Numerical Study of GasâSolid Flow in a Radial-Inlet Structure Cyclone

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Ind. Eng. Chem. Res. 2010, 49, 5450–5460

Numerical Study of Gas-Solid Flow in a Radial-Inlet Structure Cyclone Separator Jie Cui, Xueli Chen,* Xin Gong, and Guangsuo Yu Key Laboratory of Coal Gasification of Ministry of Education, East China UniVersity of Science and Technology, Shanghai 200237, China

The development of a radial-inlet structure cyclone separator is reported in this paper, which is used as a primary device for gas-particle separation in an opposed multi-burner (OMB) gasification system. The radialinlet cyclone is more suitable for a high-pressure industrial operation environment on the premise of higher efficiency. A model based on computational fluid dynamics (CFD) techniques was applied to study the performance of a new-type cyclone separator. In the approach, the turbulent flow was described by the Reynolds stress model, and the particle flow was described by the stochastic Lagrangian model. The validity of the proposed approach is verified by the good agreement between the measured and the predicted results. The results indicate that, though the velocity flow field is not geometry symmetrical and a three-dimensional unsteady state, it is quasi-periodic. Additionally, there exists a processing vortex core phenomenon in the cyclone. The particle concentration distribution is nonuniform because of the centrifugal force. The distribution area can be divided into three parts according to the particles’ motion feature. And the larger particles are easier to separate than the smaller ones. But particles with a size exceeding a critical value will not be collected at the bottom and stagnate on the conical wall of the cyclone. This will lead to serious erosion on the conical part in the cyclone. In addition, the separation efficiency increases with the particle size, and the cut-point diameter of the radial-inlet cyclone is smaller than the traditional cyclone under the same inlet conditions. 1. Introduction The opposed multi-burner (OMB) gasification system consists of coal treatment, coal gasifier, gas cleanup, and black water treatment processes.1-3 The gas cleanup process, which runs at high temperatures and pressures, plays an important role in the whole system. It removes the particles from syngas produced in the gasifier before it reaches downstream.4 Unlike the syngas purification in GE gasification technology, in the OMB gasification purification process, a combination of mixer, cyclone, and scrubber is adopted. The existence of the cyclone improves the purification effect and stabilizes the system operation. Cyclones are widely used in air pollution control and gas-solid separation for aerosol sampling and industrial applications. With the advantages of relative simplicity to fabricate, low cost to operate, and easy adaptability to extremely harsh conditions, cyclone separators have become one of the most important particle removal devices which are preferably utilized in the field of both science and engineering.5-9 In general, the conventional cyclone usually adopts a tangential inlet structure.10-13 Hoffmann and Louis14 have introduced some design points about cylinder-on-cone cyclones with tangential inlets. But the tangential inlet structure cannot be suitable for some special conditions, such as high temperature, high pressure, and so on. So, several attempts have been made in the past decades to improve cyclone performance by introducing a new inlet design.15,16 And in the OMB gasification system, the tangential inlet cyclone separator is also inapplicable. Because of that, tangential welding is hindered by the large sum of investment, high technology demand, and big risk. In this paper, a new-type cyclone separator is introduced which adopts a special radial-inlet structure shown in Figure 1. The radialinlet cyclone can adapt to the industrial operation environment well on the premise of higher efficiency. Incorrect design of * To whom correspondence should be addressed. Fax: +86 21 64251312. Tel: +86 21 64250734. E-mail: [email protected].

the separation equipment would be particularly damaging, so a better basis for design is crucial. Therefore, it is necessary to understand the gas-particle flow and separation characteristics of the radial-inlet cyclone. However, due to the complicacy of three-dimensional strong swirling flows in the cyclone, conventional research methods cannot provide the prediction accurately. With the development of modern computational fluid dynamics (CFD) techniques, it is now possible to sufficiently simulate the gas flow and particle dynamics in the cyclone.17,18 In this paper, we focus on the simulations of the gas-particle flow field in the radial-inlet cyclone with the commercial CFD software FLUENT. By analyzing the information obtained by

Figure 1. Sketch map of a radial-inlet cyclone.

