Discrete phase modeling of a Cyclone separator using Ansys Fluent

Introduction : 

Cyclone separators are separation devices that use the principle of inertia to remove particulate matter from flue gases, air or liquid steam without the use of filters. They are also known as precleaners. A high speed rotating flow is established within a cylindrical or conical container. Flow goes in a spiral pattern, beginning at the top wide end of the cyclone ending in the narrow end before exiting the cyclone in a straight stream through the center of the cyclone and out the top. Larger denser particles in the rotating stream too much inertia to follow the tight curve of the stream and strike the outside wall, falling then to the bottom of the cyclone where they can be removed.  

In this project, we have a cyclone separator model on which we perform Discrete Phase Modeling using ansys fluent. We perform simulation using the particle material as anthracite, Its a type of coal of a hard variety that contains relatively pure carbon and burns with little flame and smoke and an inlet fluid as air. The simulation is performed using 4 different boundary conditions at inlet, i.e reflect, trap, escape & walljet and on reflect b.c the number of particles that pass through the inlet are varied using mesh control to observe as a case study. 

Literature Review on empirical models used for cyclone separator efficiency : 

1. Lapple Model : 

Lapple (1951) model was developed based on force balance.  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 (1951) to calculate a 50% cut diameter, d_pc , is         

where μ is viscosity of air, b is inlet width, N_e is the number of effective turns (number of turns the flow makes from the entrance to the midpoint of the core section), V_i is inlet velocity and ρ_p is particle density. The collection efficiency (the ratio of the collected fraction to the total at the inlet) for any other size, d_pj, can then be determined by

2. Koch and Licht Model :

Koch and Licht (1977) collection theory recognized the inherently turbulent nature of cyclones and the distribution of gas residence times within the cyclone. Koch and Licht describe 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: uR n = constant.

A force balance and an equation on the particles collection
yields the grade efficiency η_i. G is a factor related to the configuration of the cyclone, n is
related to the vortex and τ is the relaxation term.

3. LI and Wang Model :

The Li and Wang (1989) 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 cyclones.
• Boundary conditions with the consideration of turbulent diffusion coefficient and particle bounce re-entrainment on the cyclone wall are:

Resultant exp of collection efficiency is given as 

4. Iozia and Leith Model :

Iozia and Leith (1990) logistic model is a modified version of Barth (1956) 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. Core length, zc , and core diameter, dc , are given as :

The addition made by Iozia and Leith on the original Barth (1956) model are the core length z c and slope parameter β expression which is derived based on the statistical analysis of experimental data of cyclone with D = 0.25 m. The collection efficiency η i of particle diameter d pi can be calculated from 

Objective : 

1. Perform analysis using 4 different b.c types at inlet i.e reflect, trap, escape, wall-jet.

2. Vary number of particles at inlet using reflect b.c. 

3. Post process and conclude results 

 Geometry & SpaceClaim : 

This on left is the main Geometry consisting of Cyclone separator & its frame. To prepare for analysis extract the Fluid volume inside the separator as shown on the right side.  

Meshing : 

Mesh Data : 

 1. Reflect boundary condition : 3 Mesh cases created to vary the number of particles passing through inlet. 

2. Trap, Escape , walljet B.c

1. Reflect Case 1 : Coarse mesh                                          2. Reflect Case 2 : Dense Mesh

 3 Reflect Case 3. Refined Mesh                                         4. Escape, Trap ,Reflect, Wall-jet : 

  Case Setup :

1. Time : Steady state

2. Pressure Based.

3. Gravity On

4. Viscous model : K epsilon ; RNG ; Swirl Dominated Flow (to enhance accuracy of swirling flow)

5. Discrete Phase modeling Turned on : Interact with Continous phase on

5. Update Dpm Sources checked. 

6. Injection particle type : Inert 

7. Material : Anthracite 

8. Injection type : Surface - Inlet

9. X-velocity : 3 m/s

10. (m) Dia : 5e-06 (5 micrometers)

11. B.c : Inlet ; Dpm - Reflect, Trap, wall-jet, Escape (case by case)

12. Standard Initialization : Compute from inlet 

Solution & Post Processing Results : 

Residuals for Different Reflect B.C cases : 

All the equations converge for all reflect boundary condition , For 1st case it converges at around 550 iterations whereas for 2nd and 3rd around 500 iterations.

Residuals for Trap, Escape & Wall-Jet B.c : 

All cases Converge at around 360 iterations and are ran for 400 iterations. 

Particle Comparison For Reflect B.c :

Particle Time shown in the plot below calculates particle residence time in seconds. This data is exported from the solution & imported in post processing where the data of anthracite particle tracks is shown. For varying Mesh sizes the number of practicles tracked are different.  

As the Mesh becomes Dense the Number of Particles tracked are more and less when mesh is coarse

1. Case 1                                                                                    2. Case 2

3. Case 3 

The tabulated & Bar graph is shown below comprising of particle history data obtained from simulation . For reflect b.c as the mesh became denser for same setup its observed that the particles escaped has reduced significantly. As the reflect boundary condition rebounds off the particle, it is observed that for the run time 600 iterations, the particles are still in the conical section of the cyclone. 

Particle Comparison For Trap, Escape, Wall-jet, Reflect B.c :

For same Mesh Size, Similarly as above Setup only varying the boundary condition we obtain following plots. They have very little changes to each other as observed from the particle residence time and Escape conditions. 

Case 4: Trap                                                              Case 5 : Escape 

Case 6 : Wall Jet 

Tabulated data & Bargraph plot showing the particle history data, i.e Particles escaped, Tracked & incomplete. All the different B.c conditions act according to their own mathematical rule ( reference 1), Theres minor 1-2% variation in the data observed. To monitor those data these all boundary conditions are necessary. 

Animation : 

Velocity Plot using Vortex Core region : 

As we have captured using Swirl Dominated flow, The vortex of the flow is captured displaying the velocity of the regions inside the Separator.

Pressure Plot :

The various pressure distribution inside the Separator, the higher pressure zone is the beginning inlet side and the conical section of the separator where the denser particles ram basically and are closer to walls causing more erosion in that area. 

Conclusions : 

1. Application of Cyclone separator is studied and simulated with 4 types of B.C . 

2. Understanding of Particle tracking & Flow zones in the separator, understanding Of efficiency models of C.separator.

3. Importance of proper meshing to track Proper particle history in ansys fluent.

4. Pressure & velocity distribution plots explain how the model needs to be modified and helps in better development. 

References : 

1. https://www.afs.enea.it/project/neptunius/docs/fluent/html/ug/node699.htm

2. https://www.che.iitb.ac.in/online/labfacility/cyclone-separator

3. https://www.afs.enea.it/project/neptunius/docs/fluent/html/ug/node1048.htm

4. https://energyeducation.ca/encyclopedia/Cyclone_separator

5. https://en.wikipedia.org/wiki/Cyclonic_separation

6. EVALUATION ON EMPIRICAL MODELS FOR THE PREDICTION OF CYCLONE EFFICIENCY [Journal - The Institution of Engineers, Malaysia (V ol. 67, No. 3, September 2006) ]