Ž .Powder Technology 112 2000 79–86www.elsevier.comrlocaterpowtec

Studies on the effect of swirl numbers on strongly swirling turbulentgas-particle flows using a phase-Doppler particle anemometer

L.X. Zhou), Y. Li, T. Chen, Y. XuDepartment of Engineering Mechanics, Tsinghua UniÕersity, Beijing 100084, People’s Republic of China

Abstract

The effect of swirl numbers on the flow behavior of strongly swirling turbulent gas-particle flows with swirl numbers of ss0.47, 1.0,Ž .1.5 and 2.1 in sudden-expansion and cyclone chambers is studied using a 2-D and a 3-D phase Doppler particle anemometers PDPA .

Ž .The axial and tangential time-averaged and the root mean square RMS fluctuation velocities of gas and particle phases and particleconcentration are measured. The result shows that the swirl number has an obvious effect on the axial velocity profiles, the Rankinevortex structure of tangential velocity profiles, the relationship between two-phase velocities, the turbulence intensity level and theanisotropy of turbulence. q 2000 Elsevier Science S.A. All rights reserved.

Keywords: Gas-particle flows; Strongly swirling flows; PDPA measurement

1. Introduction

Swirling gas-particle flows are encountered in cycloneseparators, cyclone combustors, hydrocyclones and swirl

w xburnerrcombustors 1,2 . However, most of the presentw xstudies are limited to a single gas-phase flow field 3–5 .

w xYuu et al. 3 studied the gas flow field in a cycloneseparator using probe and hot-wire systems, and the resultsshow a reduction of gas axial and tangential velocities inthe presence of particles. The probe measurements made

w xby Silva and Nebra 4 give similar results. The studies byw xOgawa et al. 5 demonstrated that there was no distinct

difference between the dust-laden gas flow and the pure airflow in a cyclone chamber. Obviously, probe measure-ments cannot give gas-particle two-phase flow field. Stud-ies using conventional and modified Laser Doppler Ve-

Ž . w xlocimeter LDV 2,6 show similar behavior of gas andŽparticle flow fields e.g., Rankine vortex structures of

.tangential velocity profiles and slip velocity between gasand particle phases in swirling flows. However, even themodified LDV measurements cannot give reliable results,because it is unable to identify particle sizes and the effectof particle size is included in the particle ‘‘turbulent’’fluctuation. The phase Doppler particle anemometer

) Corresponding author. Tel.: q86-10-6278-2231; fax: q86-10-6278-5569.

Ž .E-mail address: zhoulx@mail.tsinghua.edu.cn L.X. Zhou .

Ž .PDPA measurements allow obtaining the detailed infor-mation of particle velocity, size and concentration. ThePDPA was first used to study sudden-expansion swirlinggas-particle flow with swirling number ss0.47 by Som-

w xmerfeld and Qiu 7 . Their results demonstrate the differentbehavior of particles of different sizes in swirling flows.However, no PDPA measurements of strongly swirlinggas-particle flows are reported.

In this paper, the PDPA system is used to study stronglyswirling gas-particle flows in sudden-expansion and cy-clone chambers with tangential inlets. The swirl numberswere varied from 1.0 to 1.5 and 2.1. The results arecompared with those for the case of ss0.47 and ana-lyzed. The purpose of this study is to clarify the effect ofswirl numbers on the behavior of swirling gas-particleflows.

2. Experimental set-up and measurement methods

The experimental set-up is shown in Fig. 1a and b. Itconsists of a test section, a feeding system and a PDPA

Ž .system. The test section is a plexiglass chamber Fig.1bŽ .with one axial inlet f 60 mm plus two symmetric

Ž .rectangular tangential inlets 68=32 mm . The chamber isa tube of 812-mm length and an inner diameter of 120mm. On one side, a slot is opened and a piece of opticalglass is mounted. The exit tube is a cylinder of 1500 mm

0032-5910r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved.Ž .PII: S0032-5910 99 00308-3

( )L.X. Zhou et al.rPowder Technology 112 2000 79–8680

Ž .Fig. 1. a 1: Data processor; 2: signal processor; 3: converter; 4:transformer; 5: power supply; 6: water purifier; 7: laser; 8: laser con-troller; 9: splitting group; 10: emitter unit; 11: receiver; 12: test section;13: flow stabilizer; 14: cyclone separator; 15: powder feeder; 16: flow

Ž .meter; 17: valve; 18 and 19: compressor. b Experimental set-up.

