CFD Analysis of Cyclone Separator

Aim:

CFD analysis of Cyclone Separator

Objectives:

• To understand different empirical models and Discretize Phase Modelling role in cyclone separator
• To understand working of cyclone separator
• To recognize effects of velocity and particle size on cyclone efficiency and pressure drop

Introduction:

Cyclone separators are separation devices that use the principle of interia to remove particular 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.

Figure: Cyclone Separator

Working:

Cyclone separators work much like a cetrifugal, 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 below 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.

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.

Figure: Working of Cyclone Separator

Effectiveness:

Cyclone separators are generally able to remove somewhere between 50-99% of all particulate matter in 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 in 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.

Cyclone efficiency empirical models:

Iozia and Leith Model:

Iozia and Leith logistic model is a modified version of Barth 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. The collection efficiency `η_i`of particle diameter `d_(p.i)`can be calculated from

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

`d_(pc)` is the 50% cut size given by Barth

Where, core length, `z_c` and core diameter,`d_c`, are given as

Li and Wang Model:

The Li and Wang [3] 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 model was developed based 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 turbulent diffusion coefficient and particle bounce re-entrainment on the cyclone wall are:

• The tangential velocity is related to the radius of cyclone by:`uR^n` = constant.

The concentration distribution in a cyclone is given as:

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

Koch and 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 cyclone by: `uR^n`= constant.

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

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

Lapple Model:

Lapple modelwas 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, is

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

Simulation Procedure:

Step: 1 Geometry Preparation

Open the Space-claim and select volume extraction by edge which gives new volume which is used for simulation through which air and particles mixture flows. The outer body is considered as suppress for physics.

Figure: Volume Extraction in Space-claim

Step: 2 Meshing

In the meshing, firstly name selection is provided to model. Then, cut-cell meshing is given to model which gives best results.

Figure: Cut-shell Meshing

Step: 3 Set-up

In this set-up, first of all the gravity is given in negative Y-direction.

In this simulation, K-epsilon with RNG and Swirl dominant flow is selected which gives best results in this situation.

The Discretize Phase Modelling is selected in this study. In this, surface injections are selected for inlet with face along normal. Inert particles of anthracite with uniform diameter are selected. The size of diameter and velocity are selected as per case study but in this both fluid and particles velocity are considered as same. Coupled DPM is selected but particles size are very less it is not much more effected. DPM source is updated after 10 iterations and step length factor is 5.

The following boundary conditions are selected:

Inlet: Reflect

Outlet 1 (Upper): Escape

Outlet 2 (Lower): Trap

In this, First Order Up-wing in momentum is selected which gives good results while second order momentum gives higher order instability.

Standard Initialization from inlet is selected.

Step: 4 Results

In this simulation, the effects of particle size and velocity on cyclone separator are analyzed.

Effect of particle size

In this study, the velocity of fluid and particles are considered constant as 3 m/s. But, the particle size is varied from 1 μm to 10 μm .

The variation in particle time with change in size of particle size is given in below table which includes contours of particle time.

Particle size	1 μm	2 μm	3 μm
Particle Time
Particle Size	5 μm	8 μm	10 μm
Particle Time

In this case study, the velocity of fluid (air) and particles are taken as 3 m/s so that pressure, velocity and vortex contour are nearly same. Hence, contors of particle size of 5 μm are shown below:

Particle Size	Pressure Contour	Velocity Contour	Vortex Contour
5 μm

Animations:

In this section, the video animations created using CFD Post for diferent particle size with same velocityn of 3 m/s are displayed below:

Particle Size: 1 μm

https://drive.google.com/file/d/1aEEjXfgPeXn8gjiRiLyHqaMkLvAeUxGY/view?usp=sharing

Particle Size: 5 μm

https://drive.google.com/file/d/1aBN9jybU21pR7sU-qPWEoMxLg241aZiF/view?usp=sharing

Particle Size: 10 μm

https://drive.google.com/file/d/1_xNaq6N8Tmo5Idt3zfQBZdm6_S_jaLU-/view?usp=sharing

Graph Plot:

Figure: Effect of particle size on cyclone separation efficiency

Particle size	Tracked	Escaped	Trapped	Incomplete	Efficiency
1 μm	435	221	206	8	0.47
2 μm	435	206	228	1	0.5241
3 μm	435	203	232	_	0.5333
4 μm	435	168	267	_	0.6137
5 μm	435	119	316	_	0.7264
8 μm	435	18	417	_	0.9586
10 μm	435	29	406	_	0.9333

Effect of velocity:

In this study, the particle size is are considered constant as 5 μm. But, velocity of fluid and particles is varied from 1 m/s to 5 m/s.

Velocity	1 m/s	2 m/s	3 m/s	4 m/s	5 m/s
Particle Time
Pressure
Velocity
Vortex

Animations:

In this section, the video animations created using CFD Post for diferent particle velocity of same 5 μm particle size are displayed below:

Velocity: 1 m/s

https://drive.google.com/file/d/1A56vNkIZJ-5xdMI2ZVaQ97YKUBQl0J2C/view?usp=sharing

Velocity: 3 m/s

https://drive.google.com/file/d/1RtJIBiz5kVCmdXW_q5hhWwK8ZLckIkED/view?usp=sharing

Velocity: 5 m/s

https://drive.google.com/file/d/1Im90RC3TpRprtAJ4KdgyNJPhl2Th8Rf8/view?usp=sharing

Graph Plot:

Figure: Effect of velocity on the pressure drop and efficiency

Velocity	Tracked Particles	Escaped Particles	Trapped Particles	Incomplete Particles	Inlet Total Pressure (Pa)	Outlet 2 Total Pressure (Pa)	Outlet 1 Total Pressure (Pa)	Total Pressure drop (Pa)	Efficiency
1	435	215	125	95	1.724287	0.28901981	0.17093794	1.55	0.2873
2	435	169	266	_	7.4986466	1.1175452	0.77670181	6.72	0.6114
3	435	119	316	_	16.785933	2.534116	1.7519855	15.03	0.7264
4	435	74	361	_	32.070126	4.3155669	3.4395887	28.64	0.8298
5	435	44	391	_	51.025735	6.7174045	5.503218	45.52	0.8988

Conclusion:

There are different models used for cyclone separator but CFD is the best method to simulate the cyclone separator.

Cyclone separator have high efficiency or majorly remove particles of size above 3 μm but it is not effective for below 2 μm micrometer particle size.

Increase in velocity also increases efficiency of cyclone separation but it also increases the pressure which results into high power consumption.