Cyclone Separator Challenge

Objective:

• Simulation of a cyclone separator on Ansys Fluent using DPM.
• Study of various models used in predicting the cyclone efficiency
• Study the influence of particle size on the efficiency
• Study the efficiency of flow velocity on the collection efficiency.

Introduction:

Cyclone separators are separation devices that use the principle of inertia to remove particulate matter from flue gases. Cyclone separators are used in air pollution as pre cleaners since they generally remove larger pieces of particulate matter. Thus, there is less load on the finer filtration methods from having to deal with large, more abrasive particles.

The principle of a cyclone separators is similar to that of a centrifuge. There is a continuous feed of dirty air or flue gases. A spiral vortex like a tornado is generated inside the chamber. Lighter components of gas have less inertia and thus travel up. The larger components of particulate matter have more inertia and do not get influenced by the vortex. These large particles have difficulty following high speed spiral motion and thus hit the inside walls of the container and drop down into the collection hopper.

Cyclone separators have one inlet and two outlets, one each for the cleaned air and particulate matter.

The collection efficiency of cyclones varies as a function of density, particle size, inlet velocity and geometrical parameters of the separator.

Separator Efficiency can be predicted by the following models:

Iozia & Leith Model:

The model is based on force balance assuming that the particle carried away by the vortex experiences centrifugal force and flow resistance. The collection efficiency is calculated from

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

Beta is slope parameter based on statistical analysis of experimental data.

Li & Wang Model:

Li and Wang model includes particle bounce and re entrainment and turbulent diffusion at the walls. The collection efficiency for a particle of any size is given by:

`eta_i=1-exp{-lambdatheta_1}`

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

Kock and Licht Model:

This theory takes into account the turbulent nature of the cyclones and the gas residence times within the cyclone. The following assumptions were made:

1. The tangential velocity of a particle is equal to tangential velocity of the gas flow.
2. Tangential velocity is related to cyclone radius by uRⁿ=constant.

Lapple model:

The lapple model is developed based on force balance without considering flow resistance. Lapple assumed that a particle entering the cyclone is evenly distributed across the inlet opening.

The efficiency of collection of any size of the particle

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

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

N_e is number of revolutions

Geometry:

The geometry of the cyclone separator is given in the figure:

Only the fluid volume has to be analyzed and hence the fluid volume is extracted using the volume extract option in the prepare tab.

The outer geometry is sepressed for physics and only the fluid volume generated is used for further analysis.

Meshing:

The mesh is generating keeping in mind the number of particles at the inlet. It can be noted that the number of particles is proportional to the number of elements at the inlet.

The general mesh used is of element size 0.0075m

The named selections are created as shown in the figure.

Case Setup:

Gravity enabled steady state analysis is used.

Turbulence Model: Swirl dominated RNG K epsilon

DPM is used for particle tracking.

Boundary Conditions:

Inlet: Velocity Inlet (Value equal to particle velocity)

Outlet_air: Pressure Outlet

Outlet_Particle: Pressure Outlet

DPM Boundary conditions:

Inlet: Reflect

Wall: Reflect

Outlet_particle: Based on Case simulated

Outlet_Air: Escape

Species:

Injection Material: Anthracite

No of steps for particle tracking: 50000

Different cases are set up by changing the boundary condition, inlet velocity,particle diameter.

The solution is initialized and the particle data file is exported.

Initialisation: Standard from inlet

Results:

The following figure shows the particle path through the separator. The anthracite particle time is the variable used to color.

The following figure shows the formation of the vortex.

The number of particles generated is based on the mesh.

The maximum number of particles that can be used for post processing should be less than or equal to the number of particles generated.

The particles trapped and escaped in each of the cases are tabulated below:

The particles used for post processing are 16% of the total particles created .

Change in Number of particles at the inlet: Inlet Velocity =5m/s

Mesh Size	Number of Elements	No of Particle	Trap	Escape	Incomplete
0.0085	161106	154	157	0	0
0.007	257477	226	225	0	1
0.0065	307846	246	241	5	0
0.0055	458970	346	316	12	18

Different Boundary conditions at the particle_outlet:

Boundary Condition	Velocity (m/s)	No of Particle	Trap	Escape	Incomplete
Reflect	3	190	0	4	186
Trap	3	190	165	5	020
Escape	3	190	0	170	020
Wall Jet	3	190	0	4	186

Variation of particle size:

Diameter ( µm)	Velocity (m/s)	No of Particle	Trap	Escape	Incomplete
1	3	190	64	54	72
5	3	190	165	5	20
10	3	190	154	0	36

For 1 micro m

For 10 micro m

Variation of Particle and Gas Inlet Velocity:

Diameter ( µm)	Velocity (m/s)	No of Particle	Trap	Escape	Incomplete
5	3	190	165	5	20
5	5	190	189	1	0
5	10	190	188	2	0

For inlet velocity 5m/s for gas and particle

For inlet velocity 10m/s for gas and particle

The following animation shows the movement of particle data in the cyclone seperator:

(100/190 created particles generated are used for post processing)

Conclusion:

The mesh size is varied and thus the number of particles created at the inlet are varied accordingly and simulation is carried out for different boundary condidtions.

The simulation is carried out by applyinf different boundary conditions at particle outlet.

Simulation is carried out for different inlet velocities and the efficiency of the separator is found to increase.

The efficiency of the cyclone is higher for higher particle size.