experimental and numerical study of hydrodynamics in … ·...
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EXPERIMENTAL AND NUMERICAL STUDY OFHYDRODYNAMICS IN A CIRCULATING FLUIDIZED BED
SIRPA KALLIO+, JOHANNA AIRAKSINEN+, MATIAS GULDÉN*, ALF HERMANSON*,JUHO PELTOLA‡, JOUNI RITVANEN#, MAIJU SEPPÄLÄ+, SRUJAL SHAH#,
VEIKKO TAIVASSALO+
+VTT Technical Research Centre of Finland, P.O.Box 1000, FI02044 VTT, Finland*Åbo Akademi University, Heat Engineering Laboratory, Piispankatu 8, FI20500 Turku, Finland‡Tampere University of Technology, Department of Energy and Process Enginering, P.O.Box 527, FI33101 Tampere, Finland#Lappeenranta University of Technology, Department of Energy and Environmental Technology,P.O.Box 20, FI53851 Lappeenranta, Finland
[email protected], +358 20 7224015
ABSTRACT
Fluid dynamics of circulating fluidized beds (CFB) can be computed with differentmethods. Typically simulations are conducted as transient to fully describe thecomplicated flow patterns of dense gassolid suspensions. The computational mesh usedin these simulations must be reasonably fine, which in the case of large industrialprocesses leads to unfeasibly long computations. To improve the modelling capabilitiesit is necessary to develop faster simulation methods. The most attractive approachseems to be timeaveraged modelling facilitating steadystate simulation of afluidization process.
The model development and closure of the timeaveraged equations requires transientsimulations. A prerequisite for utilisation of the transient results is that the simulationsare validated by measurements. For that purpose, a laboratory scale 2D CFB was built atÅbo Akademi University and experiments were carried out. The apparatus and theexperiments are described in the present paper. The behaviour of the CFB was videorecorded and the videos were analysed to determine the average voidage distribution inthe CFB. The pressure distribution in the bed and the circulation rate of solids were alsomeasured. In addition, PIV measurement methods were tested to evaluate thepossibilities to analyse the velocities of individual particles and particle clusters.
Two experiments done in the laboratory scale 2D CFB were simulated as transient withthe Fluent software using the kinetic theory models of granular flow. The timeaveragedvolume fractions and velocities of gas and particles were determined for the calculationperiod in the studied cases. The simulation results were compared with measured flowproperties, observing a reasonably good agreement. The results were further analysedwith the aim to develop timeaveraged CFD models. Since CFB simulations usuallyrequire a coarse mesh, additional simulations were conducted to evaluate the meshdependence of the results with the aim to later develop closure relations that take themesh into account.
Keywords: CFB, CFD simulation, experimental, PIV
INTRODUCTION
Fluid dynamics of circulating fluidized beds (CFB) can be computed with differentmethods. Typically the simulations are conducted with EulerianEulerian models basedon the kinetic theory of granular flow. The simulations are run as transient to fullydescribe the complicated flow patterns of dense gassolid suspensions. These modelsrequire a fine computational mesh, which in the case of large industrial processes leadsto unfeasibly long computations. To improve the modelling capabilities, it is necessaryto develop faster simulation methods. The most attractive approach seems to be timeaveraged modelling facilitating steadystate simulation of fluidization.
Several attempts to develop models for steadystate simulations, and for coarsemeshsimulations that require similar equation closures, have been presented in the literature.Agrawal et al. (2001) and Andrews et al. (2005) studied the average drag and stressterms through simulations in small domains with periodic boundary conditions. Zhang& VanderHeyden (2002) suggested an addedmass force closure for the correlationbetween fluctuations of the pressure gradient of the continuous phase and fluctuations ofsolids volume fraction. De Wilde (2007) analyzed the same term from simulations andaccounted also for the drag force in the derivation of a new closure. Zheng et al. (2006)presented a twoscale Reynolds stress turbulence model for gasparticle flows.
