Week 8 - Simulating Cyclone separator with Discrete Phase Modelling

Simulating Cyclone separator with Discrete Phase Modelling

Aim:- To perform analysis on cyclone separator and calculate the separation efficiency and pressure drop.

Objective:-

1. To write a few words about any four empirical models used to calculate the cyclone separator efficiency.
2. To perform an analysis on a given cyclone separator model by varying the particle diameter from 1 μm to 5 μm and calculate the separation efficiency in each case. Discuss the results. [Use both the velocity's as 3m/sec.]
3. To perform an analysis on a given cyclone separator model by varying the particle velocity from 1 m/sec to 5 m/sec and calculate the separation efficiency and pressure drop in each case. Discuss the results. [Use particle diameter size as 5 μm for all cases & keep flow velocity same as particle velocity]

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1. To write a few words about any four empirical models used to calculate the cyclone separator efficiency.

Cyclones separator are devices that employ a centrifugal force generated by a spinning gas stream to separate partices from the carrier gas. These are basically centrifugal separators. They simply transform the inertia force of gas particle to a centrifugal force by means of a vortex generated in the cyclone body. The particle laden gas enters tangentially at the upper part & passes through the body describing the vortex. Particles are driven to the walls by centrifugal forces, losing its momentum & falling down to the cyclone leg.

Four empirical models used to calculate the cyclone separator efficiency-

a) lozia & leith model:

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

E=1/(1+(dpc/dpi)^beta)

b) li & wang model:

The li & wang model include particle bounce or re-entrainment & turbulent diffusion at the cyclone wall. A 2D analytial expression of particle distribution in the cyclone is obtained. The resultant expression of the collection efficiency for particle of any size is given as

E= 1-exp{-lambda*theta(1)}, where theta1= (2*pie(S+L))/α">α

c) koch & licht model:

koch & licht model recognized the inherently turbulent nature of cyclones & the distribution of gas residence times within the cyclone. koch & licht described particle motion in the entry & collection regions with additional following assumptions:

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

A force balance & an equation on the particles collection yield the grade efficiency E

E=1-exp*{-2[((G*tau*Q)/(D^3))*(n+1)]^0.5/(n+1)}

d) 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 cyclone is collected with 50% efficiency.

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

E=1/(1+(dpc/dpi)^2), where dpi is particle diameter & dpc is cut diameter.

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Simulation:

Geometry Setup:

As we import cyclone separator geometry, we need to carry out analysis on fluid volume, so extract fluid volume.

Preapare->volume extract->edges

Mesh Generation:

As in Ansys 2020 R2 cut cell mesh option is not available so we choose cartesian mesh method.

Name selection: name one inlet (in axial direction), outlet 1 at top & outlet 2 at bottom.

Select cutcell meshing for the best results.

Mesh element size is 5mm.

Fluent Setup:

Setting physics:

Select k-epsilon, RNG, with swirl dominated flow turbulent model.

Solver- pressure based as Mach number is less than 3.

choose steady state-solver, enable gravity in y-axis -9.81 m/s^2.

Material selection:

for gas- air

for solid particles- Anthracite

Material properties:

For air: density= 1.22 kg/m^3

viscosity= 0.0000181 kg/m.s

For anthracite: density= 1506 kg/m^3

Boundary condition setup:

for inlet:

velocity= 1,3,5 m/s

Temp.= 300K

In DPM:

Inlet & wall- reflect

Outlet:

pressure= 0 Pa

In DPM:

outlet (top)- escape

outlet (bottom)- trap

To enable interaction between gas & solid particles, turn on DPM.

Click on interaction with continuos phase.

select update DPM sources every flow iterations.

select contour plots for DPM variables.

select maximum number of steps 50,000 & step length factor 5.

Now select Injections->create->particle type->inert->material(anthracite)->injection type(surface injections)->surface release(inlet)

Point properties: x-velocity= 3m/s, diameter= 5 microns, total flow rate= 1e-20 kg/s.

Solution:

Methods

for turbulent kinetic energy-> 1st order upwind interpolation scheme.

for turbulent dissipation rate-> 1st order upwind interpolation scheme.

Controls: pressure of particle 0.1, momentum of particles 0.3

Solving: standard initialization->options->compute from inlet->initialize & calculate.

After calculations, check DPM data from console & export->particle history data from injection0 folder for CFD post.

CFD Results:

Import particle data.

double click on Fluent PT for Anthracite->color(anthracite particle time)->apply.

Enable the vortex core region.

Create an animation of particle tracking.

calculate various properties like mass flow rate, pressure, velocity.

Plots & results:

case 1- For the particle diameter from 1,3 & 5 μm respectively & velocity of gas & particle is constant 3 m/s.

Residual plot:

Anthracite particle time plot:

Vortex core region:

Velocity tracking of particles:

Pressure plots:

Efficiency & mass flow rate:

At a velocity of particle & gas is constant 3 m/s

For particle size 1 micron

Mass flow rate at accept port=-0.0126078 kg/s

Mass flow rate at reject/trash port=-0.0057678 kg/s

Particle tracked=378; particle escaped=200; trapped=133, incomplete=45

E=Number of particle trapped/number of particles tracked= 133/378=35.19%

For particle size 3 micron

Mass flow rate at accept port=-0.0126076 kg/s

Mass flow rate at reject/trash port=-0.00576779 kg/s

Particle tracked=378; particle escaped=203; trapped=137, incomplete=38

E=Number of particle trapped/number of particles tracked= 137/378=36.24%

For particle size 5 micron

Mass flow rate at accept port= -0.020851 kg/s

Mass flow rate at reject/trash port= -0.00977679 kg/s

Particle tracked=378; particle escaped=185; trapped=146, incomplete=47

E=Number of particle trapped/number of particles tracked= 146/378=38.62%

• As the particle size increases, the efficiency of cyclone separator also increases.
• The mass flow rate is negative, means mass flows out of the separator.

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case 1- For the particle & gas velocities are 1,3 & 5 m/s respectively, particle size is constant 5 μm.

Residual plot:

Anthracite particle time plot:

Vortex core region:

Velocity tracking of particles:

Pressure plots:

Efficiency & mass flow rate:

At a particle size constant 5 microns &

For particle & gas velocity 1 m/s

Mass flow rate at accept port= -0.00431663 kg/s

Mass flow rate at reject/trash port= -0.00180832 kg/s

Particle tracked=378; particle escaped=183; trapped=136, incomplete=59

E=Number of particle trapped/number of particles tracked= 136/378=35.98%

For particle & gas velocity 3 m/s

Mass flow rate at accept port= -0.0126075 kg/s

Mass flow rate at reject/trash port= -0.00576771 kg/s

Particle tracked=378; particle escaped=185; trapped=149, incomplete=44

E=Number of particle trapped/number of particles tracked= 149/378=39.42%

For particle & gas velocity 5 m/s

Mass flow rate at accept port= -0.020851 kg/s

Mass flow rate at reject/trash port= -0.00977679 kg/s

Particle tracked=378; particle escaped=157; trapped=180, incomplete=41

E=Number of particle trapped/number of particles tracked= 180/378=47.62%

• As the velocity of gas & particles increases for the same size of particle, the efficiency of cyclone separator also increases.
• The mass flow rate is negative, means mass flows out of the separator.

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Conclusion:

• As the velocity of gas & particles increases for the same size of the particle, the efficiency of cyclone separator & pressure drop also increases due to inertia.
• As the particle size increases the efficiency of cyclone separator & pressure drop also increases.

Animation: