Cyclone Separator Challenge

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

OBJECTIVE:-

1. To perform an analysis on a given cyclone separator model by varying the particle diameter and to calculate the separation efficiency in each case.
2. To perform an analysis on a given cyclone separator model by varying the particle velocity from and to calculate the separation efficiency and pressure drop in each case.

A cyclone Separator is an important particle removal device used in both engineering and process operations. It employs centrifugal force generated by a spinning gas stream to seperate particles from the carrier gas. They have simple design with low maintenance cost and can be adapted to a wide variety of operating conditions.

The behaviour and efficiency of a cyclone efficiency can be predicited by four different empirical models. They are;

• IOZIA AND LEITH MODEL:

It is a modified version of Barth model based on force balance and assumes that a particle carried by the vortex endures
the influence of two forces: a centrifugal force and a flow
resistance. The collection efficiency, `eta_i` of particle diameter `d_p` can be calculated from;

Where, `d_pc` is 50% cut size given by Barth.

• LI AND WANG MODEL:

The Li&Wang model gives a two dimensional analytical expression of particle distribution in the cyclone. It includes re-entrainment or particle bounce and turbulent diffusion at the cyclone wall. The cyclone efficiency is given by;

Where,

• KOCH AND LICHT MODEL:

Koch and Licht theory recognized the inherently turbulent nature of cyclones and the distribution of gas residence times within the cyclone. The theory describes 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

Cyclone efficiency is given by;

Where, G is a factor related to the configuration of the cyclone, n is related to the vortex and `tau` is the relaxation term.

• LAPPLE MODEL

This Lapple model is based on force balance without considering the flow resistance. The assumption of this model is 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 particles by the cyclone is given by;

Where, `d_pc` isthe 50% cut diameter.

Geometry:

Below is a basic geometry of cyclone separator;

The cyclone separator has a single inlet and two oulets.

The fluid volume, which isextracted from the cyclone separator geometry is shown below;

Mesh:

The fluid volume was meshed using cut cell mesh for a body-fitted mesh with minimal inaccuracies in the domain.

The total elemnet count was at 499135 with 510607 nodes.

SETUP:

Pressure based, absolute steady state solver was used to run the simulation with gravitational acceleration enabled in the negative y direction. K-epsilon RNG moldel with swirl dominated flow was enabled to capture the vortex inside cyclone clearly. The continous fluid was set as air.

The discreet phase model was also enabled to inject particles into the cyclone separator along with the contionous fluid phase with a step length factor of 5 and max iterations being 70000.

The injection of anthracite particles through inlet(Surface injection) normal to the surface was created. The particle properties were varied for two different case studies and simulated. The different injection cases being;

Case 1: Particle size variation & velocity constant(Velocity = 3 m/s)

1. Particle size = 1e-6 m
2. Particle size = 5e-6 m

Case 2: Particle velocity variation & particle size constant(Particle size = 5e-6 m)

1. Velocity = 5 m/s
2. Velocity = 1 m/s

Boundary Conditions:

Inlet velocity: Same as particle velocity.

Inlet DPM boundary: Reflect

Outlet pressure(both) : Zero Pascal

Outlet1(top) DPM boundary: Escape

Outlet2(bottom) DPM boundary: Trap

Wall DPM boundary: Reflect

Solutions & Observations:

The SIMPLE solution method with second order upwind turbulent kinetic energy and turbulent dissipation rate was used. The set up was standard inizialized and ran till convergence. Convergence was observed when the particle data was unchanged with each iteration.

Case 1: Particle size variation & velocity constant(Velocity = 3 m/s)

• Particle size = 1e-6 m

Residuals:

Particle time data inside cyclone:

• Particle size = 5e-6 m

Residuals:

Particle time data inside cyclone:

Particle size	Particles tracked	Escaped	Trapped	Incomplete	Fractional Separation Efficiency(%)
1 `mum`	435	220	214	1	49.3
5 `mum`	435	123	312	0	71.7

Case 2: Particle velocity variation & particle size constant(Particle size = 5e-6 m)

• Particle Velocity = 5 m/s

Residuals:

Particle time data inside cyclone:

• Particle Velocity = 1 m/s

Residuals:

Particle time data inside cyclone:

Particle Velocity	Particles tracked	Escaped	Trapped	Incomplete	Pressure drop (Pa/m^3)	Fractional Separation Efficiency(%)
5 m/s	435	44	391	0	0.524	89.8
1 m/s	435	122	313	0	0.174	71.9

Conclusions:

• A cyclone separation is an effective tool to separate different liquid phases or particles from a gas stream.
• The particle separation efficiency is proportional to the particle size. Bigger the particle size, better separation efficiency. This is due to the fact that as size increases, inertia force will also increase on the particles. Particles with lower size will have lower inertia and will be easily swept along by the gas stream. Hence, a reduction in particle size will give a corresponding reduction in efficiency.
• A higher flow velocity resulted in higher pressure drop inside the cyclone and vice versa. Also, the effciency was observed to be higher for higher velocity flow. The increase in particle velocity resulted in higher separation efficiency because, the falling velocity of particle was also high hence the particle was not swept along the gas stream. Although the pressure drop was lower for decreased flow velocity, the separation efficiency for the low velocity case was still lower than high velocity case implying that pressure drop can be ignored to a certain extent.

Reference Papers:

1. Numerical study of gas-solid flow in a cyclone separator
2. A CFD Study on the Prediction of Cyclone Collection Efficiency