10.1021/ie901962r  2010 American Chemical Society Published on Web 04/29/2010

Ind. Eng. Chem. Res., Vol. 49, No. 11, 2010

the simulation and comparing with the traditional cyclone, the gas-particle flow field of the radial-inlet cyclone can be verified. 2. Numerical Simulation Methods 2.1. Turbulence Model. It is important to apply an appropriate turbulent model to solve complicated three-dimensional strong swirling flows in the cyclone. The present work is based on the RSM model, which can describe anisotropic turbulence. The RSM has been proven to be an appropriate turbulence model for cyclone flow, although it is computationally more expensive than other unresolved-eddy turbulence models.19-21 The QUICK scheme is used to discretize the convective terms, and the pressure gradient terms are treated with a pressure staggering option (PRESTO!). The solution of pressure coupling is based on the SIMPLE semi-implicit method. In the RSM, the transport equation is written as:22

µt ) FCµ

-

∂ (Fu′u′u′ + p′u′δkj + p′uj′δkj) ∂xk i j k

(2)

the viscous diffusion term: DL,ij

[

∂ ∂ ) µ (u′u′) ∂xk ∂xk i j

]

(3)

the shear production term:

(

Pij ) -F ui′uk′

∂uj ∂u + uj′uk′ i ∂xk ∂xk

)

(4)

the floatage production term: Gij ) -Fβ(giuj′θ + gjui′θ)

(5)

(

∂uj′ ∂ui′ + ∂xj ∂xi

)

()

(6)

∂ui′ ∂uj′ ∂xk ∂xk

FD )

Fij ) -2FΩk(uj′um′ eikm + ui′um′ ejkm)

(8)

and Sij is user-defined source term. The turbulent dispersion is included in the RSM model. A scalar turbulent diffusivity is used, shown as DT,ij )

(

∂ µt ∂ui′uj′ ∂xk σk ∂xk

)

18µm d2pFp

CD

Rep 24

(12)

where b up is the particle velocity, b u is the velocity of the gas phase, Fp is the density of the particle, and dp is the particle diameter. Rep is the relative Reynolds number, and CD is the drag coefficient between gas and particles, and the drag coefficient is given as23

{

24 Rep e 1 , Re 0.687 CD ) 24(1 + 0.15Rep ) 1 < Re e 1000 , p Rep Rep > 1000 0.44,

(9)

The turbulent viscosity, µt, is computed using the following equation:

(13)

The discrete phase particles can dampen or produce turbulent eddies. In stochastic tracking, the turbulent dispersion of particles is predicted by integrating the trajectory equations for individual particles, using the instantaneous fluid velocity, uj + u′, along the particle path during the integration. The values of u′, V′, and w′ that prevail during the lifetime of the turbulent eddy are sampled by assuming that they obey a Gaussian probability distribution, so that

(14)

where ξ is a normally distributed random number, and the remainder of the right-hand side is the local RMS value of the velocity fluctuations. Since the kinetic energy of turbulence is known at each point in the flow, these values of the RMS fluctuating components can be defined (assuming isotropy) as

√u′2 ) √V′2 ) √w′2 ) √2k/3

(7)

the rotation production term:

(11)

where (F/Fp)u bp(∂u b//∂xi) is an additional acceleration term that includes thermophoresis force, Brownian force, and Saffman’s lift force/unit particle mass; FD(u b-b up), the drag force per unit particle mass, is given by

u′ ) ξ√u′2

the dissipation term: εij ) -2µ

(10)

du bp b g (FP - F) F ∂b u ) FD(u + b-b u p) + b u dt Fp Fp p ∂xi

the pressure-strain term: φij ) p′

k ε

where Cµ ) 0.09 and ε is the dissipation term. 2.2. Gas-Solid Two-Phase Flow Model. The motion of a particle is described by the so-called stochastic Lagrangian multiphase flow model. The trajectories of discrete phase particles are predicted by integrating the force balance on the particles. So, the governing equation is written as

∂ ∂ ∂ (Fu′u′) + (Fu′u′u′) ) - (Fui′uj′uk′ + p′ui′δkj + ∂t i j ∂xk k i j ∂xk p′uj′δkj) + DL,ij + Pij + Gij + φij + εij + Fij + Sij (1) where the left two terms are the local time derivative of stress and convective transport term, respectively. The right terms are the stress diffusion term:

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2

(15)

The particle-eddy interaction time and dimension should not be larger than the lifetime and size of a random eddy. And the two-way coupling method was used for considering the effects between the gas phase and discrete phase. The two-way coupling is accomplished by alternately solving the discrete and continuous phase equations until the solutions in both phases stop changing. The momentum transfer from the gas phase to the particle phase is computed by examining the change in momentum of a particle as it passes through each control volume, and the momentum change is computed as F)

∑

(

3µCDRe 4Fpd2p

)

(up - u) + Fa mp∆t

(16)

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Table 1. Configuration Parameter for Radial-Inlet Cyclone Separator (D ) 600 mm) H/D

