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INSTITUTE OFPHYSICSPUBLISHING MEASUREMENTSCIENCE ANDTECHNOLOGY
Meas. Sci. Technol.12(2001) 11091119 www.iop.org/Journals/mt PII: S0957-0233(01)22190-0
Application of electrical capacitancetomography for measurement ofgassolids flow characteristics in apneumatic conveying system
Artur J Jaworski1 and Tomasz Dyakowski2,3
1 School of Engineering, University of Manchester, Oxford Road, Manchester M13 9PL, UK2 Department of Chemical Engineering, UMIST, PO Box 88, Manchester M60 1QD, UK
E-mail: [email protected] and [email protected]
Received 22 February 2001, in final form 29 May 2001, accepted forpublication 13 June 2001
AbstractTransient three-dimensional multiphase flows are a characteristic feature ofmany industrial processes. The experimental observations andmeasurements of such flows are extremely difficult, and industrial processtomography has been developed over the last decade into a reliable methodfor investigating these complex phenomena. Gassolids flows, such as thosein pneumatic conveying systems, exhibit many interesting features and thesecan be successfully investigated by using electrical capacitance tomography.This paper discusses the current state of the art in this field, advantages andlimitations of the technique and required future developments. Variouslevels of visualization and processing of tomographic data obtained in apilot-plant-scale pneumatic conveying system are presented. A case studyoutlining the principles of measuring the mass flow rate of solids in avertical channel is shown.
Keywords:solids concentration, solids velocity, mass flow rate, tomography,correlation, pneumatic transport
1. Introduction
Transient three-dimensional multiphase flows are a character-istic feature of many industrial processes. The experimentalobservations and measurements of such flows are extremelydifficult, and industrial tomography has been developed overthe last decade into a reliable method for investigating thesecomplex phenomena [1, 2]. Various levels of visualization ofdata can be extracted from tomograms. Images can charac-terize the behaviour of the flow at a single level (or plane) asit varies with time and therefore allow distinguishing betweendifferent flow patterns. Individual images can reveal impor-tant cross-sectional information such as the concentration ofsolids in gassolids systems. A set of successive images, for
known velocities of solids, can be transformed into the three-dimensional distribution of solids along the direction of flowto provide additional body shape type information similar to
3 Author to whom correspondence should be addressed.
that obtained from conventional photography. However, there
are important differences between data obtained from the two
techniques and these are discussed in more detail in section 3.
On the micro-scale, the motion of a particle is governed
by various types of particleparticle and particlefluid
interactions. On the macro-scale, these are responsible for
the appearance of various types of macro-structures in the
flow with characteristic sizes much larger than the diameter
of the particles. Pneumatic conveying is usually classified into
two categories: dilute (or lean) and dense phase [3]. In
dilute-phase conveying the particles are usually transported
in the form of a suspension with the concentrations of solids
typically below 10%. On the other hand, dense-phase
transport is usually understood as the conveying of particles
along a pipe, which is filled with particles at one or more crosssections [4]. Flow patterns in a dense pneumatic conveying
system exhibit many interesting features and analogies with
gasliquid flows. Typically, materials conveyed in a dense-
0957-0233/01/081109+11$30.00 2001 IOP Publishing Ltd Printed in the UK 1109
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A J Jaworski and T Dyakowski
Filter unit &
air reservoir
Removable
testsection
100 L
PT
Cooler
ELECTRONIC
CONTROLLER
4-20mA
AIR
COMPRESSOR
8 bar g
oil free air
Blowerunit
100 L
Sonic
nozzle
Approx. 7 m
Rotary
feeder
Upper tank
Lower tank
Approx.
3m
PNEUMATIC
CONVEYING
FLOW LOOP
AIR SUPPLY
Removable
test section
Removable
test section
Load
cell
Load
cell
PG PS
PS - Pressure overload switch
PG - Pressure gauge
PT - Pressure transducer
Bleed
valve
4-20mA
Figure 1.A schematic diagram of the UMIST dense-phase pneumatic conveying rig.
phase system are not very cohesive and exhibit a permeability
and de-aeration rate that are either both low (for a moving bed)
or both high (for slug or plug flow). High permeability and
de-aeration rates characterize polyamide chips, so the modes
of a dense transport of these chips are discussed in this paper.
