Flow-Field Numerical Simulation of Gas-Solid Cyclone Separator Based on FLUENT

Flow-field Numerical Simulation of Gas-Solid Cyclone Separator based on FLUENT

DENG Qing-fang

School of Mechanical and Electrical Engineering, Central South University, Changsha 410083, China Department of Mechanical and Energy Engineering, Shaoyang College, Shaoyang, 422004, China

E-mail: dqf721029@sina.com

Abstract—In this paper, gas-solid flow-field, pressure changes, velocity changes and vortex properties inside gas-solid cyclone separator are studied under various conditions with software of Fluent. The research results show that velocity along Y axis direction (tangential velocity) and along Z axis (axial velocity) are dominant factors in cyclone; static pressure, velocities in all directions, and separation rate of solid particles in cyclone all increase with increase of gas treating amount; the static pressure in center of cyclone separator is the lowest and solid particles are concentrated on the inner wall of middle section; gas-flow’s velocities in all directions reduces as particles density increases. However, wear of the cyclone separator wall would aggravate at high particle density. It can be inferred that the characteristics of a cyclone separator with v=18m/s and dust content value being 1% are supposed to be mostly close to calculation result of the empirical model.

Keywords-FLUENT, mathematical model; cyclone separator; numerical simulation

Dongyi Zhou, SHEN Ai-ling

Department of Energy and Power Engineering, Shaoyang College, Shaoyang, 422004, China

E-mail: zhoudongyi2005@163.com

Much higher requirement such as properties, structure, cost and operability is brought up for dust collector by modern industry. To study cyclone merely through experiments, because of the limitation of conditions, such as a large amount of manpower, material resources and financial resources need to be expended and long period and so on. With rapid development of computer technology nowadays, numerical simulation for flow-field and particles movements inside cyclone can deal with by the aid of computer simulation technology, which has merits of strong simulating ability, quick calculation speed and less investment etc. Through simulation for two phases of gas and solid inside cyclone separator, inner flowing law could be discovered, thereby the cyclone structure could be optimized and the development period could be shortened, which is of important engineering significance.

II.

THE MATHEMATICAL MODEL OF CYCLONE

SEPARATOR

I. INTRODUCTION

Cyclone separator is the most widely used and the most representative dust collector among centrifugal dedusting equipments, and play an important role in effective dust recovery in cement production. Typical cyclone separator’s structure is generally composed of gas inlet pipe, cylinder, cone, vent pipe and dust flue. Gas flows with particles enter the separator along tangential direction from inlet, rotating and flowing downwards inside of separator. As the action of centrifugal force, solid particles are thrown away towards cylinder wall and go down into the collector along the cone part, meanwhile gas flow whirl upwards and go away from separator through outlet pipe[1].

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Figure 1. Typical schematic diagram of cone- type cyclone separator

For easy to problem solving, simplifications for the practical cyclone are deal with as follows before its mathematical model is established:

1) Fluid is incompressible gas, namely p=constant.

2) Heat effect caused by friction between fluid and wall is ignoring.

3) The flowing velocity of inlet gas is even, and flow being at turbulent state.

4) Suppose that fluid-field is constant-temperature, and energy conduction is overlook.

Feed process parameters: material rate of flow is 700-1000kg/h, material density is 1.63kg/m3, gas rate of flow is 377m3/h, material temperature is 116󰀓ć, pressure is 0.015Mpa. Material compositions of gas and solid phases coexist (dust content 20%), material density is 3*10-5Pa⋅S, dust density is 280-300kg/m3, dust granularity 󰀂10mm(30%), 3-10mm(40%), 1-3mm(27%) and 󰀃1mm(3%).

According to design requirements, separation efficiency should be not less than 98%, stainless steel iron Inconel 600 (an nickel-base alloy) is needed and saturated steam of 2bar is needed for jacket heating.

Concerned parameters is shown in Tab.1, the final model attained from calculation through Muschelknautz modeling method is indicated in figure2, all dimensions and scale are marked in figure too.

978-0-7695-4286-7/10 $26.00 © 2010 IEEEDOI 10.1109/ICDMA.2010.88

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TABLE I. PARAMETERS OF THE DESIGNED MODEL

Paraeter value paraeter value Cut particle A. Analysis for the basic characteristics of gas-solid flow-Even quality of flow

850kg/h diameter of inner 0.55mm

rate field vortex

Final speed of the cut The movement of particles inside the cyclone is very Material volume of 8.14*10-particle diameter on 1.093m/s 3

flow rate 4m/s complicated, especially those small ones’ movements cylinder wall

showing great randomness. For the same particles, their Reynolds number Gas volume of flow

