Cyclone Separator

OBJECTIVES:

1. Setup Cyclone Separator model in ANSYS Fluent.
2. Study the concept of DPM
3. Post process the results and documentation

SCOPE:

• Clean up CAD and extract fluid domain.
• Prepare and verify FV model of Cyclone Separator Model.

SOFTWARE USED:

• ANSYS, SpaceClaim - To prepare FV model and setup cases of Cyclone Separator Model.
• ANSYS, Meshing - To mesh the given model.
• ANSYS, FLUNET - To setup the boundary conditions and to solve the given model.
• ANSYS, CFD Post - To perform Post Processing.

INTRODUCTION:

Cyclones are devices that employ a centrifugal force generated by a spinning gas stream to separate particles from the carrier gas. Their simple design, low capital cost and nearly maintenance-free operation make them ideal for use as precleaners for more expensive final control devices such as baghouses or electrostatic precipitators. Cyclones are particularly well suited for high temperature and pressure conditions because of their rugged design and flexible component materials.

Cyclone collection efficiencies can reach 99% for particles bigger than 5 μm, and can be operated at very high dust loading. Cyclones are used for the removal of large particles for both air pollution control and process use. Application in extreme conditions includes the removal of coal dust in a power plant and the use as a spray dryer or gasification reactor.

Engineers are generally interested in two parameters in order to carry out an assessment of the design and performance of a cyclone. These parameters are the collection efficiency of particle and pressure drop through the cyclone. An accurate prediction of cyclone efficiency is very important because an inaccuracy in the efficiency prediction may result in an inefficient design of the cyclone separator. CFD has a great potential to predict the flow field characteristics and particle trajectories inside the cyclone as well as the pressure drop. The complicated swirling turbulent flow in a cyclone places great demands on the numerical techniques and the turbulence models employed in the CFD codes when modelling the cyclone pressure drop.

CYCLONE EFFICIENCY EMPIRICAL MODELS

1. Iozia and Leith Model

Iozia and Leith logistic model is a modified version of Barth Model, which is developed based on force balance. The model assumes that a particle carried by the vortex endures the influence of two forces: a centrifugal force, Z and a flow resistance, W. The collection efficiency η_i of particle diameter dpi can be calculated from

`eta_i = 1/(1 + (d_(pc)/d_("pi"))^(beta)`

β is an expression for slope parameter derived based on the statistical analysis of experimental data of a cyclone with D = 0.25 m given as

`beta = 0.62-0.87ln(d_(pc)/100)+5.21ln((ab)/D^2)+1.05[ln((ab)/D^2)]^2`

and `d_(pc)` is the 50% cut size given by Barth

`d_(pc) = [(9muQ)/(pi rho_pz_cv_(tmax)^2)]^0.5`

where core length `z_c`, and core diameter `d_c`, are given as:

`z_c=(H-S)-[(H-S)/((D/B)-1)][(d_c/B)-1] "for " d_c gt B`

`z_c=H-S " for " d_c ltB`

`d_c=0.47D((ab)/D^2)^-0.25((D_e)/D)^1.4`

2. Li and Wang Model

The Li and Wang model includes particle bounce or re-entrainment and turbulent diffusion at the cyclone wall. A two dimensional analytical expression of particle distribution in the cyclone is obtained. Li and Wang model was developed based on the following assumptions:

• The radial particle velocity and the radial concentration profile are not constant for uncollected particles within the cyclone.
• Boundary conditions with the consideration of turbulent diffusion coefficient and particle bounce re-entrainment on the cyclone wall are:

`c= c_0," at " theta=0`

`D_r(delc)/(delr) = (1-alpha)omegac," at " r = D/2`

• The tangential velocity is related to the radius of cyclone by: `uR^n = "constant"`.

The concentration distribution in a cyclone is given as:

`c(r,theta) = (c_0(r_w-r_n)exp{-lambda[1/(K(1+n))r^(1+n)]})/(int_(r_n)^(f_w)exp{1/(K(1+n))r^(1+n)}dr`

`K=((1-n)(rho_p-rho_g)d^2Q)/(18mub(r_w^(1-n)-r_n^(1-n))`

and `lambda = ((1-alpha)Komega_w)/(D_rr_w^n)`

The resultant expression of the collection efficiency for particle of any size is given as

`eta_i = 1 - exp{-lambdatheta_1}`

where `theta_1 = 2pi(S+L)/a`

3. Koch and Licht Model

Koch and Licht collection theory recognized the inherently turbulent nature of cyclones and the distribution of gas residence times within the cyclone. Koch and Licht described particle motion in the entry and collection regions with the additional following assumptions:

• The tangential velocity of a particle is equal to the tangential velocity of the gas flow, i.e. there is no slip in the tangential direction between the particle and the gas.
• The tangential velocity is related to the radius of cyclone by: uRⁿ = constant.