When the gas velocity is reduced below the saltation
velocity, a settled layer of solids is formed at the bottom of
a horizontal pipe. The transport of solids occurs through the
propagation of flow instabilities referred to as slugs[57].
These pick up the solids from the settled layer and convey
them along the pipe for some distance. The slug propagation
velocity ishigherthan theaxialvelocityof solidsand thesolids,
after being mixed within the slug body, are dropped off to form
a settled layer behind the slug. The slope at the back of the
slug is usually steeper than the slug front.
In vertical pipelines, the behaviour of the solid particles is
somewhat different. Theflow instabilities responsible for thenet transport of solids take theformof nearly axis-symmetrical
discreteplugs, which move upwards along the pipe [8]. The
consecutive plugs are separated from one another by an air
gap. Flowvisualizationsreveal that the solidparticlescan raindownfrom the back of the preceding onto the following plug.
In both cases, the horizontal and thevertical transport, the flow
instabilities described exhibit a quasi-periodic behaviour with
a frequency typically in the region between fractions of a hertz
and a few hertz.
Granular materials conveyed in the pipelines are usually
relatively dry and can be assumed electrically non-conductive.
Moreover, their bulk dielectric constant is rather low, typically
between 1.5 and 5. Therefore, electrical capacitancetomography (ECT) seems to be ideally suited for investigating
this type offlow [913]. Typically, ECT systems operate at a
frame capture rate of up to 200 frames s1, which allowsone to
reproduce theflow patterns observed with sufficient accuracy,
the additional advantage, of course, being the possibility of
reconstructing the internal structure of the flow from the
distribution of dielectric permittivity in the cross-sectional
image.
The main problem with measuring multiphase flows is
associated with the fact that both the phases distribution
and the velocity profile vary widely both in the temporaland the spatial sense. The development of the so-called
twin-plane tomographic instruments, potentially offers an
excellent opportunity to develop techniques for measuring
the velocity field by cross-correlating, on a pixel-by-pixel
basis, the time series of tomographic images obtained. The
concept of such systems was describedfor example in [1,2, 14]
and was successfully implemented in the area of hydraulic
conveying by Loh et al [15]. However, attempts to apply
the cross-correlation for measuring the mass flow rates ofsolids in pneumatic conveying systems achieved only limited
success [16]. A more detailed discussion on the suitability
of twin-plane tomography and correlation techniques for
measuring massflow rates is given in [17].The objective of this work was to investigate the
complexities of flow morphology in dense pneumatic
conveying systems. This was done by using two
complementary techniques: a high-speed video camera (up
to 500 frames s1) and a twin-plane ECT system (with a
capture rate of 100 frames s1). A comparison between
the flow visualizations obtained using these two techniques
is provided. The prospects of using twin-plane ECT for
measuringmassflowratesof solids, arediscussed, a theoreticalanalysis is outlined and preliminary results concerned with
flow measurement within a vertical pipe to obtain estimates of
the mass flow rate of solidsare presented. Theresults obtained
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A J Jaworski and T Dyakowski
Figure 3.The design of the twin-plane ECT sensor.
Figure 4.High-speed camera visualization of the slugflow in the horizontal pipe.
3. Results and discussion
Qualitative and quantitative data were obtained during the
experimentalprogramme. The firstconfirmed theapplicability
of the ECT system for investigations of transient and three-
dimensional gassolids flows. Here we are presenting the
gassolids flow morphology in a vertical and a horizontal
channel. The latter is congruent with the earlier publications,
e.g. [1012]. Theflow structure in a vertical channel exhibits
many interesting features, in particular a bi-directional flow
sequence. Quantitative analysis was focused on applying the
ECT system for measurement of the mass-flow of solids. The
theoretical analysis is presented and simplifications made in
the current study are introduced and discussed in some detail.
