Cyclone Separator

CYCLONE SEPERATOR

Cyclones(Cyclone separators) 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 pre-cleaners for more expensive final control devices such as bag-houses or electrostatic precipitators. Cyclone collection efficiencies can reach 99% for particles bigger than 5 μm and can be operated at very high dust loading.

Two parameters that define the design of a Cyclone separator 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. Complicated swirling turbulent flow in a cyclone places great demands on the numerical techniques and the turbulence models employed in the CFD codes when modeling the cyclone pressure drop.

[A]

The four empirical models used for cyclone separator efficiency are:

1. 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 efficiency of collection of any size of the particle is given by

η_i = 1/ [1+(d_pc/d_pi)²]

2. Koch & 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 by the

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 = 1- exp {-2[(G τ_i Q)(n+1)/ D³)]^0.5/(n+1)}

3. Iozia & Leith Model: Iozia and Leith logistic model are based on force balance. The model assumes that a particle carried by the vortex endures the influence of two forces: a centrifugal force and flow resistance.

The collection efficiency η_i of particle diameter d_pi can be calculated from

η_i = 1/[1+(d_pc/d_pi)^β

β is an expression for slope parameter derived based on the statistical analysis of experimental data of a cyclone.

4. Li & Wang Model: The Li and Wang model include 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.

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

η_i = 1- exp{-λθ₁}

Geometry:

Cyclone separator shown below is used for analysis:

Fluid Volume extracted using SpaceClaim is shown below:

Mesh Details:

Size Function: Proximity & Curvature

Min Element Size: 10mm

No of Nodes: 113728

No of Elements: 591754

Boundary Conditions:

Inlet & outlets are as shown above.

Gravity is enabled in the -ve y-direction.

The swirl dominated RNG K-epsilon model is used here to capture the flow more accurately.

The Discrete phase modeling is used to track the flow of the particles.

No of step for particle tracking = 50000

Injection Material : Anthracite (5microns in diameter)

Velocity Inlet: 3m/s

Outlet: Pressure Outlet( Gauge Pressure= 0Pa)

DPM Settings are varied b/w reflect,escape, trap & wall jet.

Reflect: The particle rebounds off the boundary in question with a change in its momentum as defined by the coefficient of restitution.

Escape: Particle escapes out when encountered by the boundary.

Trap: The trajectory calculation is terminated & fate of the particle is recorded as a trap.

Wall-Jet: The wall-jet type boundary condition is applicable for high-temperature walls where no significant liquid film is found & high Weber No impacts when the spray acts as a jet. This model is not applicable to regimes where the film is important.

[B]

When Reflect:

Residual Plot is as shown above.

Particle tracking is shown above.

Vortex Region is shown above.

DPM Tracking is shown above.

When Escape:

Residual Plot is as shown above.

Particle tracking is shown above.

Vortex Region is shown above.

DPM Tracking is shown above.

When Trap:

Residual Plot is as shown above.

Particle Tracking is shown above.

Vortex Region is shown above.

DPM Tracking is shown above.

When Wall-Jet:

Residual Plot is as shown above.

Particle Tracking is shown above.

Vortex Region is shown above.

DPM Tracking is shown above.

Comparison:

Mesh Size-10mm	Tracked	Escaped	Incomplete	Trapped	Aborted	Evaporated
Reflect	364	181	183	0	0	0
Escape	364	181	183	0	0	0
Trap	364	181	183	0	0	0
Wall-Jet	364	181	183	0	0	0

From the above comparison table, its seen that for all 4 cases with different input DPM conditions produce no change in the result at all & also the residual plot remains to be the same.

[C]

Varying number of particles at the inlet: No of particles at the inlet is varied by varying the size of the mesh at the inlet.

Here DPM boundary condition used for all the below three cases will be Reflect
& other boundary conditions also remain the same for all three cases.

Case 1:

Mesh Size: 10mm

No of Nodes: 113728

No of Elements: 591754

Mesh Metrics:

Mesh quality is well above the minimum limit of 0.1 as seen above.

Residual plot for Case 1 is shown above.

Particle Tracking for Case 1 is shown above.

Vortex Region for Case 1 is shown above.

DPM Tracking for Case 1 is shown above.

Case 2:

Mesh Size: 10mm

Inlet Face Sizing: 5mm

No of Nodes: 122563

No of Elements: 636451

Mesh Metrics:

Mesh quality is well above the minimum limit of 0.1 as seen above.

Residual plot for Case 2 is shown above.

Particle Tracking for Case 2 is shown above.

Vortex Region for Case 2 is shown above.

DPM Tracking for Case 2 is shown above.

Case 3:

Mesh Size: 10mm

Inlet Face Sizing: 1mm

No of Nodes: 419150

No of Elements: 2203150

Mesh Metrics:

Mesh quality is well above the minimum limit of 0.1 as seen above.

Residual plot for Case 3 is shown above.

Particle Tracking for Case 3 is shown above.

Vortex Region for Case 3 is shown above.

DPM Tracking for Case 3 is shown above.

Comparison:

Mesh Size Inlet Face	No of elements	Tracked	Escaped	Incomplete	Trapped	Aborted	Evaporated
10	591754	364	181	183	0	0	0
5	636451	1298	481	812	0	0	0
1	2203150	32214	7475	24712	0	0	0

It\'s evident from the above comparison table that particle tracked varies with the mesh count & hence variation in the result so obtained.