Hc/D

di/D

dr/D

hs/D

dc/D

3

0.75

0.25

0.3

1.14

0.13

where mp is the mass flow rate of the particles and ∆t is the time step. But note that the model described ignored the effect of particle-particle interaction. This treatment has been widely accepted for dilute flows.24,25 3. Grid Division and Boundary Conditions 3.1. Grid Division. The geometrical dimensions of the cyclone separator used for simulation are depicted in Figure 1. To validate the model against experimental data, the dimensions of the cyclone used in this numerical study are the same as those used by Chen,26 which are based on geometric similarity modeling method. The inlet structure of the new-type cyclone is a straight line type with a vane which imparts a circular motion. The pipe inserting into the cylinder is half cut along its axis, forming a virtual plane. The angle between virtual plane normal and the y axis is defined as inlet angle β. The direction of the mixture entering the cyclone changes with the inlet angle β. The mixtures flow through the mixer and impact on the vane. After impact, the gas with particle mixtures enters the cyclone along the vane and is separated under centrifugal force. The choice of the origin coordinates is as follows: The axial direction is the z axis, and the upward direction is positive. The normal position of the inlet is the x axis, and the y axis is vertical to the inlet center line. The coordinate’s origin is defined at the point where the normal of the inlet and the cylinder axis of the cyclone intersect. In addition, the inlet position is set as 0°, and the anticlockwise direction is positive. And the relevant dimensionless configuration parameter values are listed in Table 1. Figure 2 shows the computational domain, containing 210 000 cells. The whole computational domain is divided by structured hexahedron grids. At the zone near the wall, the grids are dense,

Figure 3. Particle size distribution.

while at the zone away from wall, the grids are refined. Griddependent tests are conducted. The specified grid is fine enough to give grid-independent solutions, suggesting that computed results are independent of the characteristics of the mesh size. In addition, when the RSM is selected with wall functions, it is important that the first cell size for wall-adjacent cells, which is defined as the distance between the cell centroid and the wall, is chosen to appropriately satisfy the dimensionless distance (Y+) requirement. First, cell size should not be too small, as that would prevent it from encompassing the viscous sublayer and the buffer region. Also, there should be a few cells inside the boundary layer, so the first cell cannot be too coarse. In this study, the first cell size has a Y+ of about 40-60, on the basis of these considerations. 3.2. Boundary Conditions. Air is used to represent the gas phase. The gas inlet velocity is set as ui ) 12.58 m/s. The initial positions of the particles are the location of the data points on the inlet surface. The particle inlet velocity is equal to the gas inlet velocity. The particle density is 1400 kg/m3, and the particle Reynolds number is about 13 500. The particles are continuously fed into the cyclone at a flow rate of 0.0168 kg/s when the gas flow field is steady. Independence tests of the particle number have been conducted. The particle size distribution uses the Rosin-Rammlar distribution method, fitting to measurement data shown in Figure 3. The boundary conditions at the gas outlet of the cyclone are described as a fully developed pipe flow. And the gradients of all variables in the axial direction are assumed to be zero. Turbulent intensity was used for the gas turbulence quantity of the inlet. The gas turbulent intensity I was calculated as I ) 0.16Re-0.125 g

(17)

where Reg is the gas Reynolds number under inlet conditions. The turbulent kinetic energy is used for the RSM model boundary condition. And the initial inlet turbulence kinetic energy k can be calculated as k ) 0.005u2i

Figure 2. Meshed geometry.

(18)

where ui is the inlet velocity of gas phase. There is no gas flow escaping the bottom outlet. Particles are assumed to be reflected elastically by walls and trapped by the bottom outlet of the cyclone. Moreover, no-slip conditions are assumed on the wall. The default wall roughness is 0.5. The standard wall function method is used to account for the near-wall effects in the flow field.


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Figure 4. Comparison of experimental data and model calculation (a, tangential velocity; b, radial velocity).

of the vortex finder. Once this flow reaches the bottom of the vortex finder, the gas flow collides with the upward flow and forms a chaotic flow close to the outside wall of the vortex finder, shown as points 1, 2, and 3. This part of the flow is also called the up eddy current or short circuit flow. As can be seen in Figure 5, the tangential velocity includes two types of vortex. The inner vortex and the outer vortex are divided at a position corresponding to the maximum tangential velocity in the separation space. The dividing positions are basically independent of the axial locations but only depend on the diameter of the vortex finder, indicating that the swirl is not attenuated in the separation space. Iozia and Leith28,29 have studied the effect of traditional cyclone dimensions on the gas flow pattern and fraction efficiency, summarizing the expression of the inner vortex diameter dt, expressed as dt ) 0.5DKA0.25 Figure 5. Contour of tangential velocity.

