CFD Analysis of a Gas Cylone Seperator

Introduction:

• A cyclonic separation is a method of removing particulates from an air, gas or liquid stream, without the use of filters, through vortex separation. When removing particulate matter from liquid, a hydrocyclone is used; while from gas, a gas cyclone is used. They work on the principle of inertia to filter particles from the fluid stream and contains no moving parts.
• In this project we will be simulating a Gas cyclone seperator. They are efficient in removing particles ranging anywhere between 2.5-10 micrometers.

Objective 1 : Four empirical models used for cyclone seperator efficiency

1. IOZIA AND LEITH MODEL:

Iozia and Leith (1990) logistic model is a modified version of Barth (1956) model which is developed based on force balance. The model assumes that a particle carried by the vortex endures the influence of two forces.

centrifugal force - Z

flow resistance - W

Core length - Zc

Core diameter - dc

The addition made by Iozia and Leith on the original Barth (1956) model is the core length Zc and slope parameter β expression which is derived based on the statistical analysis of experimental data of cyclone.

2. LI AND WANG MODEL:

• The Li and Wang (1989) 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.
• Li and Wang model was developed based on the following assumptions:

1. The radial particle velocity and the radial concentration profile are not constant, for uncollected particles within the cyclones.
2. Boundary conditions with the consideration of turbulent diffusion coefficient and particle bounce re-entrainment on the cyclone wall.

The final expression for the collection efficiency of the particles of different sizes is given by:

3.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.
• By using the additional assumptions, Koch and Licht describe particle motion in the entry and collection regions. Those are:

1. 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.
2. The tangential velocity is related to the radius of cyclone by: uR^n = constant.

4.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. The semi-empirical relationship developed by Lapple (1951) to calculate a 50% cut diameter, d_pc, is

Reference -

EVALUATION ON EMPIRICAL MODELS FOR THE PREDICTION OF CYCLONE EFFICIENCY
Jolius Gimbun1, T.G. Chuah2, Thomas S. Y. Choong2 and A. Fakhru’l-Razi2

Objective 2 - CFD analysis of cyclone seperator using 4 different boundary conditions to track the particles using DPM

Geometry -

The above picture shows the full assembly of parts found in an cyclone seperator. However our main region of interest is the fluid flow through the seperator hence it is necessary to extract the fluid volume to perform a CFD Analysis. The extracted fluid volume is shown below.

Mesh -

A baseline mesh with an element size of 15mm was generated with a total mesh size of 157185 elements. This standard mesh will be used to simulate 4 cases with different boundary condition so that the results for comparison are independent of the mesh.

Solver Setup:

• The case is solved using a steady state pressure-based solver.
• The viscous model chosen was , K-Epsilon - RNG model with the Standard wall function near-wall treatment and swirl dominated flow was enabled because of the rotational nature of the flow inside the chamber.
• An inlet-velocity of 3m/s was defined and four different DPM conditions were used.
• A total of 132 elements were chosen to be released from the inlet with a diameter of 5`mum`.
• All the 4 cases were run for 600 iterations.

Results:

a. DPM Condition - reflect:

In this boundary conditionThe particle rebounds off the boundary in question with a change in its momentum as defined by the coefficient of restitution.

• The contour of the Anthracite particle time is shown above.
• Varying time scales at which the particles are dropping down the outlet can be seen, also majority of particles are stuck in the recirculating zone.

b. DPM Condition - trap:

The trajectory calculations are terminated and the fate of the particle is recorded as \"trapped\'\' in this boundary condition.

c. DPM condition - Escape:

• The particle is reported as having \"escaped\'\' when it encounters the boundary in question. Trajectory calculations are terminated.

d. DPM condition - Wall-Jet:

• The direction and velocity of the droplet particles are given by the resulting momentum flux, which is a function of the impingement angle φ, and weber number.
• The \"wall-jet\" type boundary condition is appropriate for high-temperature walls where no significant liquid film is formed, and in high-Weber-number impacts where the spray acts as a jet.

Grid dependency study using reflect-dpm condition:

Case 1 - Coarse mesh using element size of 20mm, total cell count = 76057

Case 2 - baseline mesh using element size of 15mm, total cell count = 157185

Case 3 - fine mesh using element size of 10mm, total cell count = 407358

Comparison of results:

It can be noted from the above table that accuracy of the particles tracked increases with increasing the mesh resolution. More number of particles are caught in the recirculating region due to the swirling vertices caused by centrifugal forces. This can be corrected only by changing the flow rate of the fluid or by changing the dimensions of the cylone chamber.