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]

Cyclone separator

Cyclone separators or simply cyclones 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 precleaners 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 multicyclone.

Cyclone Separators 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).

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.

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. 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.

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.

Methods used to compute cyclone separator efficiency are

Iozia and Leith Model

Iozia and Leith model 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 dpi 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 = 0.25m given as

and dpc is the 50% cut size given by Barth

Li and Wang Model

The Li and Wang 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: uRn = 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: uRn = 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 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 to calculate a
50% cut diameter dpc, is

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

Cad model

Importing cyclone separator geometry into spaceclaim

We need to carry out analysis of cyclone separator in fluid volume. Hence, we go to prepare-volume extraction in order to extract the fluid volume.

Creating named selection

Mesh generation

After developing the geometry, generate a base mesh. The mesh needs to be refined in order to have more particles at the inlet. Cutcell mesh is used so as to give more structured form to the mesh.

Setting up physics

Solver Selection

K-epsilon model with RNG and swirl dominated flow is selected

Selection of DPM

DPM model is selected for interaction of discrete phase with the continuous phase

Injection

Boundary conditions

inlet

outlet1

outlet2

The analysis is done for 600 iterations

Post Processing

The data generated is exported to CFD post to analyze the results. After opening the CFD post, the particle history is imported.

Using both inlet and injection velocity as 3m/sec.

Case 1

Diameter of the particles = 1µm

Residuals

Vortex formation

Swirl Strength = 0.02

Swirl Strength = 0.4

Case 2

Diameter of the particles = 2µm

Residuals

Vortex formation

Swirl Strength = 0.02

Case 3

Diameter of the particles = 3µm

Residuals

Vortex formation

Swirl Strength = 0.02

Case 4

Diameter of the particles = 4µm

Residuals

Vortex formation

Swirl Strength = 0.02

Case 5

Diameter of the particles = 5µm

Using both the velocity's as 3m/sec

Residuals

inlet pressure

outlet1 pressure

outlet2 pressure

Vortex formation

Swirl Strength = 0.02

Using particle diameter size as 5 μm for all the below cases

Case 1

Using both the velocity's as 1m/sec

Residuals

inlet pressure

outlet1 pressure

outlet2 pressure

Vortex formation

Swirl Strength = 0.02

Case 2

Using both the velocity's as 2m/sec

Residuals

inlet pressure

outlet1 pressure

outlet2 pressure

Vortex formation

Swirl Strength = 0.02

Case 3

Using both the velocity's as 4m/sec

Residuals

inlet pressure

outlet1 pressure

outlet2 pressure

Vortex formation

Swirl Strength = 0.02

Case 4

Using both the velocity's as 5m/sec

Residuals

inlet pressure

outlet1 pressure

outlet2 pressure

Vortex formation

Swirl Strength = 0.02

Cyclone separator Efficiency

It is calculated as the ratio of number of particles trapped to number of particles tracked.

Cyclone separator Efficiency = Number of particles trapped/ Number of particles tracked.

Case 1 : Using both inlet and injection velocity as 3m/sec

Sr No	Particle size (μm)	Number of particles tracked	Number of particles trapped	Separator efficiency (%)
1	1	98	28	28.571
2	2	98	60	61.224
3	3	98	70	71.429
4	4	98	84	85.714
5	5	98	95	96.939

Case 2 : Using particle diameter as 5μm

Sr No	Inlet and Injection velocity (m/s)	Number of particles tracked	Number of particles trapped	Separator efficiency (%)	Pressure drop (Pa)
1	1	98	8	8.163	4.622
2	2	98	32	32.653	10.815
3	3	98	95	96.939	25.601
4	4	98	96	97.959	46.881
5	5	98	98	100	74.716

Conclusion

1. Mesh size determines the number of particles to be tracked. More number of particles can be tracked with finer mesh which increases efficiency but requires more computation power and time.

2. The cyclone separator efficiency increases with increase in particle size diameter for same injection and inlet velocity.

3. The cyclone separator efficiency increases with increase in injection and inlet velocity for same particle size diameter.

4. The pressure drop increases with increase in injection and inlet velocity for same particle size diameter.