ANALYSIS OF VORTEX FINDER GEOMETRY AND ITS INFLUENCE ON CYCLONE S EFFICIENCY AND WEARING PROCESSES BY COMPUTATIONAL FLUID DYNAMICS

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1 Volume2 Number1 March2011 pp ANALYSIS OF VORTEX FINDER GEOMETRY AND ITS INFLUENCE ON CYCLONE S EFFICIENCY AND WEARING PROCESSES BY COMPUTATIONAL FLUID DYNAMICS TomaszNowak 1,JózefGawlik 2,JerzySchmidt 2 1 ABBCorporateResearch,Poland 2 CracowUniversityofTechnology,ProductionEngineeringInstitute,Poland Corresponding author: Józef Gawlik Cracow University of Technology Production Engineering Institute Jana Pawła II 37 St., Kraków, Poland phone: jgawlik@mech.pk.edu.pl Received: 15 December 2010 Abstract Accepted: 7 January 2011 Cyclones are widely used for removal dust of gaseous flows in industrial processes, and various studies were conducted in last decades to improve their performance parameters. However, in process industry, the reliable and uninterrupted operation of the cyclone is of the same importance as performance capabilities. In this work, the effect of vortex finder geometry on wearing processes is analyzed. The work relates to the gas-solid flow, which can be found in real dry-process kiln systems used in cement plants. Numerical studies were performed using Computational Fluid Dynamics software package, and covered three different geometrical versions of analyzed object. The relevant velocities of gas-solid medium within the cyclone, aswellaspressurevalueswerecalculatedwithhelpofdesturbulencemodel.asaresult the design variant offering best balance between performance parameters and wearing resistance was pointed out. Keywords cement industry, cyclone modeling, wearing process, computational methods. Introduction Cyclone dust collectors have been used in many industrial facilities to collect solid particles from gassolid flows. Currently, with new engineering applications of cyclones as dryers or reactors, industrial production systems require a greater understanding of turbulent gas flows, which could lead to rigorous procedures capable of accurately predicting efficiency, velocity and pressure fields[1]. Therearemanytypesofcyclonesforthepurpose of solid particle separation. But this study is concentrated on most typical, centrifugal devices, which are commonly used in cement industry. In this type of separators, the dust-laden gas is initially brought into a swirling motion. The dust particles are slung outward to the wall, and are transported downwardtothedustoutletbythegasflownearthe wall. For the standard, reverse-flow cyclone with a typical rectangular entry, Fig. 1, the swirling motion is achieved by designing the inlet in such a manner, thatitforcesthegastoentertheunitatangentto theinteriorbodywall.asthegasswirls,itmoves axially downwards in the outer part of the separationspace.intheconicalpartofthecyclone,the gasisslowlyforcedintotheinnerregionofthecyclone, where the axial movement is upwardly directed.thisflowpatternisoftenreferredtoasadouble vortex(an outer vortex with downwardly directed axial flow, and an inner one with upwardly directed flow). 27

2 Fig. 1. Sketch of tangential-inlet cyclone with flow pattern indicated. The gas exits the cyclone through the so-called vortex finder(known also as central pipe or tube), which extends downward from the center of the roof. Particles entering the separation space are subjected to an inwardly directed drag and outwardly directed centrifugal force. The centrifugal force, which slings the particles outwards to the wall, is proportionaltotheparticlemassand,thereforetothecube oftheparticlediameter: x 3.Thedragforcetransportingtheparticlestothedustexit,isduetothe flowofgasfromtheoutertoinnerpartofthevortex, and is proportional to x. The largest particles are therefore the easiest to separate[2]. Thegasflowpatterniscyclonesisfairlywell known from experimental evidence collected over decades. For particle trajectories, on the other hand, very limited experimental data are available, so for this reason Computational Fluid Dynamics(CFD) simulations are involved. The first application of CFD techniques to cyclone simulation was presented byboysan[3].fromthisstudyitcameout,thatthe standard k-ε turbulence model is inadequate to simulate flows with swirl, because it leads to excessive turbulence viscosities and unrealistic tangential