Week 8 - Simulating Cyclone separator with Discrete Phase Modelling

Aim:A CFD analysis of cyclone separator is performed. The separation efficiency is found out . First the diameter of the particles is varied . then the velocity at the inlet is varied with particles of constant diameter . The physics of the problem is to set up in ANSYS FLUENT . The post processing of the result obtained from CFD - post.

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]

Models of Cyclone separator:

Cyclone separator:

Cyclone separator or simply cyclones are separation devices  (Dry Scrubbers) that use the principle of inertia to move particles matter from flue gases. Cyclone separators is one of many air pollution removers known as pre - cleaner, Since they generally remove large pieces of particulate matter.

This prevents finer filtration methods from having to deal with large more abrasive particles later on in addition , several cyclone operators in parallel and this system is known as multi cyclone .

Cyclone separators work much with a centrifugal , but with a continuous feed of dry air. In a cyclone separator, dirty flue gas is fed into chamber . the inside of the chamber creates a spiral vortex similar to tornado .This spiral formation and the separation is shown in figure .The lighter components of the gas has 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 the larger particles have difficulty following the high speed spiral motion of the gas and vortex. , the particles hit the inside wall of the container and drop down into the collection hopper . These chamber are shaped like an upside -down cone to promote the collection of the particle at the bottom of the container the cleaned flue gases escapes out at the top of the chamber.

Four imperial models used to calculate the cyclone separator efficiency:

IOZIA AND LEITH MODEL:

 

Lozia and leith model (1990) logistic model Is a modified version as Barth (1956) model which is developed based on force balance . The model assumes that a particle carried by a vortex endures the influence of two forces.

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

  Betta 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,

Li and Wang Model:

The Li and Wang [3] model includes particle  bounce or re-entertainment 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 180 on the following assumptions:

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

Boundary conditions with the consideration of turbu185 lent diffusion coefficient and the particles bounce re-entertainment on the cyclone wall are:

The resultant expression of the collection efficiency for part of any size is given as:

Where,

Koch and Licht Model:

Koch and Licht [2] collection theory recognized the inherently  turbulent nature of cyclones and the distribution of gases.

Residence times within the cyclone , Koch and Licht  described.

Particle motion in the entry and collection regions with the 200 additional following assumptions

* The tangential velocity of the 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 cyclone by u Rn=constant.

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

Lapple Model:

Lapple Model [1]was developed based on force balance without considering the flow resistance .Lapple assumed that the particle 215  entering into 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   relation ship developed by the lapple [1]

To calculate a 50% cut diameter , dpc ., is.

The collection  efficiency of cyclone varies as a function of density , particle size and cyclone design . Cyclone efficiency will generally increases with increase in particle size and or density ; inlet duct velocity ; Cyclone body length; Number of gas  revolution 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 parameters such as gas viscosity; cyclone body diameter ; gas exit diameter;gas inlet duct area; gas density; leakage of air into the duct outlet.

 

The efficiency of 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 . From the above discussion it is clear that small cyclones are more efficient than large large cyclones. Small cyclones however have a higher pressure drop and are limited with respect to volumetric flow rates. Another option is to arrange smaller cyclones in series and or in parallel to sustainability increase efficiency at low pressure drops.These gains are some what compensated , however by the increased cost and maintenance problem , Also these type of arrangements tend  to plug   easily , When common hoppers are used in such arrangements , different flow through cyclones can lead to re-entertainment problem.

 

Modelling approach:

There are five steps in the modelling approach;

* Geometry creation using space claim

* Meshing using ANSYS mesh

* Set up problem using ANSYS FLUENT

* Solution Using ANSYS FLUENT

* Result using CFD - POST

 

Geometry Preparation:

Geometry:

Fluid volume extract using Space claim is shown below:

 

Mesh Details:

 

Mesh Quality:

 

Size Function:  Proximity and Curvature

Min Element Size:6mm

No of Nodes:104406

No of Elements:93994

Boundary Conditions: 

Inlet and outlet are shown

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  ( 5 microns in diameter)

Velocity inlet = 3 m/s

Outlet= Pressure outlet ( Gauge pressure =0 pa)

DPM setting are varied b/w reflect , escape , trap and wall inject.

Reflect:The particle rebounds off the boundary in question with a change in its momentum as defined by the coefficient of restitution.

Escape:Particle escape out when encountered by the boundary

Trap: The trajectory calculation is terminated and fate of the particle is recorded as trap.

Wall - jet: The wall jet type boundary conditions is applicable for high temperature  walls where no significant liquid film is found and weber no impacts when the spray act as a jet,This model is not applicable to regimes where the film is important.