Anisotropy and the influence of the real CFB geometry, both essential for successfuldescription of CFB hydrodynamics, were largely left out in the papers listed above.These topics were, however, addressed in earlier work on steady state modelling done atLappeenranta University of Technology (LUT) and at Åbo Akademi University (ÅA) inthe 90’s but accurate equation closure was not possible at that time. New measurementtechniques and computational methods have now made a more elaborate equationclosure feasible by providing us detailed data on the fluctuations in velocities andvoidage. Transient simulations can now be utilized in development of equation closures.
With the objective to develop models for steady state simulation, work is now inprogress in a threeyear joint project at the authors’ research units. A prerequisite forutilisation of the results from transient simulations is validation of the models bymeasurements. For that purpose, a laboratory scale 2D CFB was built at ÅA andexperiments were carried out. The present paper deals with experimental and numericalstudies conducted in the joint project, which are related to this laboratory scalecirculating fluidized bed at ÅA.
EXPERIMENTAL
The 2D circulating fluidized bed
A 2D CFB unit was constructed at Åbo Akademi University (Guldén, 2008). The heightof the riser is 3 m and width 0.4 m. The distance between the riser walls is 0.015 mwhich renders the bed twodimensional. The CFB pilot is shown in Figure 1. The 0.4 mwide riser walls, the side walls of the solids separator and the walls of the standpipe aremade of polycarbonate plates and the rest of plywood and metal plates. Air distributorconsists of 8 equally spaced air nozzles. Solids separation is done in a simple separation
box from which particles fall through the return leg into a loop seal consisting of twofluidized 10 cm wide sections. The amount of solids in the loop seal and the solidscirculation rate are determined from videos taken of the loop seal region when the gasflow to the loop seal is abruptly cut off. The fluidization air flow rates to the riser andthe loop seal are measured and controlled by Bronkhorst HighTech B.V. Thermal MassFlow Controllers. Pressure was measured at several locations round the CFB with acustom made manometer.
Figure 1. The 2D CFB system: a rough sketch, the 2D CFB, and its lower part with the wind box and theloop seal.
The experimentsThe bed material consisted of spherical glass particles with material density of 2480kg/m3 and the Sauter mean diameter of 0.385 mm. In the ambient conditions of theexperiments, the terminal velocity of an average size particle is 2.9 m/s. Twoexperiments with superficial gas velocities 3.1 m/s and 3.5 m/s were conducted. Thesolids mass in the riser was in the two experiments 2.76 kg and 2.50 kg, respectively.
In the experiments, a dense vigorously fluctuating bottom region was observed withhighest particle concentration in the wall regions. Above the bottom zone, thesuspension travelled mainly upwards in form of clusters and more dilute suspensionbetween the clusters. At the side walls clusters were seen to fall down. Fig. 2 illustratesthe flow structure at the bed bottom and at 114145 cm height. A denser wall regionwith falling clusters is seen at both heights. The figures show long narrow clusters andstrands everywhere in the bed. The widths of the narrowest strands observed were about2 mm. At the higher elevation, solids concentration inside the clusters was significantlylower than in the clusters in the bottom region.
The behavior of the CFB at bed bottom and at 114145 cm height was video recorded inboth experiments. From each case a 30 s section of the video was analyzed to estimatethe average volume fractions in the studied locations. The estimate was based on acomparison between the local instantaneous grey scale values of the video image withthe reference values corresponding to an empty bed and to a packed bed. In theinterpretation of the concentration, BeerLambert law of absorbance of light wasutilized. The reference image corresponding to an empty bed at bed bottom and the
reference corresponding to a packed bed higher up were not available. Thus thereference values had to be extrapolated from other regions. Since the lighting conditionswere not fully uniform, the poor reference values reduced the accuracy of the method.
Figure 2. Images from the experiments, from the left: at the bottom at U0=3.1 m/s, at the bottom at U0=3.5m/s, at 1.141.45 m height at U0=3.1 m/s, and at 1.141.45 m height at U0=3.5 m/s.
