Kohl, Maximilian H.

Muller, Christoph R.

Prussmann, Klaas P.

Third, James R.

English

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Engineering Conferences InternationalECI Digital ArchivesThe 14th International Conference on Fluidization– From Fundamentals to Products Refereed Proceedings2013A Magnetic Resonance Imaging (MRI) Study ofthe Formation and Interaction of Spouts and JetsMaximilian H. KohlETH Zurich, SwitzerlandJames R. ThirdETH Zurich, SwitzerlandKlaas P. PrussmannETH Zurich, SwitzerlandChristoph R. MullerETH Zurich, SwitzerlandFollow this and additional works at: http://dc.engconfintl.org/fluidization_xivPart of the Chemical Engineering CommonsThis Article is brought to you for free and open access by the Refereed Proceedings at ECI Digital Archives. It has been accepted for inclusion in The14th International Conference on Fluidization – From Fundamentals to Products by an authorized administrator of ECI Digital Archives. For moreinformation, please contact franco@bepress.com.Recommended CitationMaximilian H. Kohl, James R. Third, Klaas P. Prussmann, and Christoph R. Muller, "A Magnetic Resonance Imaging (MRI) Study ofthe Formation and Interaction of Spouts and Jets" in "The 14th International Conference on Fluidization – From Fundamentals toProducts", J.A.M. Kuipers, Eindhoven University of Technology R.F. Mudde, Delft University of Technology J.R. van Ommen, DelftUniversity of Technology N.G. Deen, Eindhoven University of Technology Eds, ECI Symposium Series, (2013).http://dc.engconfintl.org/fluidization_xiv/81A MAGNETIC RESONANCE IMAGING (MRI) STUDY OF THEFORMATION AND INTERACTION OF SPOUTS AND JETSMaximilian H. Köhla, James R. Thirda, Klaas P. Prüssmannb and Christoph R. Müllera(a) Laboratory of Energy Science and Engineering, ETH Zürich, Sonneggstrasse 3,8092 Zürich, Switzerland(b) Institute for Biomedical Engineering, ETH Zürich, Gloriastrasse 35, 8092 Zürich,SwitzerlandT: +41 44 632 34 40; E: muelchri@ethz.chhttp://www.ese.mavt.ethz.chABSTRACTMagnetic Resonance Imaging (MRI) was used to image non-intrusively the formationand interaction of jets and spouts. It was found that the formation and interaction ofjets is critically affected by the particle size and shape, the bed dimensions and thefluidization history.INTRODUCTIONFluidization is the process that transforms a static, packed bed of particles into a liquid-like state. The bed becomes fluidized when the pressure drop across the bed is equalto the weight of the bed per cross sectional area. One of the major advantages offluidized beds over other gas solid reactors is their excellent heat and mass transfercharacteristics. However, despite their widespread application, fluidized beds are stillonly poorly understood on a fundamental level. The poor physical understanding offluidized beds can be explained, at least partially, by the small number of measurementtechniques that can probe non-intrusively the dynamics of these systems, e.g. positronemission particle tracking [1], electron capacitance tomography [2], X-ray attenuation[3] or magnetic resonance imaging (MRI) [4],[5],[6]. An important phenomenon ingas-fluidized beds is the formation of jets and spouts since they critically influence bedmixing as well as heat and mass transfer. Thus, it is important to acquire detailed infor-mation about the geometry and interaction of jets and and spouts. For example, Reeset al. [7] studied the geometry of jets described by their jet half angle, their initial angleof a jet and jet height, forming at multi orifice distributors containing 15 - 37 holes. Thejet parameters determined from the MRI measurements were subsequently correlatedwith the orifice velocity and the pitch size. Pore et al. [8] extended the work of Reeset al. [7] and studied jet-jet and jet-wall interactions. An attractive interaction betweenjets and jets and walls was reported for separating distances less than approximately13 mm. However, an important jetting phenomenon which has not been consideredin fluidized beds is the hysteresis in jet height. Indeed, the