4. Results and Discussion 4.1. Gas Flow Field. 4.1.1. Model Validation. Figure 4 shows the comparison of experimental and calculated velocities at the cylindrical section of Z ) -690 mm at ui ) 12.58 m/s. The flow fields of the cold model with a total height of 1800 mm are measured using a dual particle dynamics analyzer (PDA).26 The comparison between the experimental and predicted tangential and radial velocity profiles indicates that the simulated results are in good agreement with the experimental value. The tangential velocity distribution is a Rankine vortex with a quasi-free vortex in the outer part and a quasi-forced vortex in the inner part. And the discrepancy between the experimental and calculated value may be caused by the ideal hypothesis of simulation and the experimental error and so on. Despite the discrepancy observed, the results described above confirm that the RSM model can predict well the flow field in the cyclone. In the following, the model will be used to establish some general understanding of the flow field about the radialinlet cyclone. 4.1.2. Gas Flow Field in the Cyclone. Figure 5 presents the calculated tangential velocity distribution in detail. The flow field in the cyclone indicates the expected forced/free combination of the Rankine vortex, which is similar to the traditional cyclone.27 The value of the tangential velocity is zero at the wall and at the center of the flow field. Part of the gas flows up to the roof cover and then flows down along the outside wall

() dr D

1.4

(19)

where KA is the inlet section rate and dr is the vortex finder diameter. But formula 19 is not suitable for the radial-inlet cyclone. So, a revised expression from an experimental study is put forward,26 shown as dt ) 0.37DKA0.25

() dr D

1.4619

(20)

According to the expression 20, it can be calculated that dt ) 76 mm under simulation conditions. It is in good accordance with the simulation result dt ) 74 mm, which confirms the reliability of the modeling further. Figure 6 shows the effect of the vortex finder diameter rate djr ) dr/D on the tangential velocity field. djr has a large influence on the maximum value of tangential velocity and the dividing point of inner-outer separation region. It can be found that the outer swirl region increases with decreasing djr, and the tangential velocity increases thereupon. So it is beneficial to separation due to the enhancing centrifugal force. Figure 7 shows the tangential velocity profiles of different sections at the same time. From section Z ) 0 mm, it can be seen that the flow velocity accelerates up to 1.5-2.0 times that at the inlet. Then, the velocity decreases as the flow spins down along the wall. In the separation space, there exists a velocity vortex core at each section, and the velocity rotates around the vortex core center but not around the geometric center. Moreover, the position of the vortex core center is not fixed, which causes the change of the peripheral velocity field. The

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Figure 8. Illustration of the natural vortex length.14

Figure 6. Effect of vortex finder on tangential velocity.

vortex core is oscillating on the section, and the oscillating amplitude is larger at the bottom of the vortex finder. Upon reaching the cone part, the oscillating has a large range and ends at some point on the wall, shown as section Z ) -1800 mm. This phenomenon can be easily explained by the precession vortex core mentioned in the literature,14,28 illustrated in Figure 8, and is often referred to as the “natural end of the vortex”.30,31 At the point of attachment, the high velocity vortex core is therefore in direct contact with the lower cyclone walls, which, in the presence of solids, produces the observed erosion peak. This phenomenon will be discussed in the following text. So, it can be found that the flow field in the cyclone is unsteady. Figure 9a shows the axial velocity distribution. A proportion of gas is seen flowing upward to the roof first and then downward along the outside wall of the vortex finder to collide with the upward flow, escaping from the vortex finder finally and forming a short circuit flow (point A). Moreover, several recirculations can be seen in the annular space. It also can be found that, at the bottom of the vortex finder, the axial velocity reaches the peak value (point B), which may be caused by the upward flow and the short circuit flow. Figure 9a indicates that the axial velocity is directed downward in the outer region and upward in the inner region around the center. The upward flow is a helical twisted cylinder and not axially symmetric, especially in the conical section. The results are qualitatively similar to those obtained by Narasimha et al.32 for the dense medium tangential-inlet cyclone. Figure 9b presents the calculated radial velocity distribution. It can be found that the radial velocity distribution is irregular,

Figure 7. Contour of tangential velocity at different axial sections.