Evaluation of the feasibility of PIV as a tool for analysis of solids velocities
To get more detailed data for model development and validation, local solids velocitiesshould be measured along with the local voidage. A good method for velocity analysisis Particle Image Velocimetry (PIV) which was tested here to confirm the feasibility ofthe method. Different imaging methods were tested. In PIV, images of a particleladenflow are recorded with a short time delay between consecutive frames. The images arethen divided into smaller interrogation areas, the intensity profiles of which are crosscorrelated between the consequent frames. The displacement of the particles can becalculated from the correlation peak and translated to velocity. For backlit images thelocal instantaneous void fraction can be estimated from the BeerLambert law on basisof the average gray scale value of the interrogation area.
In this study a LaVision ImagerPro HS high speed camera was used with continuousand pulsed lighting. The measurement setup for pulsed light is shown in Fig. 3. Thecamera has a CMOS sensor with a resolution of 1280x1024 pixels. The maximumrecording frequency of the camera at the full resolution is 638 Hz for singleframe and518 Hz for doubleframe images. In the doubleframe mode, the two frames to becorrelated are recorded with a very short time delay followed by a longer delay beforeanother double frame. In the singleframe mode, images are recorded with even timeintervals and each pair of consecutive frames is correlated. Before the crosscorrelationthe intensities were locally normalized and inverted for the backlit images.
Figure 3. The measurement setup for the pulsed light measurements.
Pulsed light source
To control the exposure in the doubleframe mode of the camera, a pulsed light source isneeded. Due to the limitations set by the CFB geometry, the light can only be directedfrom the front or from the behind. Backlighting (light comes from behind) creates ashadow image with the particles in the focus plane sharp and those outside of it blurred.The crosscorrelation algorithm weights the sharp particles as they create the highestintensity peaks in the inverted image. However, the method can only be used if the lightcan penetrate through the suspension. In the CFB this is not the case at the lowest voidfractions.
Figure 4 shows samples of the recorded images of a 45x34 mm2 window located in themiddle of the riser at 130 cm height, and the vector fields calculated from them.Sections c and d in Figure 4 show examples of a situation in which the light doesn'tpenetrate the clusters and the velocity calculation fails.
Figure 4. Instantaneous velocity vector fields overlaid on theoriginal images recorded using a diodelaser backlight. Every 16th
of the calculated vectors is displayed. The measurement window ismarked with red in the schematic on the right.
With frontlighting the light isdirected from the direction ofthe camera. Intensity peaksare generated by the particlesclosest to the front wall. Thevelocities of these particlescan be determined at any voidfraction but these measuresonly represent the particlesright next to the wall. Anotherdrawback is that there is noeasy way to estimate the voidfraction. The frontlit methodcan be used to study walleffects and those regionswhere the backlighting failsdue to low void fractions.
In this study a diodelaser was used for its portability. The power of the laser limits thesize of the usable measurement window to around 5080 mm. With these image sizesdoubleframe imaging has to be used to achieve a short enough time delay betweencorrelated frames. As the maximum pulse length of the laser also decreases as thetriggering frequency increases, no time resolution can be obtained and the calculatedfields have to be considered as discrete samples. The sampling can be spread over a longperiod of time without collecting an unreasonable amount of data, making thecalculation of representative average fields more convenient. The spatial resolution ofthe method is around 5 times the diameter of the particles.
Continuous light source
To get a larger measurement area than the available laser could provide, high frequency
fluorescent tubes were used to provide a backlight. Tests showed that with the singleframe imaging mode, a 600 Hz imaging frequency and a measurement window over thewhole width of the experimental device is a suitable combination. The individualparticles are not distinguishable, but the PIV correlation algorithm produced anacceptable solids velocity field except in regions with the lowest voidage. The spatialresolution was around 20 mm. The light source used proved to be slightly inadequate toallow a short enough exposure time to eliminate the motion blur at the highest solidsvelocities. A more powerful a light source is needed for future measurements. For goodaverage fields the data has to be collected over a long period of time, at least for 20seconds. Figure 5 shows a sample of a velocity vector field measured over the wholewidth of the riser just below the solids return inlet. The downward flow near the wallcan be seen on the left side, while the incoming solids disturb the wall layer on the rightside.
