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$$\mu m$to $5$$\mu m$ 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 $\mu m\; for\; all\; cases\; \&\; keep\; flow\; velocity\; same\; as\; particle\; velocity$]

Boundary Conditions:

Inlet and wall:- Reflect

Outlet(Top):- Escape 

Outlet(Bottom/dustbin) - Trap

Introduction:

CYCLONE SEPARATOR

Cyclone separators, or simply cyclones, are separation devices (dry scrubbers) that use the principle of inertia to remove particles from flue gases. Cyclone separators are one of many air pollution removal devices known as precleaners, as they typically remove large particulate matter. This eliminates the need to deal with large, highly abrasive particles later on with finer filtration methods. In addition, multiple cyclone separators can be operated in parallel, this system is called multi-cyclone.

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 Figure . 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.

Four empirical models used to calculate the cyclone separator 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: a centrifugal force,

Z, and a 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. LI AND WANG MODEL:

The Li and Wang [3] model includes particle bounce or reentrainment and turbulent diffusion at the cyclone wall. A twodimensional analytical expression of particle distribution in the cyclone is obtained. Li and Wang model was developed based 180 on the following assumptions:

The radial particle velocity and the radial concentration profile are not constant for uncollected particles within the cyclone.

Boundary conditions with the consideration of turbu185 lent diffusion coefficient and particle bounce reentrainment on the cyclone wall are:

3. KOCH AND LICHT MODEL:

Koch and Licht [2] 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 ad- 200 ditional 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 cy- 205 clone by: u Rn = constant.

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

4. LAPPLE MODEL:

Lapple [1] model was developed based on force balance without considering the flow resistance. Lapple assumed that a par215 ticle 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 [1] to calculate a 50% cut diameter, dpc, is:

The collection efficiency of cyclones varies as a function of density, particle size and cyclone design.

Cyclone efficiency will generally increase with increases in particle size and/or density;

inlet duct velocity; cyclone body length;

number of gas revolutions in the cyclone; ratio of cyclone body diameter to gas exit diameter;

inlet dust loading;

smoothness of the cyclone inner wall.Similarly, cyclone efficiency will decrease with increases in the parameters such as gas viscosity;

cyclone body diameter; gas exit diameter;

gas inlet duct area; gas density;

leakage of air into the dust outlet.

The efficiency of a cyclone collector is related to the pressure drop across the collector.

This is an indirect measure of the energy required to move the gas through the system. The pressure drop is a function of the inlet velocity and cyclone diameter. Form the above discussion it is clear that small cyclones are more efficient than large cyclones. Small cyclones, however, have a higher pressure drop and are limited with respect to volumetric flow rates. Another option is arrange smaller cyclones in series and/or in parallel to substantially increase efficiency at lower pressure drops. These gains are somewhat compensated, however, by the increased cost and maintenance problems. Also these types of arrangements tend to plug more easily. When common hoppers are used in such arrangements, different flows through cyclones can lead to reentrainment problems.

GEOMETRY:  

 To create the required geometry first load the .STEP file into the SpaceClaim.

Using the extract volume tool in the prepare tab select the edges as shown below 

click on the green check to extract volume .

Now suppress for physics the Cyclone Seperator from the structure and also uncheck the box before it  

 

The required geometry for meshing is ready

 

MESHING:

For Meshing we have used the structured mesh as it helps in obtaining better results in the Discrete Phase Modelling. 

We have used a global mesh size of 4 mm and added body fitted cartesian method to obtain structure mesh, also a face sizing was intriduced to get good inlet face mesh

the mesh has a good nuber of elements further reduction in the size may lead to limitaions for the acadamic liscence of ansys.

the body fitted cartesian method details are shown below

the face meshing was done on the inlet face and the details are shown below

now the generated mesh images are shown below

mesh near the inlet

mesh in the conical region

 

FLUENT Setup:

SETUP for the Analysis of particle velocity while keeping the size of particle as 5e-6 [m]. The analysis is performed for teh particle velocity of 1, 3 and 5 [m/s] .

now first add gravity in the general setting of -9.81 [m/s^2] and keep the slover as pressure based and time as steady

For the viscous we will be using the RNG K-epsilon model with swirl dominant flow

now to set up the Discrete Phase Model select the Discrete Phase tool in the model menu of the Physics tab and DPM Window will open up 

Now here check the box for the Interaction with continuous phase this will make the model couples with both the phases and also check the box for Udate DPM sources  every flow iterations and make sure the number of iterations is set to 10.

now select the injection button and give the injection according to the required particel velocity and partivel size here the image is shown for the particle velocity of 1[m/s] and the diameter of particle as 5e-6[m].

 

also make sure to select the injection type to surface and select the inlet for the injection.

set the solution method as the COUPLED scheme and keep the turbulent kinetic energy and turbutlent dissipation rate as second order upwind for both.

now initialse the solution from the inlet usign standard initialization and run the calculation for about 500 or 600 iteration until the residual reach a stable state.

 

RESULTS :

 

Analysis For Varying Particle Size:

1. Residuals:

Size = 1e-6 m

Size = 3e-6 m

Size = 5e-6 m

2. Pressure Drop: (Area waighted avg @)

 

Particel Size	Pressrue at inlet	Pressure at outlet-top	Pressrue at outlet-bottom	Pressure drop
1e-6 m	25.83	-0.0209	-0.0121	25.8630
3e-6 m	25.83	-0.0209	-0.0123	25.8633
5e-6 m	25.83	-0.0209	-0.0129	25.8638

We can see as the particle size increase form 1 [um] to 3 [um] the ressure drop increases but for the size 3 and 5 [um] the pressure remains the same.

 

3. Separating Efficiency:

size = 1e-6 m

Size = 3e-6 m

Size = 5e-6 m

Velocity	numbers tracked	Escaped	Traped	incomplete	Separating efficiency = trapped/number tracked
1m/s	378	78	300	0	0.7936
3m/s	378	72	306	0	0.8095
5m/s	378	131	247	0	0.6534

Here we can see the trend that as the particle size is increasing the separating efficiency also increases

 

Analysis For Varying Particle Velocity:

1. Residuals:

velocity 1m/s

velocity 3 m/s

velocity 5m/s

2. Pressure Drop:

 

Velocity	Pressrue at inlet	Pressure at outlet-top	Pressrue at outlet-bottom	Pressure drop
1 m/s	2.2203	-0.0006	-0.0004	2.2213
3 m/s	25.6226	-0.0197	-0.0111	25.7533
5 m/s	74.3905	-0.0677	-0.0434	74.5016

We can see the trend that as the velocity increases the pressure drop also increases

 

3. Separating Efficiency:

velocity 1m/s

velocity 3m/s

velocity 5m/s

Velocity	numbers tracked	Escaped	Traped	incomplete	Separating efficiency = trapped/number tracked
1m/s	378	109	8	261	0.0211
3m/s	378	77	242	59	0.6402
5m/s	378	70	305	3	0.8068

Here we can see the trend that as the velcoity is increasing the separating efficiency also increases ,

the more number of incomplete in the velocity 1m/s is beacuse it required more number of mesh elements but due to the limitation we can only get this results .

Pressure contour:

Animation:

 

Conclusion -

1. As the Particle Size increases the separating efficiency of the cyclone separator increases.
2. As the particle size increases the Presure drop of the cyclone separator increases.
3. As the particle velocity increases the separating efficency of the cyclone separator increases.
4. As the particle velocity increases the pressure drop of the cyclone separator increases.
5. for better results smaller element size and high mesh quality is required which is beyond the limits of the adacamic version of the software so we have to stop our simulation here.

 

References:

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