pressure drop across thebed [9],[10],[11] and the packing density of the bed [12] are not only a function of thegas velocity but also the fluidization history. This hysteretic behavior has also beenobserved recently in numerical simulations [13],[11],[10]. In this study we extend thework of [8],[7] by systematically investigating the effect of bed and particle parameteron the jetting characteristics.Figure 1: (a,b,c,d) schematic sketches of the distributor plates and (e,f,g) optical mi-croscopy images of the seeds used: (e) Iceland poppy seeds, (f) opium poppy seedsand (g) mustard seeds.Paticle type Diameter Density Minimum Sphericity1dp [mm] ρp [kg/m3] fluidization velocity SUmf [m/s]Iceland poppy 0.74 ± 0.08 1120 0.17 0.61Opium poppy 0.94 ± 0.04 1090 0.24 0.84Mustard 1.34 ± 0.18 1130 0.41 0.98Table 1: Particle diameter, density and minimum fluidization velocity of the seeds used.SETUPThe beds (square cross section) were constructed of poly-methyl-methacrylate (PMMA).The dimensions of the distributor plates used are shown in Fig. 1(a). The size of theorifice (square cross-section) was kept constant with a side length of Lo = 3.6 mm.The properties of the seeds used i.e. their diameter, density and minimum fluidizationvelocity (Umf ) are summarized in Table 1. The projected cross-sections of the seeds,obtained using optical microscopy, are given in Fig. 1. It is worth noticing that thepoppy seeds are highly non-spherical, whereas mustard seeds are roughly spherical.Magnetic resonance imaging (MRI) was applied to image non-intrusively the forma-tion of jets in a bed of seeds. For a detailed description of the MRI technique theinterested reader is referred to e.g. [15]. Here, MRI measurements were performedin a Philips 3T Achieva system equipped with dual Quasar gradients. The maximalgradient strength and slew rate were 40 mT/m and 200 mT/m/ms, respectively. A 2Dspin echo sequence of repetition time 1500 ms, echo time 12.4 ms and flip angle 60◦was used to image the bed of particles. The voxel size was 1 mm × 1 mm × 1 mm.The MR signal was obtained from the seeds (grey-coloured voxels) whereas no signalwas obtained from the gas (black voxels).1S = [(particle volume)/(volume of circumscribed sphere)]1/3 [14]OBSERVATIONS AND DISCUSSIONShape of single jetsFig. 2(a-d) shows jets forming at a single orifice. The jets possess a highly symmet-rical shape. It was found that the height and the shape of the jets was affected by(i) the size of the bed particles (ii) the dimension of the bed (iii) and the fluidizationhistory. For example, the images shown in Fig. 2(a,b) and (c,d) were obtained in abed containing, mustard (dp = 1.3 mm) and Iceland poppy seeds (dp = 0.7 mm), re-spectively. From Fig. 2(a-d) it can be seen that jets forming in a bed of mustard seedsare wider and higher orifice velocities are required form a jet. The larger jet width inthese, large particle, systems can be explained by an increasing gas expansion anglewith increasing particle size [16], whereas the higher orifice velocities, are probablydue to increased gas leakage with increasing particle size. Additionally, Fig. 2(a-d)shows that also the dimensions of the bed can dramatically influence the jet geometry,in particular for the case that jets form in a small bed containing large particles as inthe system imaged in Fig. 2(b). We believe that in these jet height increases due to theconfinement of the gas flow by the side walls, resulting in a bed (partially) approachingminimum fluidization. Filla et al. [16] measured jet angles in 2D beds and found thatjets forming in beds of larger or particles have wider jet angles. It is assumed that theincreased friction between non-spherical particles result in more “stable” packings andconsequently in smaller jet angles. As shown in Fig. 2(e) the fluidization history of thebed also influences the jetting behaviour, i.e. if the jet height is plotted as a functionof orifice velocity a hysteresis loop is observed. The hysteresis loop can be explainedby the occurrence of different packing densities in beds subjected to increasing ordecreasing velocities.Jet-jet and jet-wall interactionFig. 