Figure 5. Velocity vectors overlaid on the original image, constant backlight. Every 4th one of thecalculated vectors is displayed. The measurement window is marked with red in the schematic on theright.
CFD SIMULATION OF THE 2D CFB AND VALIDATION
Models used
The two experiments conducted in the 2D CFB were simulated with the models basedon the kinetic theory of granular flow available in the Fluent 6.3.26 CFD software(Fluent, 2006). The continuity and momentum equations used in the transientsimulations can be written for phase q (gas phase denoted by g and solid phase by s) asfollows:
0=,
k
kqqmqqmq
xu
t ∂∂
+∂
∂ ραρα (1)
( )isiggsiqmq
qsi
q
k
ikqM
q
k
ikqq
iq
k
iqkqqmqiqqmq
uuKg
xp
xxxp
xuu
tu
qs,,
)1(
,,,,,
)1(
=
−−++
∂
∂−
∂
∂+
∂
∂+
∂∂
−∂
∂+
∂
∂
+δρα
δτατα
αραρα
(2)
where t is time, x is spatial coordinate, volume fraction, density, u velocity, p gasphase pressure, ps solids pressure, g gravitational acceleration, K drag coefficient, qsKronecker delta, laminar stress, and M local scale turbulent stress.The granular temperature was obtained from a partial differential equation using theSyamlal et al. (1993) model for granular conductivity. The solid phase granularviscosity was calculated from the model by Syamlal et al. (1993). The solids bulkviscosity and solids pressure were calculated from the formulas by Lun et al. (1984).The k turbulence model producing the local scale turbulent stress was the versionmodified for multiphase flows (“dispersed turbulence model”, Fluent (2006)). At thewalls, the partial slip model of Johnson and Jackson (1987) was used for the solids withspecularity coefficient equal to 0.001 and the free slip boundary condition was used forthe gas. For gasparticle interaction, a combination of the Wen & Yu (1966) (at thevoidage above 0.8) and Ergun (1952) equations was used. The frictional solids stresseswere calculated from the model of Schaeffer (1987). The firstorder discretization fortime stepping and the secondorder spatial discretization were employed. The 2D grid ofthe simulation consisted of 31648 elements with the mesh spacing of 6.25 mm. Thetime step in the simulation was 0.2 ms. Air inflow velocity at the bottom was describedby a function that reproduces the orifice locations.
Simulation results and comparisons with measurements
For comparison with measurements, thesimulations were first run till a steadystate and then for an extra 10 s timeperiod to obtain averages of the velocitiesand void fractions. Fig. 6 illustrates thetypical flow patterns obtained in thesimulations and the corresponding timeaverages at the two fluidization velocities.The flow patterns are similar to the onesobserved in the experiments. Due to thecoarseness of the computational mesh,however, the simulated clusters are wider,the thinnest ones being 1.5 cm wide.Otherwise the flow structure is correctwith a dense bottom bed and dense wallregions with downflow of solids.
Figure. 6. Instantaneous and average solidsvolume fractions at U0=3.1 m/s (left) and atU0=3.5 m/s.
A comparison between measured and simulated solids circulation rates was also done.Both in the experiments and in the simulations, an increase in the fluidization velocityincreased the circulation rate. Moreover, solids circulation rates obtained in thesimulations were of the same order as measured in the laboratory unit.
Gas phase static pressure was measured at several elevations at the wall opposite to thesolids return. Figure 7a shows a comparison between the measured and simulatedaverage pressure profiles at the wall. The simulated pressure profile is close to themeasured one at the lower fluidization velocity but a clear discrepancy is seen at thehigher velocity. From the measured pressure profile, the vertical voidage profile couldbe estimated. The results are depicted in Fig. 7b together with the average solids volumefraction profile obtained from the simulations. In addition, the average values obtainedfrom the analysis of the videos, taken at two elevations during the experiments, aremarked in Figure 7b. In general, the match is reasonably good considering theinaccuracies in both of the experimental methods.
a) b)Figure. 7. Comparison of measured and simulated vertical pressure (a) and solid volume fraction (b)profiles in the experiments with fluidization velocities 3.1m/s and 3.5 m/s.