Figure 9. Contour of velocity (a, axial velocity; b, radial velocity).

similar to a twisted cylinder. The radial velocity value of one side is positive, and the other is negative centered in the central axial line (shown in Figure 9b). Close to the bottom outlet, the velocity directions of both sides all change, which is different from the separation space. This indicates the unsteady flow field in the cyclone separator. Like the axial velocity distribution, there also exists a peak value at the bottom of the vortex finder. 4.1.3. Gas Flow in Vortex Finder. Figure 10 shows tangential velocity profiles in the vortex finder; they are high near the wall and low near the axis, and the velocity distribution center is constantly changing, indicating that the turbulence is still anisotropic in the vortex finder. Additionally, the maximum value of the tangential velocity decreases along the axial direction. The swirl flow in the vortex finder is bad for the separation, which is also a main factor causing the pressure drop


Figure 10. Profiles of tangential velocity distribution in the vortex finder.

in the cyclone. So, how to inhibit the gas swirl flow and to weaken the pressure drop will be taken into consideration in future work. 4.1.4. Periodicity of Gas Flow. The flow field in the cyclone is a three-dimensional unstable state. In this text, the internal law of the flow field has been studied, in the hopes of finding the flow field periodicity. The flow field is unsteadily computed after the stationary computation. The changes of tangential velocities at any two points in the separation space are shown in Figure 11. It can be seen from Figure 11 that the tangential velocity is quasi-periodic in the cyclone, indicating that the flow field is quasi-periodic and verifying the phenomenon of the vibration vortex core.30

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The rotation of the vortex core can cause a change in the ambient velocity field. So, the general movement law can be gained by studying the velocity frequency of each point in the vortex core region. The purpose of this part is to study the velocity vibration frequency at different points in the separation space in order to further explain the vortex core phenomenon. The velocity vibration frequency is computed by counting the number of vibrations in one second. The vibration frequency along the radial direction is investigated as represented by section Z ) -445 mm, as shown in Figure 12a. It shows that the vibration frequency in the radial direction changes little, which is almost around 6-7 Hz. The vibration frequency on the same section should be the same, because the vortex core can spread the wave and cause velocity vibration on the same section. So it is believed that the other sections have the same law. Figure 12b shows the vibration frequency along the axial direction. It can be found that the vibration frequency is almost the same in the separation space, which can explain the processing vortex core well. 4.2. Particles Flow Pattern. Figure 13 shows that the particles change in location with time for 1168 particles, one time step with eight particle diameters. Inside the cyclone, the two rotating streams are formed. One stream rotates close to the cyclone wall and flows down with bigger and heavier particles to the underflow. The second stream rotates near the center and flows up with small and lightweight particles to the overflow. It can be found that the trajectories of the largest particles concentrate on the wall, and the trajectories of the smallest particles are in the center. The other sized particles are largely between the two extremes. The larger particles are

Figure 11. The quasi-periodicities of tangential velocity.

Figure 12. The tangential velocity frequency (a, section Z ) -445 mm; b, axial direction).

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Figure 13. Flow pattern of particles at different times.

easy to separate through the cone due to the larger centrifugal force. Parts of smaller particles which are difficult to separate escape from the vortex finder with a short residence time (about 0.4 s), and other parts of them linger under the cover roof of the cyclone, forming a top dust ring. This is considered to be due to the effect of a secondary longitudinal vortex near the top of the annular space. The particle concentration is nonuniform because of the centrifugal force in the cyclone. Figure 14 shows the particle concentration distribution along the radial direction in the separation space. According to the particle motion feature and the concentration distribution characteristics, the distribution area is divided into three parts. Part A represents the particle trapping

area near the wall. In this part, the particles detach from the gas flow and deposit on the wall, forming a particle tape helix. So the particle radial velocity in this part is zero. Part B represents the separation area. The particle concentration is well distributed in this part. The particles move outward across the gas flow under the centrifugal force. Part C represents the particle escaping area. The particle concentration is very low because the drag forces acting on the smaller particles are larger than the centrifugal force, so the particles easily escape with the upward current. The particle traces colored by particle velocity magnitude are presented in Figure 15. It can be observed that the particles’ velocities are nonmonotonic, especially in the inlet part. Just


Figure 14. Particle concentration distributions along the radial direction.