velocities. After this pioneering work, several studies were done on turbulence modeling in order to improve the prediction of velocity and pressure fields[4, 5]. Recent research works suggest that, the accuracy of numerical solution can be improved by application of Reynolds Stress model(rsm). The today s increase in computing power and mesh generation capabilities have allowed the latest CFD models to include the full three-dimensional shape and to be used for evaluating of design modifications. Thanks to modern computer tools researchers started to analyze the influence of cyclone design parameters on its filtration efficiently[5 9]. Early simulations of Akker and co-workers[10] were limited to small scale cyclones at a moderate inlet Reynolds number, while increased computational power allowed simulations of industrial scale equipment, as stated by Ingham[11]. The capabilities of newly developed large-eddy models to simulate the turbulent flow in a cyclone separator have been reported by Shalaby[12], Pisarev[13], and others[14 16]. Whereas, with help of CFD analysis, an efficient cyclone design is today possible, one practical problem remains unsolved and still requires deeper research effort. It is the pre-diction of wearing and erosion processes of the cyclone walls. The erosion often poses a serious problem in industrial cyclone separators. It is caused by the friction between the particles and the cyclone walls due to the continuous motion of the particles along the cyclone wall. The damagingprocessoftenleadstoaphysicalfractureofthe equipment and costly unscheduled shutdowns. Some researchers assume, that wearing processes are correlatedwiththeangleatwhichtheparticleshitthe cyclone wall[17]. Some other studies suggest that erosion is proportional to the kinetic energy of the particles hitting the cyclone wall and the intrinsic strength of the particles[18, 19]. Similarly, there is no consensus on the location of places subjected to most severe wearing process. The high erosion rates areob-servedattheinlet,butalsointheareaextending180 fromtheinletpoint,aswellasnearthe lower edge of the inlet stream, and at the intersection between the cyclone cylinder and cone[20 22]. It must be pointed out that performed studies were focused on erosion of cyclone walls, and none of the research concerned wearing of vortex finder. Since, in some industrial applications, the central pipe is the most critical element of the production system, in this work the wearing effects caused by vortex finder geometry are studied. Numerical simulations of gas-solid flow phase were carried out using commercial CFD software package. The achieved results showed the influence of the pipe s design on filtration efficiency and wearing rate. Definition of the problem Industrial problem to be solved The main processes of cement production include raw meal preheating and calcination, clinker formation and cooling to achieve a crystalographic structure that meets the required cement specifications. In modern cement plants, raw meal is preheated to calcination temperature in a multi-stage cyclone preheater. The raw meal consists mainly of pulverized calcium carbonate and silicon dioxide. During its heatingattemperaturesfrom100 Cto500 C,the 28 Volume2 Number1 March2011

3 moistureevaporates,andatthelevelof840 Cthe endothermous calcination reaction begins, in which CaCO 3 isconvertedintocaoandco 2.Theinnovation in the entire pyroprocess in modern cement plantsisduetotheuseofanadditionalcalcining vessel, where the raw meal undergoes calcination to alevelof90 95%.Inthisway,thecalcinedrawmeal enters the rotary kiln at a higher temperature, Fig. 2, thus reducing the energy demand and the thermal loadonthekiln[23]. Fig. 2. Schema of cement production. [based on: Thanks to this approach the main calcination process taking place in a separately fired calciner is significantly shortened. But preheating process involves very high temperature, what, together with corrosion atmosphere and wearing processes generated in cyclone, drives to premature failure of the cyclone s components. It relates especially to the central pipe in hot preheater cyclones, marked by dashed rectangular in Fig. 2. Unplanned service actions aiming to replace faulty vortex finder elements often reduce significantly the main benefit of central pipe (the