A force balance and an equation on the particles collection yields the grade efficiency η_i.

`eta_i = 1-exp{-2[(Gtau_iQ)/D^3(n+1)]^(0.5/(n+1))}`

where `G = (8K_c)/(K_a^2K_b^2)`

`n = 1-{1-(12D)^0.14/2.5}{(T+460)/530}^0.3`

`tau_i=(rho_pd_(pi)^2)/(18mu)`

G is a factor related to the configuration of the cyclone, n is related to the vortex and `tau` is the relaxation term.

4. Lapple Model

Lapple model was developed based on force balance without considering the flow resistance. Lapple assumed that a particle entering the cyclone is evenly distributed across the inlet opening. The particle that travels from inlet half width to the wall in the cyclone is collected with 50% efficiency. The semi-empirical relationship developed by Lapple to calculate a 50% cut diameter, `d_(pc)`, is

`d_(pc) = [(9mub)/(2piN_ev_i(rho_p-rho_g))]^(1/2)`

where `N_e` is the number of revolutions

`N_e = 1/a[h+(H-h)/2]`

The efficiency of collection of any size of particle is given by

`eta_i = 1/(1+(d_(pc)/d_("pi"))^2`

GEOMETRY:

Fig 1. Cyclone Separator Geometry

Fig 2. Fluid domain

Once these methods are completed, then the model is taken for meshing.

PART I: Comparision between REFLECT, TRAP, and ESCAPE boundary conditions

MESH DETAILS:

The fluid voume once extraced is then extraced to the ANSYS Mesher to generate mesh.

Fig 3. Mesh

• Element Size : 10 mm
• Nodes : 79499
• Elements : 407308
• Mesh Quality :

1. Element Quality:

• Min : 4.1398e-002
• Max : 1.
• Average : 0.83812
• Standard Deviation : 9.4284e-002

Fig 4. Element Quality

2. Aspect Ratio:

• Min : 1.158
• Max : 46.526
• Average : 1.8487
• Standard Deviation : 0.4713

Fig 5. Aspect Ratio

3. Skewness:

• Min : 1.6722e-006
• Max : 0.96732
• Average : 0.22703
• Standard Deviation : 0.11797

Fig 6. Skewness

• Mesh Check (FLUENT):

Fig 7. Mesh Qualtity Check in Fluent

BOUNDARY CONDITIONS DEFINITION:

For the Combustion Model, the following boundary conditions are applied:

• Case 1: Escape
• Case 2: Reflect
• Case 3: Trap

Fig 8. Boundary Conditions

1. Model : Cyclone Separator

2. Condition : Steady State

3. Viscous Model : k - `epsi` Model

4. Material : Anthracite

5. Inlet Velocity : 3 m/s

6. Particle Diameter : 5e-6 m

7. Outlet : Atmospheric Outlet

RESULTS :

The Following Results are obtained:

From the above comparison we obseve that there is no much variation in the efficiency of the cyclone separator with different boundary conditions i.e., Escape, Reflect and Trap.

1. Velocity Animation:

• Case 1:

Vid 1. Case I: Escape - Particle Velocity Animation

• Case 2:

Vid 2. Case II: Reflect - Particle Velocity Animation

• Case 3:

Vid 3. Case III: Trap - Particle Velocity Animation

From the above velocity animation, they show almost identical behaviour.

2. Particle Escape Time:

• Case 1:

Fig 9. Case I: Escape - Anthracite Particle Time

• Case 2:

Fig 10. Case II: Reflect - Anthracite Particle Time

• Case 3:

Fig 11. Case III: Escape - Anthracite Particle Time

From the above contours, we observe that there is no much diifence in the residence time of anthracite particles in the cyclone separator.

3. Particle Escape Pressure:

• Case 1:

Fig 12. Case I: Escape - Pressure

• Case 2:

Fig 13. Case II: Reflect - Pressure

• Case 3:

Fig 14. Case III: Trap - Pressure

From the above contours, high pressure is obseved at the inlet and low pressue is observed at both the outlets.