length and 96 mm diameter, to which a larger sudden-ex-pansion chamber is connected for protecting the test sec-tion from the disturbance of downstream flows. A cycloneseparator is used to collect the particles. Two blowers withvariable flow rates supply the axial and tangential flowsvia the valves, whereby the flow rate may be adjusted. Theflow rates were measured by three separate flow meters.Spherical glass beads of sizes ranging from 0 to 150 mmwere used in measurements. The particle size distributionis shown in Fig. 2. Particles below 10 mm were taken asthe gas tracer. This avoided mismatched refractive indicesbetween seeding and dispersed-phase particles and alsoeliminated interference between particles. The wide sizedistribution allows obtaining the different behavior of par-

Fig. 2. Particle size distribution.

Fig. 3. PDPA instrumentation: 1, laser; 2, splitter; 3 and 4, lens; 5,measuring volume; 6, receiver; 7, filter; 8, detector.

ticles with different sizes. Particles are introduced from theaxial inlet using a screw feeder.

A 3-D PDPA, made by Dantec Inc., and a 2-D PDPAmade by Aerometrics, were used to measure sudden-ex-pansion gas-particle flows.

As for the measurement technique, the 3-D PDPA witha backward scattering arrangement is shown in Fig. 3. It isa well-known and widely used technique. The particlevelocity is measured based on the Doppler frequency shift.The particle size and concentration are measured based onthe phase difference caused by Mie scattering and thenumber of particles passing the measuring volume during acertain time interval. The parameters of the optical systemare shown in Tables 1 and 2.

The measurements were performed at five cross-sec-tions in the chamber, and for each section, there are about25–35 measuring locations along the diameter. In thesingle-phase flow measurement, more than 1000 sampleswere taken at each measuring location, while 2000–5000samples were taken for two-phase flow measurement. Theinlet flow conditions are given in Tables 3–5. The swirlnumber is defined as:

D3r2 22 r wur d rH0Ss ,

D3r2 2D ru rd rH40

where D is the inlet diameter and D is the chamber3 4

diameter.During the experiments, the tangential inlet flow veloc-

ity was constant, and the axial inlet flow velocity was

Table 1

Transmitting optics U U Ux y z

Ž .Wave length of the laser mm 514 488 476Ž .Frequency shift MHz 40 40 40Ž .Beam separation mm 74 74 74

Ž .Diameter of measuring volume mm 1.35 1.35 1.35Ž .Focal length of the laser mm 500 500 500

Ž .Fringe separation mm 3.5 3.31 3.23Fringe number 36 36 36

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Table 2

Receiving optics

The maximum detected diameter 260.24 mm5 3The maximum detected concentration 2=10 rcm

Off-axial angle 1478

Focal length of receiving length 600 mmThe valuable sign level y6 dBThe maximum phase error 258

The maximum spherical error 15%

Ž . Ž .increased from 0 case 1 to 12 mrs case 3 , and in turn,decreased the swirl number from 2.1 down to 1.0.

3. Experimental results and discussion

Figs. 4–12 show the measurement results for cyclonicŽ .gas-particle flows with ss2.1 case 1 . The comparison

between the particle-laden gas and the pure gas flow fieldis given in Figs. 4–7. Figs. 4 and 5 show the generalbehavior of cyclonic two-phase flow field — w-shapedaxial velocity profiles with an annular reverse flow zoneand Rankine-vortex tangential velocity profiles of twophases. It can be seen that the presence of particles reducesthe gas tangential velocity everywhere. The gas axialvelocity decreases in the near-wall zone, but increases inthe near-axis zone.

Ž .The measured fluctuation velocities Figs. 6 and 7show the central peak and the increased value near thewall, because of the large velocity gradient in the solid-body rotation region and the region near the wall. Thecomparison between these two figures shows that the axialfluctuation velocities are much smaller than the tangentialones, demonstrating the high anisotropy of turbulent fluc-tuations. Moreover, these figures indicate that the presence

Ž .of particles reduces gas root mean square RMS tangentialfluctuation velocity almost everywhere and axial fluctua-tion velocity in most regions but increases the latter one inreverse flow zones.

For particle concentration, a large amount of particlesŽ .almost concentrates in the near-wall region Fig. 12 due to

the strong centrifugal force.Figs. 8–11 give the comparison between the two-phase

tangential, axial velocities together with the RMS fluctua-Ž .tion velocities for two-particle sizes 60 and 100mm .