3(a) shows jets forming at a distributor containing two orifices. It can be seen thatfor these configuration the tips of the jets are pointing towards each other at higher ori-fice velocities. These particular jet shapes are probably indicative of jet-jet interactionsleading to a ‘merging‘ gas flow at the centre of the bed. Furthermore, at high orificevelocities the two jets are not of equal height, i.e. the left jet has a height of 19.3 mmwhereas the right jet is 18.2 mm high. However, reducing the orifice velocity to Uo =8.4 m/s resulted in straight jets of equal height, i.e. 25 mm Fig. 3(b). To prove that thatbending of the jets towards each other is indicative of a jet-jet interaction, one orificeof the two-orifice distributor was blocked (Fig. 3c) Interestingly, in this configuration thetip of the jet turned towards the wall, indicating a jet-wall interaction. The distancesbetween the two orifices and the orifice and the wall are 21.6 mm which is outside therange for jet-jet and jet-wall interaction given in Pore et al. [8]. Seeds were used asparticles. They differ in size and shape that makes it difficult to identify a single particleproperty effecting the interactions. The void above the orifice is induced by the dragacting on the particles. The formation of multiple jets or the presence of a wall affectsthe flow profile of the gas entering the bed consequently also the drag force and theshape of the jets. As demonstrated by Filla et al. [16] and Koehl et al. [17] also thesphericity of the particles affects the jet shape. Jets forming in beds of large sphericalparticles are characterized by a larger jet angle.Figure 2: MRI images of jets forming at a single orifice distributor: (a,b) jets forming ina small (L = 46.8 mm) and wide bed (L = 72.0 mm) containing large particles (dp = 1.3mm) and applying an orifice velocity of ∼ 63 m/s; (c,d) jets forming in a bed containingIceland poppy seeds (dp = 0.7 mm) (e) plot of the jet height in a bed of opium poppyseeds as a function of orifice velocity.z (mm)072c)d) e)Figure 3: MRI measurements of jets forming at multi-orifice distributors. The fluidizinggas was introduced via two (a,b), one (c) or three orifices (d,e). Fig. 3(a) shows thebending of the tips of at an orifice velocity of Uo = 17.8 m/s, whereas very little jetinteraction can be observed at an orifice velocity of Uo = 8.4 m/s (Fig. 3(b)) in a bedof Iceland poppy seeds. Fig. 3(c) shows a jet forming at a two-orifice distributor whenone orifice was blocked (opium poppy seeds and Uo = 17.8 m/s.) Jets forming at athree-orifice distributor (Fig. 3 d,e) using opium poppy seeds (dp = 0.9 mm), and anaverage orifice velocity of (d) Uo = 33.0 m/s and (e) Uo = 35.3 m/sFigure 4: Methods applied to induce jet and spout transitionsJet spout transitionFig. 3(d,e) shows jets forming at a distributor plate containing three orifices. In thisconfiguration, a mixture of jets and spouts can be observed. The jets are roughly halfas high as the bed height. As shown in Fig. 3(d,e) it was possible to transform jets intospouts and vice versa. Based on our observations, the use of non-spherical particlesis a requirement to allow this mechanically induced jet-spout transition. Furthermore,the the orifice velocity has to be in the range in which both jets and spouts can form, c.f.hysteresis loop. Three methods have been applied successfully to induce a jet-spoutor spout-jet transition (Fig. 4)a) Oscillation of the entire bed perpendicular to the flow direction. This induces aparticle motion “unbalancing” the spout structure and, thus, results in a jet. Theoscillation had approximately amplitude of 30 mm and a frequency of 2 hertz.b) Penetration of the bed with a cylinder with a diameter similar to that of the spout.The cylinder was then moved horizontally through the upper section of the voidand jet covering particles and subsequently slowly removed vertically from thebed.c) Tilting of the beds. this results in a bed height that varied