3.1. Characterization of flow patterns
The flowpatterns in thepneumaticconveyingwereinvestigated
both for horizontal and for vertical sections (see figure 1).
Figure 4 shows a series of six photographs illustrating the
passage of two consecutive slugs through the viewing section
in the horizontal pipe. A few interesting features are worth
mentioning. Firstly, the structure of the two slugs is slightly
different. Thefirst slugfills the pipe completely and exhibits
rather clearly defined boundaries of the front and the tail.
This is not true for the following slug. A thin layer of gas
and suspended particles can be seen in the upper part of thepipe, above the main body of the slug. The front of the
slug appears less clearly defined and it looks as though the
material at the front of the slug is trying to catch up with the
preceding slug. Secondly, referring to the front of the first
slug, it can be seen that its slope can vary rather fast (compare
photographs (a) and (b)). Thirdly, the thickness of the settled
layer separating the two slugs evolves and becomes thinner
from one photograph to another (compare photographs (b)
(e)). The latter is accompanied by an increase in the distance
separating the two slugs. Finally, it is also worth noting that
clusters of particles are present immediately before the fronts
of both slugs described.
Figure 5 shows a time series of cross-sectional
tomographic images corresponding to the slugflow presented
in figure 4. The first seven images show the transition between
a half-filled pipe and a fullyfilled pipe that corresponds to the
passage of the slug front. Similarly, the last four images showthe passage of the slugs tail through the measurement plane.
All images in between correspond to the slug passing through
the sensing plane (images betweent = 0.12 s andt = 0.29 s
are omitted to save space).
Applying a twin-plane system allows reproducing the
shape of the slugs as presented in figure 6. Here, the pixels
lying along a vertical line passing through the centre are
selected from each frame. These are combined to give a
longitudinal cross section of the slug. Figure 6 illustrates
the problems encountered by the ECT measurement. These
relate to the limited spatial resolution of the images in the
cross-sectional plane, averaging of the concentration of solidsalong thefinite length of the electrodes (in our case 3 cm) and
smearing of the sharp boundaries between the phases. This is
why figure 6 showsshadesof grey, instead of sharp boundaries.
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Gassolidsflow characteristics
Figure 5.A series of ECT images corresponding to the passage of a slug through the sensor. (Images betweent = 0.12 s andt = 0.29 s areomitted since they show fullyfilled pipe (black).)
Figure 6.The longitudinal cross section of the slugflow obtainedfrom the tomograms. The data were obtained by extracting pixelsfrom the vertical axis of each tomogram and combining them as atime series.
Figure 7.The shape of slugs obtained by thresholding the data fromfigure 6.
Of course, one of the methods used to extract sharp
boundaries between the phases from the ECT images is the so-
calledthresholding. The areas of the normalized dielectric
permittivity above the threshold (e.g. 0.5) are represented inblack, while the areas of the permittivity below threshold are
shown in white. An illustration of this method is shown in
figure 7. It can be seen that the shape of the slug body can
(a) (b)
Figure 8.Still photographs showing plugs travelling upwards (a)and material dropping downwards in between the trains of plugs (b).
be determined, of particular interest being the slopes of the
front and tail of the slug. These exhibit many similarities
to the photographs shown in figure 4. Of course, the spatial
resolution does notallowimaging of thepresence of individual
particles or particle clusters as can be seen on a high-speed
video recording.
In the experiments conducted in the vertical pipe, theflow patterns observed were generally in agreement with those
described in the literature. The conveying of solids typically
consisted of two distinctive phases. Atrainof a few plugs,
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Figure 9.The cross-sectional distribution of dielectric permittivity for upward plugflow (a) and downward return of material (b).
eachapproximately 1020 cm long, appeared on average every
34 s. It could be seen that some particlesrain downfrom
the preceding onto the following plug. At the end of each
passage some granular material (most probably from the tail
of the train) dropped downwards under the gravity. This was
collected at the bottom of the vertical section and was picked
up by the next train of plugs. Figure 8 shows four photographs
taken from the high-speed video, which show a typical flow
pattern observed.
