Week 8 - 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]

Cyclone Separator: 

Cyclone separators are separation devices that use the principle of inertia to remove particulate matter from flue gases. Cyclone separator 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 on. In addition, several cyclone separators can operate in parallel, and this system is known as a multi-cyclone.

 

It is important to note that cyclones can vary drastically in their size. The size of the cyclone depends largely on how much flue gas must be filtered, thus larger operations tend to need larger cyclones. For example, several different models of one cyclone type can exist, and the sizes can range from a relatively small 1.2-1.5 meters tall (about 4-5 feet) to around 9 meters (30 feet)—which is about as tall as a three-story building.

Working:

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, similar to a tornado. This spiral formation and the separation is shown in the Figure below. 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.

Effectiveness:

Cyclone separators are generally able to remove somewhere between 50-99% of all particulate matter in the flue gas. How well the cyclone separators are actually able to remove this matter depends largely on particle size. If there is a large amount of lighter particulate matter, less of these particles are able to be separated out. Because of this, cyclone separators work best on flue gases that contain large amounts of big particulate matter.

The various empirical models to calculate the separation efficiency of cyclone separators are:

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

β is an expression for slope parameter derived based on the
statistical analysis of experimental data of a cyclone with D = 170
0.25 m given as:

and dpc is the 50% cut size given by Barth:

where core length, zc, and core diameter, dc, are given as

2)Koch and Licht model:

Koch and Licht inherently described the turbulent nature of cyclones and distribution of gas residence times within the cyclone. They attributed by the two following assumptions:

• The tangential velocity of the particle is equal to the tangential velocity of the gas flow. There is no slip in the tangential direction between gas nad particle.
• The tangential velocity is related to the radius of the cyclone by uR^n = constant

 This is the formula for the grade efficiency

3) Li and Wang model:

• The Li and Wand 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's 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 cyclone.

  2. 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 cyclone radius by uR^n=constant

The concentration distribution is given as :

The expression for collection efficiency for particles of any size is given as:

4)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, dpc">dpcdpc is 

where Ne">NeNe is the number of revolutions

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

Discrete Phase Modelling (DPM):

Discrete Phase Modelling (DPM) is a method in which the flow of the discrete phase with a continuous flow is modeled. DPM is used to track individual particles through the continuum fluid. The  Lagrangian discrete phase model follows the Euler Lagrange approach. The fluid phase is treated as a continuum by solving the Navier Stokes equation, while the dispersed phase is solved by tracking a large number of particle, bubble, or droplet through the calculated flow field. The dispersed phase can exchange momentum, mass, and energy with the fluid phase.

1)Geometry setup:

a. Choose Fluid flow fluent and start with editing geometry.

b. Import the step file of the model into spaceclaim.

c. Volume extract the fluid model

     

d. Hide the previous geometry and supress physics since we are only interested with the fluid flow and not the external surface.

2)Mesh:

Method = Cartesian method

Cartesian element size= 5mm

After up-streaming the model to meshing, choose generate mesh to generate the base mesh but since the number of particles entering the inlet depends on the refinement of the mesh, it's better to refine the mesh appropriately.

  

Generated mesh -

   

 Assign the names by applying name selection for inlet and outlets accordingly

• Inlet:

• Outlet-1 (Top Outlet)

• Outlet 2(Bottom Outlet )

3)SIMULATION SETUP:

1. Solver: Steady State

2. Type: Pressure Based

3. Graviational Accelaration: 

• X: 0 
• Y: -9.81 m/S^2
• Z: 0 

4. Turbulance Model: k-Epsilon(RNG) with Swirl Dominated Flow

5. Cell Zone Conditions:

• Fluid: Air

6. Discrete Phase Model:

• Interaction:   

             Interaction with Continuous Phase

            Update DPM Source Every Flow Iteration

• DPM Iteration Interval: 10
• Injection Properties:

                  Injection Name: injection_0

                  Injection Type: Surface

                  Release From Surfaces: Inlet

                  Particle Type: Inert

                  Material: Anthracite

                  Point Properties: X-Velocity = 3 m/s

                  Diameter: 1e-6 m

                  Total Flow Rate: 1e-20 kg/s

 7. Boundaries:

• Inlet:

               Velocity: 3 m/s

     DPM Boundary Types:

• Inlet: Reflect
• Outlet 1: Escape
• Outlet 2: Trap

8. Solution Methods: Coupled(Default)

• Momentum: Second order upwind
• Turbulent Kinetic Energy: Second order upwind
• Turbulent Dissipation Rate: Second order upwind

9. Initialization:

• Method: Standard Initialization from the Inlet
• Number of Iterations: 600

  

DPM

     

Particle diameter and X- velocity is given in the below tab

                           

Inlet

  

Outlet1

  

Outlet2

 

Solution method is set to default and standard initialization is done

  

Part 1:  Analysis of a given cyclone separator model by varying the particle diameter from 1 μm to  5 μm at 3m/s for both particle and flow velocity and calculate the separation efficiency in each case.

