PEPT- a novel tool for investigation of

pneumatic conveying

–Powder Technology 237 (2013), pp. 87–96,

10.1016/j.powtec.2013.01.024

Prachi Middha, 1 Boris V. Balakin, Line Leirvaag,Alex C. Hoffmann and Pawel Kosinski

The University of Bergen

Department of Physics and Technology, Bergen, Norway

Abstract

Bends are integral parts of a pneumatic conveying system. However, the presence ofbends often leads to undesirable phenomena such as roping, particle segregation andattrition, particle deposition and pipe abrasion. It is important to understand theparticle motion in the vicinity of bends to better address these issues. In this work asingle radioactively labelled particle is tracked in a PET (Positron Emission Tomog-raphy) camera, as the particle traverses a horizontal to vertical 90◦ bend in a typicalpneumatic conveying system in the absence of any other particles. Experiments havebeen carried out in varying configurations at different carrier-phase velocities. Theresults show detailed spatial trajectories of the particle as it moves through thesystem. The particle exhibits various types of path through the flow loop, whichare primarily governed by particle inertia, particle-fluid and particle-wall interac-tions. The results are relevant to understanding of solids phenomena in lean phasepneumatic conveying and are unique in demonstrating the use of Positron EmissionParticle Tracking (PEPT) as a universal non-invasive visualization technique forthe study and analysis of any rapid industrial process.

Key words: pneumatic conveying, bends, elbows, PET, PEPT

1 Corresponding author. Address: Allegaten 55, 5007 Bergen, Norway. Tel:+4755582790; Fax:+4755589440. E-mail: Prachi.Chhabra@ift.uib.no

Preprint submitted to Powder Technology 8 April 2013

1 INTRODUCTION1 Introduction

1.1 Pneumatic conveying

Pneumatic conveying is an important process that is widely used for the trans-port of granular particles in several different industries ranging from food andpharmaceutical to cement and chemical plants to the power industry. It hasthe advantage of being practical and economical while still providing largeconveying capacities. Despite the widespread use of pneumatic conveying ina multitude of industrial processes, there are still a number of challenges fac-ing the process. Pipe abrasion, particle deposition and attrition, among otherthings, are some of the unresolved topics of interest. Bends and elbows arewidely used in pneumatic conveying systems even though they give rise toseveral of the aforementioned problems. A brief review of some of the studiesis provided below.

The experimental methods used to study the above phenomena include ther-mal, electrical, acoustic and optical methods in addition to light-scatteringbased techniques, capacitative sensors and radiometric ones. In their reviewpaper, Zheng and Liu [1] provide an exclusive list of experimental techniquesdeveloped and available for quantifying pneumatic conveying parameters. How-ever, they conclude that none of these methods are ideal for use in real in-dustrial settings universally. Most of them suffer from low temporal and/orspatial resolution and are unsuitable for opaque systems.

Fangary et al. [2] conclude that Laser Doppler Velocimetry (LDV) and ParticleImage Velocimetry (PIV) are not suited for the measurement of the localsuspension due to the particle size and concentration whereas PEPT can besuccessfully used to study a suspension of coarse particles. The techniquegives information on particle velocity, trajectory, and circulation. Wei et al. [3]used an optical fiber probe (OFP) to estimate the solid concentration andparticle velocity in dilute phase pneumatic conveying system. In addition,other tomographic techniques like magnetic resonance imaging (MRI) andelectrical resistance tomography (ERT), have been utilized to study pneumaticconveying [4, 5].

PTV and PIV have been used extensively by researchers to study such sys-tems [6, 7]. Recently, Rinoshika and Yan [8] have developed high-speed PIVspecifically for pneumatic conveying. Laser Doppler Anemometry (LDA) withPhase Doppler Anemometry (PDA), which is a classical technique utilized forthe measurements of velocities and velocity fluctuations using laser probes,has also been exploited [9, 10].

The development of spatially inhomogeneous particle distributions, such as in

2

1.2 PEPT 1 INTRODUCTION

particle roping or particle segregation in certain sections of a pipeline, e.g. inthe vicinity of elbows and bends, are issues that have daunted the researchersfor a long time. Huber and Sommerfeld [11] used PDA to determine the par-ticle velocity and the spatial particle distribution. They considered that thedevelopment of inhomogeneous particle distributions was influenced by sev-eral factors including the conveying velocity, the particle loading and the wallroughness. Yilmaz and Levy [12, 13] measured solids velocity and mass con-centration using a fiber optic probe.

Lee et al. [10] studied particles with different Geldart’s classification for bothdilute and dense flow and determined the solids concentration past a 90◦

bend utilizing several different techniques, including Electrical CapacitanceTomography (ECT), Particle Image Velocimetry (PIV) and Phase DopplerParticle Analysis (PDPA). Akili et al. [14] studied the particle depositionafter a 90◦ elbow using a fiber-optic probe and CFD methods.

In addition to the above-mentioned experimental studies, researchers have forlong looked at pneumatic conveying in silico as well. Levy [15] deduced fromnumerical modelling that both secondary flows and turbulence play an impor-tant role in determining the particle distribution beyond a bend. Yilmaz andLevy [13] used the commercial code CFX-Flow3D to understand roping be-yond an elbow. Bilirgen and Levy [16] investigated the effectiveness of severaldifferent flow straighteners installed after a 90◦ bend in dispersing a particlerope. They used a Lagrangian type particle tracking method together withflow solution from the commercial software package CFX.