4. Velocity:

• Case 1:

Fig 15. Case I: Escape - Velocity

• Case 2:

Fig 16. Case II: Reflect - Velocity

• Case 3:

Fig 17. Case III: Trap - Velocity

From the above contours, the particles at the core is experiencing high velocity due the centrifugal action on the particles.

5. Particle Vortex Core:

• Case 1:

Fig 18. Case I: Escape - Particle Vortex

• Case 2:

Fig 19. Case II: Reflect - Particle Vortex

• Case 3:

Fig 19. Case III: Trap - Particle Vortex

From the above vortex core generation plots, we obseve that all the three cases have an identical vortex developed.

6. Residuals:

• Case 1:

Fig 20. Case I: Escape - Residuals

• Case 2:

Fig 21. Case II: Reflect - Residuals

• Case 3:

Fig 22. Case III: Trap - Residuals

The residuals of the three cases show identical behaviour.

PART II: Comparision by varing the number the mesh size

MESH DETAILS:

The fluid voume once extraced is then extraced to the ANSYS Mesher to generate mesh.

Case 1:

Fig 23. Mesh

• • Element Size : 10 mm
  • Nodes : 79499
  • Elements : 407308
  • Mesh Quality :

  1. Element Quality:

  • Min : 4.1398e-002
  • Max : 1.
  • Average : 0.83812
  • Standard Deviation : 9.4284e-002

  Fig 24. Element Quality

  2. Aspect Ratio:

  • Min : 1.158
  • Max : 46.526
  • Average : 1.8487
  • Standard Deviation : 0.4713

  Fig 25. Aspect Ratio

  3. Skewness:

  • Min : 1.6722e-006
  • Max : 0.96732
  • Average : 0.22703
  • Standard Deviation : 0.11797

  Fig 26. Skewness

  • Mesh Check (FLUENT):

  Fig 27. Mesh Qualtity Check in Fluent

Case 2:

Fig 28. Mesh

• Element Size : 8 mm
• Nodes : 134931
• Elements : 701779
• Mesh Quality :

1. Element Quality:

• Min : 3.3647e-002
• Max : 1.
• Average : 0.83964
• Standard Deviation : 9.397e-002

Fig 29. Element Quality

2. Aspect Ratio:

• Min : 1.158
• Max : 56.489
• Average : 1.8424
• Standard Deviation : 0.46559

Fig 30. Aspect Ratio

3. Skewness:

• Min : 1.3891e-006
• Max : 0.97613
• Average : 0.22473
• Standard Deviation : 0.11731

Fig 31. Skewness

• Mesh Check (FLUENT):

Fig 32. Mesh Qualtity Check in Fluent

Case 3:

Fig 33. Mesh

• Element Size : 5 mm
• Nodes : 414279
• Elements : 2208429
• Mesh Quality :

1. Element Quality:

• Min : 1.6556e-002
• Max : 1.
• Average : 0.84257
• Standard Deviation : 9.3398e-002

Fig 34. Element Quality

2. Aspect Ratio:

• Min : 1.1577
• Max : 107.9
• Average : 1.8295
• Standard Deviation : 0.46687

Fig 35. Aspect Ratio

3. Skewness:

• Min : 4.1095e-007
• Max : 0.98465
• Average : 0.22045
• Standard Deviation : 0.11636

Fig 36. Skewness

• Mesh Check (FLUENT):

Fig 37. Mesh Qualtity Check in Fluent

BOUNDARY CONDITIONS DEFINITION:

For the Combustion Model, the following boundary conditions are applied:

Fig 38. Boundary Conditions

1. Model : Cyclone Separator

2. Condition : Steady State

3. Viscous Model : k - `epsi` Model

4. Material : Anthracite

5. Inlet Velocity : 3 m/s

6. Particle Diameter : 5e-6 m

7. Outlet : Atmospheric Outlet

RESULTS :

The Following Results are obtained:

From the above comparision, we observe that with increase in number of particles the efficiency of the cyclone separator is increased by a small value.

1. Velocity Animation:

• Case 1:

Vid 4. Case I: Mesh: 10mm - Particle Velocity Animation

• Case 2:

Vid 5. Case II: Mesh: 8mm - Particle Velocity Animation

• Case 3:

Vid 6. Case III: Mesh: 5mm - Particle Velocity Animation

Since the increase in the efficiency is increased by a small value, there is no much deviation that is observed in the velocity animation.

2. Particle Escape Time:

Case 1:

Fig 39. Case I: Mesh: 10mm - Anthracite Particle Time

• Case 2:

Fig 40. Case II: Mesh: 8mm - Anthracite Particle Time

• Case 3:

Fig 42. Case III: Mesh: 5mm - Anthracite Particle Time

With the increase in the number of particles, they escape through the separator much easily.

