Cyclone Separator Challenge

Cyclone Separator & Emperical Models :

• Cyclone performance is determined by pressure drop and collection efficiency.
• This work presents analytical and Computational Fluid Dynamics calculation to predict and to evaluate the effects of change in dimensions on static pressure and collection efficiency.
• Velocities, pressures, collection efficiency and the pressure drops have been studied according to change in inlet parameters .
• Relative Changes in inlet parameters are calculated from a modification reported in. The further study is performed with reference to this parameter.
• In the present work the pressures and collection efficiency have been generated using CFD. The pressure drops have been evaluated for the existing design and the modified design. Significant pressure drops have been observed in the optimized model.
• The study has been carried out using three standard models Lapple Model, Swift Model and Starmand Model varying the values of inlet height and inlet width.
• All the predictions proved to be satisfactory when compared with the presented literature data. CFD is used to verify the obtained results with actual and Literature.

Four empirical models for the prediction of cyclone collection efficiency:

Lapple:

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.

Iozia and Leith:

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, and a flow resistance,. Core length and core diameter,.

Koch and Licht:
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:
• 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.

Li & Wang Model :

The Li and Wang (1989) 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.

Geometry of cyclone separator :

• A cyclone separator is an economical device for removing particulate solids from a fluid system. The induced centrifugal force is tangentially imparted on the wall of the cyclone cylinder. This force, with the density difference between the fluid and solid, increases the relative setting velocity.
• The highly swirling flow field inside a cyclone is very complex, including the interactions between the particulate and fluid phases.
• Designing an “optimum” set of cyclones requires a balance between the desired collection efficiency, pressure drop, space limitations, and installation cost.

Cyclone seperator model : Model Used for simulation: Extracted Flow Volume :

Computational Meshing :

• The basic mesh methods are enough to predict the flow and efficiency of the separator. It can be accurate enough to predict the flow of the particles through the separator body & outlets.
• We can make finer meshes in same meshing method to increase the number of elements on the body of the Cyclone separator.
• As Fluent is taking the number of elements in the inlet as the number of particles that enter the separator, we can consider a finer mesh which would give a considerable amount of particles for the simulation.
• In this case we considered the element size as 10mm, which gives the number of particles to be 124.

Setup & Simulation:

After entering the Fluent setup, we have to create and specify the particles to flow along with the air.

This can be done using Descrete Phase Module. Here we can specify the type of the particle motion and using injection option we specify the particle.

In injections we specified the particle as Anthrasite & the particle velocity & Diameter are given as input arguments.

Then in the boundary conditions, we can specify the conditions as given in the problem by using the DPM boundary layer conditions. The conditions are,

Inlet and wall:- Reflect

Outlet(Top):- Escape

Outlet(Bottom/dustbin) - Trap

The inlet velocity of the air is also taken as the input parameter.

So by changing the required input parameters, we can simulate different analysis and compare them for better results.

Varying the particle diameter from 1 μm to 5 μm:

This is to compare the efficiency of the cyclone separator for different sizes of the particles.

The efficiency can be calculated by considering the amount of particles trapped at the outlet below at the end of the simulation. By considering the percentage of particles trapped we can define the efficiency of the cyclone Separator for different diameters of the particles.

Conclusion on comparison :

• We can see that the increase in the particle diameter increases the efficiency of the cyclone separator.
• The smaller particles are difficult to be removed through this model of the separator.
• A decent efficiency is obtained only when the particle size is 5 micro meters.
• Through the plot we can see the increase in efficiency. The amount of increse in efficiency reflects the cpability of the separator.
• Thus this model used here can only be used to separate particles having diameter higher than 5 micr meters for higher accuracy of cleanliness.
• Still we can see some particles escaping along with the air outlet. This can be corrected by changing the design of the Separator.

Varying the particle velocity from 1 m/sec to 5 m/sec :

Another factor that highly influence the particle motion is the velocity of the particle & inlet velocity.

Now we can change both the velocity and simulate for comparing the results.

Here we are going to compare the efficiency and also the pressure drop caused during the flow of the particles. Let us see how the pressure drop changes as we change the velocity of the particle.

Conclusion on Comparison :

• The efficiency of the separator increses as the velocity increases. We can see a poor efficiency for the first two cases which will be a big loss.
• Only after the velocity attains 3 m/s, we can see a considerable efficiency.
• After further increase in the velocity the efficiency increses in small rate.
• For 100% efficiency we can use a velocity slightly higher than 5m/s.
• But by seeing the pressure drop values we can see that the drop is too high for higher velocities. This will increase the contact speed of the particles on the walls of the separator which will damage the walls.
• By considering all the factors we can consider the best velocity to be 3m/s which have considerable efficiency and pressure drop value.

Vortex Shedding :

• Let us see how the separator creates a swirllling effect or Vortex effect inside the walls.
• We can see how the strength of the vortex is spread inside the separator.

Strength : 8e-5 Strength : 4e-3 Strength : 0.15 Strength : 0.3 Strength : 0.5

• We can see that the strength is high at the center of the separator. The vortex effect fades away as it moves farer to the center axis.
• This defines that the particles undergo the swirlling motion as the strength of the effect attracts the particles towards the center.

Particle Motion :

• We can also see the motion the particle travels when it enters the separator.

• The particles follows the swirlling effect and gets out of the outlet.
• We can aslo track the particles that gets out of the outlet at the top.
• The particles that are incomplete still present inside the separator.