Kuan et al. [17] developed a CFD model using the differential Reynolds stressmodel together with Lagrangian particle tracking. The model also accounts forparticle-wall interactions to determine the particle velocities and showed goodagreement with LDA measurements. Chu and Yu [18] developed a combinedcontinuum and discrete model (CCDM) to study pneumatic conveying systemsin three dimensions. Vashisth and Grace [19] used a CFD model based on theEuler-Discrete phase methodology (DPM) together with the re-normalizationgroup (RNG) k-ǫ model.

In spite of the work done till date, there remain outstanding challenges andquestions that must be answered to fully understand the pneumatic conveyingprocess as a whole.

1.2 PEPT

Positron Emission Particle Tracking (PEPT) is an off-shoot of the popularPositron Emission Tomography (PET) technique which has been used exten-sively in the medical environment. PET utilises an image reconstruction algo-

3

1.2 PEPT 1 INTRODUCTION

rithm to reconstruct the shape and position of a cloud of radioactive particlesor molecules, whereas PEPT is a technique for tracking individual radioactiveparticles in an experimental set-up by cross-triangulation rather than imagereconstruction. The time-resolution of the former is of the order of 1 s, whileit is of the order of 1 ms for the latter. The term PEPT was coined by re-searchers at University of Birmingham [20,21] who adapted the standard PETusing home-grown customised particle tracking algorithm to study particle dy-namics.

Both PET and PEPT are powerful tools for studying the processes in anexperimental set-up and for obtaining dynamic flow information. Both tech-niques are specifically useful for studying multiphase flows; in particular tomap fluids and particles, one phase being labelled and its behaviour beingobserved in a PET camera.

By introducing a single radioactive tracer particle into the experimental set-upand observing the spatio-temporal profile of the radioactivity one can learnabout the dynamics of the system. An isotope of the tracer has in generalfewer neutrons relative to the number of protons, and is unstable. It can attainstability by undergoing a β+-decay where a proton is converted into a neutronwhile emitting a positron and a neutrino at the same time. The neutrino isvirtually undetectable, while the positron plays a key role in PET. After beingemitted the positron is scattered along a random path before it annihilateswith an electron. The annihilation process emits two γ-photons each with 511keV energy [22,23] back-to-back. The two photons are detected by sensors anda line of response (LOR) is drawn between the two sensors assuming that thedecay was collinear. Cross triangulation for a multitude of these events givesthe accurate position of the tracer.

The history of PEPT dates back to the end of the last century, when re-searchers at Birmingham used a PET camera together with a single positronemitting particle instead of a radioactive liquid/fluid volume to understandprocess technology phenomena [24]. It was determined that PEPT is a power-ful tool for studying various industrial processes owing to the unique amount of3D information that is obtainable. The volume of interest must, however, fallwithin the range of the camera. An additional advantage is that the powerfulγ-rays are able to penetrate most metal/opaque surfaces making this tech-nique an ideal one for investigating actual process equipment. The techniqueis unmatched at displaying the particle trajectory in three dimensions.

Several advances have since been made to improve the detection process as awhole. Better, newer-generation cameras have been commissioned with time,the particle tracking algorithms have improved to increase the accuracy, bettercandidates for tracer particles have been sought, the process of generating andlabelling tracer particles has also undergone improvement [25]. PEPT has even

4

1.3 PEPT for Pneumatic Conveying 1 INTRODUCTION

been used to track multiple particles [26].

PEPT has been applied for studying various engineering applications. It con-stitutes a non-invasive method to study granular, particulate or any otherkind of multiphase flow. In particular, it has been exploited to study severaldifferent phenomena pertaining to fluidized beds and risers for circulating flu-idized beds [27–32]. In addition to fluidization it has recently been extendedto study stirred vessels [2,33], tumbling mills [34,35], hydrocyclones [36], swirltube reactors [37] and others.

Fairhurst et al. employed PEPT to study particle trajectories and velocityprofiles in a relatively high solids-concentration solid-liquid flow system [38].The paper depicted different flow patterns and showed the presence of capsuleflow. It should, however, be noted that this work is inherently different fromthat of Fairhurst et al. The process addressed in the current research is a rapidprocess in a solid-gas system as opposed to the liquid-solid systems studiedby Fairhurst et al. [38].

Another relevant study using PEPT is one by researchers in Birmingham [39]where PEPT is utilised to predict particle velocities and the residence timedistribution in the riser section of a circulating fluidised bed. A single tracerparticle is used in the presence of other particles to predict the particle velocitydistribution under the various operating regimes. There are however, certaindifferences in the approach of that study and the current study. Chan et al. [39]predicted average particle velocities in a circulating fluidised bed comprising ofa multi-particle system where as the current research aims to look at individualparticle tracks for a single tracer particle in the absence of any other particlesin a single-pass replica of a pneumatic conveying system.

1.3 PEPT for Pneumatic Conveying

This work aims to apply PEPT to the study of pneumatic conveying. Specif-ically, PEPT is used to comprehend the details of particle paths through apneumatic conveying line with the ultimate aim of quantifying pneumaticconveying parameters. As mentioned earlier, PEPT is in its essence an ex-perimental technique for Lagrangian particle tracking, which is capable ofproviding 3D information while being non-invasive. Lagrangian tracking givesinformation that a measuring technique focused on a fixed spatial point orvolume in the process cannot give.

This article aims to demonstrate that PEPT, with state-of-the-art hardwareand software is a viable measurement technique to study pneumatic conveying,or a similar fast process, in detail. We note that this idea, in itself, is not veryrecent, Lareo [40] concluded that PEPT constitutes the ideal technique for

5

2 EXPERIMENTAL SETUP

providing useful information about pneumatic conveying but that, at the timeof writing, the technique was still in its infancy. The latter is no longer the caseas PEPT has made big strides in the past fifteen years and is, as this paperwill show, capable of mapping even rapid processes like pneumatic conveying.