Again, the use of ECT allows looking into the internalstructure of the plugs, which would not be possible using
photographic techniques. Figure 9 givesexamplesof thecross-
sectional distribution of the normalized dielectric permittivity
due to material present in the sensing area. Figure 9(a)
corresponds to the plugs travelling upwards. Images 26 and
1012 correspond to the plugs present in the sensing area,
whereas images 1, 7, 8 and 9 show the spaces between the
two consecutive plugs. Figure 9(b) shows the permittivity
distribution for material dropping down between two trains
of plugs.
Figure 10 shows the axial cross section of the flow
reconstructed from the ECT measurements. It is worth noting
that the ECT reconstruction reflects the changes in porosityof the material within the plugs. It can be inferred that the
density increases towards the pipe wall, which most probably
corresponds to an increase in inter-particle stresses. The
centre of the plug, on the other hand, seems more porous,
probably due to the passage of the gas through the centre of
the plug. For the downward movement of the material the
highest concentration is usually close to the wall; however,
the location of the region of the highest concentration moves
around the circumference of the pipe.
Of course, on the fundamental level, the information
obtained from the flow visualizations (figures 4 and 8) is
differentfromthatobtainedby combiningtomographic images
such as those shown in figures 6, 7 and 10. Whereas thephotographs show thespatial information at a given instant, the
tomographic results represent the temporal changes at a given
spatial location (that of thesensor). The twoapproaches would
be equivalent only if the flow structures were frozenwhile
moving along the pipe. Although this is not necessarily true in
thepneumatic conveying system, it isworthnoting theapparent
similarities between thephotographs andthe tomographic data.
Of course, using a twin-plane tomographic system allows
obtaining the time delay between the appearance of slugs
or plugs in respective planes and therefore the propagation
velocity of slugs. The length of the slug can then be calculated
as the time that the slug was present in one of the planes
multiplied by the propagation velocity.It is apparent that, despite difficulties encountered in
the ECT measurements (reconstruction algorithm errors,
averaging along the electrodes), the technique can provide
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Gassolidsflow characteristics
(a) (b)
Figure 10.Temporal changes in the permittivity distribution acrossthe diameter of the pipe: (a) upwards travelling plugs and
(b) material falling downwards.
unique information about the structure and the evolution of
three-dimensional and unsteady gassolids flow. In order to
provide more reliable information about theflow phenomena,
some more work is required in order to relate the measured
dielectric permittivity distribution to the concentration of
solids. This is of vital importance from the viewpoint both of
theoretical modelling and of the validation of computational
fluid dynamics (CFD) codes.
3.2. Flow measurements
As discussed in the introduction, one of the attractive ideas for
multiphaseflow measurement that still has to be investigated
is the use of twin-plane tomography systems and cross-
correlation techniques. The concept itself is not novel. In
fluid mechanics cross-correlation of pressure and velocity
fluctuations has been for many decades a standard technique
for investigating the flows within a boundary layer [21] and
for tracking the movement of coherent structures shed by
aerodynamic bodies [22]. In the area of multiphase flows
the velocity field can be measured by cross-correlating the
time-varying signal arising from one phase being dispersed
in another (e.g. gas bubbles in liquid) [23]. Some obvious (butoften tacit) assumptions made while measuring the velocity
field by cross-correlation techniques are that
(i) the sensorssize is small relative to their separation;
(ii) there are measurable disturbances in the flowfield being
investigated and
(iii) the velocityfield (convection velocity) can be associated
with the propagation velocity of these disturbances
It can easily be seen that the first assumption is not strictly
satisfied by the ECT sensor. The sensing electrodes have a
finite length (in our case 3 cm), which is often comparable tothe separation (in our case 13 cm, the centre-to-centre distance
between theplanes). In simpleterms, it couldbearguedthat the
ECT system detects the disturbance for thefirst time, while it
crosses the upstream end of plane 1. Similarly, it can detect its
presence for the last time when it crosses the downstream end
of plane 2. In these circumstances, it is not clear whether the
centre-to-centredistance is themostappropriate for calculating
propagation velocities of the disturbances. In general, this
problem is caused by the spatial averaging taking place along
the electrodes.
