Analysis of Throughflow Velocity in Two-Dimensional Fluidized Bed Bubbles

1995 ◽  
Vol 117 (2) ◽  
pp. 319-322 ◽  
Author(s):  
Dinesh Gera ◽  
Mridul Gautam
Keyword(s):  
Fluidized Bed ◽  
Fluidized Beds ◽  
Gas Flow ◽  
Bubble Formation ◽  
Particle Diameter ◽  
Vertical Direction ◽  
Approximate Model ◽  
Equivalent Diameter ◽  
Low Velocity ◽  
Flow Through

The formation of gas bubbles is one of the most characteristic phenomena of fluidized beds. Many unique properties of fluidized beds can be related directly to the presence of bubbles and are dominated by their behavior. Therefore, accurate prediction of parameters such as bubble shape and size, voidage variation and throughflow are practically important. In the present analysis, an approximate model, based on a strongly idealized picture of the bubble formation has been presented. The bubbling gas fluidized bed has regions of low solids density comprised of gas pockets or voids. The observed voids exhibited a variety of shapes (Halow and Nicoletti, 1992), depending upon the material and fluidization velocity. In the low-velocity experiments with the finer materials, rounded voids are observed. However, with coarser materials, voids were typically large and bluntnosed. In the image analyses work, reported by Gautam (1989), in a bed operating slightly above the incipient fluidization, elongated bubbles (a > b, as shown in Figure 1) were observed for glass beads (sp. gravity = 2.5) of mean diameter 500 μm and flattened bubbles (a < b) were seen for mean particle diameter of 350 μm. Also, he noticed the dependence of throughflow velocity on the elongation of the bubble as it traverses up the bed. Additionally, throughflow velocity was found to be independent of the excess gas flow rate through the bed. The digitized image of a typical bubble (refer Gautam et al., 1994) which shows that the bubble were elongated in the vertical direction and were more elliptical than circular. Therefore, description of a bubble on the basis of just one diameter, either the horizontal or the vertical or an equivalent diameter, as has been done by many researchers in the past, is rather incomplete. It is inferred from the present work that the bubble aspect ratio plays an important role in predicting an accurate gas flow through the bubble.

Effect of the Shape of the Baffles on the Hydrodynamics of a Fluidized Bed

2008 ◽  
Vol 6 (1) ◽  
Author(s):  
Srinivasa Rao Venkata Naga Kaza
Keyword(s):  
Fluidized Bed ◽  
Fluidized Beds ◽  
Gas Flow ◽  
Experimental Studies ◽  
Expansion Ratio ◽  
Blockage Ratio ◽  
Minimum Fluidization Velocity ◽  
Fluidization Velocity ◽  
Minimum Fluidization ◽  
Bubbling Fluidized Bed

Gas flow in a gas–solid fluidized bed is characterized by the predominance of bubbles. When gas flow is more than the minimum fluidization velocity, the top of the fluidized bed may fluctuate vigorously leading to unstable operation. Bed fluctuation and fluidization quality are interrelated. The quality of fluidization can largely be improved by introducing baffles in bubbling and turbulent fluidized beds. In the present work three baffle geometries, i.e., circular, triangular and square are used to determine different hydrodynamic parameters such as minimum fluidization velocity, bed expansion, pressure drop across the bed, fluctuation ratio, expansion ratio, etc. in a bubbling fluidized bed. A new parameter blockage ratio is introduced to analyze the behaviour of baffled fluidized beds. It is found from the current experimental studies that the blockage ratio mainly influences the hydrodynamics of the bed rather than the shape of the baffle.

scholarly journals Cyber-Physical Observer for Fluidized Bed-Chemical Looping Control Applications

2017 ◽  
Author(s):  
Lawrence Shadle ◽  
David Tucker ◽  
Ronald Breault ◽  
Samuel Bayham ◽  
Justin Weber ◽  
...  
Keyword(s):  
Multiphase Flow ◽  
Fluidized Bed ◽  
Gas Flow ◽  
Flow Behavior ◽  
Chemical Looping ◽  
Flow Paths ◽  
Response Data ◽  
Flow Through ◽  
And Performance ◽  
And Control

A cyber-physical fluidized bed-chemical looping reactor (FB-CLR) is proposed to observe and control the multiphase flow behavior and improve process operations, stability, and performance. The cyber-physical observer (CPO) provides an opportunity to probe a duplicate, or mirrored, non-reacting, multiphase flow system in real-time and provide response data not available from the hot reacting system in order to control the hot unit. A control strategy was developed to share and integrate this information between to the two systems. During test operations the data from the shifting inventory of granular particles in the cold flow unit will be used to control some of the valves controlling the gas flow paths in the hot unit. Taken in conjunction with the inlet flows, temperatures, and pressures in the hot unit a control system is proposed to balance the exhaust flow through the various gas outlets of the different vessels. System identification studies are needed to characterize the process delays, time constants, and interactions between control parameters.

