Measuring the Gas-Solids Distribution in Fluidized Beds -- A Review

2008 ◽  
Vol 6 (1) ◽  
Author(s):  
J. Ruud van Ommen ◽  
Robert F. Mudde
Keyword(s):  
Image Reconstruction ◽  
Fluidized Bed ◽  
Spatial Resolution ◽  
Fluidized Beds ◽  
Gamma Ray ◽  
Research Effort ◽  
Pressure Fluctuations ◽  
Local Pressure ◽  
Optical Probes ◽  
Measurement Volume

In gas-solid fluidized beds, the distribution of the particles typically varies both in time and space. Since the gas-solids distribution and its variation have a strong influence on the performance of a fluidized bed for a given process, it is very important to accurately measure the gas-solids or voidage distribution. This paper reviews techniques for measuring the voidage distribution in gas-solid fluidized beds, with a focus on the developments during the last ten years. We will treat subsequently direct visualisation, tomography, optical probes, capacitance probes, and pressure measurements.Dense gas-solids flows are typically opaque to visible light. This makes optical techniques only of limited use in dense gas-solids flow. However, direct visualization can be useful for very dilute systems, pseudo 2-D beds, and the outer layer of dense, 3-D systems. Tomography is frequently used to obtain the voidage distribution in a horizontal cross-section of the bed. Electric capacitance tomography is fast, but its spatial resolution is limited and image reconstruction is still troublesome. Although some steps forward have been made, research is continuing at this point. For nuclear (X-ray and gamma-ray) tomography, the image reconstruction is much easier and the spatial resolution better, but its temporal resolution is typically much lower. Therefore, research efforts for nuclear tomography are mainly aimed at increasing the measurement frequency. Optical probes determine the voidage as a function of time in a small measurement volume, either by the degree of reflection or by the degree of transmission of a light bundle. Capacitance probes determine the voidage as a function of time in a small measurement volume by measuring the dielectric permittivity of the gas-solids suspension in the measurement volume. Both optical and capacitance probe techniques are reasonably well-developed; the current research effort spent at improving them seems limited, especially for capacitance probes. Time-averaged pressure measurements are commonly used to determine the average bed density and bed height. By sampling the local pressure at a sufficiently high frequency (typically in the order of 200 Hz), much more information can be obtained about the fluidized bed hydrodynamics. However, obtaining quantitative voidage data from pressure fluctuations measurement remains a difficult task; in-bed pressure fluctuation (and acoustic) measurements are mostly used to determine changes in the voidage dynamics and distribution.

Pressure Fluctuation Analysis in Gas-Solid Fluidized Beds Using CFD Simulations

2012 ◽  
Author(s):  
Mirka Deza ◽  
Francine Battaglia
Keyword(s):  
Binary Mixture ◽  
Fluidized Bed ◽  
Pressure Fluctuation ◽  
Pressure Fluctuations ◽  
Design Tool ◽  
Fluctuation Analysis ◽  
Valuable Insight ◽  
Turbulent Regime ◽  
Local Pressure ◽  
Long Time

Reliable computational methods provide valuable insight into gas-solid flow processes and can be used as a design tool. Of particular interest is the hydrodynamics of a binary mixture of sand-biomass in a fluidized bed. Our study interprets the hydrodynamic states of a fluidized bed by analyzing the local pressure fluctuations of beds of sand and a mixture of cotton stalks and sand over long time periods. Standard deviation of pressure drop will determine different fluidization regimes at inlet gas velocities ranging from 2 to 9 times the minimum fluidization velocity. Bode plots will present the pressure spectra and reveal characteristic frequencies that describe the bed hydrodynamics for different fluidization regimes. This works contribution will present CFD as a useful tool to perform pressure fluctuation analysis, the study of pressure fluctuations in the turbulent regime and the analysis of a binary mixture using CFD.