along the width of thebed. Thus, the pressure drop is reduced at the “lifted” side resulting in the for-mation of a spout at the orifice covered by a bed of comparatively small height.The used tilt angel was approximately 20 degree.CONCLUSIONSMagnetic Resonance Imaging (MRI) was used to image non-intrusively jets and spoutsforming at single and multi-orifice distributors. It was found that jets in beds that con-tained larger particles are wider than jets forming in beds containing smaller particles.Additionally, narrow beds led to higher jets and the fluidization history critically affectedthe jet height. With regard to multiple jets, jet-jet interaction is indicated by the tips ofjets tips towards each other and a local variation of jet height. We also observed thatit is possible to introduce a jet-spout transition by mechanically disturbing the bed.NOMENCLATUREdp particle diameter [mm]do diameter [mm] based on equivalent orifice areaL side length of the square distributor [mm]Lo side length of the square orifice [mm]Uo orifice fluidization velocity [m/s]Ums minimum spouting velocity [m/s]Usf spout formation velocity [m/s]ACKNOWLEDGEMENTSThe authors acknowledge funding from the Swiss National Science Foundation (project200021-132657). Maximilian Häberlin and Lars Kasper are acknowledged for theirhelp with the experiments.References[1] M. Stein, T. W. Martin, and D. J. Parker. Positron emission particle tracking:Particle velocities in gas fluidised beds in gas fluidised beds, mixers and otherapplications. In J. Chaouki, F. Larachi, and M.P. Dudukovic, editors, Non-InvasiveMonitoring of Multiphase Flows, chapter 10. Elsevier, 1997.[2] Bing Du, W. Warsito, and Liang-Shih Fan. Flow dynamics of gas-solid fluidizedbeds with evaporative liquid injection. China Particuology, 4(1):1–8, 2006.[3] P. N. Rowe and D. J. Everett. Fluidized-Bed bubbles viewed by X-Rays 1. ex-perimental details and interaction of bubbles with solid surfaces. Transactions ofthe Institution of Chemical Engineers and the Chemical Engineer, 50(1):42–48,1972.[4] C. R. Müller, D. J. Holland, M. D. Mantle, L. F. Gladden, J. F. Davidson, and A. J.Sederman. Magnetic resonance imaging of fluidized beds. Powder Technology,183:53– 62, 2008.[5] L. F. Gladden. Nuclear magnetic resonance in chemical engineering: Principlesand applications. 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Industrial &Engineering Chemistry Research, 31(7):1784–1792, July 1992.[10] Liangying Wen and Xiaotao T. Bi. Cavity formation and breakup forces in conicalspouted beds. Powder Technology, 206(3):227–232, January 2011.[11] Liangying Wen and Xiaotao T. Bi. Simulation of cavity formation hysteresis in thepresence of granular compaction. Chemical Engineering Science, 66(4):674–681, February 2011.[12] R. Ojha, N. Menon, and D. J. Durian. Hysteresis and packing in gas-fluidizedbeds. Physical Review E, 62(3):4442–4445, September 2000.[13] V. Singh, G.S. Gupta, and S. Sarkar. Study of gas cavity size hysteresis ina packed bed using DEM. Chemical Engineering Science, 62(22):6102–6111,November 2007.[14] Nuri N. Mohsenin. Physical Properties of Plant and Animal Materials: Structure,Physical Characteristics, and Mechanical Properties. Taylor & Francis, 1986.[15] Paul T. Callaghan. Principles of Nuclear Magnetic Resonance Microscopy. Ox-ford University Press, 1991.[16] M. Filla, L. Massimilla, and S. Vaccaro. Gas jets in fluidized beds: The influenceof particle size, shape and density on gas and solids entrainment. InternationalJournal of Multiphase Flow, 9(3):259–267, June 1983.[17] Maximilian H. Köhl, Guang Lu, James R. Third, Klaas P. Prüssmann, andChristoph R. Müller. Magnetic resonance imaging (mri) study of jet formationin packed and fluidized beds. Manuscript under review, 2013.

A Magnetic Resonance Imaging (MRI) Study of the Formation and Interaction of Spouts and Jets

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A Magnetic Resonance Imaging (MRI) Study of the Formation and Interaction of Spouts and Jets

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