Prediction of the particle flow conditions in the freeboard of a freely bubbling fluidized bed

Glicksman, Leon R.; Yule, Thomas

doi:10.1016/0009-2509(94)00208-9

Cited by 9 publications

(5 citation statements)

References 7 publications

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“…Previous studies about erupting bubbles can be found in the literature focused either on the velocity of ejected particles ,, or on the dome evolution and the gas flow through the bubble. − Although the particle velocity field was measured experimentally in bubbling beds using PIV, the few studies available about the throughflow in erupting bubbles are limited to isolated circular bubbles or artificial cavities at the bed surface rather than in real geometries.…”

Section: Experimental Setup and Piv Measurementsmentioning

confidence: 99%

Novel Approach to Characterize Fluidized Bed Dynamics Combining Particle Image Velocimetry and Finite Element Method

Almendros-Ibáñez

Pallarès

Johnsson

et al. 2009

Ind. Eng. Chem. Res.

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This work presents a new experimental−numerical method combining PIV (particle image velocimetry), digital image analysis, and FEM (finite element method) to obtain gas and particle motion around bubbles in a 2D freely bubbling fluidized bed. The bubble geometry is captured with a high speed video camera while the particle velocity is measured using a PIV technique. These experimental data are exported to a finite element software where the pressure and gas velocity fields are obtained numerically. The flow equations proposed by Davidson’s model have been chosen to exemplify the application of the method presented in this paper. Different bubble types have been analyzed: slow and fast bubbles but also erupting and interacting bubbles. The effect on the gas flow of bubbles with a nonzero horizontal velocity component has also been analyzed, and it is shown how such bubbles interchange gas with the main stream. In addition, the Darcy’s law included in Davidson’s model has been extended including a quadratic term (i.e., Ergun’s equation) to take into account non-Darcy effects and the PIV results have been used to evaluate the error committed when voidage variation around bubbles is neglected. The results obtained including non-Darcy effects show differences in the gas velocity magnitude in regions near the nose and the wake of the bubble, where the gas velocity and, consequently, the local Reynolds number are higher. Nevertheless, these local differences give no significante difference in the gas streamlines around bubbles. Finally, the study of the voidage variation around bubbles shows that this variation is important only in a region of small thickness adjacent to the bubble.

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Section: Experimental Setup and Piv Measurementsmentioning

confidence: 99%

Novel Approach to Characterize Fluidized Bed Dynamics Combining Particle Image Velocimetry and Finite Element Method

Almendros-Ibáñez

Pallarès

Johnsson

et al. 2009

Ind. Eng. Chem. Res.

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“…When the bubble nose reaches the bed surface, a cavity of depth equal to the bubble diameter is formed and the gas throughflow accelerates the particles in the bubble nose while the bubble breaks the bed surface (Glicksman and Yule, 1995). As the bubble rises, the depth of the cavity decreases and the gas flow through the bottom of the bubble is reduced.…”

Section: Theoretical Modelmentioning

confidence: 99%

A new model for ejected particle velocity from erupting bubbles in 2-D fluidized beds

Almendros-Ibáñez

Sobrino

Vega

et al. 2006

Chemical Engineering Science

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A new model is proposed for obtaining the velocity profile of the particle ejected from the bubble dome in a freely bubbling 2-D fluidized bed. Its basis is the supposition that the initial velocity of the ejected particles, with a direction perpendicular to the dome contour, depends on bubble velocity and bubble growth velocity. This model differs from those previously appearing in the literature in that it is valid not only for vertical-ascent circular bubbles.Experiments were carried out in a freely bubbling 2-D fluidized bed using a high-speed video camera to measure the velocity profile. Upon comparing these results with the proposed model, it was established that, excepting some isolated cases, the model properly predicts the magnitude and direction of the maximum particle ejection velocity and the velocity profile.Using the work of Shen et al. (2004. Digital image analysis of hydrodynamics two-dimensional bubbling fluidized beds. Chemical Engineering Science 59, 2607-2617), we obtain two general equations for the bubble velocity and the bubble growth velocity in a 2-D fluidized bed. These expressions, together with the proposed model, can be used to calculate the initial velocity of the ejected particles.