A CFD Study of Pressure Fluctuations to Determine Fluidization Regimes in Gas–Solid Beds

2013 ◽  
Vol 135 (10) ◽  
Author(s):  
Mirka Deza ◽  
Francine Battaglia
Keyword(s):  
Binary Mixture ◽  
Fluidized Bed ◽  
Pressure Fluctuations ◽  
Design Tool ◽  
Valuable Insight ◽  
Local Pressure ◽  
Cotton Stalks ◽  
Long Time ◽  
Computational Fluid Dynamics Cfd ◽  
Bode Plots

Reliable computational methods can provide valuable insight into gas–solid flow processes and can be used as a design tool. Of particular interest in this study is the hydrodynamics of a binary mixture of sand–biomass in a fluidized bed. Biomass particulates vary in size, shape, and density, which inevitably alter how well the particles fluidize. Our study will use computational fluid dynamics (CFD) to interpret the hydrodynamic states of a fluidized bed by analyzing the local pressure fluctuations of beds of sand and a binary mixture of cotton stalks and sand over long time periods. Standard deviation of pressure fluctuations will be compared with experimental data to determine different fluidization regimes at inlet gas velocities ranging from two to nine times the minimum fluidization velocity. We will use Bode plots to present the pressure spectra and reveal characteristic frequencies that describe the bed hydrodynamics for different fluidization regimes. This work will present CFD as a useful tool to perform that analysis. Other important contributions include the study of pressure fluctuations of a fluidized bed in bubbling, slugging, and turbulent regimes, and the analysis of a binary mixture using CFD.

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.

Relationship Between Unsteady Flow, Pressure Fluctuations, and Noise in a Centrifugal Pump—Part B: Effects of Blade-Tongue Interactions

1995 ◽  
Vol 117 (1) ◽  
pp. 30-35 ◽  
Author(s):  
S. Chu ◽  
R. Dong ◽  
J. Katz
Keyword(s):  
Unsteady Flow ◽  
Centrifugal Pump ◽  
Pressure Difference ◽  
Pressure Fluctuations ◽  
Primary Sources ◽  
Pressure Measurements ◽  
Local Pressure ◽  
Pressure Distributions ◽  
Flow Pressure ◽  
Tip Of The Tongue

Maps of pressure distributions computed using PDV data, combined with noise and local pressure measurements, are used for identifying primary sources of noise in a centrifugal pump. In the vicinity of the impeller pressure minima occur around the blade and near a vortex train generated as a result of non-uniform outflux from the impeller. The pressure everywhere also varies depending on the orientation of the impeller relative to the tongue. Noise peaks are generated when the pressure difference across the tongue is maximum, probably due to tongue oscillations, and when the wake impinges on the tip of the tongue.

Development of a simultaneous imaging system to measure the optical and gamma ray images of Ir-192 source for high-dose-rate brachytherapy

2021 ◽  
Vol 16 (12) ◽  
pp. T12005
Author(s):  
J. Nagata ◽  
S. Yamamoto ◽  
Y. Noguchi ◽  
T. Nakaya ◽  
K. Okudaira ◽  
...  
Keyword(s):  
Dose Rate ◽  
Spatial Resolution ◽  
Gamma Camera ◽  
Gamma Ray ◽  
Imaging System ◽  
High Dose Rate ◽  
High Dose ◽  
Cmos Camera ◽  
Absolute Position ◽  
The Absolute

Abstract In high-dose-rate (HDR) brachytherapy, verification of the Ir-192 source's position during treatment is needed because such a source is extremely radioactive. One of the methods used to measure the source position is based on imaging the gamma rays from the source, but the absolute position in a patient cannot be confirmed. To confirm the absolute position, it is necessary to acquire an optical image in addition to the gamma ray image at the same time as well as the same position. To simultaneously image the gamma ray and optical images, we developed an imaging system composed of a low-sensitivity, high-resolution gamma camera integrated with a CMOS camera. The gamma camera has a 1-mm-thick cerium-doped yttrium aluminum perovskite (YAIO3: YAP(Ce)) scintillator plate optically coupled to a position-sensitive photomultiplier (PSPMT), and a 0.1-mm-diameter pinhole collimator was mounted in front of the camera to improve spatial resolution and reduce sensitivity. We employed the concept of a periscope by placing two mirrors tilted at 45 degrees facing each other in front of the gamma camera to image the same field of view (FOV) for the gamma camera and the CMOS camera. The spatial resolution of the imaging system without the mirrors at 100 mm from the Ir-192 source was 3.2 mm FWHM, and the sensitivity was 0.283 cps/MBq. There was almost no performance degradation observed when the mirrors were positioned in front of the gamma camera. The developed system could measure the Ir-192 source positions in optical and gamma ray images. We conclude that the developed imaging system has the potential to measure the absolute position of an Ir-192 source in real-time clinical measurements.