Figure 15. Particle traces colored by particle velocity magnitude (t ) 3.5 s).

as previously stated, in the cyclone, particles with a larger size or with a larger velocity are easy to move toward the wall from the same radial positions. Though the smaller particles are easily carried by gas and the mean velocities are larger than those larger particles, many of them cannot reach the wall because of the resistance from larger particles, and the smaller particles with a larger velocity locate closer to the center. Therefore, the velocity distribution along the radial direction is out of order. In addition, it is clear that the velocity decreases along the moving direction until reaching the bottom of the cone. It indicates that the resistance on the particles is significant. The separation efficiency comparison of the radial cyclone with a traditional cyclone has been presented in Figure 16. And the structure and operation parameters used in this numerical study are same as those conducted by Ji et al.12 in the traditional cyclone. The separation efficiency is obtained by means of mass flow rates of the inlet and outlet of the cyclone for the converged cases. As shown in Figure 16, the left y axis represents the separation efficiency of the two cyclones and the right y axis represents the inlet particle size distribution. It can be observed that both separation efficiencies increase with particle size. And the cut-point diameter of the radial cyclone is smaller than that of the traditional cyclone, which means the radial cyclone is

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Figure 16. Separation efficiency comparison of the radial cyclone with a traditional cyclone.

more suitable for fine particle separation. But the particles with a diameter above 20 µm have been removed by both cyclones. Figure 17 shows the trajectories of particles with different diameters. It can be found that particles smaller than 11 µm do not attach immediately to the wall once they enter the body of the cyclone, as the larger particles do. Because the centrifugal force on the particles is not greater than the gas drag force on them, they have some chance to enter the weak short-cut flow and directly escape from the annulus between the vortex finder and the cyclone wall into the vortex finder. The larger particles (such as those in Figure 17c and d) mainly move outward to the wall of the cyclone and are captured. The separation process is also visualized for the two-way coupling simulation by Derksen et al.22 It is found that turbulence played a crucial role in the separation process. The small particles are dispersed by turbulence throughout the cyclone and are likely to get caught in the flow through the exit pipe at the top. The larger the particles are, the more they accumulate in the wall region and gradually move (due to the combined action of gas flow and gravity) to the dustbin. However, in this text, it is found that, for particles larger than a certain size, such as the simulated 29.92 and 35.88 µm particles, an obvious stagnated layer is observed near the cone. All of the particles larger than that certain size cannot be separated directly. And the larger the particle is, the higher the terminal position is, as shown in Figure 17e-g. Figure 17h illustrates the trajectories of all the different diameter particles. It is clear that the largest particles mainly remain on the bottom of the cylinder. The simulated results, as shown in Figure 17, are in accordance with Wang’s findings.27 A reasonable explanation may be that the upward and downward forces on the oversized particles are in balance. The particles’ movement state will remain changeless when the forces which are centrifugal force, gravity, and supporting force on the particle are balanced. However, this phenomenon would cause wall erosion in the operation process. Particle erosion and accretion rates can be monitored at wall boundaries. The erosion rate is defined as Nparticles

Rerosion )

∑ n)1

mpC(dp)f(R)Vb(V) Aface

(21)

where C(dp) is a function of the particle diameter, R is the impact angle of the particle path with the wall face, f(R) is a function of the impact angle, V is the relative particle velocity, b(V) is a function of the relative particle velocity, and Aface is the area of the cell face at the wall. C, f, and b are constants given by Edwards et al.33

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Figure 17. Trajectories of particles with different diameters.

Figure 18 shows the contour of the erosion in the cyclone. The wall material is 316 L stainless steel. It can be found that the erosion is serious at the bottom of the cylinder, especially at the connection of the cylinder and the cone due to the oversized particles. In addition, there are two other parts where the erosion is serious; they are the inlet part and the cover roof, which are caused by smaller particles. So some effort should be made to prevent this phenomenon. In future work, it will be necessary to study the critical particle diameter for certain cyclones and keep the cyclone running in a good state. 5. Conclusion In this paper, a radial-inlet cyclone is introduced which is more applicable for some special industrial operations than a traditional cyclone. A mathematical model has been applied to predict the gas-particle flow field of it. And the following conclusions can be obtained from the present study: (1) In the separation space and the vortex finder, there exists a velocity processing vortex core; the velocity field rotates

around the vortex core center but not around the geometric center, which may be caused by the special inlet structure. (2) The change of the velocity flow field is quasi-periodic, and the vibration frequency is almost the same in the separation space, which can explain the processing vortex core well. (3) The particle concentration is nonuniform in the cyclone. According to the particles’ motion feature and the concentration distribution characteristics, the distribution area is divided into three parts, which are the particle trapping area, separation area, and particle escaping area. The larger particles concentrate on the wall and flow down, and the smaller particles are in the center and escape from the vortex finder or linger under the cover roof of the cyclone, forming a top dust ring. (4) The particle separation efficiency increases with particle size. With a smaller cut-point diameter, the radial-inlet cyclone is more suitable for fine particle separation than the traditional cyclone.


