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]

Introduction:

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.

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

• Empirical models used for cyclone separator efficiency:

a. 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

zc=(H−S)−H−S(DB)−1[(dcB)−1]">zc=(H−S)−H−S(DB)−1[(dcB)−1]zc=(H−S)−H−S(DB)−1[(dcB)−1] for dc>B

zc=H−S">zc=H−Szc=H−S for dc<B">dc<Bdc<B

dc=0.47D(abD2)−0.25(DeD1.4)">dc=0.47D(abD2)−0.25(DeD1.4)dc=0.47D(abD2)−0.25(DeD1.4)

The addition made by Iozia and Leith on the original Barth (1956) model is the core length zc and slope parameter β expression which is derived based on the statistical analysis of experimental data of cyclone with D = 0.25 m. The collection efficiency ηi of particle diameter dpi can be calculated from

ηi=11+(dpcdπ)β">ηi=11+(dpcdπ)βηi=11+(dpcdπ)βηi=11+(dpcd¯pi)β">dpc=[9μQπPpZcV2max]0.5

where, dpc">dpcc=c0,at θ=0 is the 50% cut given by Barth (1956)

b. LI AND WANG MODEL:

The Li and Wang (1989) model include 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:

1. The radial particle velocity and the radial concentration profile are not constant, for uncollected particles within the cyclones.
2. Boundary conditions with the consideration of turbulent diffusion coefficient and particle bounce re-entrainment on the cyclone wall are:

c=c0">r=D2

Dr∂c∂r=(1−α)wc">Dr∂c∂r=(1−α)wcDr∂c∂r=(1−α)wc at

The tangential velocity is related to the radius of cyclone by:

uRn">uRnuRn= constant

The concentration distribution in a cyclone is given as:

The resultant expression of the collection efficiency for a θ1=2πS+La">θ1=2πS+Laθ1=2πS+Laparticle of our size is given as

ηi=1-exp{-λθ1}">ηi=1−exp{−λθ1}ηi=1-exp{-λθ1}

where,

θ1=2πS+La">θ1=2πS+Laθ1=2πS+La

c. KOCH AND LICHT MODEL:

Koch and Licht (1977) collection theory recognized the inherently turbulent nature of cyclones and the distribution of gas residence times within the cyclone. Koch and Licht

describe particle motion in the entry and collection regions with the additional following assumptions:

1. 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.
2. The tangential velocity is related to the radius of cyclone by uRn=constant">uRn=constantuRn=constant

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

τi=ρpdpi218μ">Ne=1a(h+H−h2)

G is a factor related to the configuration of the cyclone, n is related to the vortex and τ is the relaxation term.

d. LAPPLE MODEL:

Lapple (1951) 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 (1951) to
calculate a 50% cut diameter, dpc is

Modelling Approach:-

For this analysis we use the Cyclone Separator which provide a method of removing particular matter from air or other gas streams at low cost and low maintenance. For this case we use the Discrete phase Modelling (DPM) Setup in the Ansys Fluent for the simulation.

There are 2 phases as given Below:

• Continuous Phase: It consists of Fluid flowing within the Fluid Volume.
• Discrete Phase: It consists of smaller particles interacting with continous Phase and It also exchange momentum, mass and energy with fluid phase or continous phase.

In the Discrete Phase Modelling (DPM) we have two ways to carried out DPM

• Uncoupled-flow: In this type of flow, Discrete Phase will not interact with the continous Phase.
• Coupled-flow: It will influence the flow of continous phase.

For, this analysis we have used the coupled-flow in DPM to get accurate results.

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. To check the effect on the Separation efficiency we will perform an analysis on given Cyclone model in two different manners.

• By varrying Particle diameter: from 1 μm to 5 μm.
• By varrying Particle Velocities: from 1m/s to 5m/s.

Procedure:

PartA:

Case1:

The given model is imported into space claim and the fluid volume is extracted.

The original cyclone seperator geometry is suppressed for physics.

Mesh module is opened and the basic mesh of 10 MM is generated with body fitted cartesian method.

Now the fluent is opened.

Pressure based steady state solver is selected and gravity is enabled in the Y direction.

RNG K Epsilon swirl dominated model with standard wall functions is selected.

Under DPM interaction with continous phase and update DPM every flow iteration options are selected.

DPM Iteration interval is set to 10.

Under injection surface injection type is selected.

Inert particle is selected

Anthracite is selected as material.

X velocity is set to 3M/s

And Diameter of the particle is to 1MM.

Under bounadry condition inlet velacity is set to 3m/s

Discrete phase BC type is set to reflect.

Outlet 1 is set to escpae and the outlet 2 is set to trap.

Solution method is set as shown below

Standard initialization is done.

Solution is run for 600 iteration and the results are shown below.

Case2:The same setup is run for 5mm particle with same velocity as 5m/s.Results are shown below.

PartB:

Case1: The same set up is run with 5mm as partcile size and vleocity is set to 1M/s.

Case2: The same set up is run with 5mm as partcile size and vleocity is set to 3M/s.

Case3: The same set up is run with 5mm as partcile size and vleocity is set to 5M/s.

Conclusion:

As observed from the graphs and the partcile tracking files

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.

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