2 Experimental Setup

2.1 The experimental rig

A typical pneumatic conveying line consists of several different sections of pipeconnected to each other by elbows and bends often with transitions betweenhorizontal and vertical. Inspired by the different requirements of a pneumaticconveying system, a number of configurations are looked at in this work. How-ever, the portal of the PET scanner available is not large enough to accom-modate equipment sizes commonly used in pneumatic conveying. The designof the experimental rig is, therefore, to an extent governed by dimensions ofPET portal.

A schematic diagram of the experimental rig is presented in Fig. 1. The systemis broadly divided into two parts: the test section which is inserted into thePET portal while the rest, marked “main unit” in the diagram, comprisesof the equipment required for the operation and control of the process. Thetest section consists of 50-cm straight transparent pipeline with a diameter of45.2 mm. However, it must be noted that PEPT does not require any opticalaccess to the system or fluid, the material of construction is simply chosenbased on availability. The section is mounted on a portable stand for ease ofmodification and transportation.

The main unit, marked 3, is also mounted on a portable frame for the sakeof portability. The primary function of the main unit is the generation ofturbulent air flow in the system. The air is sucked in from the atmosphere atambient conditions through the second flow straightener, 1 in the main unit,located directly at the entrance of the installation. This is done to ensure fullydeveloped flow 30 cm downstream. Here an anemometric flow meter depictedas 2 (620S Fast-Flo from Sierra Inst.), is located to measure the carrier-phasevelocity. The main flow entrance is used as the inlet and as a tracer-particleinjection point for one of the flow configurations explored in this work.

Due to the above-mentioned size limitation the length of the pipe is not suf-ficient for the natural development of flow. A flow straightener, marked 1, inthe test section, is provided to facilitate the development of the quasi-uniformturbulent flow in the pipe. The section under investigation, referred to as the

6

2.2 Tracer particles 2 EXPERIMENTAL SETUP

test section, is mounted on a separate frame labelled as 6. It consists of two50 × 88.5◦ bends, which are used to connect a vertical pipe with two 80-cmhorizontal straight sections of the same diameter. Both horizontal pipes endin flange connections named 4. These pipes are further linked to the main unitvia two ca. 200-cm corrugated hoses with toroidal artificial roughness 3 × 3mm. The use of the flexible hoses facilitates easy connection between the testsection and the main unit. In most of the configurations investigated in thiswork, the bottom hose is disconnected and the flow entered the test sectionfrom the entrance of 80-cm horizontal pipe. This also served as the site for theinjection of activated tracer particles.

Downstream of the flow meter, the flow goes through 35-cm long straighthorizontal pipeline and enters the bottom hose, through which it enters thetest section. Having passed the test section, the particles are removed from theair in two 1-mm layers of dust filter from Insatech located 20 cm downstreamfrom the top hose. The flow then goes to the low-pressure inlet of SCL K06-MS compressor from FPZ S.p.A, marked as 11 in Fig. 1. The air is dischargedto the atmosphere after the filter.

The flow rate in the system is varied by means of a 40-11

4DN-52 ball valve

in the bypass line marked 11, connected to the system via a 50/50 × 88.5◦

T-junction, shown as 9.

[Fig. 1 about here.]

In several flow configurations the flow straighteners are removed from thepipeline. Fig. 2 shows a detailed view of one of the straighteners used. As seenin Fig. 2, it consists of a classical 6-cm cylindrical block with 14 cylindricalholes each of them 0.8 mm in diameter to simulate a honey-comb type flowstraightener. This leads to the formation of 14 high-speed flow jets whichfurther hit a 3.5-cm half-area mixing plate located 3.2 cm downstream. Theplate is mounted on the block part with the aid of four metallic rod supports.The mixing plate gives rise to the formation of a well-developed turbulentflow. CFD simulations were carried out to verify that the well-developed flowoccurs at an approximate distance corresponding to two pipe diameters.

[Fig. 2 about here.]

2.2 Tracer particles

In order to label the tracer particle, 18F (t 1

2

= 110 min) ions were produced in

solution. The 18O(p, n)18F nuclear reaction is carried out in a cyclotron for thispurpose. The ions can further be adsorbed, possibly through ion-exchange, toproduce tracer particles.

7

2.3 Scanner and data acquisition 2 EXPERIMENTAL SETUP

The radioactively labelled tracer particle plays a key role in the PEPT pro-cess. It is imperative that the particle has high capacity for fluoride adsorption.Fan et al. [41] reported that the fluoride adsorption capacity for strong-baseion-exchange resin is higher compared to their weak-base counterparts. Inaddition, the radioactivity on a single particle depends both on the 18F con-centration in the solution and the immersion time of the particle.

The particle used within this work is an Amberlyst A21 anion-exchange styrenedivinylbenzene, with a density of 1070 kg m−3. The particles are perfectlyspherical in shape and their number average size 430 ± 56 µm is determinedby microscopy. Fig. 3 shows pictures of the particles obtained using an opticalmicroscope. The particles are immersed in a 0.5 ml aqueous solution contain-ing 18F. The particles are subsequently dried on a filter paper. It has beenreported earlier [36] that beyond an optimum immersion time the decay rateexceeds the exchange rate. Hence, the immersion time is limited to 10 min inthis case. The activity on the particles varied between 300–2000 µCi on dif-ferent days depending on the 18F concentration in the aqueous solution. Also,the moisture content of the particle varied between different days. The darkerparticles in Fig. 3 are the wetter ones whereas the white ones are relativelydry.

[Fig. 3 about here.]