Table 3Ž .Inlet flow condition case 1

Axial inlet velocity 0 mrsTangential inlet velocity 10 mrsSwirl number 2.1Particle mean diameter 76.3 mm

3Particle material density 2400 kgrmParticle mass loading 0.01 kg-solidrkg-gas

Table 4Ž .Inlet flow condition case 2

Axial inlet velocity 5 mrsTangential inlet velocity 10 mrsSwirl number 1.5Particle mass loading 0.01 kg-solidrkg-gas

Figs. 8 and 9 show that the particle tangential velocitylags behind the gas tangential velocity almost everywherewith the larger particles having lower velocities due totheir higher inertia. The particle axial velocity lags behindthe gas axial velocity only in the near-wall region. It canbe seen from Figs. 10 and 11 that the particle turbulentfluctuation is lower than the gas fluctuation in most re-gions both in axial and tangential directions, and the largerthe particle size, the smaller the particle fluctuation.

Figs. 13, 15–18 give the measurement results for theŽ .sudden-expansion flows with ss1.5 case 2 . As can be

seen from Fig. 13, in this case the axial velocities of twophases have no w-shaped distributions as that for the cases

Ž . Ž .of ss2.1 Fig. 4 and 0.47 Fig. 14 , but are higher nearthe axis and lower with a small peak near the wall. Theparticle axial velocity lags behind the gas one in mostregions, and the velocity slip between two phases increaseswith the increase in particle size, but the relative slip is notvery large. In the near-wall region, the particle axialvelocity is slightly larger than the gas velocity. The parti-cle tangential velocity lags behind the gas tangential veloc-

Ž .ity Fig. 15 , and the larger the particle size, the more thelag. In the first and second sections, the structure of rigidbody rotation in tangential velocity profiles is not obviousbecause of the injection of axial non-swirling flows.

In the subsequent sections, due to the increasing effectof centrifugal force, the tangential velocity distribution oftwo phases shows an obvious Rankine-vortex structurewith the gradually vanishing effect of inlet conditions. Fig.16 gives the particle and the gas axial fluctuation velocitieswith higher values near the wall and lower values near theaxis. Unlike that in case 1, there is no peak value near theaxis in this case. The particle axial fluctuation velocity issmaller than that of the gas phase, and decreases with theincrease of particle size. These results are similar to thosefor case 1. Fig. 17 indicates that the particle tangentialfluctuation velocity is smaller than that of the gas phase inmost regions. This is similar to that for case 1 and those

w xreported previously 2,6,7 both for weakly and stronglyswirling gas-particle flows.

Table 5Ž .Inlet flow condition case 3

Axial inlet velocity 12 mrsTangential inlet-velocity 10 mrsSwirl number 1.0Particle mass loading 0.01 kg-solidrkg-gas

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Ž . Ž .Fig. 4. Gas tangential mean velocities mrs case 1 .

Ž . Ž .Fig. 5. Gas axial mean velocities mrs case 1 .

Ž . Ž .Fig. 6. Gas tangential fluctuation velocities mrs case 1 .

Ž . Ž .Fig. 7. Gas axial fluctuation velocities mrs case 1 .

Ž . Ž .Fig. 8. Two-phase tangential time-averaged velocities mrs case 1 .

Ž . Ž .Fig. 9. Two-phase axial time-averaged velocities mrs case 1 .

Fig. 10. Two-phase tangential fluctuation velocities.

Ž . Ž .Fig. 11. Two-phase axial fluctuation velocities mrs case 1 .

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Ž 3. Ž .Fig. 12. Particle number density Eq2rcm case 1 .

Comparison between Figs. 16 and 17 indicates that bothgas and particle turbulent fluctuations are anisotropic, butthe axial fluctuation exceeds the tangential fluctuation dueto the fact that the axial velocity gradient is still larger thanthe tangential velocity gradient. So, this is qualitativelysimilar to that for the swirling flows with ss0.47 re-

w xported in Ref. 7 but is opposite to the results for the caseof ss2.1. Fig. 18 gives the particle concentration distri-bution. Unlike the case of cyclonic flows, and similar tothat for the swirling flows with ss0.47, in this case,particles first concentrate in the near-axis zone due to theeffect of axial inlet velocity, then gradually move to thewall under the effect of centrifugal force and turbulentdiffusion, and finally concentrate in a thin layer adjacent tothe wall. The phenomena that the particle axial time-aver-aged velocity exceeds the gas one, and the particle axialfluctuation velocity exceeds the gas one in reverse flowzone is owing to higher inlet axial velocity of particles for

w xthe case of low swirl number ss0.47 7 . For the cases ofss2.1 and 1.5, the inlet particle axial velocity is smaller,and the swirl number is larger, so the particle axial time-averaged and fluctuation velocities lag behind the gascorresponding velocities almost everywhere.