higher throughput). Theproblemcanbesolvedbyapplicationofbetter, and much more expensive, material for the vortexfinderelements.butinthispapertheotherapproach was proposed. Authors analyzed three different geometrical variants for central pipe, and investigated its influence on material wearing process. Numerical method The calculation technique used in this work is based on the solution of the three-dimensional unsteady Navier Stokes. Since the Reynolds numbers forcycloneflowsareusuallyintheorderofre=10 6 and the flow itself is inherently very complex due to strong streamline curvature and anisotropic turbulent structures, these flows require sophisticated turbulence modeling in order to obtain reliable and trustworthy results. Computationally, the direct solution of the Navier-Stokes equation is a very intensive and time consuming task. In some cases it is performed, but mainly for academic purposes. Traditionally, turbulent flows have been computed with Reynolds-averaged Navier-Stokes(RANS) equations.theflowisaveragedoveratimetoobtain a statistically steady flow. However, with the increased computer power, Large Eddy Simulation (LES)hasbecomemoreandmorepopular.InLES the large-scale motion is resolved and the small scales are modeled. But even using small scale modeling, LES is too expensive for most engineering computations today. Approach applied in this study combines methods solving the Reynolds-averaged Navier Stokes equations with large-eddy simulation, forming detached eddy simulation (DES). Because DES resolves the turbulence spectrum up to a certain cut-off wavenumber, it is capable of capturing helical vorticeswhichformaroundthecentrelineofacyclone, afeaturecommonformanyswirlingflows.onthe other hand, by avoiding the necessity for fine mesh structure near the walls, DES is substantially cheaperthanles. Besides analysis of cyclone efficiency, the aim of thisworkwasalsotostudythewearingprocesstaking place on the surface of vortex finder. Generally, wear can be split into two majority categories: wear dominated by the mechanical behavior and wear dominated by the chemical behavior of materials. Since geometry of vortex finder almost does not affect the chemical processes within the cyclone, these typesofwearwillnotbediscussedinthispaper. As a starting point for any discussion of mechanicalwearonthemacroscale,thearchardequationis often used. It states that: δv = kpδl, (1) where δv isthewornvolume, δlistheslidingdistance, Pistheappliedloadand kisthecoefficient of wear. The worn volume δv is the product of contact area A and worn thickness δh, and applied load can bedescribedasstaticpressure σ N multipliedbycontact area A. In additional, the sliding distance δl maybecalculatedastangentialvelocity v T intime incrementdt.thewearrate(lossofthevortextube thickness due to wearing process) is finally given by: δh dt = kσ Nv T. (2) Volume2 Number1 March

4 Assumingthecoefficientofwear kisthesame for all analyzed geometrical variants, the estimation of the wear rate for different vortex finder designs can be managed simply comparing the value of staticpressure σ N multipliedbytangentialvelocity v T nearthevortexfinderwall.itmustalsobestated that although it is widely used, the Archard s equation only serves as the estimation, providing an order of magnitude for the wear process. Geometrical configuration For convenience, the dimensions of various cyclone parts are usually stated in dimensionless form asaratiotothecyclone souterdiameter, D.This method allows a comparison between the cyclone designs. The cyclone under instigation features: d/d = 0.5; H/D = 2.0; h/d = 0.37,andahalf-angleof α = 25 intheconicalregion,fig.3. T in = 922 C(temperature), v in = 13m/s(velocity). The thermal boundary conditions were applied based on the infrared measurements of the isolation shield, which covers the cyclone: topplate=80 C(const.temperature), cylindricalsurface=80 85 C(varying), conicalsurface= C(varying). The boundary conditions used in CFD calculations are schematically shown in Fig. 5. Fig. 5. Boundary conditions: temperature of the cyclone s isolation shield. Fig. 3. The reference geometry of the cyclone. The whole numerical model generated for this case consisted of near volume elements: almost elements were used to model the rawmealinsidethecyclone,while forthe cyclone structure itself(steel