2.3 Scanner and data acquisition

As described earlier, the radioactive decay leads to the production of a positronand a neutrino. The positron subsequently annihilates with an electron givingrise to a back-to-back emission of two gamma photons of 511 keV.

The emitted photons are detected by the PET scanner, a Siemens TruePoint.The scanner consists of three rings each of which has 48 sensor blocks, andeach of those blocks comprise of 13 × 13 lutetium oxyorthosilicate (LSO)scintillator crystals [36] which are further read out by arrays of PMTs (PhotoMultiplier Tubes). The diameter of the rings constituting the sensor cylinderis 855.2 mm and the axial dimension of this cylinder 164 mm. However, it isimportant to note that the field of view of the scanner is smaller compared toits actual physical dimensions.

If the scanner detects two photons within the co-incidence time window of thescanner (4.5 ns) they are considered to have emanated from one annihilation.The data from the PET scanner are stored in the list mode format. The detailsof the method can be found elsewhere [36]. Information about the two collinearsensors is stored in the form of event words. Cross-triangulation of the LORs(lines of response) helps in determining the exact location of the tracer particle

8

2.4 Experimental procedure and parameters 2 EXPERIMENTAL SETUP

at a particular instance of time.

2.4 Experimental procedure and parameters

The experimental procedure consists of the following steps:

Placement and alignment of the rig. The location of the test section plays acrucial part in determining the quality of data obtained. It is important thatthe region of interest falls completely within the field of view of the camera.The field of view of the camera is pre-determined in accordance with theabove-mentioned dimensions of the sensor rings and the alignment of the rigwithin that region is ensured by the use of lasers.

Still PET measurements. At the beginning of each set of experiments, stillPET measurements of tracer particles in a small test-tube which was glued tothe wall of the rig are acquired to determine the location of the walls.

Starting the pump. The start-up time for the pump is calculated to be around3 s. The pump is started about 1 min before particle injection to avoid anystart-up effects. The experiments are carried out for different carrier phasevelocities, which were set by the use of a ball-valve.

Particle injection. A single radioactively charged particle is then carefullyinjected into the system in the absence of any other particles.

Data acquisition. The PET scanner and the data acquisition algorithm need tobe initiated to track the tracer particle. The PET measurements are continuedfor 5 minutes every time.

Removal of the particle from the rig. At the end of each experiment, the finallocation of the tracer particle is determined with the use of portable radiometerof Geiger-Mueller type from SigmaAldrich, measurement range 0.1–10 mR/h.It is ensured that the rig is free of any tracer particles and the tracer issuccessfully trapped in the filter unit. If the particle is found to have depositedsomewhere in the rig it is dislodged mechanically with gas flow to ensure aclear rig before the start of next injection.

2.5 Carrier phase velocity measurements

The measurements of volumetric flow acquired from flow meter were convertedinto mean flow velocity. The flow velocity measurement is done for each flowconfiguration taking into account the temporal evolution of the flow field dur-

9

3 RESULTS AND DISCUSSIONS

ing the start-up of the pump. The flow meter sent a digital signal to a computervia an RS-232. The frequency of the digital data acquisition is in the intervalfrom 3 to 6 Hz.

The flow meter is located 15 cm downstream of the top bend of the test sectionwhile the entrance part of the rig is disconnected. This made the measurementto be influenced by the non-uniform velocity profile after the bend. The datahence obtained gives the temporal evolution of the mean flow velocity for thedifferent flow configurations. The start-up time is, as mentioned, estimated tobe approximately 3 s using this method. It is also observed that the presence ofa flow straightener gives a reduction in the mean velocity owing to the increasein head losses. It must be noted that there occur unavoidable variations in theplacement of the meter, both in terms of the cross-sectional position of thewire itself and the position of the meter in the rig, which may give rise to asmall percentage of uncertainty in the flowrates measurements.

3 Results and Discussions

3.1 Particle Trajectory

As mentioned earlier an in-house algorithm is used to obtain the detailedtrajectory of the tracer particle from the data obtained from the scanner. Thebasic concept of the positioning algorithm is as follows.

Cutpoints between the LORs registered within the given time windows (often1 ms) are considered in two dimensions, first the x-y plane and then the y-zplane. In the plane under consideration a first estimate of the particle positionmay be obtained by averaging the positions of all the cutpoints within the fieldof view. Subsequently the cutpoints farther than a given distance from the firstestimation are excluded from the next averaging. The process of excluding andaveraging is carried out several times, each time halving the linear dimensionof the circular window containing the accepted cutpoints. The size of the finalwindow is chosen to minimize the standard deviations of millisecond positionsfor a stationary tracer, which turned out to be a window with a radius of48 mm enclosing hundreds to tens of thousands cutpoints in a millisecond,depending mainly on the radioactivity of the particle. Several factors inherentto the process cause scatter in the position determined for the particle centroid:

• The positrons travel a certain distance through the medium (on average 0.6mm in water) [42] before annihilating with an electron.

• The finite dimensions of the sensors.• The depth to which the photon penetrates into the sensor before detection

10

3.1 Particle Trajectory 3 RESULTS AND DISCUSSIONS

varies. If incidence is at an angle, the photon may even pass from one sensorto the other.

• The two photons may not be entirely collinear, depending on the combinedkinetic energies of the positron and the electron at annihilation.

• In spite of the narrow energy window handled by the camera, a photon mayhave undergone Compton scattering on the way to the sensor and remainwithin the energy window

• “Random coincidences” where two annihilation events take place within thesame coincidence window, and only one of the photons from each is detected.