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“…Despite significant effort over the past few decades, there continues to be a lack of understanding of mixing dynamics, especially at commercial-scales, because (a) most diagnostic techniques are only feasible in small laboratory setups, where the hydrodynamics are significantly affected by the walls 2 and (b) mixing metrics are sensitive to several operating parameters such as bed geometry, superficial gas velocity, particle characteristics and temperature. 3,4,5,6,7,8,9,10 Nevertheless, both experimental 11,12,13,14,15,16 and numerical 2, 4, 3 studies have concluded that solids mixing is driven by bubble motion through three mechanisms 17,18,19, 1 -(a) wake effect-particles get drawn into bubble wakes and accelerate upwards, (b) emulsion drift-particles rain down along bubble walls to replace voids left by rising particles and (c) bubble eruptions-particles are disbursed into the freeboard. Each of these phenomena has been investigated extensively.…”

Section: Introductionmentioning

confidence: 99%

“…20,13,14 Meanwhile, the ejection mechanism of solid particles is quite complex and depends on the bubble shape, trajectory as well as near-ejection coalescence events. 19,21,22 This mechanism is also largely responsible for lateral mixing. 10 Overall, these studies enable the fundamental understanding of bubble-particle interactions even though quantitative inferences are often not applicable to commercial operation because (a) isolated bubbles are seldom realized and/or (b) the hydrodynamics in thin lab-scale beds are significantly affected by the front and back walls.…”

Section: Introductionmentioning

confidence: 99%

Mixing dynamics in bubbling fluidized beds

2017

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Solids mixing affects thermal and concentration gradients in fluidized bed reactors and is, therefore, critical to their performance. Despite substantial effort over the past decades, understanding of solids mixing continues to be lacking because of technical limitations of diagnostics in large pilot and commercial‐scale reactors. This study is focused on investigating mixing dynamics and their dependence on operating conditions using computational fluid dynamics simulations. Toward this end, fine‐grid 3D simulations are conducted for the bubbling fluidization of three distinct Geldart B particles (1.15 mm LLDPE, 0.50 mm glass, and 0.29 mm alumina) at superficial gas velocities U/Umf = 2–4 in a pilot‐scale 50 cm diameter bed. The Two‐Fluid Model (TFM) is employed to describe the solids motion efficiently while bubbles are detected and tracked using MS3DATA. Detailed statistics of the flow‐field in and around bubbles are computed and used to describe bubble‐induced solids micromixing: solids upflow driven in the nose and wake regions while downflow along the bubble walls. Further, within these regions, the hydrodynamics are dependent only on particle and bubble characteristics, and relatively independent of the global operating conditions. Based on this finding, a predictive mechanistic, analytical model is developed which integrates bubble‐induced micromixing contributions over their size and spatial distributions to describe the gross solids circulation within the fluidized bed. Finally, it is shown that solids mixing is affected adversely in the presence of gas bypass, or throughflow, particularly in the fluidization of heavier particles. This is because of inefficient gas solids contacting as 30–50% of the superficial gas flow escapes with 2–3× shorter residence time through the bed. This is one of the first large‐scale studies where both the gas (bubble) and solids motion, and their interaction, are investigated in detail and the developed framework is useful for predicting solids mixing in large‐scale reactors as well as for analyzing mixing dynamics in complex reactive particulate systems. © 2017 American Institute of Chemical Engineers AIChE J, 63: 4316–4328, 2017

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Prediction of the particle flow conditions in the freeboard of a freely bubbling fluidized bed

Cited by 9 publications

References 7 publications

Novel Approach to Characterize Fluidized Bed Dynamics Combining Particle Image Velocimetry and Finite Element Method

Novel Approach to Characterize Fluidized Bed Dynamics Combining Particle Image Velocimetry and Finite Element Method

A new model for ejected particle velocity from erupting bubbles in 2-D fluidized beds

Mixing dynamics in bubbling fluidized beds

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