The detection of disturbances caused by gassolids
interaction is not straightforward. Two facts need to beremembered: firstly, that an ECT system has a spatial
resolution of about 10% of the pipe diameter; and secondly,
that the electrodes are rather long. Consequently, the sensing
volume within which detection takes place is large and there is
no way of detecting individual particles. Instead, an average
concentration of solids in a rather large control volume is
measured. This, of course, creates problems from the point
of view of correlation analysis. For example, it is not possible
to detect whether the settled layer at the bottom of the pipe is in
motion or stationary (tomography provides a constant signal).
Similarly, situations in which the system is nearly blocked (i.e.
the pipe isfilled completely with stationary, or slowly moving,
material) can be ambiguous for the purposes of correlation.Finally, the problems are compounded by the fact that
the gassolids structures do exhibit wavelike behaviour. The
classic example here is the propagation of slugs in the
horizontal pipeline. Here the velocity of material (plastic
pellets) cannot be directly linked to the propagation velocity
of the disturbances in theflow (slugs). Of course, it is possible
to measure the propagation velocity of the slugs (or, in other
words, how quickly the wavefronts are moving), but, strictly,
no inferences about theassociatedmass transportcan bedrawn.
The above mentioned issues should be borne in mind
while attempting to use the cross-correlation techniques for
measuring the mass flowrate of solids in pneumaticconveyors.It is not the intention of this paper to prove that such
measurements are not possible. It is simply to draw attention
to the fact that very often the results may be flow regime, or
indeedflow rig dependent and some serious caveats regarding
the underlying theoretical approach used by cross-correlation
techniques should be mentioned.
For the above mentioned reasons the experimental work
conducted at UMIST focused in thefirst instance on theflow
in the vertical section of the flow rig. It was thought that
theflow patterns could be assumed to a good approximation
axis-symmetrical and would be free of situations in which
a stationary settled layer of solids affects the correlation
analysis. The experiments focused on relatively lowconveyingvelocities (gas velocities between 1.5 and 2.0 m s 1 for an
empty pipe) and a feed of solids generally between 700 and
900 kg h1. This was a compromise between obtaining
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A J Jaworski and T Dyakowski
well-defined plug flow and ensuring that the ECT system
can still follow the passage of the flow structures with
the 100 frames s1 capture rate. The experimental results
presented are preliminary in character and illustrate the level
of analysis conducted.
Theinstantaneousmassflowratethrough thecross-section
of a pipe can be written as
m(t) =
A
(x,y,t)v(x,y,t) dxdy (1)
where (x,y,t) stands for the instantaneous density at
point (x,y) of cross section A and v(x,y,t) denotes
an instantaneous velocity at point (x,y) in the direction
perpendicular to the cross-sectional plane. It is then possible
to define averages of both density and velocity over the cross-
section of the pipe Aas follows:
A(t) = A
(x,y,t) dxdy (2)
vA(t) =
A
v(x,y,t) dxdy. (3)
Of course, the instantaneous density and velocity can now be
expressed in the following way:
(x,y,t) = A(t)+ (x,y,t) (4)
v(x,y,t) = vA(t)+v(x,y,t) (5)
where, by definition,
A
(x,y,t)dxdy = 0. (6)
A
v(x,y,t)dxdy = 0. (7)
Consequently,
m(t) =
A
[A(t)+ (x,y,t)][vA(t)+v(x,y,t)] dxdy
=
A
A(t)vA(t) dxdy+
A
(x,y,t)vA(t) dxdy
+
A
A(t)v(x,y,t) dxdy
+
A
(x,y,t)v(x,y,t)dxdy. (8)
Of course, the second and third terms on the right-hand side
are zero by definition. Thefirst term can be easily integrated
and therefore
m(t) = AA(t)vA(t)+
A
(x,y,t)v(x,y,t)dxdy. (9)
The above equation highlights the reasons why tomography is
required for highly non-uniform flows to correctly calculate
the mass flow rate of solids. It clearly shows that the more
spatially non-uniform theflow (both in the velocity sense and
in the density sense) the larger the second term on the right-
hand side of equation (9). Therefore, considering theflow on
a pixel-by-pixel basis becomes essential.
