Chemical Engineering Science 61 (2006) 1395–1400www.elsevier.com/locate/ces

The turbulent characteristics of the gas–solid suspension in asquare cyclone separator

Yaxin Su∗

Department of Environmental Science and Engineering, Donghua University, No. 1882, West Yan’an Road, Shanghai 200051, PR China

Received 13 November 2004; received in revised form 31 August 2005; accepted 3 September 2005Available online 10 October 2005

Abstract

Three-dimensional particle dynamics analyzer was employed to study the gas–solid flow in a square-shaped cyclone separator which wasdesigned for large CFB application. Distribution of flow vector, fluctuating velocity, turbulent kinetic energy, turbulent intensity and particleconcentration were discussed. The swirling flow inside the cyclone showed the Rankine vortex characteristics, i.e., strong swirling vortex atthe central region of the cross-section and weak swirling quasi-free vortex near the wall. The quasi-laminar motion of particles enhanced theturbulent movement at the corners due to particle–particle/wall collision, which led to the local peak value of the turbulent kinetic energy andturbulent intensity. The corner is one of the major region to cause pressure drop because the suspension at the corners consumed more energyof the flow. The corners were found to be beneficial to particle separation mainly because the strong fluctuating flow consumed much of thekinetic energy of both the particle and the gas.� 2005 Published by Elsevier Ltd.

Keywords: Multiphase flow; Separations; Suspension; Turbulence; Square cyclone separator; 3D-PDA

1. Introduction

Circulating fluidized bed (CFB) combustion technologyhas been developing very fast during the last two decades.Besides the researches on the bed-to-wall heat transfer andthe flow dynamics of the suspension in the riser, much ef-forts were given to the study on the separator, which is akey component of a CFB boiler. Many authors have reportedtheir research results on different types of separators for CFBboiler application (Belin et al., 1995; Cen et al., 1997; Chenet al., 1989, 1999; Darling, 1995; Gamblt et al., 1993; Herbet al., 1992; Liu et al., 1991; Makkonen, 2000; Shibagaki andNishiyama, 1999).

With the development of large CFB boilers, the huge bodyof the cyclone separator became to be a major shortcomingbecause of the thick refractory wall that needs a long periodto start the boiler. Foster Wheeler developed a water-/steam-cooled cyclone separator which needs a thin refractory wall

∗ Tel.: +86 21 62379916; fax: +86 21 62378952.E-mail address: suyx@dhu.edu.cn.

0009-2509/$ - see front matter � 2005 Published by Elsevier Ltd.doi:10.1016/j.ces.2005.09.002

and therefore shorten the start hours (Darling, 1995; Gambltet al., 1993; Makkonen, 2000; Shibagaki and Nishiyama, 1999).But it is very difficult to manufacture because the water-cooledwall structure is very complex. Hence there is still the prob-lem for large CFB boiler design and the CFB boiler withthis kind of separator can only be designed up to 350 MW.This problem can be solved by lowering the separator op-eration temperature and introducing a mid-temperature sep-arator at about 450

◦C (Chen et al., 1989, 1999; Liu et al.,

1991). Furthermore, other types of inertia separator were de-veloped, such as U-beam, shutter. A square cyclone separatorwith downward exhaust exit was designed for large CFB ap-plication (Cen et al., 1996), which has a high separation ef-ficiency with a cut diameter around 15 �m (Qiu et al., 1999)and it was easy to install heat transfer surface at its innerwalls.

We used a three-dimensional particle dynamic analyzer ( 3D-PDA) to investigate the turbulent flow field inside the cycloneto study the mechanism of particle separation and provide aguidance for the optimization of its structure. This paper wouldreport the results on the gas–solid suspension turbulent flowcharacter.

1396 Y. Su / Chemical Engineering Science 61 (2006) 1395–1400

2. Experimental setup

The experiment setup is shown in Fig. 1. The air is suppliedby an induced blower of 3.5 kW. Particles were fed into the riserby a screw feeder which was controlled by an electromagneticengine. The CFB was a three-dimensional rectangular duct of60 × 80 × 1900 mm. The suspension goes through a CFB riserand a horizontal duct and enters into the separator. The inlet ofthe separator is 20 × 60 mm. The separator is of square cross-section of 120×120 mm, its body height is 180 mm. The vortexfinder and the exhaust exit both have a diameter of 60 mm.The height of the vortex finder is 90 mm. The distance between

1

2

3

4

5

6

7

8

9

10

11

particle collection

gas exit to cyclone and inducer

glass window for measurement

air inlet

12

13

14

15

16

Fig. 1. Test rig—1: transformer, 2: power source, 3: water cleaner, 4: laser, 5: laser controller, 6: beam splitter, 7: laser transmitter, 8: receiver, 9: A/D converter,10: signal processors, 11: computer, 12: square separator, 13: CFB riser, 14: particle feeder, 15: air control valve, 16: flow meter.

