Noninvasive Tomographic and Velocimetric Monitoring of Multiphase

4476

Ind. Eng. Chem. Res. 1997, 36, 4476-4503

Noninvasive Tomographic and Velocimetric Monitoring of Multiphase Flows Jamal Chaouki,† Faı1ç al Larachi,*,‡ and Milorad P. Dudukovic´ § Biopro Research Center, Department of Chemical Engineering, Ecole Polytechnique de Montre´ al, P.O. Box 6079, Station “Centre-Ville”, Montre´ al, Que´ bec, Canada H3C 3A7, Department of Chemical Engineering & CERPIC, Laval University, Que´ bec, Canada G1K 7P4, and Chemical Reaction Engineering Laboratory, Washington University, Campus Box 1198, One Brookings Drive, St. Louis, Missouri 63130

A condensed review of recent advances accomplished in the development and the applications of noninvasive tomographic and velocimetric measurement techniques to multiphase flows and systems is presented. In recent years utilization of such noninvasive techniques has become widespread in many engineering disciplines that deal with systems involving two immiscible phases or more. Tomography provides concentration, holdup, or 2D or 3D density distribution of at least one component of the multiphase system, whereas velocimetry provides the dynamic features of the phase of interest such as the flow pattern, the velocity field, the 2D or 3D instantaneous movements, etc. The following review is divided into two parts. The first part summarizes progress and developments in flow imaging techniques using γ-ray and X-ray transmission tomography; X-ray radiography; neutron transmission tomography and radiography; positron emission tomography; X-ray diffraction tomography; nuclear magnetic resonance imaging; electrical capacitance tomography; optical tomography; microwave tomography; and ultrasonic tomography. The second part of the review summarizes progress and developments in the following velocimetry techniques: positron emission particle tracking; radioactive particle tracking; cinematography; laser-Doppler anemometry; particle image velocimetry; and fluorescence particle image velocimetry. The basic principles of tomography and velocimetry techniques are outlined, along with advantages and limitations inherent to each technique. The hydrodynamic and structural information yielded by these techniques is illustrated through a literature survey on their successful applications to the study of multiphase systems in such fields as particulate solids processes, fluidization engineering, porous media, pipe flows, transport within packed beds and sparged reactors, etc. Contents Abstract 1. Introduction 2. Tomography and Radiography Techniques 2.1. γ-ray and X-ray Transmission Tomography 2.2. X-ray Radiography 2.3. Neutron Transmission Tomography and Radiography 2.4. Positron Emission Tomography 2.5. X-ray Diffraction Tomography 2.6. Nuclear Magnetic Resonance Imaging 2.7. Electrical Capacitance Tomography 2.8. Optical Tomography 2.9. Microwave Tomography 2.10. Ultrasonic Tomography 3. Velocimetry Techniques 3.1. Positron Emission Particle Tracking 3.2. Radioactive Particle Tracking 3.3. Cinematography 3.4. Laser Doppler Anemometry 3.5. Particle Image Velocimetry 4. Summary 5. Nomenclature

6. Literature Cited 1 1 2

23

1. Introduction

12 12 13 13 13

Multiphase flow technology plays an important role in the chemical and process industry. Handling systems involving two or more phases is commonplace in areas from the processing of fuels and chemicals to the production of feed, food, pharmaceuticals, and specialty materials. Despite the wide usage of multiphase systems, the methodology adopted for their design is by and large based on intuition and rules of thumb rather than on first principles. The main reason for this state of affairs is that the local flow structure is extremely complex and the link between the micro and macro has not been clearly established. Consequently, our understanding of the numerous hydrodynamic problems encountered with such systems remains incomplete. The lack of detailed structural and dynamic information at the microscale and the mathematical difficulties associated with the methods for handling the randomness of the multiphase media are the prime reasons for the inability to treat these flows purely from a theoretical basis.

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* Author to whom correspondence is addressed. † Ecole Polytechnique de Montre ´ al. Phone: (514)-340-47114034. Fax: (514)-340-4159. E-mail: jchaouki@ mailsrv.polymtl.ca. ‡ Laval University. Phone: (418)-656-3566. Fax: (418)-6565993. E-mail: [email protected]. § Washington University. Phone: (314)-935-7187, (314)-9356021. Fax: (314)-935-4832. E-mail: [email protected].

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© 1997 American Chemical Society

Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997 4477

The successful approach toward the understanding of such complex flows requires reliable experimental data, which, in turn, depends on the implementation of sophisticated measuring techniques capable of noninvasive investigation as well as the ability to provide the required information over the entire flow field. In addition, it is desirable that such techniques are amenable for automation to reduce extensive human involvement in the data collection process. Progress in modeling the transport phenomena with or without chemical reaction in multiphase reactors depends on the availability of such experimental tools which provide the data for model evaluation and their subsequent improvement. Advances in instrumentation technology as well as in computer control have led to spectacular progress in the development of noninvasive measurement and flow visualization techniques for multiphase flows. However, in spite of the rapidly growing number of articles written on the subject, the process engineering community is often unaware of the advantages of these new techniques. One of the major reasons for this lack of awareness is that the publications on the subject are scattered, since most of them have been published in nonchemical engineering journals. Therefore, this paper provides an overview of the latest advances realized in the noninvasive measurement of multiphase systems by means of various tomographic and velocimetric techniques. Although the majority of the techniques described here are best-suited for laboratory-scale process equipments and specialized laboratory facilities, the scaleup to industrial pilot plants and/or commercial facilities is possible for some of them. Information on successful applications to process engineering equipments, and advantages and inherent limitations of these techniques are provided as a guide to engineering disciplines that deal with multiphase systems. The following topics are described in this review: γ-ray and X-ray transmission tomography; X-ray radiography; neutron transmission tomography and radiography; positron emission tomography; X-ray diffraction tomography; nuclear magnetic resonance imaging; electrical capacitance tomography; optical tomography; microwave tomography; ultrasonic tomography; positron emission particle tracking; radioactive particle tracking; cinematography; laser-Doppler anemometry; particle image velocimetry; and fluorescence particle image velocimetry. Although references to major contributions are included in this paper, no attempt is made to provide an in-depth review of each of the above techniques for which a few monographies have already appeared and may be consulted (Scott and Williams, 1995; Williams and Beck, 1995; Chaouki et al., 1997). 2. Tomography and Radiography Techniques The tomography and radiography techniques described in this section are of three types: (1) Nuclear-based imaging techniques using ionizing radiations: γ-ray and X-ray transmission tomography, positron emission tomography, X-ray diffraction microtomography, and X-ray and neutron transmission radiography. (2) Nuclear-based but nonionizing imaging techniques: nuclear magnetic resonance imaging. (3) Non-nuclear-based imaging techniques: electrical capacitance tomography, optical tomography, ultrasonic tomography, microwave tomography.

Salient features of the various tomography and radiography techniques such as the type of sensors, the achieved spatial and time resolutions, the size of the field of view, and some applications are summarized in Table 1. 2.1. γ-ray and X-ray Transmission Tomography. The transmission of X-rays or γ-rays through a heterogeneous medium is accompanied by attenuation of the incident radiation, and the measurement of this attenuation provides a measure of the line integral of the local mass density distribution along the path traversed by the beam. The measurement of several such beams at different spatial and angular orientations with respect to the test section or volume, followed by an image reconstruction procedure, provides a density distribution of phases to a high degree of spatial resolution. Since the data collection is automated and since the reconstruction process is performed using a computer, the process is referred to as either computer-assisted tomography (CAT) or computed tomography (CT). Scanners for transmission tomography employ radiation sources, such as an X-ray tube or an encapsulated γ-ray source, positioned on one side of the object to be scanned, and a set of collimated detectors arranged on the other side. Scanners of the first generation consist of a source emitting a single pencil beam of radiation and a detector (Figure 1a). They move on opposite sides of the test section, measuring the attenuation at each position. This method of scanning is very time consuming and is not capable of collecting data for fast movements without introducing important distortions in the reconstructed image. In scanners of the second generation, an array of detectors, facing a single source, move around the object and provide a number of projections equal to the number of detectors. Sometimes, to reduce the scanning time, second-generation scanners using multiple radioactive sources (Figure 1b) can be employed (Hosseini-Ashrafi and Tu¨zu¨n, 1993). Data acquisition time can be less than 1 min in this method. The source used in scanners of the third generation is collimated in such a way that the pattern of the beam is a fan (Figure 1c). A single view is obtained for a given position of the source-detectors arrangement, after which it is rotated to get the next view. Another method, referred to as the fourthgeneration scanning configuration, is the fixed-detector, rotating-source arrangement in which a large number of detectors are mounted on a fixed ring (Figure 1d). Inside, a fan beam source is sampled every few milliseconds. Samples for any one detector are referred to as detector-vortex fans and constitute one view of the object. There are two main types of detectors depending on the principle used in the detection process. In the detectors of the ionization chamber type, the sensors react to the ionization produced in them by the radiation. In the scintillation type, excitation or molecular dissociation induced by the radiation produces the measured effect. The sources most commonly used for X-rays are tungsten and molybdenum. The characteristic energy of the photons emerging out of an X-ray tube is relatively low. Since the mass absorption coefficient for an X-ray is inversely proportional to the one-third power of the X-ray energy, a suitable choice of the energy is required to obtain optimum attenuation of the beam (Kumar and Dudukovic´, 1997). To study large test sections, radioisotopes that emit more penetrative γ-rays are preferred over X-ray tubes as they have photons of

thermal neutrons/photographic film, with an intermediate conversion screen

positron emitter tracers/ positions camera as detector

X-ray source/two detectors, one for the primary beam and one for the diffracted X-ray external magnetic field gradient and radiofrequency pulses

electrodes energized sequentially

neutron transmission tomography and radiography

positron emission tomography

X-ray diffraction tomography

electrical capacitance imaging

spatial resolution

temporal resolution

III faster than X-ray and γ-ray CT and NMR

2 (1 × 2.54 cm2) resolution limited by uniformity and alignement of electrodes

4 not suited for opaque systems II γ/20 to γ/5 (γ, wave frequency) depends on size of the test object 4 (≈1 mm) I

II combine velocimetric, morphologic capabilities

I

II

I

II

II

II provides time averaged profiles

5 (0.1 mm)

4 (≈1 mm) better material discrimination than X-ray CT

3 (8 mm)

5 (0.05 mm) suited for specific materials

4 (4 mm)

4 (5 mm) allows studies on large test sections

4 (0.25 × 2 mm2) suited for small test sections

ref for more details

liquid and solid foams silica/water slurry two-phase flow

laboratory scale laboratory-scale vessel laboratory scale D ) 0.12 m

pneumatic conveying systems

Breden et al., 1995 Hauck, 1991

Williams and Beck, 1995 Darton et al., 1995 Bolomey, 1995

Xia et al., 1992 Heath et al., 1990 Pangrle et al., 1992 Kose, 1990-1992 Chung et al., 1993 Halow 1997; Halow and Nicoletti, 1992; Halow et al., 1993 Beck et al., 1993