construction, shamot bricks, isolation shield, etc). About elements were used for generation of vortex finder model, Fig.6(right). Beside the basic, straight geometry of vortex finder, two other design variants were considered: the centralpipewaswiden +7 andnarrowed 7,as schematically presented in Fig. 4. Fig. 6. Real vortex finder before assembly within the cyclone(left), and its numerical representation(right). Results and discussion Fig. 4. Geometrical variants of central pipe: +7 (left), 7 (right). Boundary conditions and numerical grid Based on the documentation of industrial process taking place in analyzed cyclone, the inlet parameters were set in following way: All three analyzed design cases were compared in terms of achieved velocities(radial, tangential and axial components), static and dynamic pressure levels,aswellasvaluesofwearrateestimate.thetemperature distribution was also studied, but it was realized that introduced design modifications of the vortex finder did not affect the temperature field significantly(the difference between cases was be- 30 Volume2 Number1 March2011

5 low2 C).Thetypicaltemperaturefieldisshownin Fig. 7. Fig.9.Velocity magnitude[m/s]:variant 7. Fig.7.Temperature[ C]:referencedesignvariant. Velocities The flow pattern within the cyclone is presented in Fig. 8. As expected, the double-vortex swirling motion is evident. Maximal values of velocity, about 36m/s,werereportedfordesigncase 7.Thisvelocityvalueisabout5m/shigherthanfortwoother design variants. Moreover, major differences can be noticed in the velocity field distribution. In variant +7 aswellasthereferencedesign,theflowofthe medium is equalized across the whole cross-section ofthetube,whileforcase 7 thecentreofthe vortex finder is almost motionless, Fig. 9. Pressure The results of static pressure calculations show that the most advantageous is the reference design of vortex finder(with straight pipe). In this case max. pressure level reaches about 720 Pa(gauge value). Two other design versions generate up to 10% higher values of static pressure. Highly unfavorable behaviorcanbeobservedinversion 7,where large dynamic pressure gradient can be found near thetube sedge.itgenerallydrivestomuchlowerefficiency of the cyclone filtration, since the particles, which just entered the inlet, can be sucked directly intothevortexfinderoutlet,fig.10.inaddition,it contributes to the increased wearing rate in this area. Fig.10.Dynamicpressure[Pa]:variant 7. Fig. 8. Flow pattern within the cyclone: reference design. Wearing rate estimation Based on the achieved results for proposed wearingrateestimationonecanstate,thatthemost Volume2 Number1 March

6 promising design is just the reference one with straightvortexfinder.thedesignvariant +7 also offers good wearing resistance, close to the reference solution.incontrast,thetubeversion 7 exhibits about 20% higher wearing rate of pipe s external surface, Fig. 11, therefore is not recommended. some contribution to this topic is already provided, more systematic research, addressing especially the experimental aspect of cyclone s wearing, is needed. Authors continue working on this topic. This work was supported by Polish Ministry of Science and Education Project KB/65/13730/IT1- B/U/08. References [1] Meier H.F. and Mori M., Anisotropic Behavior of the Reynolds Stress in Gas and Gas-Solid Flows in Cyclones, Powder Technology, 101, , [2] Hoffman A.C. and Stein L.E., Gas Cyclones and Swirl Tubes: Principles, Design, Operation, Springer-Verlag, Berlin, Fig. 11. Wearing estimate(near tube walls)[mpa/s]: variant 7. InthispapertheCFDanalysisofthereverseflow cycloneusedinkilnprocessoftherealcementplan has been presented. Three different design variants of cyclone s vortex finder have been analyzed. The performed study covered analysis of the temperature field within the cyclone, as well as velocity- and pressure distributions. In addition, the model for wearing rate of the central pipe s surface has been proposed and relevant calculations were managed. It was generally found that temperature of the centralpipeisverylittleaffectedbyitsdesignand differences in temperature results for different variantsareverylow(below2 C).However,basedonthe velocity and pressure outcome, it can stated that designversion 7 worksgenerallytheworst.there is almost motionless state in the