30

60

30

90

60

section 1

section 2glass window for measure

inlet

120 mm

vortex finder

60 mm

glass window x (u)

y (v)

(a) (b)

Fig. 2. (a) Measure section and (b) cross-section of the separator.

the bottom of the vortex finder and the top of the exhaust is60 mm. The separated particles are collected by a discharge bin.The exhaust gas goes through the downward exit to open airby an induced blower. The front side of the separator is glasswindow for PDA measurement. Fig. 2 shows the separator andthe measure sections.

The optical arrangement of the 3-D PDA is a backward-scattered-light system supplied by Dentec, including a 5 W(max. power) argon-ion laser source, laser transmitting and re-ceiving systems, signal processor and a computer. A frequencyshift of 40 MHz is added to the green and blue beam of thelaser. The measure point coordinate is automatically controlled

Y. Su / Chemical Engineering Science 61 (2006) 1395–1400 1397

by the transverse system of the PDA following the given coor-dinate before the running.

The particle used was glass beads of mean diameter30–40 �m and of density 2400 kg/m3. The glass bead hasgood physical properties, e.g., sphericity 0.95 and refractionindex 1.5. Particles of diameter 0–5 �m were selected as thegas tracer. The PDA measures the velocity of a certain num-ber of particles and calculates the statistically averaged valueas the result. Hence the more particles measured, the betterthe result is. At each measure point, 5000 particles is enoughfor the present study and the time limit is 1 min. The positivedirection of the velocity W (vertical velocity) is set upwardaccording to the frequency shift of the laser beam.

3. Results and discussion

Eight cases with different suspension concentration and par-ticle diameter altogether were investigated at different inlet ve-locity. A typical case with a mean inlet gas velocity at 20 m/sand particle load of 0.343 kg/m3 at the inlet will be discussedfor Section 2 as shown in Fig. 2a.

3.1. Vector of the flow field

The flow vector plot for Section 2 is shown in Fig. 3. Fromthe vector plot it is seen that the suspension flow field consistsof the strong swirling flow in the center and weak flow nearthe wall and local small vortex of anomalous motion existed atthe corner. The flow in the center region was regarded as theforced vortex region and the flow near the wall was regarded asthe pseudo-free vortex region. Therefore, the turbulent swirlingflow inside the square cyclone has the features of Rankine vor-tex, which is very favorable for the particle separation. Thestrong swirling flow in the center produces the relatively strongcentrifugal force which carries the particles by viscous forcefrom gas to flow with the gas phase. The pseudo-free vortexnear the wall results in relatively weaker centrifugal force andlower ability to carry the particles to flow with the gas, espe-cially the larger particles which are too heavy to trace the gasand fall down along the wall surface and get separated. It is theinertia that makes the particle move to the wall in corner andthe particle separate from gas flow. There are always the localsmall vortex at the corners where the particle motion is veryanomalous and substantially random due to the particle to par-ticle or particle to wall impact. It is confirmed (Su et al., 2001a)that at the same or similar condition of inlet particle concen-tration, the higher the inlet velocity, the higher the swirling in-tensity inside the separator and the more anomalous the vortexflow at the corner and the flow around the wall surfaces.

3.2. Fluctuating velocity and particle concentration

The turbulent fluctuating velocity is the root-mean valueof the fluctuating velocity by statistical calculation and is

-5 15 35 55 75 95 115x (mm)

-5

15

35

55

75

95

115

y (m

m)

gas

u

v

Fig. 3. Vector of the gas phase.

defined as

u′ =√√√√ N∑

i=1

(ui − U)2/(N − 1). (1)

Fig. 4 is the experimental results of the fluctuating velocity ofgas phase. It is seen that the fluctuating velocity at x =0 is verysmall and that at x = 120 mm is much larger. The fluctuatingvelocity near the front wall is smaller than that near the rearwall. The difference between the fluctuating velocity along thethree coordinates indicated the anisotropy of the flow inside theseparator.

The gas–solid suspension flow actually should be classifiedinto three kinds of flow (Liu, 1993), i.e., laminar flow, turbu-lent flow and the so-called quasi-laminar flow which includestwo types of fluctuating motion, one is the irregular motionof particles due to the inter-impact between particles or par-ticle and wall surfaces as well as the fluid fluctuating motioninduced by the particle fluctuating motion. This kind of fluc-tuating flow is almost random and has no coherence structure.It is similar to the thermal motion of gas molecular, but itis not completely chaotic. The other is the fluctuating motionin the particle wakes due to the relative motion of fluid andparticles. This kind of fluctuating flow has coherence charac-teristics, but it is of very small scale, the order of particle di-ameter or smaller. There is an obvious wake condition behindthe particles, namely the condition for vortex shedding at theparticle surface, generally when the particle Reynolds numberexceeds 100. The condition for particle impact is high particleconcentration, especially for the particle flow of asymmetricaldiameter. The impact between particle and wall surface mainly

1398 Y. Su / Chemical Engineering Science 61 (2006) 1395–1400

0 20 40 60 80 100 120

x (mm)

00.5

11.5

22.5

33.5

44.5

u' (

m/s

)

y=2y=10y=30

y=60y=90y=110

0 20 40 60 80 100 120x (mm)

0.5

1

1.5

2

2.5

3

3.5

4

4.5

w' (

m/s

)

y=2y=10y=30y=60y=90y=110

(a) (b)

Fig. 4. Fluctuating velocity for gas phase: (a) X-direction, (b) Z-direction.