Figure 18. Contour of the erosion.

(5) Particles with a size exceeding a critical value will not be collected at the bottom and stagnate on the conical wall of the cyclone. This leads to serious erosion on the conical part in the cyclone. Acknowledgment This work is financially supported by National High-tech R&D Program (2006AA05A101 and 2007AA050301). Nomenclature Aface ) the area of the cell face at the wall (mm3) b(V) ) a function of relative particle velocity C(dp) ) a function of particle diameter CD ) the drag coefficient between gas and particles D ) cylinder diameter (mm) di ) inlet diameter (mm) dp ) the particle diameter (mm) dr ) the vortex finder diameter (mm) djr ) dr/D ) rate between vortex finder diameter and the cylinder diameter f(R) ) a function of impact angle H ) cylinder height (mm) hs ) inserted height of vortex finder (mm) KA ) the inlet section rate Rep ) the relative Reynolds number ui ) inlet gas velocity (m/s) b u ) the velocity of the gas phase (m/s) b up ) the particle velocity (m/s) V ) the relative particle velocity (m/s) Greek Symbols R ) the impact angle of the particle path with the wall face (deg) β ) inlet angle (deg) Fp ) the density of the particle (kg/m3)

Literature Cited (1) Dai, Z. H.; Gong, X.; Guo, X. L.; Liu, H. F.; Wang, F. C.; Yu, Z. H. Pilot-trial and Modeling of a New Type of Pressurized Entrainedflow Pulverized Coal Gasification Technology. Fuel 2008, 87, 2304.