From the videos, the average lateral solid volume fraction profiles were determined at20 cm and 130 cm heights. Figure 8 shows a comparison between the measured and thesimulated lateral solids volume fraction profiles. In the profiles determined from thevideos there is clear asymmetry. This is very likely caused by the uneven lighting in theexperiments combined with the lack of good reference images for fixing theconcentration scale. Otherwise the measured and simulated profiles are in goodagreement. The thin dense wall region observed in the simulations could also bedetected in the experimental results.
Figure 8. Comparison of simulated and measured lateral solid volume fraction profiles at 20 cm and 130cm heights at fluidization velocities 3.1 m/s and 3.5 m/s.
From the simulationresults, the average lateralvelocity profiles could becalculated. Figure 9 showsthe profiles obtained in thetwo simulations at 130 cmheight. A downflow regionis seen at the walls. Thiscorresponds to the visualobservations and thevelocity fields determinedby PIV. No averagevelocity profiles weremeasured at this stage.
Figure 9. Lateral profiles of the solids vertical velocitycomponent at fluidization velocities 3.1 m/s and 3.5 m/sdetermined from the simulations 130 cm height.
Simulations of particle mixtures
In fluidized bed combustors the solidphase consists of particles with differentsizes, densities and compositions. Thusany model used to describe CFBhydrodynamics should also beapplicable to mixtures of particles.Experiments are planned to beconducted in the 2D CFB usingmixtures of particles to serve asvalidation of multiparticle models. Atthis stage, preliminary simulations weredone to test the capability of the Fluentsoftware to simulate mixtures. In thefirst simulation, a single solid phase wasdivided into two and Fluent was provedto correctly treat this division. Next,different mixtures of two types ofparticles were simulated. An example ofresulting segregation of the two particlesizes is shown in Fig. 10. The resultsseem reasonable but validation throughmeasurements is required and will bedone in the future.
a) b) c) d)Figure 10. Simulation results for a particle mixturewith 10 mass% of particles with dp=0.650 mm and90 mass% with dp=0.270 mm. Fluidization velocityU0 = 2 m/s. a) volume fraction of particles withdp=0.650 mm, b) volume fraction of particles withdp=0.270 mm, c) 10 s average volume fraction ofparticles with dp=0.650 mm, d) 10 s average volumefraction of particles with dp=0.270 mm.
TRANSIENT CFD MODELING USING COARSER MESHES
As long as timeaveraged models are not available, the most feasible alternative tosimulate gassolid flow in a large scale CFB is to use a coarse mesh in a transientsimulation and to timeaverage the results. In coarse mesh simulations information on
the local flow structures is lost when the flow gets filtered by the mesh. The lostinformation must be brought back into the system through closure models to predict thehydrodynamics correctly. This corresponds to what has to be done in timeaveragedmodelling where all information on time variations of the flow needs to be fed to themodel through equation closures.
In this study, the effect of mesh size was evaluated by simulating the experimentconducted in the 2D CFB at fluidization velocity 3.1 m/s in three different meshes withspacings 0.625 cm, 2.5 cm and 5 cm. The time step in all simulations was 0.2 ms. Themodels were the same as used in the validation simulations of the previous section. Theonly difference is in the description of the gas inlet. With the coarse meshes used here, itis not possible to describe accurately the gas flow through the separate orifices. Thus theentire bottom of the riser was defined as an inlet with a constant gas inflow velocity.