Ž .Fig. 13. Axial time-averaged velocity case 2 .

Fig. 14. Axial time-averaged and RMS fluctuation velocities for ss0.47Ž w x.taken from Ref. 7 .

Figs. 19–22 give the measurement results for the sud-Ž .den-expansion flows with ss1.0 case 3 . For two-phase

Ž .axial velocities Fig. 19a , it can be seen that the gasvelocity is higher near the axis and lower near the wall,similar to the case of ss1.5, but different from the cases

Ž .of ss0.47 and 2.1 the last case has a contracted exit .Particle velocity has a more uniform profile, and is lowerthan the gas one near the axis but higher than the gas onenear the wall. At downstream cross-sections, the gas veloc-

Ž .Fig. 15. Tangential time-averaged velocity case 2 .

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Ž .Fig. 16. Axial fluctuation velocity case 2 .

ity profiles become flat, and the particle velocity becomeslarger than the gas velocity in the near-axis region andsmaller than the gas velocity in the near-wall region.Smaller particles of 45 mm have small slip velocity to thegas, while larger particles of 90 and 135 mm have largeslip velocity. The measured axial velocity profiles in thiscase is different from those in the cases of ss0.47 and2.1, but is similar to that in the case of ss1.5.

w x Ž .For swirling flows with ss0.47 7 Fig. 14 , there isan annular recirculation zone and the particle velocityexceeds the gas one in most regions. In the case of

Ž .swirling flows with ss2.1 case 1 , due to the contractionat the exit, the gas axial velocity profile also has an

Ž .Fig. 17. Tangential fluctuation velocity case 2 .

Ž 10 3. Ž .Fig. 18. Particle number density 10 rm case 2 .

annular recirculation zone with the peak near the axis, butthe particle axial velocity lags behind the gas one. For thepresent case with ss1.0, no reverse flow was observed.Similar axial velocity profiles are obtained for the case of

Ž .ss1.5 case 2 , and the particle axial velocity also lagsbehind the gas axial velocity.

As in other cases, the two-phase tangential velocities inŽthis case have the typical Rankine-vortex structure Fig.

.20a . The particle tangential velocity always lags behindthe gas tangential velocity, and the larger the particles, themore they lag. This is similar to the cases of ss0.47, 1.5and 2.1. However, the relative size of the solid-body

Ž . ŽFig. 19. Axial time-averaged and fluctuation velocity case 3 . — gas;.-B- 45 mm; -`- 90 mm; -'- 135 mm .

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Ž . ŽFig. 20. Tangential time-averaged and fluctuation velocity case 3 . —.gas; -B- 45 mm; -`- 90 mm; -'- 135 mm .

rotation zone in the present case is larger than that of thecases ss0.47 and 2.1, but is smaller than that of the casess1.5. It seems that in the case of exits without contrac-tion, the relative size of the solid-body rotation zoneincreases with the increase of swirl number. Due to thetangential inlet, the size of the potential vortex zone issmall at the first and second cross-sections, and in thedownstream region, it becomes larger owing to the walleffect. The tangential velocity profiles of large particleshave only the solid-body rotation zone.

Ž . ŽFig. 21. Radial time-averaged and fluctuation velocity case 3 . — gas;.-B- 45 mm; -`- 90 mm; -'- 135 mm .

Ž .Fig. 22. Particle number density case 3 .

Fig. 19b gives the particle and gas axial fluctuations.Their profiles have two peaks due to the large velocitygradient of the inlet axial velocity profiles, and the profilesbecome flat in the downstream region. The particle axialfluctuation lags behind the gas axial fluctuation in theupstream region, but in the downstream region, it exceedsthe latter in the near-wall region. For the case of ss0.47,the particle axial fluctuation is larger than the gas axialfluctuation in the reverse-flow zone, whereas in cases ofss1.5 and 2.1, the particle axial fluctuation is smallerthan the gas axial fluctuation almost everywhere. Thus,with the increase of swirl number, the particle axial fluctu-ation will change from exceeding the gas axial fluctuationin some regions to lagging behind the gas axial fluctuationeverywhere.