On the other hand for uniform flows the second term will
disappear, leading to a simplified equation:
m(t) = AA(t)vA(t). (10)
This level of simplification was applied in the study of the flow
in the vertical pipe presented here. Applying equation (10) in
discrete form for the tomographic measurements yields
m Ak=n
k=1 (tk)v(tk)t
tn t1(11)
where discrete times t1, . . . , t n correspond to consecutive
tomographicimages. To applyequation(11)someestimatesof
the instantaneousdensity andvelocitymustbe introduced. The
former could be found if the average permittivity distribution
in the sensor control volume could be linked to the average
density (orconcentration). Thisisnot straightforward, since no
appropriate mixing law is known a priori, so the concentration
of solids cannot be obtained in a reliable fashion. For the
work described here, a linear relationship between density and
permittivity hasbeenassumed. Moresophisticatedmodels can
be applied [18], but in the current work these did not seem to
provide too much improvement.
Finding the instantaneous velocity poses another type of
problem. In the vertical pipe, theflow is bi-directional and
a method to decide the instantaneous flow direction needs
to be devised. Here, it has been decided that a short-time
window cross-correlation could be a practicable candidate,although applying this concept may raise some questions of
a fundamental nature. At this stage, the spacing between the
sensors for the correlation analysis was assumed to be 13 cm.
A sample result showing the average concentration of
solids as a function of frame number is plotted in figure 11.
The upper graph shows its variation over 40 s. It can be seen
that theoccurrence of plugs is quasi-periodic. The lowergraph
shows the variation with time over a shorter time of 4 s. The
two phases of the flow, i.e. upward plug flow and return of
material, can be easily recognized.
Figure 12(a) shows a typical power spectrum obtained
for theflow in the vertical section. In this example, the dataset consisted of 32768 concentrations of solids (over 5 min
offlow rig operation). There are two maxima on the graph.
Thefirst, in the region of 0.3 Hz, most probably corresponds
to the frequency of the large scale flow patterns (i.e. a train
of plugs every few seconds). The second maximum, in the
region between 6 and 10 Hz, corresponds most probably to the
individual plugs within the train. It can be seen, however, that
the whole spectrum is of broad-band character, indicating a
quasi-periodic rather than regularflow character. Figure 12(b)
shows the cross-correlation of the signals obtained from two
planes of the sensor. Here again the plot contains low- and
high-frequency components. The maximum of the correlation
function for this example falls around +40 ms.However, the cross-correlation function shown in
figure 12(b) would indicate only the upward direction of
the transport of solids. In order to study this in more
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Gassolidsflow characteristics
2000 2500 3000 3500 4000 4500 5000 5500 60000
20
40
60
80
100
frame #
averageconcentration[%]
2850 2900 2950 3000 3050 3100 3150 3200 32500
20
40
60
80
100
frame #
ave
rageconcentration[%]
upward plug flow material falling down
Figure 11.The average concentrations of solids obtained from the tomograms for a gas velocity of 2 m s1 and a feed of solids at 900 kg h1.
0.1 1 101E-011
1E-010
1E-009
1E-008
1E-007
1E-006
frequency [Hz]
powe
rdensity
(arbitraryunits)
time [s]
correlationcoefficient
-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 10
0.2
0.4
0.6
0.8
1
(a) (b)
Figure 12.The power spectrum (a) and correlation function (b) obtained for plugflow in the vertical pipe, for a gas velocity of 2 m s1 anda feed of solids at 900 kg h1.