In each of the simulations, computations start from a situation where solids areuniformly distributed throughout the riser. After 10second simulation, a stable solutionis reached, with solids leaving the riser from the outlet and entering the bed through thereturn leg. The mass flow rate of solids entering the system through the return channel isadjusted in such a way that, at any instant of time the mass in the riser remains the sameas in the experiment. Averaging is then performed for another 20 seconds. Timeaveraged contours of solid volume fraction and the Favreaveraged gas y velocity fordifferent mesh sizes are shown in Figure 11.
Figure 11. Simulations with mesh spacings 0.625 cm, 2.5 cm and 5 cm: a) Timeaveraged solid volumefraction and b) Favreaveraged gas y velocity.
As seen from Fig. 11a, with the smaller mesh spacing of 0.625 cm, the time averagedsolid volume fraction contours shows presence of small scale clusters. When the meshspacing is increased, the timeaveraged solids volume fraction shows a much more
uniform distribution and the mesoscale structures of the flow get filtered. This is alsoseen in Fig. 11b. When the mesh spacing is increased, channelling in the middle of theriser and wider wall layers are observed in the averaged velocity profiles. Thus thesimulations show that the results are mesh dependent. To obtain the same results indifferent meshes, closure equations that take the mesh into account need to bedeveloped. A fine mesh is computationally expensive. For the above gassolid flowcalculations of 30 s real time, the CPU time consumed by a single processor was around8 hours with the 5 cm mesh spacing, with 2.5 cm mesh spacing it was 14 hours and with0.625 cm mesh spacing 129 hours.
TOWARDS TIMEAVERAGED MODELING
A transient simulation in a coarse mesh is one alternative for simulating large CFBs.Long simulation times are required to obtain a representative average flow field throughaveraging of the transient simulation results. A faster alternative would be to directlycompute the average flow field from steady state models. Unfortunately, such modelsare not available to date. For that purpose, models for CFBs are developed in the currentresearch project.
As an initial step in the development, the instantaneous continuity and momentumequations, Equations (1) and (2), are averaged over time. First we need to define theaverage quantities. A time average, also called Reynolds average, of a variable φ isdefined as
∫∆+
∆=
tt
tdt
tφφ 1 (3)
The instantaneous values can now be written as 'φφφ += . This average is used for thevolume fraction q and pressure p. Thus, e.g., 'qqq ααα += and 'ppp += , and,consequently, 0' =qα and 0' =p . A Favre average or a phaseweighted average isdefined as follows
q
q
αφα
φ =⟩⟨ (4)
Favre averaging is applied on velocities and we denote the average velocity by⟩⟨≡ iqiq uU ,, . For the instantaneous velocity we have then ",,, iqiqiq uUu += . Note that
0" , =⟩⟨ iqu but practically always 0" , ≠iqu .
We obtain now the timeaveraged continuity equation for phase q ( qm is a constant):
0=,
k
kqqqmqmq
xU
t ∂∂
+∂
∂ αρρα (5)
and timeaveraged momentum equation for a phase q
( )k
iqkqqqmqs
i
qsigigs
k
ikqM
q
k
ikqq
iq
iqiqmq
k
iqkqqmqiqqmq
xuu
xp
uuKx
xxp
xpg
xUU
tU
qs
∂
′′′′∂−
∂
∂−−−+
∂
∂+
∂
∂+
∂′∂′−
∂∂
−∂
∂+
∂
∂
+ ,,)1(,
,,,,
)1(
=
αρδ
τα
ταααρα
ραρα
δ
(6)
The terms on the right hand side are the gravitation term, pressure term, pressurefluctuation term, laminar stress, turbulent stress, drag force, solid pressure term, and theReynolds stress term. The gravitation and pressure terms can be calculated from thebasic average flow properties but the rest of the terms need to be modelled. To evaluatethe importance of the different terms, a long (about 15 min) simulation of theexperiment at U0=3.5 m/s was conducted and the computation results were timeaveraged. Fig. 12 shows the terms in the timeaveraged balance equation for the solidphase vertical velocity component. The time derivative in the timeaveraged equationcan be discarded in this case. In order to simplify the comparison of the terms, theconvection term in Equation (6) is moved to the right hand side.