The algorithm should filter out most of the two latter effects, but the formereffects cause a degree of scatter that depend on the activity on the particleand the location of the particle relative to the field of view. The scatter inthe particle centroid, in general, has a standard deviation of no more than acouple of hundred microns.

More detailed description of the positioning algorithm can be found elsewhere[36]. The positioning algorithm is continually being developed and this workwill be published separately.

A Gaussian filter with a standard deviation of one time step is subsequentlyapplied to the data obtained from the positioning algorithm. It filters out thespread in the particle position with a characteristic time of one time step,and thereby largely eliminates the spread due to the factors listed above,inherent to the PEPT process, while preserving the true, physical features ofthe particle track.

In order to understand and interpret properly the results shown below it isessential to estimate the extent to which the tracer particle follows the gasflow in the rig. This is described by the particle Stokes number, interpreted asthe particle dynamic, or momentum, response time relative to a characteristictime for the process:

Stk =τpτs, (1)

where τp is the particles momentum response time and τs is some time char-acteristic of the flow field. τp is also called the relaxation time, since it is ameasure of the time it takes for a particle to adjust to changes in the flowvelocity in the fluid surrounding it. The momentum response time is definedas:

τp =ρpD

2

18µg

, (2)

where ρp is the material density of the dispersed phase material, D is thediameter of the particles and µg is the viscosity of the gas. A characteristictime for the process can be the length of the test section, L, divided by the

11

3.1 Particle Trajectory 3 RESULTS AND DISCUSSIONS

mean gas velocity, U , such that the Stokes number can be defined as:

Stk =ρpD

2

18µg

U

L. (3)

Thus, if the particles move with the velocity of the gas, the Stokes numberindicates the extent to which the particles have time to react to changes inthe particle-gas slip velocity, e.g. due to the particle impacting on a wall orentering a different turbulent eddy, during their residence time in the testsection.

As mentioned earlier, the average particle diameter is determined by mi-croscopy and the particle density is known. With these parameters the par-ticles will have a response time of 0.5 seconds in air. This time is significantcompared to the time spent by the gas even at the lowest velocities within the50 cm long test section, which is of the order of 0.1 seconds. Thus large val-ues of Stokes number are obtained, ranging between 5–20 for different carrierphase velocities, and it is clear that the particle trajectories depicted in thiswork are affected by the inertia of the particle as well as its interactions withthe fluid and walls.

In the following, examples of particle trajectories through the test sectionare shown. It is found that the particle can exhibit several different types ofbehaviour. In this discussion, the section of the tube wall furthest from theorigin of the coordinate system, i.e. the part of the wall that the particle willtend to impact on in the bends due to its inertia, is referred to as the ”outer”wall and the part of the wall closer to the origin is denoted as the ”inner”wall. The tracks shown are chosen so as to be representative of the differenttypes of behaviour observed. Most of the features discussed are visible inthe plots shown, but some could only be gleaned by rotating 3-D plots of thetrajectories. Note that the tracks after being subjected to the above-mentionedGaussian smoothing, show the true path more precisely as the smoothingfilters out the spread due to the inherent errors in the PEPT process butwhile doing so it also smooth the sharp angles occurring in the trajectorieswhen the particle impacts on the wall somewhat.

Fig. 4 shows the four configurations that were used during the experiments,they will be discussed separately below. In each configuration three numberedtracks are shown, corresponding to high, medium and low gas velocity, respec-tively. The gas velocities are given later in the paper in Table 1.

[Fig. 4 about here.]

As seen in Fig. 4, configuration A consists of a relatively long horizontal pipebefore the bend. Particle trajectories in this configuration are shown in Fig. 5.

12

3.1 Particle Trajectory 3 RESULTS AND DISCUSSIONS

Also the approximate outline of the tubing of the test section is shown in thesefigures. Please note that these are 2-D plots of 3-D trajectories.

[Fig. 5 about here.]

At the highest gas velocity, marked A1 in the figure, the particle enters andimpacts on the outer, the inner and the outer parts of the wall, after which itproceeds through the test section, hitting opposite ”sides” of the wall twice.In the second bend, it hits the outer wall twice.

At the intermediate gas velocity, A2, the hits the outer wall once in the firstbend, and proceeds through the test section travelling fairly straight almostwithout interaction with the wall, maybe hitting the wall once. It hits theouter wall when arriving at the second (top) bend twice and bounces backonto the inner wall.

At the lowest gas velocity, A3, the particle enters with a relatively low speedconsistent with the gas velocity, and impacts on the transition from bendto straight tubing (the straight section has a slightly smaller cross-section)evidenced by the sharp angle in the trajectory, bouncing back and followingthe curve of the outer wall on its way back but also moving towards the centreof the tube. It decelerates as it moves against the flow of the gas, turns aroundunder the influence of the gas flow and is again accelerated in the directionof the flow. The particle moves more or less straight through the straightsection without interacting with the wall. In the second bend it hits the wallthree times, one of which is not visible from this angle. In the right-mostimage the path for the A3 case, as determined from the positioning algorithmwithout Gaussian smoothing, is also shown. There is evidently more scatterin this path, especially right at the beginning and right at the end where theparticle enters and exits the camera field of view, and the triangulation in allof the coordinate directions becomes inferior. In some of the plots shown afew spurious points as the particle enters and exits the field of view have beenremoved.

Particle trajectories in configuration B are shown in Fig. 6. Notice from Fig. 4that configuration B differs only slightly from configuration A, the differencebeing that a flow straightener is installed in a relatively short pipe at the inletupstream of the first bend. The flow straightener, as discussed earlier, ensuresa fully developed flow as the flow approaches the bend leading to the verticalpipe.

[Fig. 6 about here.]