Characteristics of Pressure Fluctuations in a Fluidized Bed of Binary Mixtures

2005 ◽  
Vol 38 (12) ◽  
pp. 960-968 ◽  
Author(s):  
Zhanyong Li ◽  
Noriyuki Kobayashi ◽  
Masanobu Hasatani
Keyword(s):  
Fluidized Bed ◽  
Binary Mixtures ◽  
Pressure Fluctuations

A Brief Review of Wind Effects on Buildings and Structures

1966 ◽  
Vol 70 (665) ◽  
pp. 553-560 ◽  
Author(s):  
C. Scruton
Keyword(s):  
Air Flow ◽  
Pressure Fluctuations ◽  
Ground Level ◽  
Vertical Gradient ◽  
Proximity Effects ◽  
Wind Tunnels ◽  
Wind Loading ◽  
Local Pressure ◽  
Wind Speeds ◽  
Random Forcing

SummaryPresent day structural forms and methods of fabrication have considerably increased the importance of wind as a design consideration. For estimations of the overall stability of a structure and of the local pressure distribution on the cladding, a knowledge of the maximum steady or time-averaged wind loads is usually sufficient. Mind tunnel tests to determine the wind loading coefficients are often made in smooth uniform flow, but for more accurate data account must be taken of the effects of the vertical gradient of wind speed and the turbulence of natural winds. Further research is needed into these effects and also into methods of obtaining a sufficient representation of the natural wind in the wind tunnel.There are a number of ways by which wind excites structures into oscillation; among these are vortex excitation, galloping, proximity effects including buffeting, stalling flutter and classical flutter. The vortex and galloping excitation might be expected to be especially sensitive to the turbulence properties of the air flow. Also, in the absence of any mechanism for instability, atmospheric turbulence may directly excite oscillations through the random forcing by the pressure fluctuations which it produces. Further understanding of this problem must come through research into the effects of turbulence (and to the extent to which these effects may be disregarded), but the range of the conditions is so vast and complicated that it seems unlikely that sufficient aerodynamic and wind data will be accumulated to permit the response of a proposed structure to be calculated with reasonable certainty, and for major projects it is anticipated that comprehensive tests on aeroelastic models in wind tunnels with appropriate turbulent air flow will continue to offer the more reliable predictions.The air flow around buildings is of concern inasmuch as it influences the dispersal of combustion and other gases from the smokestack and also in its effect on the speeds and turbulence of the wind over areas used by pedestrians. The erection of a tall building may cause an increase in wind speeds and gustiness at ground level. These problems of the external flow over buildings are readily examined in wind tunnels. For this purpose tunnels with large working sections are desirable to permit a sufficiently wide area of the neighbourhood to be represented.

Identification of the State of a Magneto-Fluidized Bed by Pressure Fluctuations

2006 ◽  
Vol 4 (1) ◽  
Author(s):  
Sachin Dahikar ◽  
Ram Sonolikar
Keyword(s):  
Fluidized Bed ◽  
Autocorrelation Function ◽  
Pressure Fluctuations ◽  
Spectral Density Function ◽  
Functional Relationships ◽  
Power Spectral ◽  
Data Points ◽  
Instantaneous Pressure ◽  
Conventional Signal ◽  
Mutual Information Function

Local instantaneous pressure signals obtained through a magneto-fluidized bed have been analyzed using both classical and advanced signal analysis methods, which can deliver the necessary information about the presence of the bubbling and turbulent flow pattern. The conventional signal processing tool such as autocorrelation and cross correlation were used as preliminary tools to analyze the data. Evaluation of the dominant bubble frequency was completed using the autocorrelation function and power spectral density function. Mutual information function was used to identify the periodicity and the predictability of the local instantaneous pressure signals. Since it does not demand any particular functional relationships between the data points, it is a better method (compared to autocorrelation function) for measuring the predictability of nonlinear systems.

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