Ramaswamy et al., 1997

Kumar and Dudukovic´, 1997 packed columns Toye et al., 1994, 1997 fluidized and trickle beds Kantzas, 1994, 1996 carbonate oil reservoirs Hicks et al., 1990 porous media Jasti et al., 1990 bubble column Kumar and Dudukovic´, 1997 hopper rig Hosseini-Ashrafi and Tu¨zu¨n, 1993 fluidized beds, spouted beds Seville et al., 1986; Simons and Williams, 1993 multiphase flow horizontal pipe Fincke et al., 1980 gas fluidized bed Yates et al., 1994; Yates, (rectangular cross section) 1997 air fluidized beds (rectangular) Rowe and Everett, 1972a-c; Rowe et al., 1978 discharging bunker Jones et al., 1985a,b fast fluidization Weinstein et al., 1992 metal-filled wood dissolution Fredd et al., 1997; Jasti and of limestone, carbonate Fogler, 1992; Lindsay et porous media al., 1990; Bernadiner et al., 1992 porous oil reservoir, Benton and Parker, 1997; extrusion of powders Hawkesworth et al., 1991 into moulds slurry mixtures in McKee et al., 1995 mixing tanks timber industry Grant et al., 1997; Wells architecture of et al., 1994; Martens osteoporotic bone et al., 1994

applications

visualization of pipe pulp flow capillary flow hallow-fiber bioreactor laminar flow in porous tubes turbulence in pipe flow D ) 3.9 mm, L ) 0.08 m dean vortices D ) 0.154 m fluidized beds

V ) 6.6 × 10-3 m3 (L ) 12 m) laboratory scale D ) 1.1 mm, H ) 0.58 m V ) 2.9 × 10-3 m3

V ) 0.005 m3

V ) 7 × 10-4 m3 V ) 1.2 × 10-4 m3

D ) 0.076 m V ) 0.12 m3 (0.2 × 0.3 × 2 m3) Vmax ) 0.063 m3 (0.3 × 0.3 × 0.7 m3) V ) 0.0095 m3 V ) 0.07 m3 (H ) 4 m)

D ) 0.10, 0.145 m

0.1 < D < 0.3 m Dmax ) 0.1 m

0.035 < V < 1.69 m3 Vmax ) 0.078 m3

size of the tested system

a Spatial resolution: 1 ) very low; 2 ) low; 3 ) medium; 4 ) good; 5 ) high. Time consuming for data acquisition: I ) long, II ) medium, III ) fast. D: ) diameter. V: volume. H: height. L: length.

optical tomography visible light source/camera microwave tomography microwaves generators/ antennas ultrasonic tomography ultrasonic transducers

NMR imaging

sources same as in X-ray CT; detector, sheet of film or image intensifier camera

X-ray radiography

or 153Gd photon emitter sources/ionization chamber type or scintillation type detectors

137Cs, 241Am,

γ-ray transmission CT

sensor installed

tungsten or molybden sources/ ionization chamber type or scintillation type detectors

X-ray transmission CT

general measurement basis

Table 1. Summary of Tomography and Radiography Techniquesa

4478 Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997


Figure 1. Four CT scanning modes in transmission tomography: (a) translate-rotate parallel-beam scanning mode (first-generation scanner); (b) translate-rotate multiple-source parallel-beam scanning mode (modified second-generation scanner, after Hossein-Ashrafi and Tu¨zu¨n, 1993); (c) rotating fan beam scanning mode (third-generation scanner); (d) fixed detector rotating source scanning mode (fourth-generation scanner); (e) coordinate system for image reconstruction in transmission tomography and definition of the ray-sum. D ) detectors, S ) source, C ) collimator, O ) object to be imaged (adapted from Kumar and Dudukovic´, 1997). Each beam angle represents one single view (or projection or scan) made up of a series of paths of photon-attenuated beams. Arrows indicate degrees of freedom of the scanner.

higher characteristic energy. The sources commonly used are 137Cs, 241Am, and 153Gd which emit, through β- decay, photons of about 100 keV. However, X-ray tomography allows a better spatial resolution because of the smaller size of the detectors. It is also a safer method as X-ray sources emit radiations only when they are powered on (Toye et al., 1997) and their energy may be controlled by varying the input voltage. Each measurement of an attenuated beam of radiation at given spatial and angular orientations is digitized and stored by a computer and constitutes a projection of the object. The reconstruction method applies to any two-dimensional object containing an unknown space-dependent property, f(x,y) (which is here the image function or the attenuation coefficient distribution), about which information can be obtained by measuring projections of that function on lines which pass through the object. If P(l,θ) represents a set of projections (called also ray-sum) of f(x,y) along a beam at angle θ (0 e θ e π) and distance l from the origin (-r e l e r, r ) dimension of the test section; see Figure 1e), they are interrelated via the following integral equation:

P(l,θ) )

∫Lf(x,y) ds

The basic problem in tomography is the inversion of the above integral equation along a linear path through a scalar field. The solution of this equation has been developed along different lines, and a good account of these is given in the review of Brooks and DiChiro (1976) and recently by Kumar and Dudukovic´ (1997). A very frequently used algorithm for reconstruction in commercial CT scanners is the filtered backprojection or its equivalent, the convolution backprojection system. For image reconstruction from the measurements obtained with a fan-beam scanning configuration, instead of simple backprojection as used for parallel beams, a weighted backprojection is implemented (Herman et al., 1976). Another method known as rebining (Dreike and Boyd, 1976) first converts the fan-beam data to equivalent parallel-beam projection measurements before reconstruction. Other algorithms of reconstruction such as the algebraic reconstruction technique (ART) and maximum likelihood based methods are described by Kumar and Dudukovic´ (1997). The main factors that determine the imaging capabilities of a CT scanner used in engineering applications are its achievable spatial, temporal, and density resolution. Spatial resolution is the minimum distance that two high-contrast point objects can be separated, tem-


poral resolution refers to the frequency with which the images can be obtained, and density resolution refers to the smallest difference in mass attenuation coefficients that the system is able to distinguish. With X-ray and γ-ray tomography the data obtained for the concentration distribution of phases are almost always time-averaged since a significant period of time is required to obtain the photon count rates for all the projections (necessary to rotate the sensors and the source). Depending on the design of the scanner, this period can range from a few minutes to close to 1 h. This is a basic limitation of these techniques in studies of time-evolving flow phenomena. However, dynamic information can be obtained when the dispersion rate or the flow velocities are sufficiently slow (HosseiniAshrafi and Tu¨zu¨n, 1993). Banholzer et al. (1987) conducted a feasibility study for direct imaging of time-averaged flow patterns in a model fluidized bed using a medical X-ray CT scanner which consisted of a 88 kW X-ray tube placed in one side of the reactor and an array of 517 gas-filled detectors on the opposite side. The test section was square and its dimensions (7.3 × 7.3 × 30.48 cm3) were dictated by the largest scannable section (patient) by the scanner. A spatial resolution of 1.5 mm and a density resolution of better than 30 kg/m3 were achieved. Hicks et al. (1990) used a commercial X-ray CT scanner for studying the heterogeneities of carbonate cores with the objective of targeting the bypassed oil in carbonate oil reservoirs for improved oil recovery. Porosity and residual oil saturation distributions were of interest in this study. Coles et al. (1991) made a similar study of the core to determine the saturation and distribution of fluids using a commercial X-ray CT scanner. Jasti et al. (1990) used a cone-shaped diverging X-ray beam along with a two-dimensional X-ray detector to directly obtain a 3D reconstruction of the flow through porous media. Using a medical CT scanner, Lutran et al. (1991) investigated the liquid distribution in a squared 7.3 × 7.3 × 30.5 cm3 trickle bed packed with glass beads and under zero gas flow rate. As the scanner was of commercial type, no details were provided regarding hardware, software, and reconstruction aspects. The CT scanner allowed recording of flow patterns at the bed scale as a function of liquid flow rate, particle size, liquid surface tension, liquid feeding distributor, and bed prewetting/flooding conditions. Modi et al. (1992) observed the void fraction distributions downstream from a centerline point injection of air into water flowing turbulently in a horizontal 3.75 cm i.d. pipe. A thirdgeneration X-ray CT medical scanner, EMI 5005 X-ray tomography unit, was used to image planes downstream from the injection point and perpendicular to the flow direction. Kantzas (1994, 1996) and Holoboff et al. (1995) used a EMI 7070 commercial X-ray CT scanner for obtaining the holdup distribution in fluidized and trickle beds. The fourth-generation medical-type scanner was modified to perform scans in both horizontal and vertical directions. Toye et al. (1994, 1997) determined the spatial distribution of gas, liquid, and solid phase saturations in packed columns. An X-ray generator providing a collimated flat fan beam and a detector bank, a 1.7 m long linear array of photodiodes, were fixed on two vertical pillars embedded in a rigid metallic structure. Their vertical displacement was controlled by a motor, and the rotation was achieved around a fixed plate supporting the packed column. A fourthgeneration Technicare 2060 CT scanner (120 keV, 75

mA X-ray source) was used by Lu et al. (1994) to characterize the gas storage and transport in unconventional natural gas reservoirs such as Devonian shales. From imaging the shale samples, several important characteristics, such as fracture widths, adsorption isotherms, matrix porosities, and permeabilities, were obtained from X-ray CT. Fincke et al. (1980) used γ-ray tomography to determine the time-averaged density and its distribution in horizontal multiphase flows. They were able to differentiate between flow regimes ranging from stratified flow to annular flow. The CT scanning system used had nine detectors arranged in an arc and a source 0.5 Ci 243Am collimated into a fan beam with a subtended angle of 32°. De Vuono et al. (1980) and Schlosser et al. (1980) developed a γ-ray tomography system for twophase flow studies. Parametric analysis was presented based on the requirements set for the spatial and density resolution, the size of the test section to be scanned along with constraints on the maximum available count rate (source strength), and the allowable scan time (i.e., the desired temporal resolution). The scanning system consisted of a fan-beam fourth-generation scanner source (137Cs) rotating on a circle between 48 NaI scintillation detectors and the test section. A generalized filter reconstruction scheme was proposed by Seshadri et al. (1986) to improve reconstruction of air and water density values obtained by this scanner. The air-water system density values were predicted with a maximum absolute error of 0.01 g/cm3 for emulsion densities between 0.6 and 1.0 g/cm3. A single monoenergetic γ-ray beam (59.6 keV from a 100 mCi 241Am source) and a single-collimated NaI(Tl) detector were used to produce tomographic images in the entry region of a 51 mm diameter and 200 mm height gas fluidized bed by McCuaig et al. (1985). Later, voidage measurements in fluidized beds using γ-ray tomography were obtained by Seville et al. (1986). The feasibility of using CT scanning was demonstrated by comparing the experimentally determined jet entrance length with theoretical predictions. An improved version of the scanner from the same group was reported by Simons and Williams (1993). The system had six 153Gd sources with six collimated CsI scintillation detectors. The assembly was mounted on a fixed rotation stage, and the test section had to be lowered or raised through the scanning assembly. The scanner was used to study the differences in behavior of a spouted fluidized bed with dry and slick particles in terms of penetration of the inlet jet as well as the voidage profiles. Hosseini-Ashrafi and Tu¨zu¨n (1993) adapted the same system, which was modified to rotate around the test section, for studying granular flows in a model hopper rig. The cross-sectional profiles of solids fraction and the plane mean values of void fraction were obtained in the hopper cylindrical and conical sections. Recently, a dual-mode tomography system was built and tested for imaging each one of the three components of a gas-liquid-liquid flow by a research group at the University of Bergen, Norway (Johansen et al., 1995). A γ-ray tomograph consisting of five sources and 85 detectors and a capacitance tomograph consisting of eight electrodes and high-sensitivity capacitance detectors were used as components in a dual mode for imaging water-oil-gas flow in pipes. Void fraction distributions in bubble columns are currently carried out at Chemical Reaction Engineering Laboratory, Washington University (Kumar et al., 1995;


Figure 2. γ-ray CT images of void fraction distribution in the fully developed zone of a 0.19 m i.d. bubble column from three distributors: (a) bubble cap (4 holes, 5 mm diameter), (b) cone distributor (lower opening diameter 12.7 mm, upper opening diameter 190 mm), (c) perforated plate (156 holes, 0.25 cm2 pitch). Reprinted with permission from Kumar and Dudukovic´ (1997). Copyright 1997 Elsevier Science-NL.

