon suspension evolves as a result of the segregation prevalence in the bed.

Particle pressures generated around bubbles in gas-fluidized beds

2002 ◽  
Vol 455 ◽  
pp. 103-127 ◽  
Author(s):  
KHURRAM RAHMAN ◽  
CHARLES S. CAMPBELL
Keyword(s):  
Fluidized Bed ◽  
Bubble Size ◽  
Fluidized Beds ◽  
Gas Flow ◽  
Solid Particles ◽  
Particle Phase ◽  
Fluid Mixture ◽  
Maximum Value ◽  
Particle Pressure ◽  
Gas Fluidized Beds

The particle pressure is the surface force in a particle/fluid mixture that is exerted solely by the particle phase. This paper presents measurements of the particle pressure on the faces of a two-dimensional gas-fluidized bed and gives insight into the mechanisms by which bubbles generate particle pressure. The particle pressure is measured by a specially designed ‘particle pressure transducer’. The results show that, around single bubbles, the most significant particle pressures are generated below and to the sides of the bubble and that these particle pressures steadily increase and reach a maximum value at bubble eruption. The dominant mechanism appears to be defluidization of material in the particle phase that results from the bubble attracting fluidizing gas away from the surrounding material; the surrounding material is no longer supported by the gas flow and can only be supported across interparticle contacts which results in the observed particle pressures. The contribution of particle motion to particle pressure generation is insignificant.The magnitude of the particle pressure below a single bubble in a gas-fluidized bed depends on the bubble size and the density of the solid particles, as might be expected as the amount of gas attracted by the bubble should increase with bubble size and because the weight of defluidized material depends on the density of the solid material. A simple scaling of these quantities is suggested that is otherwise independent of the bed material.In freely bubbling gas-fluidized beds the particle pressures generated behave differently. Overall they are smaller in magnitude and reach their maximum value soon after the bubble passes instead of at eruption. In this situation, it appears that the bubbles interact with one another in such a way that the de uidization effect below a leading bubble is largely counteracted by refluidizing gas exiting the roof of trailing bubbles.

The growth, saturation, and scaling behaviour of one- and two-dimensional disturbances in fluidized beds

1998 ◽  
Vol 362 ◽  
pp. 83-119 ◽  
Author(s):  
M. F. GÖZ ◽  
S. SUNDARESAN
Keyword(s):  
Fluidized Bed ◽  
Growth Rates ◽  
Fluidized Beds ◽  
Secondary Growth ◽  
Two Dimensional ◽  
Small Perturbations ◽  
One Dimensional ◽  
Gas Fluidized Beds ◽  
The Waves ◽  
Primary Instability

It is well-known that fluidized beds are usually unstable to small perturbations and that this leads to the primary bifurcation of vertically travelling plane wavetrains. These one-dimensional periodic waves have been shown recently to be unstable to two-dimensional perturbations of large transverse wavelength in gas-fluidized beds. Here, this result is generalized to include liquid-fluidized beds and to compare typical beds fluidized with either air or water. It is shown that the instability mechanism remains the same but there are big differences in the ratio of the primary and secondary growth rates in the two cases. The tendency is that the secondary growth rates, scaled with the amplitude of a fully developed plane wave, are of similar magnitude for both gas- and liquid-fluidized beds, while the primary growth rate is much larger in the gas-fluidized bed. This means that the secondary instability is accordingly stronger than the primary instability in the liquid-fluidized bed, and consequently sets in at a much smaller amplitude of the primary wave. However, since the waves in the liquid-fluidized bed develop on a larger time and length scale, the primary perturbations need longer time and thereby travel farther until they reach the critical amplitude. Which patterns are more amenable to being visually recognized depends on the magnitude of the initially imposed disturbance and the dimensions of the apparatus. This difference in scale plays a key role in bringing about the differences between gas- and liquid-fluidized beds; it is produced mainly by the different values of the Froude number.

Particle pressures in gas-fluidized beds

1991 ◽  
Vol 227 ◽  
pp. 495-508 ◽  
Author(s):  
Charles S. Campbell ◽  
David G. Wang
Keyword(s):  
Fluidized Bed ◽  
Bubble Size ◽  
Fluidized Beds ◽  
Particle Density ◽  
Surface Force ◽  
Gas Velocity ◽  
Fluid Forces ◽  
Minimum Fluidization ◽  
Particle Pressure ◽  
Gas Fluidized Beds

The particle pressure is the surface force that is exerted due to the motion of particles and their interactions. This paper describes measurements of the particle pressure exerted on the sidewall of a gas-fluidized bed. As long as the bed remains in a packed state, the particle pressure decreases with increasing gas velocity as progressively more of the bed is supported by fluid forces. It appropriately reaches a minimum fluidization and then begins to rise again when the bed is fluidized, reflecting the agitation of the bed by bubbles. In this fully fluidized region, the particle pressure scales with the particle density and the bubble size.

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