Cyclone Separator Challenge

Project Title: Cyclone separator

Objective:

1. Study the cyclone separator

2. Study the different numerical methods for finding out the efficiency of a cyclone.

3. Study the cyclone efficiency at different velocities.

4. Study the cyclone efficiency at different particle size.

5. Perform cfd simulation of cyclone separator.

Theory:

Cyclonic separation

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 hydro cyclone is used; while from gas, a gas cyclone is used. Rotational effects and gravity are used to separate mixtures of solids and fluids. The method can also be used to separate fine droplets of liquid from a gaseous stream.

A high speed rotating (air)flow is established within a cylindrical or conical container called a cyclone. Air flows in a helical pattern, beginning at the top (wide end) of the cyclone and ending at the bottom (narrow) end before exiting the cyclone in a straight stream through the centre of the cyclone and out the top. Larger (denser) particles in the rotating stream have too much inertia to follow the tight curve of the stream, and thus strike the outside wall, then fall to the bottom of the cyclone where they can be removed. In a conical system, as the rotating flow moves towards the narrow end of the cyclone, the rotational radius of the stream is reduced, thus separating smaller and smaller particles. The cyclone geometry, together with the volumetric flow rate, defines the cut point of the cyclone. This is the size of particle that will be removed from the stream with a 50% efficiency. Particles larger than the cut point will be removed with greater efficiency and smaller particles with a lower efficiency as they separate with more difficulty or can be subject to re-entrainment when the air vortex reverses direction to move in direction of the outlet.

Large scale cyclones are used in sawmills to remove sawdust from extracted air. Cyclones are also used in oil refineries to separate oils and gases, and in the cement industry as components of kiln preheaters. Cyclones are increasingly used in the household, as the core technology in bagless types of portable vacuum cleaners and central vacuum cleaners. Cyclones are also used in industrial and professional kitchen ventilation for separating the grease from the exhaust air in extraction hoods. Smaller cyclones are used to separate airborne particles for analysis. Some are small enough to be worn clipped to clothing and are used to separate respirable particles for later analysis.

Problem statement

1. To perform an analysis on a given cyclone separator model by varying the particle diameter from 1 μm-10 μm and calculate the separation efficiency in each case. Discuss the results. [Use both the velocity's as 3m/sec.]
2. 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]

Boundary condition

Inlet and wall:- Reflect

Outlet(Top):- Escape

Outlet(Bottom/dustbin) - Trap

Solution

Geometry

a = Cyclone inlet height (m)

b = Cyclone inlet width (m)

B = Cyclone dust outlet diameter (m)

H = Cyclone height (m)

h = Cyclone cylinder height (m)

s = Cyclone gas outlet duct length (m)

D = Cyclone body diameter (m)

De = Cyclone gas outlet diameter (m)

Model

Meshing

Mesh outline

Mesh count

Different numerical methods

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: a centrifugal force, Z, and a flow resistance, W. Core length, zc, and core diameter, dc, are given as

The addition made by Iozia and Leith on the original Barth (1956) model are the core length zc and slope parameter β expression which is derived based on the statistical analysis of experimental data of cyclone with D = 0.25 m. The collection efficiency ηi of particle diameter dpi can be calculated from

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's model were developed based on the following assumptions:

• The radial particle velocity and the radial concentration profile are not constant, for uncollected particles within the cyclones.
• Boundary conditions with the consideration of turbulent diffusion coefficient and particle bounce re-entrainment on the cyclone wall are:
• The tangential velocity is related to the radius of cyclone by: uRn = constant. The concentration distribution in a cyclone is given as:

Koch and Licht:

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. A force balance and an equation on the particles collection yields the grade efficiency ηi.

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

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, dpc, is

Setup

Pressure based solver is used

gravity turn on in negative y-direction 9.81 m/sec2

Steady-state simulation

Viscous Model

To capture the swirl motion swirl dominated flow turn on

Discrete phase model

anthracite is used particle material

particles enter through inlet

injection type surface selected

diameter distribution considered uniform

Boundary condition

Inlet

Outlet (top)

Outlet (bottom)

Wall

Results

Convergence

solution run for 600 iterations

Particle information

total particle tracked 82

escaped 25

trapped 55

incomplete 2

Particle movement inside the cyclon

Eddy viscosity

Vortex formation

Pressure contour

Away from the central pressure is high

At the centre, a vacuum is created to collect the particles

Pressure Drop

The pressure drop between inlet and outlet is around 2.35054 for 1 m/s velocity with 5-micron particle size

As the velocity and particle size increases pressure drop also increases

Velocity contour

Velocity vectors

Cyclone efficiency

Considering Lapple model:

Cyclone efficiency Vs Particle velocity

As the particle velocity increases cyclone efficiency also increase

Cyclone efficiency Vs Particle diameter

As the particle diameter increases cyclone efficiency also increase

Conclusion

The most important economical parameters of a cyclone separator are separation efficiency and pressure drop. Generally, the increase of gas inlet velocity will increase the separation efficiency, but it will also increase the pressure drop.