k
yskssms
xUU
∂
∂− ,,:1
ρα
ysms gρα:2
y
sxp
∂∂
− α:3
ys x
p∂
′∂′−α:4
k
ykss
x∂
∂ ,:5τα
k
ykqM
s
x∂∂ ,:6 τα
( )sygygs uuK −:7
y
s
xp
∂∂
−:8
k
ysksssm
xuu
∂
′′′′∂− ,,:9
αρ
Figure 12. The different terms in the timeaveraged balance equation of the solids vertical velocitycomponent plotted on the riser centre line as a function of height.
Although the simulated time period was very long, the average velocities are still notperfectly smooth functions of the spatial coordinates. Consequently, the velocityderivatives are even more restless which is seen in the large spatial variations in theconvection term. However, a comparison of the magnitudes of the different terms canstill be made. In the upper part of the riser, the largest terms are the drag term and thegravitation term. The Reynolds stress term and the term originating from the correlation
between fluctuations in gas phase pressure and solids volume fraction are significant inthe bottom region. The turbulent stress term and solid pressure term are small over thewhole riser length on the centre line. However, a wider analysis of the terms in thebottom bed showed that solids pressure can be significant in the dense wall regionswhereas the turbulent stresses are insignificant everywhere in the CFB. The largestterms to be modelled are the drag forces and the Reynolds stresses. Modelling of thepressure fluctuation term was considered in De Wilde (2007) and a similar approachcould be considered also here. The average drag forces have typically been described bymeans of a correction to the drag coefficient as a function of suspension density, see e.g.Kallio et al. (2008). Analysis of the average drag forces in the transient simulation couldbe used to modify the earlier drag correction models.
In Kallio et al. (2008), the different components of the Reynolds stress tensors wereanalysed from another transient simulation. An order of magnitude difference wasobserved between the different components. Thus no assumption of isotropy, as oftenmade in modelling of single phase turbulence, can be made in modelling of turbulencein dense gassolid flows. The different components need to be modelled separately andpreferably through separate transport equations for each Reynolds stress component.Transport equations for the Reynolds stresses of a phase q can be obtained from theaveraged momentum equations and they can be written as follows:
ijqqsijqijgijqijqijqijqijqk
qjqiqqkqmqjqiqqm SFGMDPx
uuUt
uu,,,,,,,,= δε
αραρ+++−++Π+
∂
′′′′∂+
∂
′′′′∂ (7)
where Pq,ij is the production of the Reynolds stresses
∂
∂+
∂
∂−=
k
iqjqkq
k
jqiqkqqqijq x
Uuu
xU
uuP ,,,
,,,, """"ρα (8)
q,ij is the pressurestrain covariance (or redistribution) term
∂
∂+
∂
∂=Π
j
iqq
i
jqqijq x
uxu
p ,,,
""'
αα (9)
Dq,ij represents the turbulent, pressure and molecular diffusion
( )jqikqqiqjkqqikjqqjkiqqkqjqiqqqk
ijq uuuuupupuuux
D ,,,,,,,,,, ")"(")"("'"'""" ταταδαδαρα −−++∂∂
−=
(10)
q,ij is the dissipation term
k
jqikqq
k
iqjkqqijq x
uu
xu
u∂
∂+
∂∂
= ,,
,,,
")"(
")"( ταταε (11)
Fq,ij is the turbulent mass flux
(12)k
ikqqjq
k
jkqqiqijq x
Uu
xU
uF∂
∂+
∂∂
=)(
")(
" ,,
,,,
τατα
and Mq,ij arises from the modelling the local scale turbulence
k
Mikqq
jqk
Mjkqq
iqijq xu
xuM
∂
∂+
∂
∂= ,
,,
,, ""τατα (13)
The phase interaction term Gq,ij reads
( ))()1(
)()1(2
,,,)1(
,,,)1(
,,,,,,,
isigjqgs
jsjgiqgsjqiqisjgjsiggsijq
UUuK
UUuKuuuuuuKGqs
qs
′′−+
′′−+′′′′−′′′′+′′′′=+
+
δ
δ
(14)
The twophase term Ss,ij is
i
sjs
j
sisijs x
puxpuS
∂∂
−∂∂
−= ,,, "" (15)
These terms in the balance equation for the vertical normal component of the solidphase Reynolds stress are plotted in Fig. 13 on the centreline of the riser. Here again wesee that the simulation is too short to produce smooth averages. As shown in Fig. 13, thelargest term is the phase interaction term Gq,ij and a couple of other terms like theproduction and dissipation terms are also significant. In a timeaveraged CFDsimulation, only the stress production terms ijqP , needs no modelling. All the other termsneed to be modelled. Performance and applicability of various modelling alternativesfor the Reynolds stresses will next be examined and tested.