0 20 40 60 80 100 120x (mm)

101

102

103

104

105

106

107

Co

nce

ntr

atio

n (

1/cm

3 )

y=2

y=10

y=30

y=60

y=90

y =100

Fig. 5. Particle number concentration distribution.

happens when the channel shrinks, bends or turns sharply. Ingeneral the fluctuating motion due to the above reasons existsonly at local position.

The suspension flow in the cyclone is mainly the strongly tur-bulent flow. At the corners, the flow was forced to turn sharplyand the impact between particles and particle–wall surface hap-pens frequently. From the vector flow we have seen there is thelocal vortex (Su et al., 2001a). It is at the corner that the quasi-laminar fluctuating motion comes into being and enhances thegas or particle fluctuating velocity.

Both the gas and particle fluctuating velocity increase alongthe x coordinate and reach their maximum value near the rightwall. The particle number concentration showed an inverse dis-tribution along the x coordinate and reached its minimum valuenear the right wall, as shown by Fig. 5.

3.3. Turbulent kinetic energy

The turbulent kinetic energy is defined as

K = 12 (u′2 + v′2 + w′2), (2)

where u′, v′, w′ are the root-mean fluctuating velocity at eachmeasure point.

Fig. 6 gives one example of the turbulent kinetic energydistribution of the gas and particles of diameter 30–40 �m. Itis deduced by the distribution of the fluctuating velocity thatthe turbulent energy inside the cyclone is larger at the rightpart than that at the left part. Fig. 6 shows the feature visuallyby the three-dimensional distribution of the turbulent kineticenergy. Both the turbulent fluctuating velocity and the kineticenergy proves that the flow field is asymmetrical and the max-imum value of the turbulent kinetic energy is found at thefront-right corner. The motion at the four corners is differentfrom each other mainly due to the different intensity of thequasi-laminar fluctuating motion there. You (1996) applied thehot-wire anemometer to investigate the turbulent flow in an-other type of square cyclone separator with upward exhaust exit(Yue et al., 1995) and found that the maximum turbulent kineticenergy was at the center in the upper part of the cyclone. Healso found the kinetic energy had a larger value at the corners.

3.4. Turbulent intensity

The local turbulent intensity is defined to be

Tu =√

13 (u′2 + v′2 + w′2)

/ √U2

mean + V 2mean + W 2

mean, (3)

where u′, Umean are the local root-mean fluctuating and meanvelocity, respectively.

Fig. 7 gives one example of the local turbulent intensity dis-tribution of the gas and particles of diameter 30–40 �m. Al-though the turbulent kinetic energy has its maximum value nearthe front-right corner, the local maximum turbulent intensity isnot found there, but at the rear part. The turbulent intensity oflarger particles, in general, has a relatively equable distributionalong the x–y plane in comparison with that of the gas andsmaller particles.

The fact that the peak value of the turbulent kinetic energyand the local turbulent intensity exists at the corner shows thatthe suspension consumes more energy of the flow there and it isone of the main zones to cause the pressure drop of the separa-tor. Qiu et al. (1999) found that when arc-like flow-transmittingdevices were fixed at the corners to eliminate the local vortex

Y. Su / Chemical Engineering Science 61 (2006) 1395–1400 1399

Fig. 6. Turbulent kinetic energy distribution: (a) gas phase, (b) particle (d = 30–40 �m).

Fig. 7. Local turbulent intensity distribution: (a) gas phase, (b) particle (d = 30–40 �m).

and let the flow field become similar to that in a conventionalcyclone of circular cross-section, the pressure drop decreased,but the separation efficiency also decreased. The author foundin his experiment (Su et al., 2001b) that at the corner, especiallythe corner that faces the inlet, the downward particle velocitycomponent in the vertical coordinate was maximum. Hence, thecorner gives contribution for particle separation mainly due tothe strong fluctuating there which consumed the kinetic energyof both the gas and the particle.

4. Conclusion

Three-dimensional particle dynamics analyzer was employedto study the gas–solid flow in a square-shaped cyclone sepa-rator with a downward exhaust exit. The swirling flow insidethe cyclone showed the Rankine vortex characteristics, i.e.,strong swirling vortex at the central region of the cross-sectionand weak swirling quasi-free vortex near the wall. The quasi-laminar motion of particles enhanced the turbulent motion atthe corners due to particle–particle/wall collision, which led tothe local peak value of the turbulent kinetic energy and turbu-lent intensity. The corner is one of the major regions to causepressure drop because the suspension at the corners consumedmore energy of the flow. The corners were found to be benefi-cial to particle separation mainly because the strong fluctuatingflow consumed much of the kinetic energy of both the particleand the gas.

Acknowledgements

The support for this work by the fund for Candidate ofOutstanding Young University Teachers in Shanghai, No.03YQHB076, China, is gratefully acknowledged.

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