Kumar and Dudukovic´, 1997). A third-generation configuration was adopted for their CT scanner. It consists of a rotation stage (gantry) on which an array of 32 NaI scintillation detectors is mounted on one side and a source holder containing 132Cs is placed on the opposite side of the test section. The effects of operating conditions, such as superficial gas velocity, liquid properties, column diameter, and distributor configuration, on void fraction distributions were studied. Typical crosssectional distributions of void fraction obtained in the fully developed zone of a 0.19 m i.d. bubble column equipped with three types of distributors are shown in Figure 2a-c (Kumar and Dudukovic´, 1997): (a) bubble cap (4 holes, 5 mm diameter); (b) cone distributor (lower opening diameter 12.7 mm, upper opening diameter 190 mm); (c) perforated plate (156 holes, 0.25 cm2 pitch). It is seen that the perforated plate results in a uniform distribution of the gas, and this is reflected in the gradual variation in the colors for the void fraction from the column center to the wall. For the cone and the bubble-cap distributors the gas moves up the column as large bubbles in a region close to the column center. The side walls are almost unaerated, as shown by the light shades in the annulus. 2.2. X-ray Radiography. X-ray radiography is a technique based on the same principle as X-ray tomography, but the attenuation of the beam emitted by the X-ray source is registered by sheets of film or an imageintensifier camera. The registered images are then recorded on a cine camera or a video-recorder and

transferred to a computer for processing and analysis. X-rays have been used for many years to probe the interior of fluidized beds and to enable direct observations of bubble motion. The first reported use of X-ray radiography was by Grohse (1955), who measured the variation in density of a bed of silicon powder as a function of the fluidizing gas velocity. Romero and Smith (1965) used flash X-ray radiography to study the internal structure of fluidized beds. Their system used two flash tubes, operating at 300 and 600 keV each and producing a square pulse energy of 1000 mA for 0.2 ms. The tubes were mounted on opposite walls of a lead-lined room which housed a Plexiglas column. The X-ray beam passing through the bed was recorded on 8 × 10 in. sheets of film. Data on bed density distribution and the shape, size, and velocity of bubbles were obtained. Rowe and Everett (1972ac) described the design and operation of an X-ray system which incorporated a cine camera to record the rise of bubbles in fluidized beds. Their system consisted of a conventional medical tube emitting a beam registered by a 22 cm diameter intensifier which was filmed at up to 50 frames/s using a 70 mm cine camera. Bubble size, number, and velocity distributions were measured at various heights up to 70 cm in beds of alumina, carbon, quartz, ballotini, and crushed glass fluidized by air. The same system was later used to make the first detailed measurements of emulsion-phase voidage in a fluidized bed (Rowe et al., 1978). Recently, Yates and Cheesman


(1992), Yates et al. (1994), and Yates (1997) obtained enhanced X-ray radiography images of fluidized beds and were able to visualize the regions close to the boundaries of the rising bubbles. The radiography system used was composed of an X-ray tube operating at 150 keV, which had dual focal spots of 1 and 2 mm and a rotating anode constructed from molybdenum faced with a rhenium-tungsten alloy. X-ray images were registered on an image intensifier located on the opposite side of the bed from the X-ray source. Versatility of the system in terms of radiation source and detector mobility allowed different positions of the bed to be examined. A similar technique using a rotating anode was applied by Jones et al. (1985a) to determine isochrons and vertical velocity distributions of silica powder within discharging bunkers. This technique was limited to bunkers up to 1 m in diameter and for powders relatively transparent to X-rays. The same technique was also used by Gamblin et al. (1993) to characterize flow patterns and gas jet penetration lengths from various designs of aeration nozzles employed in largescale FCC regeneration units. Weinstein et al. (1992) employed an X-ray attenuation technique to study gassolid flow behavior in fast fluidized beds. Their apparatus consisted of an X-ray generator operated with a 150 keV tubehead mounted on a carriage moving along the 4 m high riser section. Plate or movie film exposures were used to provide “instantaneous” snapshots of the radial solid fractions at selected axial positions. Their data were then converted to crosssectional averaged values of solid fraction using a chordal absorptometry technique. 2.3. Neutron Transmission Tomography and Radiography. Neutron transmission imaging is a technique ideally suited for space-resolved imaging of various flow phenomena when more than two phases are present in the system, and wherein only one phase produces a contrast due to its high content in absorbers of thermal neutrons, e.g., hydrogen or boron. Neutron imaging has found widespread applications in the reactive flow phenomena occurring in consolidated porous media where hydrogen-containing fluids provide a contrast in the image due to their large thermal neutron cross sections. The object to be imaged is placed in the path of a neutron beam, produced for instance in a nuclear reactor, and the transmitted neutron flux is detected by film neutron radiography or film neutron radioscopy. A third imaging mode, referred to as the transfer film neutron radiography, is used in high γ-ray fields or when the object itself is radioactive; see von der Hardt and Ro¨ttger (1981) for the details. Film neutron radiography and film neutron radioscopy are used at the Phoenix Memorial Laboratory (PML) of The University of Michigan (Lindsay et al., 1990; Fredd et al., 1997). In the film radiography method (Figure 3a), the neutron flux is absorbed by an intermediate screen to generate a secondary form of radiation (such as electrons, γ-rays, or visible light) which is recorded onto a photographic film. Both the conversion screen and the film are placed in an aluminum front cassette to hold them in close contact and to prevent light contamination during imaging process. In the neutron radioscopic imaging system (Figure 3b), the gadolinium oxysulfide-containing screen converts the neutron intensity to light intensity through a scintillation process. The light intensity is amplified (×105) using a microchannel plate image intensifier tube in a

Figure 3. Neutron scanning modes: (a) film neutron radiography system (2D time-averaged imaging); (b) neutron radioscopic imaging system (real-time imaging). Adapted from Fredd et al. (1997).

LIXI Neutron Imaging Detector (LIXI NID). The output of the LIXI NID is viewed by an extended red newvicon videocamera. The neutron flux can also be converted into a light image, which is amplified using a magnetic focusing imaging system. Both first and second methods can be used in conjunction with computerized tomography for 3D imaging. A collimator is then used to generate a near-parallel beam of thermal neutrons. A series of exposures at uniform angular intervals are taken around the object to be imaged. A computer program transposes the data and reconstructs the internal cross section of the object at a right angle to the plane of the radiograph. Methods of image reconstruction in neutron imaging are similar to those employed in X-ray or γ-ray imaging (see section 2.1). More details on the neutron imaging facility and the image processing system may be found in the literature (Jasti et al., 1987; Jones et al., 1985b; Lindsay et al., 1990). A few investigators have applied neutron radiography to the study of reactive dissolution of consolidated porous media. Lindsay et al. (1990) and Jasti and Fogler (1992) were among the first to apply the technique to imaging of porosity patterns created by the reactive dissolution of limestone by hydrochloric acid. Rege and Fogler (1989) imaged the porosity patterns resulting from the flow, dissolution, and precipitation of ferric hydroxide in carbonate porous media. Bernadiner et al. (1992) imaged the patterns created by the flow and reaction of foamed acids in carbonate porous media. These studies focused on acidization, which is a common technique used for the stimulation of oilbearing porous media. Application of neutron transmission tomography was used to image the wood’s metal-filled porosity patterns of acidized porous media by Jasti and Fogler (1992) and Lindsay et al. (1992). These investigators used the LIXI NID system to obtain neutron radioscopic images of wormholes formed by HCl in limestone cores such as those shown in Figure 4 (Jasti and Fogler, 1992; Fredd et al., 1997). The neutron images of Figure 4 allowed for the determination of geometrical properties such as the volume fraction, surface area, and tortuosity of the wormholes.


Figure 4. LIXI NID neutron radioscopic image of wormholes or porosity patterns formed during the dissolution of limestone by 0.1 N HCl injected at 1.7 cm3/min. Projection images averaged over 4 s. Reprinted with permission from Fredd et al. (1997). Copyright 1997 Elsevier Science-NL.

2.4. Positron Emission Tomography. The positron emission tomography technique (PET) uses radioactive tracers that decay via the emission of a positron. Radioactive decay occurs when the nucleus of these tracers, rich in protons, converts one proton into an uncharged neutron, expelling a positron (β+ decay), the antiparticle of the electron. Any positron expelled from the nucleus will combine with an electron and can annihilate each other, and their total rest mass energy is converted into two 511 keV back-to-back γ-rays. By detecting both of these γ-rays simultaneously with positron-sensitive detectors, a line of sight along which the decay must have taken place can be inferred. By detecting many such decays, the distribution of tracer activity can be determined (Benton and Parker, 1997). A nonmedical positron emission detector using a positron camera was developed at the University of Birmingham (Hawkesworth et al., 1991). A schematic illustration of the design and operating principles of the Birmingham PET system is shown in Figure 5. The camera consists of two large positron-sensitive detectors giving a quantitative 2D projection of the 3D tracer distribution. Each detector is a multiwire containing a stack of 20 cathode planes. Each plane consists of a series of lead strips, where the γ-rays are most likely to interact, and between the cathode planes are planes of tungsten anode wires in isobutane gas. When a γ-ray is absorbed by a cathode strip, an electron is ejected into the gas which causes local ionization in the region of the nearest anode. Detection of an event in each detector is considered coincident if the time difference falls within 25 ns. The low detection efficiency of such a system (between 10-4 and 10-5 of events occurring within camera) limits the rate of data acquisition by the camera or necessitates longer imaging times (Benton and Parker, 1997). The radioactive tracers used are produced in a nuclear accelerator. In medical applications, considerations of patient dose lead to the use of short-lived tracers such as 15O (half-life ) 2 min), 13N (10 min), 11C (20 min), and 18F (110 min). Nonmedical applications have seen tracers of longer lifetimes such as 64Cu (12.7 h) and 22Na (2.6 yr). For more details on the characteristics

Figure 5. Layout of the Birmingham PET/PEPT imaging/particle tracking system. The camera consists of a pair of multiwire proportional chamber detectors with 20 anode-cathode planes. Each 511 keV γ-ray absorbed by a cathode strip leads to the ejection into the gas of electrons; a Townsend avalanche in the gas leads to voltage peaks. The timing signal for coincidence recognition of two antiparallel γ-rays is provided by the anodes, while the position of the γ-rays impacts are detected by the cathodes. Adapted from Hawkesworth et al. (1991).

of these tracers the reader is referred to the work of Benton and Parker (1997). Hawkesworth et al. (1991) used PET in different practical applications. In the geological field, they studied liquid percolation phenomena in porous oilreservoir sandstone by injecting aqueous KF solutions with a sample labeled with 18F. Similar experiments have been performed by a group from Shell, Amsterdam, who have used a medical ring PET scanner to examine oil displacement (Benton and Parker, 1997). Hawkesworth et al. (1991) applied PET to study the extrusion of powders into small molds. The experiment consisted of continuous extrusion of 68Ga-labeled flour dough through a 10 mm inlet port into a sealed mold. PET allowed observation of dead zones and blockages, making it difficult to fill the corners of the mold without the use of sufficient pressure (see Figure 6). This figure illustrates PET diagnostics of flour dough blockage due to corner geometry during the extrusion of 68Ga-labeled dough into an aluminum mold (125 mm high × 48 mm wide × 20 mm thick, 10 mm inlet port); each PET 2D image was averaged over 1 min. Recently, PET was used by McKee et al. (1995) to obtain concentration gradients and solids distribution in slurry mixtures. Single and bimodal sand-water slurries were agitated in a mixing tank of capacity 5 L, and the effect of impeller speed on the homogeneity of solids was examined. 2.5. X-ray Diffraction Tomography. The technique of X-ray diffraction tomography (XDT) is based on the Rayleigh (or coherent) scattering of low-energy X-rays through matter. This relatively new technique uses the interference phenomena of the radiation scattered from the primary beam to discriminate many materials based on the characteristic directional diffraction which depends on the material nature. The use of scattered radiation as a basis for tomography imaging is particularly effective when the constituents in the test object scatter into different, well-defined angular ranges. When the arrangement of scattering sites is of a


Figure 6. PET diagnostics of powder blockage due to corner geometry during the extrusion process of 68Ga-labeled dough into an aluminum mold (125 mm high × 48 mm wide × 20 mm thick, 10 mm inlet port). Each PET 2D image was averaged over 1 min. Reprinted with permission from Benton and Parker (1997). Copyright 1997 Elsevier Science-NL.

