Performing an analysis on a cyclone separator using Discrete phase Modeling DPM -- ANSYS FLUENT

OBJECTIVE:

1. To perform an analysis on the given cyclone separator model and apply for four different boundary condition types at the inlet i.e. reflect, trap, escape and wall-jet.
2. Vary the number of particles through the inlet and run the simulation for 3 cases.

Cyclone separator:

• Cyclone separators are gas cleaning devices that utilize a centrifugal force created by a spinning gas stream to separate particular matter from the gas.They are used for collection of medium sized and coarse particles.Their relatively simple construction with no moving parts result in lower capital and maintenance costs.
• Cyclone separators are usually used for removing particles 10 μm in size and larger. However, conventional cyclones seldom remove particles with an efficiency greater than 90% unless the particle size is 25 μm or larger. High-efficiency cyclones can remove particles down to 5 μm.

Four empirical models used for cyclone separator efficiency:

IOZIA AND LEITH MODEL:

It 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.

LI AND WANG MODEL:

• The Li and Wang (1989) model includes particle bounce orre-entrainment and turbulent diffusion at the cyclone wall. Atwo-dimensional analytical expression of particle distributionin the cyclone is obtained. Li and Wang model was developedbased on the following assumptions:
• The radial particle velocity and the radial concentrationprofile are not constant, for uncollected particles within the cyclones.
• Boundary conditions with the consideration of turbulentdiffusion coefficient and particle bounce re-entrainment on the cyclone wall are:
  c = c0, at θ = 0 .

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: uRn = constant.

LAPPLE MODEL:

Lapple (1951) 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.

METHODOLOGY:

1.Geometry-space claim:

fig.Geometry model

fig.Fluid volume for calculation

2.Mesh:

For all the cases default method is used for mesh generation.

3.Setup:

• Solver: Pressure based,Steady state,Gravity:-9.81 `m^2/s` (-y axis).
• Models: K-epsilon,RNG(swirl dominated flow),Standard wall function. Discrete phase model turned on

• Boundary condition: Inlet: velocity-3m/s and Dpm(set for appropriate cases).
• Solution Methods:

• Initialization: Standard,compute from inlet.
• 600 iterations were made to run the calculation.

SOLUTION:

1.Perform analysis for four different boundary condition at inlet :

Mesh: 30 mm element size for all the four BC\'s.

Nodes: 6680

Elements: 31346

1.Discrete phase BC type: reflect:

2.Discrete phase BC type: trap:

3.Discrete phase BC type: escape:

4.Discrete phase BC type: wall-jet:

Inference:

It has been observed that all the four BC\'s produce similar results.

Number tracked particle: 40

Escaped particle: 39

Incomplete : 1

Therefore,we conclude by saying that changing the Boundary condition at inlet has no effect on solution.

Animation:

2.Perform analysis by varying the number of particles through the inlet:

Boundary condition: DPM- \"Reflect\" for all the following three cases.

case 1:

Mesh element size 25mm:

fig.Mesh

2.Anthracite particle time:

3.Animation:

case 2:

Mesh element size 15mm:

fig.Mesh

2.Anthracite particle time:

3.Animation:

case 3:

Mesh element size 10mm:

fig.Mesh

2.Anthracite particle time:

3.Animation:

Results and Discussion:

• As we see from the table,Decreasing the mesh element size leads into more number of particles tracked.TThis means that number of mesh cells at inlet proportional to number of particles tracked.
• Incomplete particles(stuck in a recirculation region) for the case3 is higher than rest of the cases.
• It is also observed that reducing the mesh element size gives changes in results.In 10mm mesh element size,the vortex region is clearly seen compared to rest of the cases.

Conclusion:

Changing the Bc\'s will not give significant changes in result and reducing the mesh element size gives more number of elements which results in higher number of particles tracked.