32.7 0.015 m3/s

of particle rate movements differ from each other with same dust content

Material density 2719kg /m3 Resistance factor 36.2 but inlet speed being different, or with different dust content Inlet contraction factor Tangential velocity of wall surface VoȦ Axial velocity of wall

surface vzω Total friction factor Total vortex inner area with friction Tangential velocity at radius Res of inner

vortex

0.897 15.61m/s 1.67m/s 0.14 0.741

Feed density limit Friction loss of gas and solid phases with wall Loss of vortex Vs raising pipe Acceleration pressure loss Total pressure drop

0.0001kg/kg 1.03Pa 2008.4Pa 0.14 2009Pa

IV. NUMERICAL SIMULATION RESULT AND ANALYSIS OF

CYCLONE SEPARATOR MODEL

but inlet speed being same, and the final position would be different as well, some being captured but some escaping from vent pipe. In this paper the cloud image of all parameters is analysed to comprehend the basic motion of flow-field.

4.28m/s 󰀃

Figure 4. Static-pressure cloud image with dust content=1% and inlet

speed=18m/s

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Figure 2. Final model size of the cyclone separator

III. THE MODEL ESTABLISH OF CYCLONE GRID The generation of grid in numerical simulation of flow-mm

field is the pretreatment procedure. In view of the inner flow-field characteristics of cyclone separator, 3-D grouped divisional zone grid is generated using algebra generation method in this paper.

The grid system built is quite adaptive to geometrical structure and flow characteristics of the model, which is beneficial to the dealing with inter-zone coupling conditions of divisional numerical simulation. Meanwhile, thus generated grid has good orthogonality, which could meet certain difference format applicable for cyclone separator.[3] The total 3-D calculation domain is divided into 316436 hexahedron grid units for the model in this paper.

Figure 5. Cloud image of solid velocity along Y axis with dust content =

1% and inlet speed = 18m/s

Figure 3. grid model

As can be seen from static-pressure cloud image shown in Fig. 4, the pressure is larger in the cylinder and cone (close to inner wall) of cyclone separator, while smaller in center zone, but the difference between them is not remarkable.

Then we analyse the velocity along Y axis shown in Fig. 5, it is shown that cylinder and cone take on the color of blue and green in the left half, which means the particles move along negative direction of Y axis, namely inwardly perpendicular to paper surface, while which is red and yellow in the right half exactly opposite, meaning the particles move along positive direction of Y axis. It also show that just like gas-flow does, solid particles flow with clockwise looking in the positive direction of Z axis, and speed decrease from wall to center in turn . with a enveloping line of zero speed within which is the center zone, where speed is almost zero. There are corresponding velocity variance in the lower zone of dust collecting, with direction inwardly perpendicular to paper surface in the left half while outwardly in the right half, and the absolute value

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of velocity is relatively smaller compared with above cylinder section. contradictions among velocity components of flow-field inside cyclone[4].

Figure 6. Cloud image of solid velocity with dust content = 1% and inlet

speed = 18m/s along Z axis

Figure 8. The cloud image of solid phase with dust content = 1% and inlet

speed = 18m/s

The cloud image of solid velocity for dust content value being 1% and inlet speed value being18m/s is indicated in Fig.6. It is shown that the motion speed is larger with the negative direction of Z axis in the zone of gas rising pipe, namely upwards. For the zone of cylinder and cone of cyclone separator, it is yellow-green color near the wall which means particles’ speed is larger with the positive direction of Z axis namely straight down, especially in the area near inlet port, with yellowish color, means the speed is larger, which may be the result of driving by inlet gas. In zone of dust colleting container below the cylinder and cone, there is yellow area, which means the speed get faster when particles get into the dust collection part. As for the center zone of cylinder and cone, green is gradually turning deep and blue along the positive direction of Z axis, namely upwards ( towards gas rising pipe ), which means that the particle movement change in direction and speed get larger and larger, concentrating towards gas rising pipe and flowing out quickly. However, as shown in vector drawing of Fig7, in the zone near gas rising pipe, there are some irregular vortexes which changed the motion trajectory of the nearby gas with dust content, so that some just-go-into gas directly enter the gas rising pipe, and lead to the reduction of separation efficiency.

It can be seen from Fig. 8 that solid particles are distributed at cone and dust colleting container, tightly against the wall, which tells that the solid particle in the gas-flow are separated under centrifugal force of whirling gas-flow. As can be seen from Fig.8 that there is no solid particle in other zones except wall, with a small contribution and small area distribution on dust collecting container, which is the result of small dust content in the gas-flow. B. Effect of particle density on gas-solid flow-field

Figure 9. Static-pressure cloud image with different density and v =

18m/s

Figure 7. The locally enlarged vector drawing of solid speed in the

direction of Z axis

As shown in Fig.9, the static-pressure cloud image are obtained for the particle density 0.1%, 1% and 5% respectively from left to right while inlet speed is 18 m/s and other conditions remaining the same. From the figure we can know the total pressure is increasing when the particle density increase, but the pressure distribution law keep constant basically, namely which is large near wall and small at center zone, and reach the minimum in the gas rising pipe. However, the pressure difference between wall and center decreases as density increases.