Figure 7. Scanning geometry used in XDT showing detectors (D) in transmission and diffraction modes, an X-ray source (S), a set of collimators and diffractometer slits (C1, C2 and C3, C4), the object to be imaged (O) in the field of view represented by the broken circle, and nearby arrows show the rotation-translation movements of the object. Adapted from Grant et al., 1993.

statistical nature (as in liquid or amorphous materials), the height and angular position of the associated diffuse X-ray diffraction peaks (Blum, 1971; Steele and Pecora, 1965) may be used as a basis for materials discrimination (Grant et al., 1997). The experimental arrangement used to obtain XDT images is analogous to the first-generation mode of transmission CT scanning already discussed in section 2.1, except that in the diffraction mode the detector is placed off-axis of the primary transmitted beam (Figure 7). The object to be imaged is placed on a rotation stage mounted on a translation table traveling in the horizontal plane perpendicularly to the incident beam direction. Two detectors are used in this configuration: a single diffraction detector is placed to measure X-ray intensity at a scattering angle θ and a similar detector is placed in the primary transmitted beam (Grant et al., 1997). The latter may be thought of as an XDT arrangement at a scattering angle of 0°. XDT images were reconstructed using summation-filtered backprojection (SFBP) as in transmission CT. SFBP

Figure 8. XDT and X-ray CT images of a glycerol-filled cylinder and an oil-filled cylinder put inside a water-filled cylinder, with a bubble trapped in the oil-filled cylinder: (a) transmission X-ray CT image; (b-d) X-ray diffraction tomography images acquired with XDT scatter detectors located at 6.5°, 8.5°, and 11.5°. These angles were chosen to coincide with the oil, glycerol, and water diffraction maxima, respectively. Reprinted with permission from Grant et al. (1997). Copyright 1997 Elsevier Science-NL.

has been shown to provide images of good quality provided the photon mean free path far exceeds the size of the object to image and the detector is placed at a distance excessively larger than the beam path across the object (Grant et al., 1993). For practical situations where larger objects need to be imaged, SFBP suffers from peripheral gray-scale artifact, and Grant et al. (1995) devised an attenuation-corrected iterative reconstruction procedure for image reconstruction (see also Grant et al., 1997). In order to compare the sensitivities of an X-ray transmission CT technique and an XDT technique, Grant et al. (1997) used a sample composed of two cylindrical polyethylene vessels enclosed by a larger cylindrical polypropylene container. One inner container was filled with glycerol and the other with lubrication oil, and the space between these and the outer container was filled with water. Figure 8 shows four images reconstructed using a backprojection method from (a) a transmission CT sinogram and (b-d) XDT sinograms acquired with detectors located at θ values of 6.5°, 8.5°, and 11.5° respectively. The figure demonstrates the failure of transmission CT imaging to distinguish between water and glycerol on the basis of image gray scale and thus linear attenuation coefficient. The XDT images exhibit a definite gray-scale contrast separation between liquid components, and this does not result from image processing but rather from the appropriate setting of the scanning geometry. Many applications of XDT are reported by Grant et al. (1997) in a range of disciplines. Simultaneous transmission and diffraction CT studies of normal and osteoporotic bones are currently being pursued by these authors. These studies are aimed at investigating the strength and architecture of osteoporotic bones. Moisture distribution studies to evaluate the finished wood product in the timber industry are another successful XDT application. A complementary technique to XDT that provides related image information is referred to as energydispersive scatter CT (Wells et al., 1994). This method uses a polyenergetic incident X-ray beam, with the detector arranged to cover a broad section of the scattering plane behind the sample to be imaged. The X-rays falling on the detector at a particular scatter


angle are then analyzed to provide an energy spectrum of the scattered radiation. This form of scatter CT has been investigated as a possible means of rapidly identifying materials of low-density contrast. Other applications of this CT technique have included imaging the presence and location of contaminants within foodstuffs (Martens et al., 1994). 2.6. Nuclear Magnetic Resonance Imaging. Nuclear magnetic resonance imaging (NMRI) has received a great deal of attention particularly due to the remarkable advances that have been made in the field of medical imaging (Hausser and Kalbitzer, 1991). NMRI is a noninvasive method based on the paramagnetic properties of the nuclei. Atomic nuclei are characterized by states which are quantum mechanical in their behavior. Each nucleus has a spin quantum number associated with it and a fixed quantity characterizing its stable ground state. A hydrogen nucleus has a spin quantum number I ) 1/2, and a deuterium nucleus has I ) 1. An angular momentum and a magnetic dipole moment, proportional to the angular momentum, are associated with the spin of the nucleus. The constant of proportionality relating the angular momentum and the magnetic dipole moment is known as the gyromagnetic ratio. In NMRI experiments, signals are obtained by having radio-frequency pulses (Callaghan, 1991) and magnetic field gradient pulses interact with the spin system positioned in a static magnetic field. There have been numerous applications of NMRI in fields ranging from solid-state physics to biological systems and, more recently, applications to laminar and turbulent flow systems (Caprihan and Fukushima, 1990; Powell et al., 1992; Gladden, 1994). For flowing systems, the phase information of the signals can be used to encode the velocity of the fluid. This is carried out by applying magnetic field gradient pulses of bipolar form so that the phase of the 1H magnetization is sensitive only to the velocity of the spins. This technique has been used in Powell’s group at the University of California, Davis (Ramaswamy et al., 1997) for the visualization of pulp flow in a 26.6 mm i.d. pipe, 12 m long. Radial distributions of timeaveraged streamwise velocity and turbulent velocity fluctuations of the liquid-solid suspension, as well as the influence of suspension flow rate on the cellulose fibers-water flow structure, were reported. Figure 9 is an example of joint spatial-velocity NMRI images of a 0.5% by weight hardwood pulp suspension at 12 suspension flow rates. To obtain these images, observation times in each phase flow encoding step were on the order of a few seconds and a rendition of the joint spatial-velocity 1H spin density distribution images was performed using a 2D Fourier transformation (Li et al., 1994). Bright boundaries defining sharp suspension velocity profiles of a plug-flow type are visible from Figure 9A,B for the low suspension flow rates. Increasing the suspension flow rate decreases the diameter of the core plug and enlarges the fiber-depleted regions characteristic of the mixed flow regime (Figure 9C-H). A further increase in the suspension flow rate leads to blurred velocity profiles in the turbulent regime (Figure 9I-L). High noise-to-signal ratios due to partially averaged velocity fluctuations give rise to artefactual bright regions not associated with the velocity profiles which renders distinction of the profiles very difficult in turbulent flow (Li et al., 1994). Other NMRI studies were reported on single-phase flows (Caprihan and Fukushima, 1990). Callaghan and

Xia (1991) and Xia et al. (1992) imaged velocity and liquid diffusion profiles in liquid laminar flow through an abrupt contraction and expansion and in capillary flows using the spin-echo technique. Heath et al. (1990) and Hammer et al. (1990) measured velocity profiles in the extracapillary space of a hollow-fiber bioreactor. Kose (1990, 1991, 1992) measured turbulence properties in a transitional pipe flow using an echo-planar imaging technique. Pangrle et al. (1992) studied the laminar flow of incompressible fluids through porous tubes using a time-of-flight technique. Chen et al. (1994a) used NMRI for the quantification of porosity and saturation distributions in porous media. Chung et al. (1993) visualized Dean vortices in liquid laminar flows in curved tubes using field encoding spin warp imaging NMRI. A detailed report on NMRI of twophase flows is available in Gladden (1994). 2.7. Electrical Capacitance Tomography. Electrical impedance tomography (EIT) is a noninvasive technique for imaging the distribution of an electrical property within a medium using electrical measurements from a series of electrodes flush-mounted with the medium surface. Electrical properties which may be employed in EIT include capacitance, resistance, inductance, and eddy current. Capacitance sensing is suitable for electrically insulating multiphase systems, while resistance and inductance sensing are useful for detecting electrically conducting materials or for materials which generate eddy currents or affect magnetic permeability (Halow, 1997). Among the above EIT techniques, capacitance imaging tomography (CIT) is better developed in chemical reactor applications and will thus be described exclusively hereafter. For readers interested in the applications and principles of resistivity-based EITs, the recent paper of Dickin et al. (1993) can be consulted. CIT uses as a contrasting property for imaging the electrical capacitance (or the dielectric constant), which is a function of the volume fraction of materials. In point measurement devices, capacitance sensors usually consist of two electrodes placed on opposite sides of the section to be imaged. When such electrodes are sequentially energized by a voltage, they accumulate charge which can be measured as an electrical current. Capacitance is the ratio of the integral of the current over time divided by the applied voltage. It is sensitive to particle size and shape, moisture content, and other conditions in the process, so it is preferable to calibrate the capacitance device while it is on the process stream to assume a relationship based on this calibration. For tomographic measurements, an array of electrodes around the boundary of the process stream is required. Each measurement can be used to determine the value of capacitance in one region or pixel in the sensed volume. The research group at the Morgantown Energy Technology Center (METC) began development of CIT systems for imaging fluidized beds in the mid-1980s. Preliminary observations with a first version of the system were reported in Halow et al. (1990). A higher resolution system was later constructed, and the results were reported by Halow and Nicoletti (1992) and Halow et al. (1993). The upgraded CIT system consists of a number of electronic circuits which energize and then sense and record electrical current in electrodes imbedded in the vessel walls of the region of interest to be imaged. Fast images are obtained through rapid switching of the energized pairs of electrodes which girdle the


Figure 9. Joint spatial-velocity NMR images of a 0.5 wt % cellulose fiber suspension prepared from hardwood Kraft pulp at increasing suspension flow rates: (A, B) suspension plug flow; (C-H) transitional flow with an intact plug in pipe core and shear flow in the annular region; (I-L) the flow of suspension is turbulent and the central suspension plug is destroyed. Reprinted with permission from Ramaswamy et al. (1997). Copyright 1997 Elsevier Science-NL.

vessel. The high speed of this technique allows the study of highly evanescent phenomena such as the rise velocity and diameter of bubbles, the formation of voids

near minimum fluidization, and the behavior of a gas jet in the entrance region of a fluidized bed. CIT application to fluidized beds is summarized by Halow


Figure 10. Time sequence of CIT images illustrating the behavior of a gas jet entering a gas fluidized bed. In this experiment a bed of 700 µm plastic spheres is fluidized at minimum fluidization velocity and a 6.4 mm diameter tube located along bed axis to simulate a continuous nozzle injection of jets. Reprinted with permission from Halow (1997). Copyright 1997 Elsevier ScienceNL.

