Week 8 - Simulating Cyclone separator with Discrete Phase Modelling
AIM: To simulate the flow through a cyclone separator and showing the results for the different particle size and for the different velocities. To run the simulation for the six cases
1) Set the velocity constant to 3 m/s and do simulation for the varying size of the particle.
2) Set the particle size to constant and simulate for the different velocities.
THEORY:
Cyclone Separators
Cyclone separators or simply cyclones are separation devices (dry scubbers) that use the principle of inertia to remove particulate matter from flue gases . Cyclone separators is one of many air pollution removers known as precleaners 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 multicyclone.
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 :
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.
Most cyclones are built to control and remove particulate matter that is larger than 10 micrometers in diameter. However, there do exist high-efficiency cyclones that are designed to be effective on particles as small as 2.5 micrometers. As well, these separators are not effective on extremely large particulate matter. For particulates around 200 micrometers in size, gravity settling chambers or momentum separators are a better option.
Out of all of 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 can be seen as "rough separators" before the flue gas reaches the fine filtration stages.
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.
There are several advantages and disadvantages of using cyclone separators. First, cyclone separators are beneficial because they are not expensive to install or maintain, and they have no moving parts. This keeps maintenance and operating costs low. Second, the removed particulate matter is collected when dry, which makes it easier to dispose of. Finally, these units take up very little space. Although effective, there are also disadvantages in using cyclone separators. Mainly because the standard models are not able to collect particulate matter that is smaller than 10 micrometers effectively and the machines are unable to handle sticky or tacky materials well
Models of the cyclone separator:
1. Iozia & 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 & Wang Model :
The Li and Wang 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 & 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 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 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 semiempirical 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 re-entrainment problems.
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, = Total inlet pressure - Total Outlet Pressure (escape)
Simulation:
There are 5 steps in the modeling approach.
- Geometry correction(using space claim).
- Meshing. (using Ansys mesher).
- Setup problem.(using Ansys fluent).
- Solution using(Ansys fluent).
- Results.(using CFD post).
Geometry Preparation:
Fluid Volume extracted using SpaceClaim is shown below:
Mesh Details:
Size Function: Proximity & Curvature
Min Element Size: 10mm
No of Nodes: 113728
No of Elements: 59175
Boundary Conditions:
Inlet & outlets are as shown above.
Gravity is enabled in the -ve y-direction.
The swirl dominated RNG K-epsilon model is used here to capture the flow more accurately.
The Discrete phase modeling is used to track the flow of the particles.
No of step for particle tracking = 50000
Injection Material : Anthracite (5microns in diameter)
Velocity Inlet: 3m/s
Outlet: Pressure Outlet( Gauge Pressure= 0Pa)
DPM Settings are varied b/w reflect,escape, trap & wall jet.
Reflect: The particle rebounds off the boundary in question with a change in its momentum as defined by the coefficient of restitution.
Escape: Particle escapes out when encountered by the boundary.
Trap: The trajectory calculation is terminated & fate of the particle is recorded as a trap.
Wall-Jet: The wall-jet type boundary condition is applicable for high-temperature walls where no significant liquid film is found & high Weber No impacts when the spray acts as a jet. This model is not applicable to regimes where the film is important.
Solution approach and solver requirement:
Results:
PART - 1: Varying Particle size with constant velocity i.e, 3m/s.
Case(1) - The particle size of 1 micron.
Residuals graph:
Pressure at inlet:
Pressure at outlet:
Particle Tracking:
Separation Efficiency in % = (52 / 98) * 100
Separation Efficiency = 53.06 %
Case(2) - The particle size of 2 micron.
Residuals graph:
Pressure at inlet:
Pressure at outlet:
Particle Tracking:
Separation Efficiency in % = (75 / 98) * 100
Separation Efficiency = 76.53 %
Case(3) - The particle size of 5 micron.
Residuals graph:
Pressure at inlet:
Pressure at outlet:
Total Pressure:
Now,
Pressure drop = Total inlet pressure - Total Outlet Pressure
= 33.93 – 4.099
Pressure drop = 29.83 Pa
Particle Tracking:
Separation Efficiency in % = (98 / 98) * 100
Separation Efficiency = 100 %
PART - 2: Varying velocity with constant Particle size i.e., 5 microns.
Case - (1) : Inlet Velocity of 1m/s
Residuals graph:
Pressure at inlet:
Pressure at outlet:
Total Pressure:
Pressure drop = Total inlet pressure - Total Outlet Pressure = 3.15 – 0.36
Pressure drop = 2.79 Pa
Particle Tracking:
Separation Efficiency in % = (7 / 98) * 100
Separation Efficiency = 7.14 %
Case - (2) : Inlet Velocity of 3m/s
Residuals graph:
Pressure at inlet:
Pressure at outlet:
Total Pressure:
Pressure drop = Total inlet pressure - Total Outlet Pressure = 33.93 – 4.09
Pressure drop = 29.83 Pa
Particle Tracking:
Separation Efficiency in % = (98 / 98) * 100
Separation Efficiency = 100 %
Case - (3) : Inlet Velocity of 5m/s
Residuals graph:
Pressure at inlet:
Pressure at outlet:
Total Pressure:
Pressure drop = Total inlet pressure - Total Outlet Pressure = 98.93 – 12.04
Pressure drop = 86.89 Pa
Particle Tracking:
Separation Efficiency in % = (98 / 98) * 100
Separation Efficiency = 100 %
CONCLUSION :
From the above pressure velocity plots, we can infer that the pressure is higher near the wall region which forms the outer vortex. Due to higher pressure near the wall low-pressure region is created at the center of the cyclone separator because of which the lighter particles escape from the top outlet and heavier particles are collected at the bottom outlet.
Simulation is done for particle size varying from 1 micron to 5 micron with air and particle velocity as 3m/s. From the video, it is observed that heavier particle is trapped at the bottom outlet and lighter particles are escaped at the outlet 1. But we can also see that lighter particles also get trapped at the bottom outlet because of interaction between lighter and heavier particles and there is an exchange of inertial forces between them as a result there is a transfer of Kinetic energy from heavier particle to the lighter particle, as a result, we can observe a certain number of heavier particles escapes through the top outlet.
The collection efficiency of cyclones varies as a function of particle size, density, and cyclone design. Cyclone efficiency will generally increase with the increase in particle size, density, inlet duct velocity, cyclone body length, number of gas revolutions in the cyclone, the ratio of cyclone body diameter to gas exit diameter, inlet dust loading, smoothness of the cyclone inner wall.
The efficiency of the cyclone will decrease with an increase in the parameters such as gas viscosity, cyclone body diameter, gas exit diameter, gas inlet duct area, gas density, leakage of air into dust outlet.
With the increase in inlet velocity or particle size the efficiency increases.
The collection efficiency of the cyclone separator is related to the pressure drop across the cyclone. Here pressure drop is the indirect measure of the energy required for the gas to move the particles. Pressure drop is related to the inlet velocity of air and particle size.
Pressure drop occurs because of the following components:
1. Loss due to the expansion of gas in the cyclone chamber.
2. Loss due to rotational kinetic energy.
3. Loss due to wall frictional forces.