Flow Properties of Moisturized Powders in a Couette Fluidized Bed Rheometer

2012 ◽  
Vol 10 (1) ◽  
Author(s):  
Giovanna Landi ◽  
Diego Barletta ◽  
Paola Lettieri ◽  
Massimo Poletto
Keyword(s):  
Fluidized Bed ◽  
Gas Flow ◽  
Vertical Direction ◽  
Variable Load ◽  
Air Humidity ◽  
Flow Properties ◽  
Minimum Fluidization Velocity ◽  
Fluidization Velocity ◽  
Minimum Fluidization ◽  
The Difference

This work investigates on the effect of air humidity on the flow properties of powders. The moisture of a powder sample (50 µm glass beads) was conditioned by fluidization with humid air. Air humidity was kept between 0 and 70% at ambient temperature. A Schulze shear cell was used to measure the bulk flow properties of the moisturized samples. A Couette fluidized bed rheometer was used to measure the torque necessary for the rotation of the inner cylinder when the fluidized powder had been moisturized with the same procedure. These experiments show a certain continuity of the results below and above the minimum fluidization velocity, suggesting a similar continuity of the role that interparticle interactions play in the fixed and in the fluidized bed. Experiments below the minimum fluidization velocity were interpreted with a rheological model in which the variable load along the vertical direction in the Couette was calculated with a modified Janssen equation. In this approach the apparent weight of the powder is given by the difference between the gravity and the upward body force determined by the rising gas flow. The agreement between the model and the experiments supports the proposed approach.

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.

Experimental study of gas flow through isolated bubbles of various shapes in a fluidized bed

1976 ◽  
Vol 12 (9) ◽  
pp. 712-718
Author(s):  
I. A. Vakhrushev ◽  
V. M. Tolkachev ◽  
N. Yu. Krymov
Keyword(s):  
Experimental Study ◽  
Fluidized Bed ◽  
Gas Flow ◽  
Flow Through

Bubble Formation Due to a Submerged Capillary Tube in Quiescent and Coflowing Streams

1970 ◽  
Vol 92 (4) ◽  
pp. 705-711 ◽  
Author(s):  
S. C. Chuang ◽  
V. W. Goldschmidt
Keyword(s):  
Gas Flow ◽  
Capillary Tube ◽  
Bubble Formation ◽  
Viscous Drag ◽  
Proposed Model ◽  
Mass Coefficient ◽  
Small Tube ◽  
Flow Through ◽  
Added Mass Coefficient ◽  
Good Agreement

The case of bubble formation in both quiescent and moving streams due to the injection of a constant gas flow through a small tube is considered. Relationships predicting the expected size and quantity of bubbles generated are proposed. These are compared with measurements taken with stream velocities up to 9 ft/sec, while generating gas bubbles from 40 to 700 microns in diameter. For the case of generation in a quiescent stream the forces due to the virtual mass, surface tension, viscous drag, buoyancy, and the wake formed by the preceding bubble are accounted for. There still remains some question (only partly answered by a comparison with measurements) as to the proper added mass coefficient and the geometry of the bubble previous to detachment, as well as an adequate estimate of the interaction with a preceding bubble’s wake. The proposed model for generation in a moving stream is in good agreement with actual measurements for co-flowing velocities between 1 and 9 fps and capillary tubes in the order of 10−3 cm in dia.

Particle Fluctuation Velocity in Gas Fluidized Beds - Fundamental Models Compared to Recent Experimental Data

2000 ◽  
Vol 627 ◽  
Author(s):  
G. D. Cody
Keyword(s):  
Fluidized Beds ◽  
Gas Flow ◽  
Vibrational Energy ◽  
Particle Diameter ◽  
Power Spectral Analysis ◽  
Recent Experimental Data ◽  
Granular Temperature ◽  
Fluctuation Velocity ◽  
Gas Fluidized Beds ◽  
Random Particle

ABSTRACTThe first measurements of the mean squared fluctuation velocity, or granular temperature, of monodispersed glass spheres in gas fluidized beds were recently obtained by two independent techniques: Power Spectral analysis of wall vibrational energy excited by random particle impact or Acoustic Shot Noise (ASN), and Diffusing Wave Spectroscopy (DWS) of reflected laser light multiply scattered by random particle motion. We explore the relevance of this data to the initial stability of the uniform fluidized state and to recent fundamental models for the magnitude, gas flow, and particle diameter dependence of the steady state granular temperature.

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