(Halow, 1997). As an illustration of CIT capabilities, Figure 10 illustrates a time sequence showing the evolution of a gas jet entering a bed of 700 mm plastic spheres near Umf. A 0.64 cm diameter tube, located along the axis of the bed, was used to continuously inject gas. In the first frame of Figure 10, a gas voidage channel is formed. Subsequent frames show the increasing size of the void and the collapse of the voidage channel. The lighter shades represent higher void fractions. Because of the limited resolution of the system, the jet could have a smaller diameter than that indicated in the figure. In general, electrical sensing techniques, such as CIT, have the advantage of being faster and safer than the hard-field shorter-wavelength nuclear-based tomographies, e.g., X-ray, γ-ray, and NMR. Additionally, since the primary signal issued from electrical sensing techniques is itself of electric nature, these techniques are easily amenable for automation and digital computer control (Dickin et al., 1993). Besides the cheaper commercial implementation, CIT and electrical imaging techniques alike accommodate large and small vessel geometries with greater flexibility than nuclear-based tomographies. However, even though very powerful in tracing evanescent phenomena and fast-flowing streams exhibiting intense composition fluctuations, the spatial resolution of the CIT images is considerably poorer than that with the nuclear-based tomographies. The spatial resolution which corresponds to voxels of 1 cm2 × 2.54 cm (METC CIT, 32 electrodes) and 3.4 cm2 × 10 cm (UMIST CIT, 12 electrodes) is limited by the uniformity, alignment, and size of the electrodes used to contain and shape the electrical flux lines (Halow, 1997). As a general rule, a baseline resolution of approximately 5% of vessel or pipe diameter is typical for a 32-electrode imaging system (Dickin et al., 1993). The spatial resolution can be further improved if the vessel contains internals (draft tube, impeller shaft, etc.) on which additional electrodes could be inserted. Due to poor resolutions, electrical sensing techniques determine an average contrasting property within each voxel which is assumed to depend linearly upon the density or volume fraction of the target material (Halow, 1997). At about the same time as the METC group, a group at the University of Manchester (UMIST) developed another electrical capacitance facility for imaging multiphase flows in oil pipelines and pneumatic conveying systems. Their work and other tomography techniques

are summarized by Williams and Beck (1995), Beck et al. (1993), and Salkeld et al. (1991). The UMIST CIT has also been applied to investigate the distribution of solids inside hydrocyclones (Williams et al., 1997) for the purpose of controlling the cyclone discharge; imaging gas bubbles in the entry region of a gas fluidized bed for bubbling, slugging, and close to turbulent regimes was also attempted (Wang et al., 1996). A dual-mode tomograph using capacitance tomography was employed by Johansen et al. (1995) in their study of water-oilgas flows (see section 2.1). Reineke and Dewes (1994, 1995a,b) developed a CIT technique to image the pulse flow regime in trickle-bed reactors (Reineke and Dewes, 1996) and reported a detailed structure and composition of the liquid slugs as a function of operating conditions. 2.8. Optical Tomography. Optical or visible light tomography using interferometry is a sensitive technique for detecting spatial variation in a gas mixture, suitable for use in a laboratory environment. The use of visible light as the imaging radiation has received very little attention, probably because this technique does not allow one to “see” inside opaque and dense systems. Different methods are described in the literature based on the transmission of the light through a test section. The source and sensors vary depending on the system to be imaged (Darton et al., 1995). For measuring the 3D concentration field in a gas jet, several methods were reported (Snyder and Hesselink, 1985) whereby optical path length, concentration, and gas density were measured. Faris and Hertz (1989) used a differential interferometer to visualize an oxygen jet in air. In this technique, both interfering beams passed through the flow field, with the two beams having different polarizations. Fischer and Burkhardt (1990) used the visible radiation emitted by a flame to form an image which is recorded by a videocamera. A color camera with lens and filters recording three distinct images at different wavelengths was used to construct temperature maps of the flame. Burkhardt (1993) showed how such images could be used to control a combustion process. Application of an optical tomograhy technique for characterizing the structure of solid and liquid foams was recently reported by Darton et al. (1995). The liquid foam sample is placed in a vessel on a turntable which rotates at a fixed slow rate. The foam was illuminated by a visible light reflected by a mirror to prevent important reflections in the foam. Twenty-four frames per second were sampled by the video system at a resolution of 160 × 120 pixels. To optimize image clarity, thresholding was achieved manually for each reconstruction. The algorithm used for image reconstruction was simple backprojection. Visualization of foams by visible light has the advantage of not introducing scattering and image distortion effects as in X-ray tomography. Light is scattered much more uniformly over the spectrum. The disadvantage of using visible light attenuation lies in the impossibility of creating binary Radon transforms with a loss of information per image when the sample is not translucent. For the details, see Darton et al. (1995). 2.9. Microwave Tomography. The microwave spectrum extends from 300 MHz to 300 GHz, corresponding to wavelengths between 1 m and 1 mm. This dimensional aspect is responsible for strong diffraction effects, which characterize microwave interactions with structures comparable in size to this wavelength domain. Furthermore, unlike other tomographic techniques based on X-rays, ultrasound, or nuclear magnetic


resonance, microwave interactions depend primarily on the dielectric and magnetic constants of the material under inspection. Such constants are highly frequencydependent for many materials, making the selected operating frequency important in the optimization of the global performances of the microwave tomographic equipment (Bolomey, 1995). The diagram of microwave-based process tomography equipment is very similar to any tomographic system. It consists of three parts: the sensors and the associated electronics or optoelectronic equipment to perform data acquisition, the tomographic reconstruction process, and the translation of the reconstructed data in quantities of interest. Two basic arrangements of the process equipment are being used. The first consists of reflection probing or inspection of media accessible by only one side. This configuration is characteristic of applications aimed at the detection of buried objects in civil engineering or investigations with security or military contexts (Bolomey, 1995). For these applications probing can be performed from the analysis of the reflected field corresponding to multifrequency interrogations. The second arrangement deals with cylindrical geometries such as applications in vessels or in biomedical imaging (Bolomey, 1995; Bolomey and Hawley, 1990). In these configurations, the reconstruction process is often based on monofrequency and multi-incidence transmission data (Bolomey, 1995). Spatial resolution is severely limited by diffraction phenomena to a fraction of the wavelength. Contrast depends directly on the sensitivity of practical relevance, which varies with the frequency. Finally, time resolution is related to the number of data to be measured, which also depend on the dimension of the test object in terms of wavelength. 2.10. Ultrasonic Tomography. Ultrasonic tomography reconstitutes the morphology of an object by using as the contrasting property the refractive index or the ratio of the ultrasonic velocities in water and locally in the test object. A few studies are reported in the literature on the application of this type of tomography for multiphase systems. Measurement times in ultrasonic systems are intrinsically limited by the speed of sound, which is very low compared with electromagnetic waves. Li and Hoyle (1995) presented a brief review on different configurations of sensors and results on investigations of multiple active sensors to enhance the data acquisition performance of ultrasonic tomography systems. Xu and Xu (1995) developed an ultrasonic computerized tomography system which was used for monitoring the movement of a bubble (or multibubble group) rising from the bottom of a static liquid column. Some preliminary results were provided in their paper. Real-time acoustic planar imaging of dense slurries was carried out by Breden et al. (1995) using a unique liquid film as a detector. The longitudinal ultrasonic waves were converted to an optical diffraction grating at the interface of the liquid film. A complex flow of a dense silica/water slurry up to the maximum packing solid fraction was imaged through a depth of 2.5 cm. The attenuation was used for determining the concentration distribution. Using spatial cross-correlation, these authors attempted to determine the velocity of the moving textures in the heterogeneous flow field. An ultrasonic time-of-flight tomography technique was applied by Hauck (1991) for imaging the three-dimensional velocity field of a moving fluid. Their objective was to experi-

mentally verify a reconstruction algorithm proposed for mapping the velocity field of a two-phase flow system in a cylindrical tube with a 120 mm diameter. 3. Velocimetry Techniques Fluid or particulate flow velocities and patterns within complex multiphase mixtures are obtainable by velocimetric techniques. Such techniques are based on tracking the motion of radioactive or optically active individual particles or a clump of a small number of particles. Velocimetric techniques reported here use electromagnetic phenomena relevant to nuclear radiations, mere light, and laser or fluorescence light. The nuclear particle tracking techniques referred to are positron emission particle tracking (PEPT) and γ-ray emission radioactive particle tracking (RPT), while optical particle tracking techniques referred to are cinematography, laser Doppler anemometry (LDA), particle image velocimetry (PIV), and fluorescent particle image velocimetry (FPIV). An outline of most salient features of these techniques is given in Table 2 in terms of sensor type, space/time resolution, size of field of view, and some successful applications. 3.1. Positron Emission Particle Tracking. PEPT was developed at the University of Birmingham a decade ago (Bemrose et al., 1986, 1988; Field et al., 1991) as a variant of the positron emission tomography (see section 2.4). Positron-emitting tracers may be produced by direct irradiation of a particle in a cyclotron beam, by adsorbtion of the radioisotope into the tracer surface, or by fabrication of a particle out of radioactive material. The direct irradiation technique is limited to radiation-resistant materials and cannot be used for producing plastic tracer particles. As already mentioned in section 2.4, the detector systems for positron emitters are complex and expensive since their aim is to record and discriminate the genuine and coincident γ-rays that originate from the same annihilation and to locate their positions in the detector (Figure 5). Detecting both γ-rays from one annihilation defines a line or an “annihilation vector”. If instead of a clump of radioactive tracers, only a single tracer particle is present in the field of view, its position could, in principle, be determined as the point of intersection of two annihilation vectors. In practice, a number of physical factors such as positron range, γ-ray acolinearity, and the finite spatial resolution of the detectors introduce small errors in the determination of the annihilation vectors. Actually they are distributed around the emitting tracer and may not intersect (Stein et al., 1997). Other effects such as γ-ray scattering and “random coincidences” produce “corrupt” annihilation vectors that do not pass close to the position of the tracer (see, for example, Benton and Parker, 1997). Nevertheless, given a large enough set of annihilation vectors, the cluster of valid vectors can be distinguished from the corrupt ones and used to give an accurate estimate of particle location. The PEPT technique employs an iterative algorithm (Parker et al., 1993) to discard the corrupt annihilation vectors. For a moving particle, there is an optimum set size, large enough to give an adequate number of vectors to locate the particle accurately but not too large that the tracer has moved a significant distance during the sampling time period. At a speed of 1 m/s, a 10 MBq tracer particle would be located to within 5 mm in a parallelepipedic field of view of 600 mm × 300 mm × 300 mm, 50 times/s (Stein et al., 1997). At a speed much above 2 m/s, accurate

γ-ray emitting tracer particle/scintillation detectors

colored particle/video camera

laser source/particles or bubbles scattering light/light detector

laser sheet/light scattering particles or bulles/video camera

laser sheet/fluorescent seeds/video camera

γ-ray emission RPT

cinematography

laser Doppler anemometry (LDA)

particle image velocimetry (PVI)

fluorescent PIV

5

5 (0.2 mm)

5 (≈0.1 mm) inadequate for opaque and dense systems

4 (7 mm)

3 1 mm @ vp ) 0.01 m/s 5 mm @ vp ) 1 m/s vpmax ) 2 m/s 4 7 mm @ 100 Hz vpmax ) 3-4 m/s

spatial resolution

III

III instantaneous full-field information

III

II

III

I

temporal resolution

spouted bed fluidized bed 2D fluidized bed cohesive shale particle flow in pipe, threephase circulating bed gas and liquid velocity profile in bubble column velocity profiles in G/L and G/L/S fluidized bed, bubble columns velocity/concentration fields, dispersion coefficients in packed columns intersticial velocities in porous media

