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 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.

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.

• Empirical models used for 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 flow resistance, W. Core length, zc, and core diameter dc, are given as

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

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

1. 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:

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

The concentration distribution in a cyclone is given as:

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

1. 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 uR^n=constant

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

τi=ρpdpi218μ">Ne=1a(h+(H−h)/2)

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

1. 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

• 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, ΔP = 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.

Content:

Geometry Setup:

• Import the model file into the Spaceclaim
  
• Extact the fluid volume of the model to analys the fluid flow of the cyclone separator and subtract the Previous geometry of the model.
  

Meshing Setup:

• method = Cartesian method
• Element size = 5mm
• Number of nodes = 456604
• Number of elements = 427873

Select the whole body and apply the Cartesian method to the selected body with an element size of 5mm and generate the mesh. By this method the model is refined

And the Mesh metrics of element quality are shown below.

And give the name selection to the selected faces as Inlet, outlet_1, outlet_2.

Fluent setup:

PART 1:-

Physics:

Solver - Pressure based

Time - Steady time.

velocity Formulation - Absolute

Gravitational Acceleration - -> Enabled in y-direction = -9.81.

Viscous Model :

Model - k-Epsilon

k-epsilon Model - RNG

RNG Option - Swirl Dominated Flow

Near Wall Treatment - Standard Wall Functions

Discrete Phase Modelling (DPM):

In this case, solid particle size ranging from 1µm to 5µm i.e.., 1e-6 to 5e-6 mm being simulated as a discrete phase in a continuum.

• Now, Enabled the interaction with continuous phase - to enable coupled flow
• Then, Enabled the update DPM sources every flow iteration - Enabling this option gives chances to calculate particle source term at every iteration.
• Contour plots for DPM variables - unenabled - Allows cell averaged variable value.
• Tracking parameters - Max. No. of steps - 50000 - It is the no. of timesteps that are used to compute a single particle trajectory.
• Specify length scale - This option is also used to control the timestep size.
  

Boundary conditions:

a. Inlet: - Velocity-Inlet

Velocity Magnitude -

Constant velocity of 3m/s.

DPM - Reflect

b. Outlet 1- Pressure-Outlet

Pressure - 0 Pa

DPM - Escape

c. Outlet 2 - Pressure-Outlet

Pressure - 0 Pa

DPM - Trap

4. Solution:

Standard Initialisation:

Then Run the iterations, after completion of the calculation export the partical History data into the local file and save the data.

• Results and analysis:

At one micrometer 1µm:

Residuals:

Flow contour and vortex formation:

Animated file link: https://drive.google.com/file/d/1Pu3XZrO0hp-RLz_0YIh9LVhSlOU18F7d/view?usp=sharing

At two micrometer 2µm:

Residuals:

Flow contour and vortex formation:

Animated file link: https://drive.google.com/file/d/1JzG6NubG1kWrGiLV_aavPpJx88-wrjzQ/view?usp=sharing

At three micrometer 3µm:

Residuals:

Flow contour and vortex formation:

Animated file link: https://drive.google.com/file/d/1XSQ2v5gvhTaKpiE3ef_hlSg7Dzud73VO/view?usp=sharing

At Four micrometer 4µm:.
Residuals:

Flow contour and vortex formation:

Animated file link: https://drive.google.com/file/d/1-Ul26BpNoXpb1ujbOAx0tMbyyi3s4-Mz/view?usp=sharing

At Five micrometer 5µm:

Residuals:

Flow contour and vortex formation:

Animated file link: https://drive.google.com/file/d/1H7Ft9uwCb8gJ_zLeKij1ypS2AygrVOeH/view?usp=sharing

Particle Size (m)	Particles Tracked	Particles Escaped	Particles Trapped	Particles incomplete	Separation Efficiency(η)(%)
1µm	378	136	149	69	39.4
2µm	378	156	142	80	37.5
3µm	378	128	165	85	43.6
4µm	378	96	180	102	47..6
5µm	378	67	159	152	42.06

Part 2:
Analysis of a given cyclone separator model by varying the particle velocity from 1 m/sec to 5 m/sec at 5 micrometre and calculate the separation efficiency and pressure drop in each case.

At 1m/s:

Total Pressure at inlet:

total pressure at oulets:

Animated file link: https://drive.google.com/file/d/1RZHn-uBhXJpa7qgl2PK9NZHBy8xpf1bj/view?usp=sharing

At 2m/s:

total pressure at oulets:

Total Pressure at inlet:

At 3m/s:

Total Pressure at inlet:

total pressure at oulets:

Animated file link: https://drive.google.com/file/d/1qcPv3InVA0q2Bw0JvE59FVqOScHqm5Fv/view?usp=sharing

At 4m/s:

total pressure at oulets:

Total Pressure at inlet:

Animated file link: https://drive.google.com/file/d/1EW61LflCbh26PO3MRfCmSn1VaqS5RIY_/view?usp=sharing

At 5 m/s:

Total Pressure at inlet:

total pressure at oulets:

+

Animated file link: https://drive.google.com/file/d/1j032ifntevKhtDOXk6-VJr5i2OZTkOpK/view?usp=sharing

inlet velocity	Particles Tracked	Particles Excaped	Particals Trapped	Particals Incompleted	Separation Efficiency(η)(%)	total inlet pressure	total pressure at outlets	Pressure drop
1 m/sec	378	162	17	199	44.9	2.0567402	0.27068868	1.78605152
2 m/sec	378	130	76	172	20.1	9.4016892	1.2673189	8.134379
3 m/sec	378	67	59	152	17.7	21.42546	2.9677628	18.45769
4 m/sec	378	45	295	38	78.04	39.4926	5.4468092	34.0457
5 m/sec	378	36	341	1	90.2	62.059375	8.4592219	53.5467

Conclusion:

Here, we calculated the separation Efiiciency and Pressure drop in both the cases by varying the inlet velocity and Particle diameter. We observe that the Results depend on the both of the Parameters as the increase in diameter and velocity gives increased separation efficiency. Velocity shows a greater impact as compared to the diameter of the particle. Pressure drop is dependant on velocity. For constant diameter and by varying the Velocity the number of particles Trapped are increased in Pressure drop i.e.., Increased velocity causes a higher pressure drop.