At the highest gas velocity, B1, the particle enters slightly towards the sideof the tube with a relatively high speed, and impacts on the transition frombend to straight tubing similar to case A3. This impact is again shown by a

13

3.1 Particle Trajectory 3 RESULTS AND DISCUSSIONS

sharp angle in the trajectory. It bounces back, and is turned by the influenceof the gas flow as in case A3. The particle passes through the test section in aspiralling motion completing a bit more than one turn in the straight section,travelling rather close to the wall throughout and in the second bend.

At the intermediate gas velocity, B2, the particle path is very similar to B1in the first bend, but that particle can be seen to pass straight through thestraight section without spiralling and without interacting with the wall of thetube. The particle hits the second bend twice.

At the lowest gas velocity, B3, the particle hits the outer wall of the first bendtwice. It proceeds through the straight section, hitting the inner part of thewall once about in the middle and continues thereafter in a straight line (notinteracting with the wall any more) to the second bend where it again impactstwice on the outer part of the wall before exiting. The speed of the particle isrelatively low consistent with the low gas velocity.

It is evident from Fig. 4 that configuration C is quite different from configura-tions A and B as the particle in configuration C travels a long distance beforeit enters the test section. The tracks are shown in Fig. 7.

[Fig. 7 about here.]

At the highest gas velocity in this configuration, C1, the particle is seen toenter a strongly spiralling motion after the first bend, taking many revolutionsin the straight tube section. Particularly in the second part of this section thepitch of the spiral is decreased and the particle travels in the axial directionat much lower speed than the gas velocity would indicate. At the intermediatevelocity, C2, the particle bounces back and forth between the outer and innerwall section a number of times just after the bend, before being acceleratedin the axial direction by the gas flow. At the lowest gas velocity, the particleappears to bounce back from the curved-to-straight transition, as also seenabove, before proceeding into the straight section. This particle is movingvery much in the axial plane viewed in the 2D view, since it is moving veryclose to the outline of the tubing indicated.

Configuration D, as shown in Fig. 4, is drastically different from the otherconfigurations in the sense that it incorporates an obstruction in the form of aflow straightener right after the first 90◦ bend. The tracks in this configurationare shown in Fig. 8.

[Fig. 8 about here.]

At the highest gas velocity, D1, the particle again appears to impact on thebend/straight transition and rebound like in some of the previous cases. Afterdoing so it can be seen to interact with the flow straightener assembly, bounc-

14

3.1 Particle Trajectory 3 RESULTS AND DISCUSSIONS

ing off the mixing plate before proceeding into the straight section, which ittraverses more or less directly. An interesting feature here is that this particleenters a spiralling motion downstream of the second bend, supporting the ideathat such spiralling motion downstream of a bend is likely due to the paththat the particle takes into the bend rather than a feature of the gas flow field.

At the intermediate velocity, D2, the particle negotiates the first bend withouthinder, and interacts with the mixing plate before entering the straight section.As in the previous case, the particle enters a spiralling motion downstream ofthe second bend.

In the low-velocity case, D3, the particle interacts severely with the bend-to-straight transition, but does not seem to be hindered at all by the flowstraightener, passing the mixing section at the side wall. It passes straightthrough the test section without apparently interacting with the wall, butappears to impact twice on the outer part of the second bend.

As mentioned, one problem with presenting 3D data using 2D projections isthat some features of the trajectories are lost. In order to show more detail,3D plots of three of the trajectories, namely A3, B1 and C1, have been plottedand turned so as to show the details of each trajectory as well as possible. Theresult of this is shown in Fig. 9, wherein the tubing has been shown as tubesegments, while the bend sections themselves are not shown. For these plotsthe results without Gaussian smoothing have been used.

[Fig. 9 about here.]

The features described earlier in the text accompanying Fig. 5 for the case A3are vivid in Fig. 9. Also the three collision with the wall in and just after theupper bend are visible. It can be seen that the particle enters the lower bendclose to the wall on the left-hand side in the current view.

In the case B1 the movement of the particle both in the lower and upper bendis more evident. It is also seen how the particle spirals through the straighttest section close to the wall.

In case C1 it can now be clearly seen that the particle enters the lower bendtangentially, and this is likely to give rise to the spiralling motion that persiststhroughout the straight test section. This figure shows more accurately thatthe particle appears to enter the space between the inner and outer tubesections in the upper straight-to-bend transition.

Summarising the observations, it can be said that different types of behaviourof a particle downstream of a bend are seen: the particle could move moreor less straight through the straight section downstream of the bend, or itmight impact repeatedly on the wall while flowing through, or it could enter

15

3.1 Particle Trajectory 3 RESULTS AND DISCUSSIONS

into a spiralling motion. Many of the particles are observed to drift towardthe wall in some way or the other. The tendency of particles to drift towardthe wall has been observed previously by many researchers [10,11,14,15] andhas been found to be responsible for particle segregation, roping and otherunwanted phenomena and hence has been a subject of interest for severalresearchers [13,17–19]. The centrifugal forces acting on a particle brought intospiralling motion by its entry into the bend and fluid-solid interactions areresponsible for this motion.

The spiral motion observed under different conveying velocities for almost allof the configurations in this work has also been reported in previous stud-ies [11,16,43,44]. In general one would expect the formation of two symmetri-cal vortices (Dean vortices) behind bends, but Levy et al. [15] proposed that,in horizontal pipes conveying fluids with particles, asymmetrical vortices mayform due to the presence of the particles and the effect of gravity. One vor-tex could dominate so much that this could give rise to a spiralling motiondownstream of the bend.