Through the analysis of dual-direction flow-field for cyclone, it is concluded that the dominant velocity components are the tangential and axial direction for its separation efficiency. Radial acceleration generated former makes the dust particle has a centrifugal settling velocity from inside to outside along radius direction, and the solid particle is pushed to the wall to be separated; While the later drives the dust particle into dust colleting container along the axis from top to bottom, then the dust particle escape from inner tube with the rising gas flow. From this it can be seen that these two components are a pair of principal

Figure 10. Cloud image of solid velocity along Z axis with different

density and v = 18m/s

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Figure 11. Cloud image of solid velocity along Y axis with different

density and v = 18m/s

Finally a comparison of solid-phase cloud images under different particle densities is made. According to the solid-phase cloud images with density being 0.1%, 1% and 5% respectively from left to right, it is clear that with the increasing of particle density, solid particle zones area clinging to wall also increases, and particle densities in distribution zones increases obviously, which are concentrated at cylinder, cone and part area of dust collecting container.

V.

CONCLUSIONS

In this paper, numerical simulation research is carried out for standard type cyclone separator with CFD-oriented commercial software FLUENTT.1.2. Based on deep understanding of the model and structure of cyclone separator, gas-solid flow-field, pressure changes, velocity changes and vortex properties etc. are studied under a variety of circumstances.

1) Velocity along Y axis (namely tangential velocity) and along Z axis (namely axial velocity) play a dominant role, the generating centrifugal force, motion state of gas flow and separation process of solid particles are all affected by tangential velocity, but the effect of local small vortex and the secondary convection can’t be neglected.

2) The static-pressure, velocities of all directions and separation rate all increase with the increasing of gas treating amount. Fluids rotate downwards along the wall with the shape of vortex, and pressure at the outer vortex is higher, pressure at the inner one is lower, and pressure at center is the lowest. Solid particles concentrate at the middle section of wall.

3) The motion speed in all directions would decrease with particle density increasing, however, the capturing ability of the total particle group is enhanced in some extent under high density, but the wear of cyclone separator wall would be aggravated.

4) It is inferred that the characteristics of a cyclone separator with v = 18 m/s and dust content 1% are supposed to be mostly close to calculation result of the empirical model.

5) The larger the particle is, the higher the separation efficiency is. As the turbulences current has more remarkable effect on those smaller particles, hence their separation efficiency is more prone to be affected by operation conditions.

REFERENCES

[1] Xiaogang Luo. Numerical Simulation of Gas-Solid Two Phase Flow

in Cyclone Separator. SHANGHAI:Shanghai Jiaotong University, 2006. (in Chinese)

[2] Alex C. Hoffmann, Louis E. Stein. Gas Cyclones and Swirl Tubes:

Principles, Design and Operation, Springer-Verlag Berlin Heidelberg, 2002.

[3] Zhou L.X, SL. Soo. Gas-Solid Flow and Collection of Solids in a

Cyclone Separator. Powder Technology, 1990 (63): 45-53. (in Chinese)

[4] Liang-Shih Fan and Chao Zhu. Principles of Gas-Solid F1ows.

Cambridge University Press, 1997. (in Chinese)

Fig.10 is the cloud image of solid velocity along axis Z on condition that particle densities are 0.1%, 1% and 5% respectively from left to right, as can be seen from Fig.10, with the same inlet speed, velocity along Z axis reduces with particle density increasing, and its form distribution varies too. For the low density, for example 0.1%, the speed along the walls vicinity of cylinder and cone is fast with positive direction in Z axis (downwards), while the speed at center zone, is increasing along the negative Z axis, which lead to the forming ofing along the nagative axis Y, backflow with larger absolute value of velocity both above and below. Compared with cloud image of density 5%, it can be seen that the particle movement direction remain unchanged basically but the absolute value of velocity become less when the density increases. Related to the above analysis for static pressure figure, it can be inferred that it may be the reduction of pressure difference of flow-field that cause the particle motion speed to become slow.

Second we research the cloud image of solid velocity in Y axis direction with particle density being 0.1%, 1% and 5% respectively from left to right. For the cylinder part, both direction and absolute value of the velocities distribute almost the same with different densities; while for the cone part, the absolute value of velocity along wall decrease with density increase. Overall, under condition of different particle densities, velocity distribution and motion tendency remains almost the same along Y axis, in the left half moving along the Y axis negative direction against wall namely inwardly perpendicular to the paper surface, while in the right half in the Y axis positive direction namely outwardly. And looking from the inlet port of gas rising pipe, it takes on a clockwise rotation flow. There is no obvious linear relation between particle motion speed and density variation in Y direction.

Figure 12. Cloud image of solid phase with different density and v = 18m/s

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