As seen from Fig. 20b, the profiles of gas and particletangential fluctuations also have two peaks near the inlet.The particle tangential fluctuation lags behind the gas one,which is similar to the cases of other swirl numbers. Itseems that the swirl number does not affect the generalfeatures of the two-phase tangential fluctuation. Compar-ing the axial and tangential fluctuations shows that at first,they are near each other, then, starting from Zs148 mm,the axial fluctuation becomes larger than the tangentialone. In the case of ss0.47, the axial fluctuation is alwayslarger than the tangential one, but in the cases of ss1.5

Table 6X X XŽ . Ž . Ž . Ž .S U mrs u mrs Õ mrs w mrsm

0.47 1.6 4 3 21.0 6.0 2 3 1.51.5 4.0 1.5 r 1.02.08 3.5 0.4 r 0.7

( )L.X. Zhou et al.rPowder Technology 112 2000 79–8686

Table 7X X XS u rU Õ rU w rUm m m

0.47 2.5 2.0 1.251.0 0.33 0.5 0.251.5 0.37 r 0.252.08 0.11 r 0.20

and 2.1, the axial fluctuation is always smaller than thetangential one. So, for swirling flows, the two-phase turbu-lence is always anisotropic, and increasing the swirl num-ber leads to an increase in tangential fluctuation, whichgradually exceeds the axial one.

Ž .The two-phase radial velocity profiles Fig. 21a showthe tendency of gas and particles to move toward the wallfrom the near-axis region due to the effect of the centrifu-gal force. In the near-wall region owing to bouncing effect,the radial velocity changes rapidly from moving to thewall to departing from the wall. There is only a slightradial slip velocity between two phases. The radial fluctua-tion of the two phases is larger than the axial and tangen-

Ž .tial fluctuations Fig. 21b , and is higher in the near-wallregion.

As in the cases of ss0.47 and 1.5, since powder isintroduced from the axial inlet, particle concentration has

Ž .at first, a peak in the near-axis region Fig. 22 , then,owing to the centrifugal force and turbulent diffusion,particles are transported outward, and finally accumulatenear the wall. In the case of ss2.1, there is only near-wallconcentration peak due to pure tangential inlets.

The effect of swirl numbers on turbulence fluctuationand anisotropy of gas phase is shown in Tables 6 and 7,where U denotes the cross-sectional-averaged velocity inm

the chamber. Obviously, as the swirl number increases,both turbulent fluctuation velocity and turbulence intensitydecrease, and the tangential velocity fluctuation changesfrom lagging behind the axial one to exceeding the axialone.

4. Conclusions

Ž .1 For all swirl numbers, the particle velocity field isqualitatively similar to the gas one.

Ž .2 For ss0.47 and 2.1, there are w-shaped axialvelocity profiles with annular reverse-flow zones. For ss1.5 and 1.0, there are U-shaped axial velocity profiles withno reverse-flow zones.

Ž .3 For all swirl numbers, there are Rankine-vortextangential velocity profiles.

Ž .4 For all swirl numbers, whether strongly or weaklyswirling flows, both gas and particle fluctuations areanisotropic. As the swirl number increases, the axial fluc-tuation changes from exceeding the tangential to laggingbehind the tangential.

Ž .5 For all swirl numbers, the two-phase tangentialvelocity profiles have the Rankine-vortex structure and the

particle tangential velocity always lags behind the gastangential velocity.

Ž .6 With the increase in swirl number, the size of thesolid-body rotation zone increases.

Ž .7 With the increase in swirl number the recirculationŽzone in axial velocity profiles will disappear however, the

.contracted exit may cause recirculation and the particleaxial velocity changes from exceeding the gas axial veloc-ity to lagging behind the gas axial velocity in most regionsor everywhere. This is due to the decreasing initial axialmomentum of particles with an increase in centrifugaleffect.

Ž .8 With the increase in swirl number, the two-phaseturbulence fluctuation in axial and tangential directionsdecreases, but the turbulence fluctuation in radial directionmay increase.

Ž .9 With the increase in swirl number, the particle axialfluctuation changes from exceeding the gas axial fluctua-tion in reverse-flow zones to lagging behind the gas axialfluctuation everywhere.

Ž .10 With the increase in swirl number, the maximumparticle concentration at the axis near the inlet disappearsand it shifts to the wall.

Acknowledgements

This paper is in memory of the late Prof. S.L. Soo, whoinitiated the interest of the first author in studying stronglyswirling gas-particle flows and did the first cooperative

w xresearch with the first author in cyclonic flows 2 . Thisstudy is part of the research results of the National KeyProject of Fundamental Research, sponsored by the StateScience and Technology Commission, PRC, and the Pro-ject, sponsored by the National Natural Science Founda-tion of China.

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