-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 10
0.2
0.4
0.6
0.8
1
upward
downward
time [s]
correlationcoefficient
Figure 13.Two sample correlation results corresponding to theupward and downward transport of solids in the vertical pipe for agas velocity of 2 m s1 and a feed of solids at 900 kg h 1.
detail, the signal was divided into shorter sequences (typically
windows of the order of 128 samples). These were
subsequently correlated between two planes. Figure 13
presents the correlation function for the data taken for the
upward movement (filled circles) and material falling down
the vertical section (empty circles). It can be seen that thedirectionof theflowcannow be determined, a positive time lag
indicatingupward movement anda negative timelag indicating
the downward transport of solids.
The calculations of the mass flow rate of solids carried
out using formula (11) and estimates of density and velocity
as explained above typically underestimated the actual mass
flow rates by 2030%. For the two benchmark flow rates
of 700 and 900 kg h1, the typical results from calculations
would fall into the regions of 500600 and 700750 kg h1,
respectively. On the one hand, the results are encouraging,
because they prove that a formalized approach to calculating
the mass flow rate can give answers of the same order of
magnitude. On the other hand, it is apparent that there isstill a lot of scope for improvement in applying this approach.
In particular, the method described uses concentrations of
solids and therefore propagation velocities averaged over the
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cross-section of the pipe. However, in principle it is possible
that, while some solids travel upwards in one part of the pipe,
other particles travel downwards in another part. Therefore,
the cross-correlation analysis should be performed on a pixel-
by-pixel basis, rather than for the whole cross-section. This
could also include thecorrelation between pixels in two planes
which do not correspond to one another, to allow for lateralmovement of solids. Further work in this area is planned.
4. Conclusions
The paper presented is concerned with the application of
ECT for investigating the unsteady and three-dimensional
characteristics of gassolids flow associated with pneumatictransport both in horizontal and in vertical pipes. It has been
demonstratedthat ECTis able to image thedynamicsof macro-
structures (slugs and plugs) in a manner that is consistent
with high-speed photographic techniques. Moreover, unlike
photographic methods, ECT can give valuable insight into
the internal structure offlow instabilities such as slugs andplugs. This is important from the viewpoint offlow modellingand validating of CFD codes, which makes ECT an excellent
research tool.
Furthermore, the application of a twin-plane tomography
system to the measurement of multiphase flow is discussed.This is usually attempted by using well-known cross-
correlation techniques. Fundamental assumptions lying
behind these techniques are discussed in some detail and
these are related to the prospective application of ECT for
measurement offlow rates of solids in pneumatic conveyingsystems. Several potential weaknesses are identified, inparticular the finite length of electrodes, ability to detect theflow in a large control volume only and wavelike character ofdense gassolidsflow.
The theoretical analysis underlying the mass flowmeasurements is presented and appropriate simplifications arediscussed. This is applied to aflow in the vertical channelfor measuring theflow rate of solids in the presence of plugflow and preliminary results are presented. However, it isgenerally apparent that, before the tomographic techniques
can successfully be applied to multiphase measurement, a few
further developments need to take place. These should include
the following.
(i) The capture rate of the tomographic equipment should be
increased to the region of 5001000 frames s
1 to allowmore accurate estimation of the propagation velocity of
the disturbances in theflow. In the current work, theflowwas selected such that ECT could cope with representing
theflow sequences correctly, but it was felt that time lagestimates were not accurate enough.
(ii) Thedevelopment ofsuitablemodelsto relatethe measured
dielectric permittivity of the gassolids mixture to thedensity (or concentration). This is an issue that ECT
equipment manufacturers are well aware of, but it needs
to be addressed more strongly by the research community.
(iii) Theeffects of spatial filtering dueto finiteelectrode lengthon correlation analysis should be considered. At present,
a centre-to-centre distance between the sensor planes is
taken into account in calculations, but some more detailed
investigations are needed in order to justify the use of
correlation techniques for measurements.
Acknowledgments
We would like to gratefully acknowledge the support obtained
from the European Commission BRITE EURAM programme
(BRST-CT98-5402) and the UK Engineering and Physical
Sciences Research Council (GR/M31910). We would also
like to thank Process Tomography Ltd for providing a twin-
plane ECT systemandtheRutherford Appleton Laboratory for
access to the high-speed camera used forflow visualizations.
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