D ) 0.152 m V ) 0.002 m3 V ) 0.000 48 m3 V ) 5 × 10-3 m3

V ) 0.003 m3 D ) 0.102 m (bubble column) V ) (1.5-3.7) × 10-4 m3

V ) 0.009 m3

0.14 < D < 0.292 m

fluidized beds rotating drum ploughshare mixer fluidized beds circulating fluidized bed bubble columns

applications

V ) 2.6 × 10-3 m3 V ) 11 × 10-3 m3 V ) 0.005 m3 D ) 0.1 m D ) 0.082 m, H ) 7 m

size of the tested system

Northup et al., 1993

Rashidi et al., 1996a,b; Peurrung et al., 1995

Chen and Fan, 1992; Reese and Fan, 1994, 1997; Lin et al., 1996

Hassan et al., 1992

Dudukovic´ and Devanathan, 1993; Moslemian et al., 1992 Roy et al., 1994, 1996 Agarwal et al., 1997 (1 × 0.2 × 0.01 m3) Grbavcˇic´ et al., 1990 Evstropeva et al., 1972 Arastoopour and Shao, 1997

Stein et al., 1997 Seville et al., 1986, 1994, 1995 Parker et al., 1995 Bridgwater et al., 1993; Broadent et al., 1995 Larachi et al., 1996, 1997 Godfroy et al., 1996

ref for more details

a Scale of resolution: 1 ) very low; 2 ) low; 3 ) medium; 4 ) good; 5 ) high. Time consuming for data acquisition: I ) long, II ) medium, III ) fast. D: diameter. V: volume. v : particle p velocity.

positron emitting tracers/positron camera

sensor installed

positron emission particle tracking (PEPT)

general measurement basis

Table 2. Summary of Velocimetric Techniquesa



tracking is not possible because of the low sensitivity and limiting counting rate of the positron camera. Parker et al. (1995) used PEPT to study the mixing of granules in rotating drums filled to approximately 30% volume with 1.5 mm glass ballotini, one of which was activated to provide the tracer labeled with 18F. Dynamic characteristics such as the residence time and the transaxial mean velocity field of the solid for different rotational speeds were measured. Particle motion in “ploughshare” mixers was analyzed by Bridgwater et al. (1993) and Broadbent et al. (1995) with the aim of optimizing the geometry of the blades and the inlet feed injections of the solids in the mixer. PEPT experiments were performed using long grain rice (5 mm × 2 mm, density ) 1500 kg/m3) with a cylindrical silica tracer particle labeled with 18F (2 mm × 2 mm, density ) 2600 kg/m3). Also, the motion of a large cylindrical 22Na-labeled radioactive tracer among 100 µm sodium phosphate powder in a Lo¨dige-type mixer was studied by Broadbent et al. (1993). The influence of rotation frequency on the transaxial residence time and velocity field was studied. Applications of PEPT to gas fluidized beds were reported by Bemrose et al. (1986), Seville et al. (1994, 1995), and Stein et al. (1997). Solids motion experiments were carried out in a 150 mm i.d. cylindrical perspex tube filled with coarse sand (850-1000 mm) up to the height of 150 mm. A 2-mm 18F-labeled silica particle was used. These same authors also used PEPT for the determination of particle velocities during the discharge of a hopper and compared the velocity profiles obtained with the predictions of the kinematic theory (Tu¨zu¨n, 1987; Nedderman, 1992). Garncarek et al. (1994) used PEPT to measure an index of the degree of inhomogeneity in the tracer recirculation inside a rectangular fluidized bed. It was concluded that this index afforded a quantitative measure of how the fluidization parameters impact the particle kinematics. 3.2. Radioactive Particle Tracking. Similar to PEPT, RPT takes advantage of one single particle introduced in the flow with the ability to mimic the motion in the phase of interest. The majority of radionuclides used in RPT are β--emitters produced by neutron capture. These emitters often lead to the formation of an excited state in the daughter nuclei, which decay to the ground state by the emission of one or several characteristic γ-rays. The emitted γ-rays cross the material contained in the vessel and interact through photoelectric absorption and Compton scattering with an array of inorganic scintillation detectors (generally thallium-activated alkali halide scintillation crystals) positioned around the flow vessel. The amount of radiations falling on the detectors mainly depends on the point tracer-detector effective solid angle, radiation attenuation in the process equipment which is related to the depth at which the tracer lies inside the vessel with respect to the column wall, and the straight path length the γ-ray would cross in the detector crystal without being scattered. Figure 11 shows the main geometrical variables affecting the γ-ray counts registered by a NaI(Tl) scintillation crystal of an RPT system. Typical RPT radiolabeled tracers contain 24Na, 46Sc, 60Co, 99Mo, and 198Au (Kondukov et al., 1964; van Velzen et al., 1974; Lin et al., 1985; Godfroy et al., 1997) and are fabricated using techniques similar to PEPT fabrication techniques (section 3.1). Early application of RPT to determine “full-flow-field” velocities in multiphase reactors can be traced back to

Figure 11. Main components of RPT: NaI(Tl) detector and radioactive tracer inside a process equipment, and geometrical variables affecting the genuine photopeak γ-rays striking the detector. Adapted from Larachi et al., 1994.

the 1960s with the pioneering work of Kondukov et al. (1964). Six scintillation detectors fixed in pairs along three mutually perpendicular axes around a gas fluidized bed were used to measure the 3D trajectory of the tracer. Later, Borlai et al. (1967) devised a singledetector system placed beneath the distributor to monitor the vertical displacements of the tracer inside a gas fluidized bed. van Velzen et al. (1974) preferred the option of mounting a single detector above the fountain region of their gas spouted bed to record the tracer longitudinal motion. Using two vertically staggered and partially collimated scintillation detectors, Masson et al. (1981) outlined some qualitative features of the recirculation of a large, light radioactive plastic sphere in the bulk of a gas fluidized bed of small dense glass beads. Bascoul et al. (1993) used a similar detector assembly to study the particle movements in monolayer liquid fluidized beds. In all the above studies, accuracy and spatial resolution of measured tracer trajectories were poor and were unable to provide meaningful quantitative data. Lin et al. (1985) were the first who reported space-resolved three-dimensional trajectories of tracers in gas fluidized beds. Enhancement of RPT performances was attained by using many more detectors (12) and an accurate triangulation method. Later, refinement of RPT electronics and circuitry was pursued by Moslemian et al. (1989) and resulted in drastic reduction in tracer location errors. Two more RPT upgrades were constructed for further quantification of the solids movements in gas fluidized beds and for the tracking of liquid flow behavior in bubble columns (Devanathan et al., 1990; Moslemian et al., 1992; Yang et al., 1993). Finally, a fourth RPT facility was developed which yielded velocity information on the granular motion in three-phase and liquid fluidized beds and spouted and circulating fluidized beds (Larachi et al., 1995a-c, 1996; Roy et al., 1994; Godfroy et al., 1996). Three triangulation methods have been developed to determine the 3D coordinates of the radioactive tracer from the γ-ray counts recorded by the detectors. In the first method, each detector is represented by a hypothetical virtual center whose distance to the tracer is a polynomial of the recorded counts. The polynomial parameters are fitted by calibrating the RPT system by


positioning the tracer at many known locations within the field of view of interest in the vessel. To unambiguously locate the tracer coordinates, use is made of detectors’ redundancy and a weighted least-squares method (Lin et al., 1985; Moslemian et al., 1992). In the second method, an exact model that describes interactions of radiation with matter for the system geometry is developed to map by a Monte Carlo technique each detector’s response versus location in a finite grid within the vessel. Subsequently, the target location of the tracer is determined by a least-squares search for the grid point on the map which best matches the counts registered in the detectors (Larachi et al., 1994). The third method uses an enhanced algorithm where the searches of the second method are replaced by a direct and faster neural network model trained on the Monte Carlo generated count maps (Larachi et al., 1997; Godfroy et al., 1997). Dudukovic´ and co-workers (Devanathan et al., 1990; Dudukovic´ et al., 1991; Moslemian et al., 1992; Yang et al., 1992, 1993; Dudukovic´ and Devanathan, 1993; Kumar et al., 1994; Degaleesan and Dudukovic´, 1995) performed experiments on the liquid phase recirculation in liquid-batch bubble columns from the bubble to the churn turbulent flow regimes. The RPT data were mainly utilized to quantify the liquid recirculation patterns, the liquid velocity profiles, the Reynolds shear stress, and the eddy dispersion coefficients. An extensive experimental work was performed at University of Illinois (Chen et al., 1983; Lin et al., 1985; Sun et al., 1988; Moslemian, 1987; Moslemian et al., 1989, 1992) to study the solids movements in gas fluidized beds. Shallow and deep beds equipped with restricted and unrestricted porous plate distributors and internals such as rod bundles simulating heat transfer tubes or immersed spheres were studied. RPT data were utilized to identify the recirculation patterns and to measure the turbulent shear and normal stresses, Lagrangian autocorrelations and cross-correlations of fluctuating velocities, turbulent radial and axial velocities, integral time scales, radial and axial eddy dispersion coefficients, etc. Chaouki and co-workers (Roy et al., 1994, 1996; Roy, 1996; Godfroy et al., 1996; Larachi et al., 1995a-c; Cassanello et al., 1995, 1996) studied the solids movements in conical-base spouted beds, circulating fluidized beds, monolayer and binary liquid fluidized beds, and monolayer and binary three-phase fluidized beds. In a recent work by Limtrakul (1996), three-phase fluidized beds equipped with draft tubes and liquid fluidized beds were also studied. RPT measurements were utilized for the (i) quantification of the solids flow structure and solids motion mechanisms, (ii) evaluation and modeling of solids mixing and circulation times, and (iii) mapping of the averaged full-flow-field velocity vectors and turbulence fields. Figure 12 illustrates RPT capabilities to measure the mean Eulerian velocity fields of the solids flow (a) in a gas spouted bed (3 mm glass beads spouted in a 152 mm i.d. acrylic conical-base column under overdeveloped fountain conditions) and (b) in a three-phase fluidized bed (3 mm glass beads fluidized in a 100 mm i.d. three-phase column in the vorticalspiral flow regime). 3.3. Cinematography. Cinematography is an optical particle tracking technique based on the contrast of the color between the tracer and the phase to be studied. In liquid-solid systems, cinematographic measurements require a refractive index matching of the solid

