Week 8 - Simulating Cyclone separator with Discrete Phase Modelling

SIMULATION OF A CYCLONE SEPERATOR USING DISCRETE PHASE MODELING

The following study consists of analysis of cyclone separator and calculating separation efficiency for different cases by varying particle size and velocities.

CYCLONE SEPERATOR

Cyclone separators are separation devices that use the principle of inertia to remove particulate matter from flue gases. Cyclone separators is one of many air pollution control devices known as pre-cleaners since they generally remove larger pieces of particulate matter. This prevents finer filtration methods from having to deal with large, more abrasive particles later. In addition, several cyclone separators can operate in parallel, and this system is known as a multicyclone.

How It Works

Cyclone separators work much like a centrifuge, but with a continuous feed of dirty air. In a cyclone separator, dirty flue gas is fed into a chamber. The inside of the chamber creates a spiral vortex, like a tornado. The lighter components of this gas have less inertia, so it is easier for them to be influenced by the vortex and travel up it. Contrarily, larger components of particulate matter have more inertia and are not as easily influenced by the vortex.

Since these larger particles have difficulty following the high-speed spiral motion of the gas and the vortex, the particles hit the inside walls of the container and drop down into a collection hopper. These chambers are shaped like an upside-down cone to promote the collection of these particles at the bottom of the container. The cleaned flue gas escapes out the top of the chamber.

Most cyclones are built to control and remove particulate matter that is larger than 10 micrometres in diameter. However, there do exist high efficiency cyclones that are designed to be effective on particles as small as 2.5 micrometres. As well, these separators are not effective on extremely large particulate matter. For particulates around 200 micrometres in size, gravity settling chambers or momentum separators are a better option.

Out of all the particulate-control devices, cyclone separators are among the least expensive. They are often used as a pre-treatment before the flue gas enters more effective pollution control devices. Therefore, cyclone separators be "rough separators" before the flue gas reaches the fine filtration stages.

CYCLONE EFFICIENCY EMPIRICAL MODELS

Iozia and Leith Model

Iozia and Leith logistic model is a modified version of

Barth Model, which is developed based on force balance.

The model assumes that a particle carried by the vortex endures 165.

the influence of two forces: a centrifugal force, Z, and a flow

resistance, W. The collection efficiency η_i of particle diameter

d_pi can be calculated from

Li and Wang Model

The Li and Wang model includes 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:

• The radial particle velocity and the radial concentration

profile is not constant for uncollected particles within the cyclone.

• Boundary conditions with the consideration of turbulent diffusion coefficient and particle bounce re-entrainment on the cyclone wall are:

c = c₀ at θ =0

D­_{r = (1 − α)wc, at r = D/2}

• The tangential velocity is related to the radius of cyclone by: u R = constant.

The concentration distribution in a cyclone is given as:

Koch and Licht Model

Koch and Licht collection theory recognized the inherently turbulent nature of cyclones and the distribution of gas residence times within the cyclone. Koch and Licht described 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 η­_i

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 the cyclone is collected with 50% efficiency. The semi

empirical relationship developed by Lapple to calculate a 50% cut diameter, d_pc, is

CFD ANALYSIS

Analysis is done using ANSYS Workbench 2020. The geometry is modified using Spaceclaim and meshed. The solution is set up using ANSYS Fluent and the results are further analysed using CFD Post.

GEOMETRY

The volume is extracted, and the exterior geometry is suppressed for physics.

MESH

The geometry is named as shown in order to specify boundary conditions and to obtain various results.

Cell cut approach is used for better mesh quality and accuracy.

The mesh created has an average element quality of 91% hence the created mesh is suitable for the simulation.

SETTING UP PHYSICS

• Pressure based, steady solver. Gravity=9.81m/s^2 in the negative y direction.
• Model: K-epsilon, RNG, Swirl dominated flow
• Model: Discrete Phase Model

• Boundary conditions

Inlet- Velocity 3m/s, DPM- reflect

Outlet top- DPM-escape

Outlet Bottom- DPM- trap

Walls- DPM- reflect.

