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]

Theory:

Cyclone Seperator:

A cyclone separator is an economical device for separating solid particles from contaminated gas streams.It has long been used in industrial applications such as power generations, gas turbines, chemical processes and many more.
A high speed air flow is established that follows a spiral pattern. Denser particles caught in the stream posses a high inertia and are forced to rotate at higher radii this causes them to strike the wall of the separator and leave the separator via collection device.

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.

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 Mode:

he 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:

Following image is obtained after extracting volume from given geometry in spaceclaim for to perform simulation.

Dimension of cyclone seperator:

Mesh:

Cartesian method is used for meshing with mesh element size 5mm.

Named selection is done as follows

1-Inlet

2-Outlet 1(top)

2-Outlet 2(bottom)

Total no of nodes: 427528

Setup and Solvers:

Steady simulaton is performed with gravity enabled.

Pressure based solver is used as flow is incompressible.k-epsilon RNG turbulence model with Swirl dominated flow has been used as it provide improved result in case of swirl associated flow where vorticity and recirculation is taken into consideration.

Descrete phase model is used and material used for discrete phase is anthracite with maximum no of steps used are 50000.

Flow is simulated for three particle diameter as 1μm 3μm and 5μm with discrete phase and continuous phase velocity as 3m/s.

Results:

Case1-When particle size is 1μm and both velocities are 3m/s.

sepperation efficiency, η=Total Number of particle traped/ Total No of pparticle tracked

η=323/378=85.4%

https://drive.google.com/file/d/1HVd6dFiIxdY9SZRJJWVm_QD25ntHrSNu/view?usp=share_link

Case2-When particle size is 3μm and both velocities are 3m/s.

331/378=87.5%

https://drive.google.com/file/d/1j4W8Ge-z6wiYz4qRw0yY1FDJ82u2HIaD/view?usp=share_link

Case3-When particle size is 5μm and both velocities are 3m/s.

https://drive.google.com/file/d/1WIwwns1xqlApe9OEddKuT5Mhhx1NqKXf/view?usp=share_link

Particle size(micrometer)	Inlet velocity(m/s)	Inlet/wall DPM	Outlet (top) DPM	Outlet (bottom) DPM	Viscous Model	No of Particle tracked	No of Particle escaped	No of Particle traped	Seperation Efficiency
1	3	reflect	escape	trap	k-epsilon-RNG Enhanced wall treatment	378	55	323	85.4%
3	3	reflect	escape	trap	k-epsilon-RNG Enhanced wall treatment	378	47	331	87.5%
5	3	reflect	escape	trap	k-epsilon-RNG Enhanced wall treatment	378	68	310	82%

Particle size(micrometer)	Inlet velocity(m/s)	Inlet/wall DPM	Outlet (top) DPM	Outlet (bottom) DPM	Viscous Model	Inlet press(Pa)	outlet top press(Pa)	Outlet bottom press(Pa)	Press drop(Pa)
1	3	reflect	escape	trap	k-epsilon-RNG Enhanced wall treatment	22.51	3.09	2.9	19.42
3	3	reflect	escape	trap	k-epsilon-RNG Enhanced wall treatment	22.5	3.1	2.3	19.4
5	3	reflect	escape	trap	k-epsilon-RNG Enhanced wall treatment	22.54	3.1	2.29	19.4

INFERENCES:

seperation effficiency is maximum for 3micro meter particle size for constant velocity of 3m/s

Pressure drop across device(cyclone separator) decreases on increase in particle size for a constant inlet gas velocity in device.

Separation efficiency decreases on decreasing the particular particle size for particular inlet velocity .

Case1-When particle size is 5μm and both velocities are 1m/s.

https://drive.google.com/file/d/1UYmyjJRqjQct-QZ17R8Dhyt_TeK0oOiV/view?usp=share_link

Case2-When particle size is 5μm and both velocities are 3m/s.

Case3-When particle size is 5μm and both velocities are 5m/s.

https://drive.google.com/file/d/1ekgU19j4Hr1TxXqKZjnihKvS7-TvIcnL/view?usp=share_link

Particle size(micrometer)	Inlet velocity(m/s)	Inlet/wall DPM	Outlet (top) DPM	Outlet (bottom) DPM	Viscous Model	No of Particle tracked	No of Particle escaped	No of Particle traped	Seperation Efficiency
5	1	reflect	escape	trap	k-epsilon-RNG Enhanced wall treatment	378	74	304	80.4%
5	3	reflect	escape	trap	k-epsilon-RNG Enhanced wall treatment	378	47	331	87.5%
5	5	reflect	escape	trap	k-epsilon-RNG Enhanced wall treatment	378	132	245	64%

Particle size(micrometer)	Inlet velocity(m/s)	Inlet/wall DPM	Outlet (top) DPM	Outlet (bottom) DPM	Viscous Model	Inlet press	outlet top press	Outlet bottom press	Press drop
5	1	reflect	escape	trap	k-epsilon-RNG Enhanced wall treatment	1.96	0.24	0.23	1.72
5	3	reflect	escape	trap	k-epsilon-RNG Enhanced wall treatment	22.5	3.1	2.9	19.4
5	5	reflect	escape	trap	k-epsilon-RNG Enhanced wall treatment	65.37	9.52	6.36	55.85

INFERENCES:

Pressure drop across device(cyclone separator) increases on increase in inlet gas velocity in device.

Separation efficiency increases on increasing the inlet velocity for particular particle size.

CONCLUSION:

The numerical analysis for cyclone separator model under steady state conditions for different input flow and particle velocities, and particle diameter sizes, using the RNG k-ε turbulence model and discrete phase model in ANSYS Fluent 2023R1 was well understood.

The creation of dispersed phase particles of different properties using particle injections as well as appropriate boundary conditions greatly assisted in recreating a commonly employed industrial level filtration or pre-filtration technique.

Moreover, it was realised that the particle and flow properties, both particle diameter sizes and particle and flow velocities, impacted the efficiency and performance of the cyclone separator and investigations into the geometrical features of the device, as parameters for future research work, can greatly aid in optimising the design configuration aimed towards requirements of different applications.