particles and of the fluid. The difficulty in performing cinematographic studies in liquid systems is to find optically clear particles that have refractive indices close to that of the liquid. Distortion effects introduced by light diffraction limit the applicability of cinematography to flows in thin two-dimensional transparent vessels (Evstropeva et al., 1972; Gbavcˇic´ et al., 1990) or in the vicinity of the wall in three-dimensional geometries (Massimila and Westwater, 1960). Cylindrical geometries can be studied provided they are embedded in rectangular optical boxes (Kmiec´, 1978). Last but not the least, conventional cinematographic techniques require laborious manual effort and consequent tediousness that hamper collection of a meaningful amount of data statistically representative of the flow being studied. Cinematography has been used by Handley et al. (1966), Volpicelli et al. (1966), Richardson and colleagues (Carlos and Richardson, 1968; Latif and Richardson, 1972), Kmiec´ (1978), and Gbavcˇic´ et al. (1990) to measure the circulation patterns and the angular, axial, and radial velocities and turbulent velocities for solids in cylindrical and thin rectangular monolayer liquid fluidized beds. Mathur and Gishler (1955), Thorley et al. (1959), Lefroy and Davidson (1969), Lim and Mathur (1978), Suciu and Patrascu (1977, 1978), Kutluoglu et al. (1983), Rovero et al. (1985), and Day et al. (1987) used cinematography to measure solids particle velocities in the spout and the fountain regions of semicylindrical and cylindrical gas spouted beds. The annulus solids flow in spouted beds was studied by Sullivan et al. (1987). Massimila and Westwater (1960) measured bubble and solid particle velocities in the vicinity of the wall of a gas fluidized bed. Solids circulation flow rates in tapered gas fluidized beds were reported by Toyohara and Kawamura (1993). Evstropeva et al. (1972) determined velocity and turbulence intensities of the liquid and solid phases in a rectangular gas-liquid-solid fluidized bed; polystyrene particles and glass spheres have been used to seed the liquid and the solids. By using flow visualization, qualitative aspects of the dynamic macroscopic structure of the liquid and the solids in three-phase fluidized beds and two-dimensional bubble columns have been obtained by Fan and colleagues (Fan et al., 1992; Tzeng et al., 1993). The properties of ascending large bubbles in swarms of small bubbles in a three-phase fluidized bed have been measured by Mihayara and Fan (1992) using a video camera moving at the same speed as the large bubbles. Recently, Agarwal and colleagues (Lim and Agarwal, 1992, 1994; Lim et al., 1992, 1993; Hull and Agarwal, 1995; Agarwal et al., 1997) developed an automated noninvasive digital imaging facility for following the course of solids motion and mixing and for studying the bubble dynamics in 2D gas fluidized beds. Owing to a high level of automation of their analysis procedure, inherent drawbacks of previous cinematography techniques, such as poor statistics, have been dramatically alleviated. The specific applications that were tackled by these authors were as follows: (i) Measurement of bubbles characteristics (size, rise velocity, and angle) in beds with and without immersed obstacles. (ii) Measurement and interpretation of solid tracer concentration profiles in fluidized beds of uniform particles. The technique used colored solid tracers, and tracer concentration was inferred from the spread of a slug of tracers placed within identified regions. Tracer con-


Figure 12. Mean Eulerian velocity fields of the solids flow (a) in a gas spouted bed and (b) in a three-phase fluidized bed as measured by RPT. 3 mm glass beads were spouted in a 152 mm i.d. acrylic conical-base spouted bed under overdeveloped fountain conditions, and 3 mm glass beads fluidized in a 100 mm i.d. three-phase column in the vortical-spiral flow regime. Reprinted with permission from Larachi et al. (1995a). Copyright 1995 American Institute of Chemical Engineers.

centration was calculated after a frame grabber had digitized each pixel of the field of view into a gray scale. Calibration of the gray level intensity versus colored tracer concentration allowed pixel-to-pixel concentration rendition and also phase discrimination. (iii) Measurement and interpretation of jetsam concentration profiles in segregating binary fluidized beds. (iv) Study of the kinematics of a larger and lighter “active” particle in fluidized beds of smaller and heavier particles. As an example of application of this technique, the overall recirculation pattern (Figure 13) of the motion of the light large particle is given. Time averaging of the vertical and horizontal particle velocity components suggests that the particle moves downward near the edge of the bed and rises through the central region. The pattern looks similar to that measured by Lin et al. (1985) in 3D gas fluidized beds by RPT. The corresponding particle speed clearly shows (Figure 13) a higher magnitude for the particle velocity in the central region of the bed where the tracer interacts strongly with the bubble wakes. 3.4. Laser Doppler Anemometry. A change in the frequency of a wave motion due to the relative motion of the wave source and/or the wave receiver is referred to as the “Doppler effect”. To produce this effect, either

a moving source vs a stationary receiver or a moving receiver vs a stationary source can be used. Laser Doppler anemometry (LDA) uses the Doppler effect to conduct noninvasive measurements of velocity, particle size, and concentration, with a high degree of spatial resolution in multiphase flow systems. In LDA, beams of a laser source are scattered by particles, droplets, or bubbles suspended in the flowing fluid and detected by a light detector. On the one hand, the scattering particles act as moving receivers for the laser source, and on the other hand, they are moving sources for the light detector, causing Doppler shifts or interference fringes (Figure 14). The resulting frequency is related to the particle velocity (speed, direction), the laser wavelength, and the geometrical arrangement of the incident beam(s) and the light detector. Techniques such as the reference beam mode (Figure 14a), the dual beam mode (Figure 14b), the Rudd (1969) fringe technique (Figure 14c), and the large particles LDA method (Durst and Zare, 1975) are used for particle velocity and fluctuating velocity measurements. In the literature many investigations were conducted using LDA to measure velocities of dispersed or continuous phases, some of which are briefly reported hereafter. LDA measurements of the vertical velocity


Figure 14. Doppler models for the interpretation of LDA signals: reference beam mode (a), dual-beam mode (b), and fringe model (c). Adapted from Arastoopour and Shao, 1997.

Figure 13. Angular orientation, recirculation pattern (a), and magnitude of speed in various regions (b) of a large light active tracer among small heavy fluidized grains in a 2D gas fluidized bed. Tracer trajectory measured by the automated digital imaging system of Agarwal and colleagues. Reprinted with permission from Agarwal et al. (1997). Copyright 1997 Elsevier Science-NL.

component of the solids were reported by Boulos and Waldie (1986) in the dilute regions of a semicylindrical gas spouted bed, namely, the spout and the fountain regions. Nouri et al. (1987) used LDA to measure liquid and solid particle mean and root-mean-square axial velocities, both in the fully-developed turbulent vertical pipe flow and for the flow downstream of a baffle. Both bubbles and liquid root-mean-square velocities were measured by Sun and Faeth (1986a,b) and Loth and Faeth (1989), who applied LDA to the study of bubbly, vertical turbulent jets mixing with a still liquid. The three-component velocities and turbulent velocities of bubbles and liquid in the same geometry were also reported by Gross and Kuhlman (1992). LDA measurements of the longitudinal profiles of tangential, axial, and radial mean and root-mean-square velocities of the solids in a high-efficiency cyclone separator were conducted by Bankel and Olsson (1993). LDA studies were

attempted by Dybbs and Edwards (1984) to measure the local liquid velocity and flow regimes in porous media. Davuluri (1990) traced the gas flow of a gas transmission system with 0.94 mm latex seeds and measured with LDA the gas velocity for both steady and unsteady pressure buildup. Later, Arastoopour and Yang (1992) modified this LDA system to measure the velocity of cohesive shale particles in a vertical pipe. The fringe model was used to find the frequency, velocity, and fluctuating velocity of the particles. A similar procedure was used by Berkelmann and Reviz (1989a) to measure particle and gas velocities in the freeboard region of a bubbling fluidized bed. LDA particle, bubble, or droplet sizing are most frequently performed using techniques based on pedestal and visibility, phase Doppler (Saffman et al., 1986), shape discrimination (Arastoopour and Yang, 1992), and, for large particles, flight time (Shao and Arastoopour, 1995). The characteristic features (pedestal, visibility, envelope) of a typical LDA signal are illustrated in Figure 15. The pedestal of a Doppler signal can be obtained by passing the original signal (received by the detector) through a low-pass filter (Chu and Robinson, 1977; Durst and Eliasson, 1975), or it may be calculated by taking the arithmetic mean of the peaks and the valleys of the cycles in the Doppler burst (see Figure 15). The envelope of the Doppler burst is the difference between the peaks and the valleys divided by 2 (Farmer, 1972; Brayton, 1974; Adrian and Orloff, 1977). The visibility is the ratio of the envelope to the pedestal. Generally, large tracing particles give large pedestals and low visibilities, whereas small tracing particles give small pedestals and high visibilities. Such signatures were used by Pfeifer (1984) to discriminate the components of a two-phase flow. Because the distribution of light intensity in lasers is Gaussian, amplitude techniques based on pedestal or on visibility suffer from the so-called “trajectory ambiguity”, where a particle penetrating the measuring volume at different positions will produce Doppler signals with different pedestals and visibilities. To minimize this effect, some techniques are used to change the light intensity distribution from Gaussian to uniform (Hishida et al., 1984; Hadded et al., 1982; Berkelmann and Reviz, 1989b). In phase Doppler sizing methods, it is shown that the phase difference between Doppler signals


Figure 16. Combination of two LDA variants: threshold technique for phase identification and matched refractive index method for bubble velocity profile measurements in a circulating gasliquid-solid circulating fluidized bed at different solids loadings. To ensure optically identical liquid and solids, the liquid-solid slurry was made up of a mixture of organosilicone oils (Dybbs and Edwards, 1984) and poly(methyl methacrylate) (PMMA) cylindrical pellets. Reprinted with permission from Arastoopour and Shao (1997). Copyright 1997 Elsevier Science-NL.

Figure 15. Extraction from Doppler bursts of pedestal, envelope, and visibility for particle, droplet, and bubble sizing by LDA. Pedestal (respectively envelope) is obtained by low (respectively high) pass filtering of the Doppler burst, whose ratio gives the visibility. Reprinted with permission from Arastoopour and Shao (1997). Copyright 1997 Elsevier Science-NL.

observed by two detectors is related linearly to the particle size. Bauckhage and colleaques (1984, 1985) and Saffman et al. (1986) used this method for simultaneous measurements of droplet size and velocity in nozzle spray flows and in gasoline and diesel fuel systems. Hardalupas et al. (1988), Saffman (1988), Jackson and Samuelsen (1988), and Martin et al. (1989) identified the limitations of the phase Doppler technique and made some improvements. Tadrist and Cattieuw (1993) used the same technique to analyze the size and velocity of solid particles in the freeboard of circulating fluidized beds. Another area of LDA measurements in multiphase flows concerned the development of threshold techniques able to discriminate and measure simultaneously velocities within each phase. Pedestal and visibility or a combination of the two were generally employed to discriminate Doppler signals issued from each phase. As large seeds scatter more light and give large pedestals, while small tracing seeds scatter less light and give small pedestals, a pedestal thresholding of Doppler bursts can give access to velocities of both phases’ seeds (Arastoopour and Shao, 1997). LDA also permits accessibility to particle, droplet, or bubble concentration in the multiphase flow. There are basically two LDA approaches: (a) The first uses the data rate of the signal processor in the LDA system (McDonell and Samuelsen, 1989; Maeda et al., 1989; Nouri et al., 1988). This type of technique is best suited for dilute two-phase flow systems (b) The second uses the time ratio between the dispersed phase and total

sampling time to find the volume fraction of the dispersed phase (Sekoguchi et al., 1982; Yang, 1991). These two techniques are thoroughly discussed by Arastoopour and Shao (1997). For multiphase flows with high solids loadings, a modified LDA technique called “matched refractive index method” (Zisselmar and Molerus, 1979) can be used to measure velocities and turbulence parameters for individual phases such as in liquid-solid slurries (Abbas and Crowe, 1987) or in flow through porous media (Northrup et al., 1991). The threshold technique and the matched refractive index method were used by Shao and Arastoopour (1994) to measure bubble flow parameters in a gas-liquid-solid circulating fluidized bed. A mixture of organosilicone oils was used as the refractive index matching fluid, and PMMA cylindrical pellets were used as the refractive index matching solids. In this way, a clear optical path for the laser beam was guaranteed for the measurement of the bubble flow parameters at different solids loadings. It was shown that, as the solids loading increases, the bubble velocity distribution flattens and the bubbles slow down (Figure 16), due to the intensification of particle-particle and particle-bubble interactions at higher solids loading. 3.5. Particle Image Velocimetry. Particle velocimetry is a flow visualization technique which provides instantaneous full-field information of a multiphase system for each of its components as it flows through a planar laser-sensed region. The flow is seeded with neutral density light scattering particles or with rising dispersed gas bubbles, which scatter the incident sheet of laser, while refractive index matching is necessary to eliminate scattering at the multiphase interfaces and to render the entire system transparent. Any particle velocimetry system consists of laser sheeting, image recording, and image processing. A typical particle velocimetry arrangement is shown in Figure 17. Laser sheets are formed by expanding the circular Gaussian laser emission through a system of mirrors and lenses. Image recording is ensured by CCD video cameras placed perpendicular to the laser propagation beam. Image processing allows evaluation of properties of interest such as velocity or holdup distributions.