• Method

• Initialisation

RESULT

CASE 1(Velocity kept constant and particle size varied)

• Velocity=3m/s; Diameter=1e-6m

Residual

Particle Resident Time and Separation Efficiency

number tracked = 120, escaped = 36, trapped = 76, incomplete = 8

Separation Efficiency = (76/120)*100=63%

Pressure Contour and Pressure drop

Max Pressure =24.828 Pa Min Pressure =-4.41201 Pa

Pressure Difference = 29.24 Pa

• Velocity = 3m/s; Diameter = 2e-06m

Residual

Particle Resident Time and Separation Efficiency

number tracked = 120, escaped = 13, trapped = 86, incomplete = 21

Separation Efficiency = 71.66%

Pressure Contour and Pressure Drop

Max Pressure = 23.682 Pa Min Pressure = -4.887 Pa

Pressure Difference = 28.569 Pa

• Velocity = 3m/s; Diameter = 3e-06

Residual

Particle Resident Time and Separation Efficiency

number tracked = 120, escaped = 13, trapped = 88, incomplete = 19

Separation Efficiency = 73.33%

Pressure Contour and Pressure Drop

Max Pressure = 23.4808 Pa Min Pressure = -5.0346 Pa

Pressure Difference = 28.51 Pa

• Velocity = 3m/s; Diameter = 4e-06m

Residual

Particle Resident Time and Separation Efficiency

Number tracked = 120, escaped = 6, trapped = 93, incomplete = 21

Pressure Contour and Pressure Drop

Max Pressure = 23.4789 Pa Min Pressure = -5.03777 Pa

Pressure Difference = 28.51 Pa

• Velocity = 3m/s; Diameter = 5e-06

Residual

Particle Resident Time and Separation Efficiency

number tracked = 120, trapped = 103, incomplete = 17

Pressure Contour and Pressure Drop

Max Pressure - 23.4789 Pa Min Pressure -5.03777 Pa

Pressure Difference = 28.51 Pa

CASE 2(Particle size kept constant and Velocity varied)

Particle size = 5e-06m

• Velocity = 1m/s

Residual

Particle Resident Time and Separation Efficiency

number tracked = 120, trapped = 102, incomplete = 18

Separation Efficiency = 85%

Pressure Contour and Pressure Drop

Max Pressure -23.4789 Pa Min Pressure -5.03777 Pa

Pressure Difference = 28.516 Pa

• Velocity = 2 m/s

Residual

Particle Resident Time and Separation Efficiency

number tracked = 120, trapped = 101, incomplete = 19

Separation Efficiency = 84%

Pressure Drop and Pressure Contour

Max Pressure -23.4776 Pa Min Pressure - -5.03801 Pa

Pressure Difference = 28.515 Pa

• Velocity = 3m/s

Residual

Particle Resident Time and Separation Efficiency

number tracked = 120, trapped = 103, incomplete = 17

Separation Efficiency = 85.8%

Pressure Difference and Pressure Contour

Max Pressure -23.159 Pa Min Pressure - -5.27355 Pa

Pressure Difference = 28.432 Pa

• Velocity = 4m/s

Residual

Particle Resident Time and Separation Efficiency

number tracked = 120, trapped = 100, incomplete = 20

Separation Efficiency = 83%

Pressure Difference and Pressure Contour

Max pressure - 23.159 Pa Min Pressure --5.273 Pa

Pressure Difference = 28.432 Pa

• Velocity = 5m/s

Residual

Particle Resident Time and Separation Efficiency

number tracked = 120, trapped = 118, incomplete = 2

Separation Efficiency = 98%

Pressure Difference and Pressure contour

Max Pressure -22.767 Pa Min Pressure - -5.499 Pa

Pressure Difference = 28.266 Pa.

CONCLUSION

• Efficiency of cyclone separator decreases as particle size decreases
• Higher velocity yields higher separation efficiency.
• Pressure drop only depends on velocity of particle