However, as mentioned before, the bends in the present work are vertical,and there is only one particle present. The spiral motion behind the bend istherefore more likely due to the position of the particle upon entry into thebend and its large response time. It is interesting, however, that the spirallingmotion persists so far downstream as it does.

Li and Shen [45] and Akilli et al. [46] conclude that the roping characteristics ofsolids in gas-phase flows are a function of the gas velocity while Lee et al. [10]observed insignificant changes in solids velocity pattern with increasing gasvelocities. In this work spiralling in the straight part of the test section isseen at the highest gas velocity, but spiralling after the second bend is alsoobserved at the intermediate gas velocity.

It is difficult to reach firm conclusions about the parameters that give rise todifferent trajectories due to lack of sufficient statistics in this study.

The presence of a flow straightener right after the bend somewhat dampensthe effect of the bend and the spiralling motion and impacts on the wall in thestraight section are reduced. There is little in the literature about the effect ofa mixing element after a bend, Bilirgen [16] observed that due to the presenceof a mixing element after the bend the particle motion is reduced within arope region, possibly due to increased collisions.

16

3.2 Particle and gas velocities 3 RESULTS AND DISCUSSIONS

3.2 Particle and gas velocities

Given the precise particle location at known times the velocity of the particlecan readily be calculated. The average absolute and axial velocities are deter-mined in the part of the test section where the influence of the bends is notsignificant. Also the gas velocity is determined from the flow measurements.Table 1 lists the carrier gas velocity, average absolute velocity of the particlein the test section together with the axial velocity in the straight test section,the Reynolds number for the gas flow in the tube, the particle Stokes number,and the relative particle Reynolds number for all the cases considered in thisstudy.

[Table 1 about here.]

The particle velocities are always lower than the average flow velocity exceptin the case of the lowest velocity for the D configuration, D3. That the particlevelocity is slightly higher in D3 may be due to any of a number of factors, onebeing that the particle is shot out from the half-area mixing zone, withouthaving impacted on it, in a gas velocity twice that in the test section, anotherpossibility could be that the particle ends up in the middle of the tube wherethe velocity is a maximum, and finally there may, as mentioned, be some errorin the gas velocity measurement.

In two of the cases with the highest gas velocity B1 and C1, the particle goesthrough the test section in a spiralling path, while staying close to the wall. Inthese cases it is remarkable that the particle velocity is much lower than thegas velocity, and also lower than at the particle velocities for the correspondingintermediate gas velocity cases, B2 and C2.

The straightener assembly has eliminated this spiralling motion at high veloc-ity in D1, where the particle can be seen to move very fast through the middleof the tube.

In these present experiments the particle velocities are typically between 20–80% of the gas velocities. Other researchers have also reported particle veloc-ities in the range of 10–70% of the conveying gas velocities [6, 10, 14, 15, 44].The loss of kinetic energy by the particles in the bend can be attributed tocollisions with the wall, which are clearly visible in the PEPT visualization.This is responsible for significant slip occurring between the two phases.

It can be observed from the particle trajectories corresponding to intermediategas velocities in configurations A, B and C that the particle collides with thewall more times as it passes a bend than it does at higher gas velocities. Thisstands to reason, and Wadke et al. [47] reached a similar conclusion from theirnumerical study of the trajectory a single particle in a bend.

17

4 CONCLUSIONS

Although the evidence for the long particle response time, mentioned before, isclear in the figures and discussion above, the response time is not quite as longas the formal derivation indicates. Given a sudden change in the relative veloc-ity between particle and surrounding fluid, the particle momentum responsetime gives the time it takes for the particle to bridge a fraction (1−1/e) of thegap between the previous and the new terminal velocity assuming Stokesian

drag. Slip velocities between particle and gas may, in the present experiments,be anywhere between above 15 and 0 m/s, the former e.g. when the particle isrebounding from an obstruction, the latter when it follows the flow. The dragforce acting on the particle will be 3.5, 5.2 and 8.2 times as high as that calcu-lated from Stokes law at 2, 5 and 10 m/s slip velocity, respectively, reducingthe effective response time of the particle.

4 Conclusions

This work has demonstrated that PEPT is the visualization technique that iscapable of giving detailed information of the flow for a rapid process like pneu-matic conveying. The results show a spectrum of different particle behavioursthat one may expect in similar set-ups. These different types of behaviourcould be relevant to various aspects of pneumatic conveying, such as roping,wear of conveying tubes and plugging.

Having shown that it is possible to study such a fast process with the PEPTtechnique, various lines of inquiry are interesting, including:

• Gathering actual statistical information about the particle flow by studyinga large number of tracks in a recirculating system. This work is being carriedout at the moment

• Varying the physical parameters for the tracer particle, expressed as theStokes number.

• Introducing a variety of relevant obstacles, such as valves and flow meters tostudy the particle behaviour around these and draw inferences to improvetheir design.

• Introducing other particle and varying the concentration of particles throughthe system to observe the influence of other particles on the behaviour ofthe tracer particle.

Another priority is to clearly identify the parameters that give rise to one orother of the identified types of motion by gathering enough statistical evidencestudying a large number of tracks. As mentioned, the point of entry of theparticle into the bend seems to be an important factor, but this has to beconfirmed and the effects of other possible factors have to be studied.

18

4 CONCLUSIONS

This is a first, exploratory study which reveals PEPT as a very useful toolfor studying pneumatic conveying or any other similar fast processes. Longer,systematic and detailed studies will provide detailed information on solidsdistribution, particle velocity variations, residence times and other relevantparameters.

Acknowledgements

The authors would like to thank Dr Tom C. Adamsen, Jan Selbu and ØyvindSteimler at Haukeland University Hospital for their support during the exper-iments. We also wish to thank Yu-Fen Chang for fruitful discussions regardingthe analysis algorithm. Funding for this work from the Norwegian ResearchCouncil under the FRINATEK programme is gratefully acknowledged.