Solution approach and Solver Requirements:

 

 

Result:

Case 1: In this case , we are making the velocity constant and we are visualizing the difference in results for different particle size.

 Result for the particle size of 1 micron with a velocity of 3m/s.

Residual:

Pressure at inlet:

Pressure at outlet:

 

 

Vortex Shading:

Particle Tracking:

number tracked = 406, escaped = 246, trapped = 150

 

Separation particle size= No of particle Trapped/No of particles Tracked

Separation Efficiency in % = ( 150/406 ) *100

Separation Efficiency = 36% 

 

Result for the particle size of 2 micron with a velocity of 3m/s.

Residual:

Pressure at inlet:

Pressure at outlet:

Vortex Shading:

Particle Tracking:

number tracked = 406, escaped = 231, trapped = 175

Separation particle size= No of particle Trapped/No of particles Tracked

Separation Efficiency in % = ( 175/406 ) *100

Separation Efficiency = 43%

 

Result for the particle size of 5 micron with a velocity of 3m/s.

 

Residual

Pressure at inlet:

Pressure at outlet:

 

Vortex Shading:

Particle Tracking

number tracked = 406, escaped = 156, trapped = 250

Separation particle size= No of particle Trapped/No of particles Tracked

Separation Efficiency in % = (250 /406 ) *100

Separation Efficiency =  61%

 

Case 2:

Result for varying velocity and keeping the particle size constant  I.e 5 Micron

Inlet velocity of the particle and the discrete phase is same I.e 1 m/s

 

Residual:

Pressure at inlet:

Pressure at outlet:

 

Vortex shading:

 

Particle Tracking:

number tracked = 136, escaped = 23, trapped = 113

Separation particle size= No of particle Trapped/No of particles Tracked

Separation Efficiency in % = (113/136 ) *100

Separation Efficiency = 83%

 

Inlet velocity of the particle and the discrete phase is same I.e 3 m/s

Residual:

Pressure at inlet:

Pressure at outlet:

Vortex shading:

Particle Tracking:

number tracked = 136, escaped = 12, trapped = 119

Separation particle size= No of particle Trapped/No of particles Tracked

Separation Efficiency in % = (119 /136 ) *100

Separation Efficiency = 87%

 

Inlet velocity of the particle and the discrete phase is same I.e 5 m/s

Residual:

Pressure at inlet:

Pressure at outlet:

Vortex shading:

Particle Tracking:

number tracked = 136, escaped = 4, trapped = 132

 

Separation particle size= No of particle Trapped/No of particles Tracked

Separation Efficiency in % = ( 132/136 ) *100

Separation Efficiency =97%

Case 1: Final efficiency tabulation

Particle Diameter	velocity in m/s	Efficiency
1e-6	3	36%
2e-6	3	43%
5e-6	3	61%

 

Case 2: Final efficiency tabulation

Particle Diameter	velocity m/s	Efficiency
5e-6	1	83%
5e-6	3	87%
5e-6	5	97%

 

 

Separation Efficiency:

It is defined as the fraction of particles of a given collected in the cyclone, compared to those of that size going into 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 the solids through the cyclone legs.

 In this case depending upon the particle history data- separation Efficiency 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 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 due to the entry and exit losses, and friction and kinetic energy  losses in  a 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)

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

Plot of efficiency of the cyclone separator vs the diameter of the particles:

The separation efficiency is plotted along the Y- axis and the particle diameter along the X-axis

Interference :

From the above pressure velocity plots , we can infer that the pressure is higher near the wall region which performs the outer vortex. Due to the higher pressure near the wall low-pressure region is created at the center of the cyclone separator, because of which the lighter particle escape from the top outlet and heavier particle are collected at the bottom outlet.

Simulation are performed for various particle size from m to 5m with air and particle velocity are 3 m/s. From the video, it is observed that heavier particle is trapped at the bottom outlet and lighter particle are escaped at top - outlet , But we can also see lighter particles are 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 cyclone 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; No of gas revolution in the cyclone; the ratio of cyclone diameter to gas exit diameter ; inlet dust loading; smoothness of the cyclone inner wall.    

The efficiency will decreases with in increase in parameters such as gas viscosity; Cyclone body diameter; gas exit diameter; gas inlet duct area ; gas density ; leakage of air into dust outlet.

With in increase in inlet velocity or particle size the efficiency increases.

The collection of efficiency of the cyclone separator is related to the pressure drop across the cyclone . Here pressure drop is indirect to the energy required for a gas to move the particles. Pressure drop is related to the inlet velocity and particle size.

Pressure drop occurs because of the following assumptions:

* Loss due to the expansion of gas in the cyclone chamber

* Loss due to the rotational kinetic energy

* Loss due to wall friction forces.