1. At 1 μm :

 a. Flow contour and vortex formation:

 

b.Residuals:

 

Animation

2. At 2 μm :

a. Flow contour and vortex formation:

 

b.Residuals

Animation

 https://youtu.be/P9mVSCLD_ak

3. At 3 μm :

a. Flow contour and vortex formation:

 

b.Residuals

Animation

 https://youtu.be/EMeCbZ1UXgs

4. At 4 μm :

a. Flow contour and vortex formation:

 

b.Residuals

Animation

 https://youtu.be/o_VIRIERjgI

5. At 5 μm :

a. Flow contour and vortex formation:

 

b.Residuals

Animation

 https://youtu.be/84WGBmii6FM

Part 2: Analysis of a  given cyclone separator model by varying the particle velocity from 1 m/sec to 5 m/sec at 5 micrometre and calculate the separation efficiency and pressure drop in each case

1. At 1 m/s :

a. Flow contour and vortex formation:

 

b.Residuals

c.pressure

Animation

 https://youtu.be/f5NdA3_st-g

2. At 2 m/s :

a. Flow contour and vortex formation:

 

b.Residuals

 

c.Pressure

Animation

 https://youtu.be/gW4rs3dypW0

3. At 3 m/s :

a. Flow contour and vortex formation:

 

b.Residuals

c. pressure

Animation

 https://youtu.be/RM36T-3ZgAg

4. At 4 m/s :

a. Flow contour and vortex formation:

 

b.Residuals

c.pressure

Animation

https://youtu.be/DKxaFE7S-Cw

5. At 5 m/s :

a. Flow contour and vortex formation:

 

b.Residuals

c.pressure

Animation

 https://youtu.be/x2yclElihv4

• Separation efficiency:

It is defined as the fraction of particles of a given size collected in the cyclone, compared to those of that size going into the cyclone. Experience shows that the separation efficiency of cyclone separator increases with increasing particle mean diameter and density; increasing gas tangential velocity; decreasing cyclone diameter; increasing cyclone length; extraction of gas along with solids through the cyclone legs.

In this case, depending upon the particle history data - Separation Efficiency is defined as the ratio of concentration that has been removed from the feed stream to the initial concentration in the feed stream. For this case ratio of the number of trapped particles to the total number of particles tracked.

Separation efficiency = no. of particles trapped / no. of particles tracked

• Pressure Drop: 

Pressure drop across the cyclone is of much importance in a cyclone separator. The pressure drop significantly affects the performance parameters of a cyclone. The total pressure drop in a cyclone will be due to the entry and exit losses, and friction and kinetic energy losses in the cyclone. Normally the most significant pressure drop occurs in the body due to swirl and energy dissipation.

Pressure drop is defined as the difference in total pressure between two points of a fluid carrying network.

                          Pressure drop, ΔP">ΔP = Total inlet pressure - Total Outlet Pressure (escape)

As the escape DPM condition is given to the outlet-top in this case, so outlet-top is considered in pressure drop calculation.

1. For varying particle diameter at 3m/s for both the velocities:

	Particle size	Tracked	Escaped	Trapped	Incomplete	Separation Efficiency
Case1	1μm	190	68	102	20	53.68
Case2	2μm	190	68	118	4	62.1
Case3	3 μm	190	54	136		71.57
Case4	4 μm	190	37	149	4	78.42
Case5	5 μm	190	37	151	2	79.47

 2. For varying velocities at 5 micrometres: 

5 μm	velocity	Tracked	Escaped	Trapped	Incomplete	Separation Efficiency	Pressure drop
Case1	1	190	49	16	125	8.42	2.22
Case2	2	190	35	149	6	78.42	10.88
Case3	3	190	30	160		84.21	25.932
Case4	4	190	31	159		83.68	47.94
Case5	5	190	31	159		83.684	76.75

Conclusion:

1. Both the parts simulated above show that Separation efficiency depends on both the diameter and velocity of the particles. As increased diameter and velocity gives increased separation efficiency.
2. Velocity has a greater impact as compared to the diameter of the particle.
3. Pressure drop is dependant on velocity. For constant velocity with varying diameter sizes, the pressure drop remains almost constant.
4. Increased velocity causes a higher pressure drop.