2000
1000
0
1000
2000
3000
4000
5000
0 0.5 1 1.5 2 2.5 3y [m]
12345678910
k
ysysksssyys x
UuuP
∂∂
−= ,,,, ""2:1 ρα
y
yssyys x
up
∂∂
=Π ,,
"'2:2
α
( )ksysysssk
Tyys uuu
xD ,,,, """:3 ρα
∂∂
−=
( )yssy
pyys up
xD ,, "'2:4 α
∂∂
−=
( )ysykssk
myys uu
xD ,,, ")"(2:5 τα
∂∂
=
k
ykssysyys x
UuF
∂
∂=
)("2:6 ,
,,
τα
k
ysykssyys x
uu
∂
∂= ,
,,
")"(2:7 ταε
( ) )(22:8 ,,,,,,,, ysygysgsysysysyggsyys UUuKuuuuKG ′′+′′′′−′′′′=
y
sysyys x
puS∂∂
−= ,, "2:9
k
Mykss
ysyys xuM
∂
∂= ,
,, "2:10τα
Figure 13. The terms in the balance equation for the vertical normal component of the solid phaseReynolds stress [kg/ms3].
CONCLUSIONS
Fluid dynamics of circulating fluidized beds (CFB) can be computed with a variety ofmethods. Typically the simulations are conducted with EulerianEulerian models based
on the kinetic theory of granular flow. These models require a fine computational meshand, in addition, that the simulations are run as transient, which in the case of largeindustrial processes leads to unfeasibly long computations. To improve the modellingcapabilities it is necessary to develop faster simulation methods. The most attractiveapproach seems to be timeaveraged modelling facilitating steadystate simulation offluidization. An order of magnitude slower but still feasible alternative would be to usea coarse mesh and special meshdependent closure equations in a transient simulation.
A prerequisite for utilisation of transient simulations in steadystate model developmentis validation by measurements. Thus, a 0.4 m wide and 3 m high laboratory scale 2DCFB was built at Åbo Akademi University. Experiments were carried out for validationof the CFD simulations. The behaviour of the process was video recorded and thevideos were analysed to determine the average voidage distribution in the CFB. Thepressure distribution in the bed and the circulation rate of solids were also measured.The measured flow properties were compared with simulation results, observing areasonably good agreement. In addition, PIV measurements were conducted to evaluatethe possibilities to determine the velocities of individual particles and particle clusters.The high speed imaging, together with time resolved velocity fields and void fractionestimates based on the gray scale value, proved to provide an excellent visualization ofthe velocity of the clusters in the CFB.
Results from a long simulation were analysed to evaluate the requirements for theclosure of the timeaveraged equations. The largest terms to be modelled are the gassolid interaction and the Reynolds stress. In addition, the correlation between pressurefluctuations and voidage can be significant in dense bottom bed conditions and solidspressure in the wall layers at riser bottom. Modelling of the momentum equation termswas discussed in the paper.
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ACKNOWLEDGEMENTThe financial support of Tekes, VTT Technical Research Centre of Finland, FortumOyj, Foster Wheeler Energia Oy, Neste Oil Oyj and Metso Power Oy is gratefullyacknowledged.