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(2) Guo, X. L.; Dai, Z. H.; Gong, X.; Chen, X. L.; Liu, H. F.; Wang, F. C.; Yu, Z. H. Performance of an Entrained-flow Gasification Technology of Pulverized Coal in Pilot-scale Plant. Fuel Process. Technol. 2007, 88, 451. (3) Higman, C.; van der Burgt, M. J. Gasification, 2nd ed.; Elsevier: Boston, 2008. (4) Choi, J. H.; Moon, Y. S.; Ryu, H. J.; Yi, C. K.; Son, J. E.; Kim, S. D. Modeling the Desulfurization Characteristics of a Continuous Bubbling-Fluidized-Bed Hot-Gas Cleanup System. Ind. Eng. Chem. Res. 2004, 43, 5770. (5) Peng, W.; Hoffmann, A. C.; Dries, H. W. A.; Regelink, M. A.; Stein, L. E. Experimental Study of the Vortex End in Centrifugal Separators: The Nature of the Vortex End. Chem. Eng. Sci. 2005, 60, 6919. (6) Salcedo, R. L.; Pinho, M. J. Pilot- and Industrial-Scale Experimental Investigation of Numerically Optimized Cyclones. Ind. Eng. Chem. Res. 2003, 42, 145. (7) Chan, C. W.; Seville, J. P. K.; Fan, X. F.; Baeyens, J. Particle Motion in CFB Cyclones as Observed By Positron Emission Particle Tracking. Ind. Eng. Chem. Res. 2009, 48, 253. (8) Liu, C.; Wang, L.; Wang, J.; Liu, Q. Investigation of Energy Loss Mechanisms in Cyclone Separators. Chem. Eng. Technol. 2005, 28, 1182. (9) Vegini, A. A.; Meier, H. F. Computational Fluid Dynamics (CFD) Analysis of Cyclone Separators Connected in Series. Ind. Eng. Chem. Res. 2008, 47, 192. (10) Narasimha, M.; Brennan, M. S.; Holtham, P. N. Numerical Simulation of Magnetite Segregation in a Dense Medium Cyclone. Miner. Eng. 2006, 19, 1034. (11) Zughbi, H. D.; Schwarz, M. P.; Turner, W. J.; Hutton, W. Numerical and Experimental Investigations of Wear in Heavy Medium Cyclones. Miner. Eng. 1991, 4, 245. (12) Ji, Z. L.; Xiong, Z. Y.; Wu, X. L.; Chen, H. H.; Wu, H. X. Experimental Investigations on a Cyclone Separator Performance at an Extremely Low Particle Concentration. Powder Technol. 2009, 191, 254. (13) Avci, A.; Karagoz, I. Effects of Flow and Geometrical Parameters on the Collection Efficiency in Cyclone Separators. J. Aerosol Sci. 2003, 34, 937. (14) Hoffmann, A. C.; Louis, E. S. Gas Cyclones and Swirl Tubes Principles, Design and Operation; Springer: New York, 2004; p 342. (15) Zhao, B. T.; Shen, H. G.; Kang, Y. M. Development of a Symmetrical Spiral Inlet to Improve Cyclone Separator Performance. Powder Technol. 2004, 145, 47. (16) Qian, F. P.; Wu, Y. P. Effects of the Inlet Section Angle on the Separation Performance of a Cyclone. Chem. Eng. Res. Des. 2009, 87, 1567. (17) Peng, W. M.; Boot, P. J. A. J.; Hoffmann, A. C.; Dries, H. W. A.; Kater, J.; Ekker, A. Flow in the Inlet Region in Tangential Inlet Cyclones. Ind. Eng. Chem. Res. 2001, 40, 5649. (18) Wan, G. J.; Sun, G. G.; Xue, X. H.; Shi, M. X. Solids Concentration Simulation of Different Size Particles in a Cyclone Separator. Powder Technol. 2008, 183, 94. (19) Shi, L. M.; Bayless, D. J.; Kremer, G.; Stuart, B. CFD Simulation of the Influence of Temperature and Pressure on the Flow Pattern in Cyclones. Ind. Eng. Chem. Res. 2006, 45, 7667. (20) Zhao, B.; Su, Y.; Zhang, J. Simulation of Gas Flow Pattern and Separation Efficiency in Cyclone with Conventional Single and Spiral Double Inlet Configuration. Chem. Eng. Res. Des. 2006, 84, 1158. (21) Hoekstra, A. J.; Derksen, J. J.; Van Den Akker, H. E. A. An Experimental and Numerical Study of Turbulent Swirling Flow in Gas Cyclones. Chem. Eng. Sci. 1999, 54, 2055. (22) Derksen, J. J.; Sundaresan, S.; Van den Akker, H. E. A. Simulation of Mass Loading Effects in Gas-solid Cyclone Separators. Powder Technol. 2006, 163, 59. (23) Shi, L. M.; Bayless, D. J. Comparison of Boundary Conditions for Predicting the Collection Efficiency of Cyclones. Powder Technol. 2007, 173, 29. (24) Wang, B.; Chu, K. W.; Yu, A. B. Numerical Study of Particlefluid Flow in a Hydrocyclone. Ind. Eng. Chem. Res. 2007, 46, 4695. (25) El-Behery, S. M.; Hamed, M. H.; El-Kadi, M. A.; Ibrahim, K. A. CFD Prediction of Air-solid Flow in 180° Curved Duct. Powder Technol. 2009, 191, 130. (26) Chen, X. L. Study on the Primary Purification Process of the Coal Entrained-Bed Gasification. Ph.D. Thesis, East China University of Science and Technology, 2003. (27) Wang, B.; Xu, D. L.; Chu, K. W.; Yu, A. B. Numerical Study of Gas-solid Flow in a Cyclone Separator. Appl. Math. Model. 2006, 30, 1326. (28) Iozia, D. L.; Leith, D. Effect of Cyclone Dimensions on Gas Flow Pattern and Collection Efficiency. Aerosol Sci. Techonl. 1989, 10, 491. (29) Iozia, D. L.; Leith, D. The Logistic Function and Cyclone Fractional Efficiency. Aerosol Sci. Techonl. 1990, 12, 598.

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(30) Cortes, C.; Gil, A. Modeling the Gas and Particle Flow Inside Cyclone Separators. Prog. Energy Combust. Sci. 2007, 33, 409. (31) Derksen, J. J.; Van den Akker, H. E. A. Simulation of Vortex Core Precession in a Reverse-Flow Cyclone. AIChE J. 2000, 46, 1317. (32) Narasimha, M.; Brennan, M. S.; Holtham, P. N.; Napier-Munn, T. J. A Comprehensive CFD Model of Dense Medium Cyclone Performance. Miner. Eng. 2007, 20, 414. (33) Edwards, J. K.; McLaury, B. S.; Shirazi, S. A. Evaluation of Alternative Pipe Bend Fittings in Erosive Service. In Proceedings of ASME

FEDSM’00; ASME 2000 Fluids Engineering Division Summer Meeting, June 2000, Boston, MA.

ReceiVed for reView December 10, 2009 ReVised manuscript receiVed March 29, 2010 Accepted April 21, 2010 IE901962R

Numerical Study of GasâSolid Flow in a Radial-Inlet Structure Cyclone

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