Figure 17. Typical particle velocimetry arrangement showing main constituents such as the laser sheeting and the image recording system. Reprinted with permission from Rashidi (1997). Copyright 1997 Elsevier Science-NL. Table 3. Principles of the Three Approaches Used in Particle Velocimetry Techniques [Reprinted with Permission from Rashidi (1997). Copyright 1997 Elsevier Science-NL]

Particle image velocimetry (PIV), particle streak velocimetry (PSV), and particle tracking velocimetry (PTV) are the three variants of the operation of particle velocimetry. These variants differ only in the manner in which images are recorded, and the way in which velocities are computed. In PIV, the laser is pulsed many times and instantaneous images of all the seeds in the measurement plane are obtained through double or multiple exposures on the same piece of film. The

positions of the moving seeds are recorded as pairs or multiple spots, and local instantaneous velocity is obtained by dividing the spatial separation of the spots by the time between light pulses. In PSV, the flow is continuously illuminated for a fixed length of time. As the particles pass by during the exposure, they produce streaks, and particle velocities are calculated by dividing streak lengths by the illumination time (Rashidi and Banerjee, 1988, 1990). In PTV individual particles


rather than groups of particles are tracked. PTV employs continuous illumination and generates a series of single-exposure images rather than one composite multiple-exposure image in contrast with PIV or PSV images. Each seed’s position history is then identified, and particle velocities are calculated frame to frame. The imaging methods employed in PIV, PSV, and PTV are compared in Table 3. In previous PIV image processing techniques, velocity information was obtained by relying upon optomechanical processing techniques of photographs which involved Younge’s fringe (Northrup et al., 1991, 1993) combined with the Fraunhofer diffraction concept (Chen and Fan, 1992) or fast Fourier transform (Adrian, 1986). In recent PIV image processing, techniques of cross-correlation involved interrogation of two sequentially captured digital image frames in terms of smaller subgrid regions in which the average displacement and, hence, average velocity of the seeds in the subgrid region is determined (Hassan et al., 1992). Okamoto et al. (1995) provided a brief review of studies conducted with modified cross-correlation techniques, in addition to presenting a new algorithm for calculating displacements between particle clusters which are supposed to be connected by invisible elastic springs. Direct digitization methods in which separate flow images are captured in sequence represent another approach to calculate the velocity field. Procedures to locate the digitized particle images and to compute the displacements between image pairs to determine the velocity are logical computer operations (Wernet and Edwards, 1990) for which the processing time required is by far less than in the optomechanical and cross-correlation interrogation methods. Conversely, less accuracy in the velocity measurement is achieved due to the loss of spatial resolutionsno interrogation spots (Reese and Fan, 1994). Besides the velocity information, PIV allows measurements of local phase holdups in two- or threephase systems. Local holdups are calculated using pixel connectivity algorithms to identify for each spot the boundaries, the corresponding area and centroid, and the average gray level (Chen and Fan, 1992). Based on knowledge of the size distribution of each phase, the spots are allocated to the phase to which they belong. Uemura et al. (1990) were among the first who applied PIV to gas-liquid systems. Local liquid velocity in a gas injection mixing flow was calculated with a binary cross-correlation method (Uemura and Yamamoto, 1989; Yamamoto et al., 1988). Difficulties to discriminate liquid seeds from bubbles precluded liquid velocity measurement in the vicinity of the bubbles. Later, using the binary cross-correlation method, Yamamoto et al. (1991) succeeded in measuring by PTV gas and liquid velocities in a cylindrical bath with a bottomblowing bubbling jet. For trustful traceability of the liquid, 0.9 mm polystyrene seeds were introduced in a saline water solution. Fluorescent light and air mixed with smoke allowed clear visualization of the bubbles. Hassan et al. (1992) applied PIV to the simultaneous two-phase and two-dimensional velocity measurement in a rectangular bubble column. The heavy mineral oil was seeded with 70 mm plastic spheres, and the gas phase, by its constituent millimeter-sized air bubbles introduced by a single bubble injector. Instantaneous and average fluid properties such as velocities, streamline, and vorticity fields were obtained. Liu and Adrian (1993) applied PIV to a gas-liquid system where 0.10.5 mm sized bubbles were injected, through a single

Figure 18. (a) PIV image of air-water flow in a 2D bubble column, 80 cm above the gas injectors. (b) Corresponding liquid velocity vectors shown together with Pliolite seeding particles. (c) Corresponding gas velocity vectors and bubble images. The bubbles move upward in clusters, no coalescence. Field of view: 15.3 cm horizontal × 11 cm vertical, rectangular bubble column, 15.2 cm width × 160 cm height × 1.27 cm depth, superficial gas velocity of 1 cm/s corresponding to transition between dispersed and coalesced bubble flow regime. Reprinted with permission from Reese and Fan (1997). Copyright 1997 Elsevier Science-NL.


Figure 19. Representative data slice of velocity and concentration fields in the central region of a porous medium using FPIV. The data include (i) demarcation of the pore and packing geometry, (ii) the 2D velocity components of the moving fluorescent seeds in the liquid, (iii) the normalized solute concentration within the pore spaces, 24 min after dye introduction at the column bottom. 6 mm fluorescent latex microsphere seeds; packing made of 3.1 mm PMMA spherical beads. Reprinted with permission from Rashidi (1997). Copyright 1997 Elsevier Science-NL.

injector, into a water jet seeded with 4 mm diameter polystyrene particles. Using PIV, Horio et al. (1993, 1994) studied the three-dimensional solids cluster structures in a 0.2 m i.d. circulating fluidized bed. A set of three laser sheets intersecting at right angles was applied to observe the three-dimensional behavior of the suspension and the cluster shape and size. Extension of PIV to three-phase systems, such as gas-liquid-solid fluidization, was reported by Chen and Fan (1992). The velocity fields of the gas, liquid, and solids phases, the bubble size distribution, the gas and liquid holdups, and slip velocities were simultaneously measured in the freeboard region of a three-phase fluidized bed. Other studies by Reese et al. (1995), Chen et al. (1994b), Reese and Fan (1994, 1997), and Lin et al. (1996) focused on the characterization of each phase in more details in 2D and 3D gas-liquid and gas-liquid-solid systems. Gas-liquid flow in a 2D bubble column seeded by gas bubbles and 350 mm Pliolite particles is illustrated in Figure 18a-c (Reese and Fan, 1997). Figure 18a is a PIV image showing air-water flow in an area 15.3 cm horizontal × 11 cm vertical of a rectangular bubble column, 80 cm above the gas distributor. From this image, it can be seen that the liquid seeds (Pliolite grains: 0.23-1 mm) can be easily differentiated from the gas bubbles (2.2-4.4 mm) to allow quantitative utilization of the image. Phase discrimination is not always easy, and generally seed ambiguity is eliminated by sound selection of the tracing particles, knowing the size distribution of each phase in the multiphase system (Reese and Fan, 1997). The liquid (Figure 18b) and gas (Figure 18c) instantaneous velocity fields can be computed by differentiating the displacements of the liquid seeds and the bubbles from two successive images as Figure 18a. In conventional particle velocimetry, images are created from light scattered by the seeds themselves. This light may be easily overwhelmed by residual scattering from interface inhomogeneities and may render particle

velocimetry information useless. Use of fluorescent seeds alleviates this problem by allowing spectral discrimination against scattered laser light. Seed particles are excited at the laser wavelength and emit at a longer wavelength. Thus, when appropriate narrowband filters centered at the particle emission frequency are adjusted on the camera, the laser-scattered radiation is rejected and the seeds are better resolved. Northrup et al. (1991, 1993), Rashidi et al. (1996a,b, 1997), and Peurrung et al. (1995) from Lawrence Livermore National Laboratory at University of California devised a fluorescence particle image velocimetry technique (FPIV) to measure interstitial velocity fields and transport of a liquid in a porous medium. Packed porous columns with a refractive index-matched fluid seeded with fluorescent latex tracer particles were used to obtain velocity measurements. A neutrally buoyant organic dye was also used for interstitial chemical concentration measurements. Various geometric, flow, and transport quantities have been evaluated at the pore scale such as velocity and concentration fields, dispersive fluxes, dispersion coefficients, and pore geometry (Rashidi, 1997). Concentration measurements using fluorescent dye allowed the pore geometry on each slice to be measured. An overlaid vertical slice of velocity and normalized concentration data obtained in the central region of the porous bed imaged by FPIV is shown in Figure 19, 24 min after dye introduction in the column bottom. 4. Summary This review paper is an outgrowth of the latest advances accomplished in the field of nondestructive evaluation of structural and dynamic characteristics of multiphase systems and reactors using various tomographic and velocimetric techniques. The detailed structural and dynamic information obtained in multiphase problems using such techniques provides valu-


able tools for progress in the comprehension and the modeling of the transport phenomena occurring in these systems. For each technique, the theoretical backgrounds and applications as well as the limitations/ advantages were tentatively presented. The techniques were categorized into two broad categories: (i) Nuclear and nonnuclear tomographies and radiographies which include γ-ray and X-ray transmission tomography, positron emission tomography, X-ray diffraction microtomography, X-ray and neutron transmission radiography, nuclear magnetic resonance imaging, electrical capacitance tomography, optical tomography, ultrasonic tomography, microwave tomography. (ii) Velocimetric techniques which include positron emission particle tracking, γ-ray emission particle tracking, cinematography, laser Doppler anemometry, particle image velocimetry, and fluorescent particle image velocimetry. Acknowledgment Financial support from the Natural Sciences and Engineering Research Council and the Fonds pour la Formation de Chercheurs et l’Aide a` la Recherche du Que´bec is gratefully acknowledged. M.P.D. is grateful for the support received from DOE Contract DE FC 95 22 PC 95051 and Grant DE FG 95 22 PC 95212 that made his contribution to this study possible. J.C. and F.L. acknowledge Dr. L. Belfares for her help during the preparation of this manuscript. Nomenclature f ) space-dependent contrasting property, e.g., attenuation coefficient distribution l ) distance between origin and beam direction P ) ray-sum of the space-dependent contrasting property r ) vessel radius Umf ) minimum fluidization velocity x ) Cartesian coordinate y ) Cartesian coordinate q ) orientation of ray-sum with respect to x-axis Acronyms ART ) algebraic reconstruction technique CT ) computer tomography CAT ) computer-assisted tomography CIT ) capacitance imaging tomography EIT ) electrical impedance tomography FCC ) fluid catalytic cracking FPIV ) fluorescent particle image velocimetry LDA ) laser Doppler anemometry NID ) neutron imaging detector NMRI ) nuclear magnetic resonance imaging PET ) positron emission tomography PEPT ) positron emission particle tracking PIV ) particle image velocimetry PSV ) particle streak velocimetry PTV ) particle tracking velocimetry RPT ) radioactive particle tracking SFBP ) summation-filtered back-projection XDT ) X-ray diffraction tomography

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Received for review March 12, 1997 Revised manuscript received July 14, 1997 Accepted July 22, 1997X IE970210T

X Abstract published in Advance ACS Abstracts, October 1, 1997.

Noninvasive Tomographic and Velocimetric Monitoring of Multiphase

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