3. Particle Escape Pressure:

• Case 1:

Fig 43. Case I: Mesh: 10mm - Pressure

• Case 2:

Fig 44. Case II: Mesh: 8mm - Pressure

• Case 3:

Fig 45. Case III: Mesh: 5mm - Pressure

With increase in the number of particles, we observe a decreasing trend in the pressure of the particles at the core.

4. Velocity:

Case 1:

Fig 46. Case I: Mesh: 10mm - Velocity

• Case 2:

Fig 47. Case II: Mesh: 8mm - Velocity

• Case 3:

Fig 48. Case III: Mesh: 5mm - Velocity

From the above contours we observe a increasing trend of the velocity of the particles at the core with increasing in the number of particles.

5. Particle Vortex Core:

• Case 1:

Fig 49. Case I: Mesh: 10mm - Particle Vortex Core

• Case 2:

Fig 50. Case II: Mesh: 8mm - Particle Vortex Core

• Case 3:

Fig 51. Case III: Mesh: 5mm - Particle Vortex Core

Because of the increase in the number of particles, we obseve a distoted vortex core.

6. Residuals:

• Case 1:

Fig 52. Case I: Mesh: 10mm - Residuals

• Case 2:

Fig 53. Case II: Mesh: 8mm - Residuals

• Case 3:

Fig 54. Case III: Mesh: 5mm - Residuals

The above images show the residual plots obtained by running a variation in the number of particles through the inlet.

PART III: Effect of efficiency due to changes in particle diameter

MESH DETAILS:

The fluid voume once extraced is then extraced to the ANSYS Mesher to generate mesh.

Fig 55. Mesh

• Element Size : 10 mm
• Nodes : 79499
• Elements : 407308
• Mesh Quality :

1. Element Quality:

• Min : 4.1398e-002
• Max : 1.
• Average : 0.83812
• Standard Deviation : 9.4284e-002

Fig 56. Element Quality

2. Aspect Ratio:

• Min : 1.158
• Max : 46.526
• Average : 1.8487
• Standard Deviation : 0.4713

Fig 57. Aspect Ratio

3. Skewness:

• Min : 1.6722e-006
• Max : 0.96732
• Average : 0.22703
• Standard Deviation : 0.11797

Fig 58. Skewness

• Mesh Check (FLUENT):

Fig 59. Mesh Qualtity Check in Fluent

BOUNDARY CONDITIONS DEFINITION:

For the Combustion Model, the following boundary conditions are applied:

Fig 60. Boundary Conditions

1. Model : Cyclone Separator

2. Condition : Steady State

3. Viscous Model : k - `epsi` Model

4. Material : Anthracite

5. Inlet Velocity : 3 m/s

6. Outlet : Atmospheric Outlet

7. DPM : Reflect

RESULTS :

The Following Results are obtained:

From the above comparion we obseve that the efficiency of the separator decreases with increase in particle diameter.

PART IV: Effect of efficiency due to changes in Inlet Velocity

MESH DETAILS:

The fluid voume once extraced is then extraced to the ANSYS Mesher to generate mesh.

Fig 61. Mesh

• Element Size : 10 mm
• Nodes : 79499
• Elements : 407308
• Mesh Quality :

1. Element Quality:

• Min : 4.1398e-002
• Max : 1.
• Average : 0.83812
• Standard Deviation : 9.4284e-002

Fig 62. Element Quality

2. Aspect Ratio:

• Min : 1.158
• Max : 46.526
• Average : 1.8487
• Standard Deviation : 0.4713

Fig 63. Aspect Ratio

3. Skewness:

• Min : 1.6722e-006
• Max : 0.96732
• Average : 0.22703
• Standard Deviation : 0.11797

Fig 64. Skewness

• Mesh Check (FLUENT):

Fig 65. Mesh Qualtity Check in Fluent

BOUNDARY CONDITIONS DEFINITION:

For the Combustion Model, the following boundary conditions are applied:

Fig 66. Boundary Conditions

1. Model : Cyclone Separator

2. Condition : Steady State

3. Viscous Model : k - `epsi` Model

4. Material : Anthracite

5. Particle Diameter : 5e-6 m

6. Outlet : Atmospheric Outlet

7. DPM : Reflect

RESULTS :

The Following Results are obtained:

With the increase in the velocity at inlet, a decraseing trend of efficiency is observed for the given cyclone seperator.