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22

LIST OF FIGURES LIST OF FIGURES

List of Figures

1 Schematic diagram of the experimental system (1 - flowstraightener, 2 - flow meter, 3 - main frame, 4 - flangeconnection, 5 - corrugated hose, 6 - secondary frame, 7 - bend,8 - filter, 9 - T-junction, 10 - bypass line, 11 - compressor) 24

2 Drawing of the flow straightener 25

3 Picture of the Amberlyst A21 particles used in the presentstudy 26

4 Schematics of the four configurations used in the experiments. 27

5 Tracks obtained for configuration A. 28

6 Tracks obtained for configuration B. 29

7 Tracks obtained for configuration C. 30

8 Tracks obtained for configuration D. 31

9 3D plots of three of the tracks turned so as to show the pathsof the particles as clearly as possible 32

23

FIGURES FIGURES

1 2

8

1

11

10

3 6

4 4

7

7

main unit

44

5

59

test s

ectio

n

flow in

flow out

bypass

30 cm 35 cm

~ 240 cm

~ 210 cm

80 cm

20 cm

48

cm

~ 180 cm

injection

injection

Fig. 1. Schematic diagram of the experimental system (1 - flow straightener, 2 -flow meter, 3 - main frame, 4 - flange connection, 5 - corrugated hose, 6 - secondaryframe, 7 - bend, 8 - filter, 9 - T-junction, 10 - bypass line, 11 - compressor)

24

FIGURES FIGURES

14 x 0.8 cm

4.2

cm

6 cm

3 c

m

4.2 cm

Fig. 2. Drawing of the flow straightener

25

FIGURES FIGURES

Fig. 3. Picture of the Amberlyst A21 particles used in the present study

26

FIGURES FIGURESFilter Start of "hose"

Test

secti

on

Inlet

"Long pipe"

Pump

Filter Start of "hose"

Test

secti

on

Inlet

"Short pipe"

Pump

Flow

Straightener

Filter

Test

secti

on

Inlet

Pump

Flow

Straightener

"Long pipe"

"Hose"

"Hose" Filter Start of "hose"

Test

secti

on

Inlet

Pump

Flow

Straightener

"Long pipe"

A B

C D

Fig. 4. Schematics of the four configurations used in the experiments.

27

FIGURES FIGURES

z-coordinate (horizontal) [mm]

y-c

oord

inate

(vert

ical)

[m

m]

-250

-200

-150

-100

-50

0

50

100

150

200

250

0 50 100

A1 -250

-200

-150

-100

-50

0

50

100

150

200

250

0 50 100

A2 -250

-200

-150

-100

-50

0

50

100

150

200

250

0 50 100

A3 -250

-200

-150

-100

-50

0

50

100

150

200

250

0 50 100

A3 -

no G

aussia

n s

mooth

ing

Fig. 5. Tracks obtained for configuration A.

28

FIGURES FIGURES

-250

-200

-150

-100

-50

0

50

100

150

200

250

0 50 100

-250

-200

-150

-100

-50

0

50

100

150

200

250

0 50 100

-250

-200

-150

-100

-50

0

50

100

150

200

250

0 50 100

z-coordinate (horizontal) [mm]

y-c

oord

inate

(vert

ical)

[m

m]

B1 B2 B3

Fig. 6. Tracks obtained for configuration B.

29

FIGURES FIGURES

-250

-200

-150

-100

-50

0

50

100

150

200

250

0 50 100

-250

-200

-150

-100

-50

0

50

100

150

200

250

0 50 100

-250

-200

-150

-100

-50

0

50

100

150

200

250

0 50 100

z-coordinate (horizontal) [mm]

y-c

oord

inate

(vert

ical)

[m

m]

C1 C2 C3

Fig. 7. Tracks obtained for configuration C.

30

FIGURES FIGURES

-250

-200

-150

-100

-50

0

50

100

150

200

250

0 50 100

-250

-200

-150

-100

-50

0

50

100

150

200

250

0 50 100

-250

-200

-150

-100

-50

0

50

100

150

200

250

0 50 100

z-coordinate (horizontal) [mm]

y-c

oord

inate

(vert

ical)

[m

m]

D1 D2 D3

Fig. 8. Tracks obtained for configuration D.

31

FIGURES FIGURES

Fig. 9. 3D plots of three of the tracks turned so as to show the paths of the particlesas clearly as possible

32

LIST OF TABLES LIST OF TABLES

List of Tables

1 Particle and carrier gas velocity data for different flowconfigurations 34

33

TABLES TABLESTable 1Particle and carrier gas velocity data for different flow configurations

Configuration Gas velocity Absolute particle Axial particle Re Stk Rer

[m/s] velocity [m/s] velocity [m/s]

A1 15.9 10.01 9.95 46000 18.23 156.8

A2 11.3 10.05 9.97 33000 12.95 35.73

A3 5.0 3.72 3.70 14000 5.73 34.13

B1 16.9 5.09 4.8 49000 19.38 314.93

B2 10.4 7.00 6.99 30000 11.93 90.67

B3 4.6 3.43 3.33 13000 5.28 31.2

C1 17.1 2.98 0.75 49000 19.61 376.53

C2 13.4 4.95 4.73 39000 15.37 225.3

C3 5.8 2.72 2.70 17000 6.65 82.13

D1 15.7 15.16 15.09 45000 18.00 14.4

D2 10.6 7.45 7.21 31000 12.15 84

D3 4.4 5.15 5.14 13000 5.05 20

34