center of the central pipe,andmostoftheflowismanagednearthetube walls. It enhances(about 20%) the wearing processes happening on the tube s external surface, and additionally drives to high pressure gradient near the tube s edge. In consequence, one can also expect lower filtration efficiency of the cyclone with this kind of the vortex finder. Two other design options(referenceone,and +7 )generatesimilarresultsandboth of them can be equally recommended. Itmustbealsoconcluded,thatinspiteofsimple cyclone s construction, the flow pattern is quite complex, and not all physical phenomena are fully understood. It refers especially to the estimation of the wearing processes, which affect the reliable and uninterrupted operation of the cyclone. Although, [3] Boysan F., Ayers W.H., and Swithenbank J.A., Fundamental Mathematical Modeling Ap-proach to Cyclone Design, Trans. Inst. Chem. Eng., 60, , [4] Hoekstra A.J., Derksen J.J., and Van Den Akker H.E.A., An experimental and numerical study on turbulent swirling flow in gas cyclones, Chemical Engineering Science, 54, , [5] Sommerfeld M., Ho C.H., Numerical calculation of particle transport in turbulent wall bounded flows, Powder Technology, 131, 1 6, [6] WangB.,XuD.,XiaoG.andYuA.,Numerical study on gas-solid flow in a cyclone separator, Proceedings of 3rd International Conference of CFD in the Minerals an Process Industry, Melbourne, Australia, December, [7] SinghV.,SrivastavaR.,VitankarV.,andBasuB., Simulation of gas-solid flow and design modifications of cement plant cyclones, Proceedings of Fifth International Conference of CFD in the Process Industry, Melbourne, Australia, December, [8] NorilerD.,VeginiA.,SoaresC.,BarrosA.,and MeierH.,Anewroleofreductioninpressuredropin cyclones using computational fluid dynamics techniques, Journal if Chemical Engineering, 21(01), , [9] FiciciF.,AriV.,andKapsizM.,Theeffectsofvortex finder on the pressure drop in cyclone separators, International Journal of the Physical Sciences, 5(6), , [10] Hoekstra A.J., Derksen J.J., and Van Den Akker H.E.A., An experimental and numerical study of turbulent swirling flow in gas cyclones, Chemical Engineering Science, 54, , Volume2 Number1 March2011

7 [11] Ingham D.B. and Ma L., Predicting the performance of air cyclones, International Journal of Energy Research, 26(7), , [12] Shalaby H., On the potential of large eddy simulation to simulate cyclone separators, PhD Dissertation, Chemnitz University of Technology, [13] Pisarev G.I., Hoffmann A.C., Peng W., and Dijkstrac H.A., Large Eddy Simulation of the vortex end in reverse-flow centrifugal separators, Applied Mathematics and Computation, 217 (11), , [14] Boysan F., Swithenbank J.A., and Ayers W.H., Mathematical modeling of gas-particle flows in cyclone separators, Encyclopedia of Fluid Mechanics, Volume 4, Gulf Publishing Company, Houston, Texas, [15] UtikarR.,DarmawanN.,TadeEvansG.,andGlenny M., Hydrodynamic Simulation of Cyclone Separators, Computational Fluid Dynamics, InTech Publisher, [16] Peng W., Hoffmann A.C., Dries H.W.A., Regelink M.A., and Stein L.E., Experimental study of the vortex end in centrifugal separators: The nature of the vortex end, Chemical Engineering Science, 60(24), , [17] FinnieI.,Somereflectionsonthepastandfutureof erosion, Wear, 186(1), 1 10, [18] Khalil Y.K., and Rosner D.E, Erosion rate prediction and correlation technique for ceramic surfaces exposed to high speed flows of abrasive suspensions, Wear, 201(1 2),64 79, [19] FanJ.,YaoJ.,ZhangX.,andCenK.,Experimental and numerical investigation of a new method for protecting bends from erosion in gas-particle flows, Wear, 251(1 12), , [20] Sommerfeld M., Validation of a stochastic Lagrangian modelling approach for inter-particle collisions in homogeneous isotropic turbulence, International Journal of Multiphase Flow, 27(10), , [21] Schade K. et al., Experimental and numerical investigation of particle erosion caused by pulverized fuel in channels and pipework of coal-fired power plant, Powder Techn., 125, , [22] CortésC.andGilA.,Modelingthegasandparticle flow inside cyclone separators, Progress in Energy and Combustion Science, 33, , [23] Fidaros D.K., Baxevanou C.A., Dritselis C.D., and Vlachos N.S., Numerical modelling of flow and transport processes in a calciner for cement production, Powder Techn., 171, 81 95, Volume2 Number1 March