Cyclone Separator simulation for different number of particles

A cyclonic separation is a method of removing particulates from an air, gas or liquid stream, without the use of filters, through vortex separation. A high speed rotating (air)flow is established within a cylindrical or conical container called a cyclone. Air flows in a helical pattern, beginning at the top of the cyclone and ending at the bottom end before exiting the cyclone in a straight stream through the centre of the cyclone and out the top. Larger (denser) particles in the rotating stream have too much inertia to follow the tight curve of the stream, and thus strike the outside wall, then fall to the bottom of the cyclone where they can be removed.

Cyclone separators are separation devices (dry scrubbers) that use the principle of inertia to remove particulate matter from flue gases. Cyclone separators are 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.

Cyclone separators are generally able to remove somewhere between 50-99% of all particulate matter in the 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 of 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 micrometres effectively and the machines are unable to handle sticky materials well.

EFFICIENCY:

Lapple Model: It is amongst the earliest of the model proposed and is still considered the benchmark for design of cyclone separators in many industries. This is comparatively a simple model based on force balance with considering the flow resistance. It assumes that the particles are evenly distributed across the inlet while entering the cyclone. It is based on the assumption of a plug flow (i.e. laminar flow with no axial or radial mixing). In order to calculate the efficiency, first the particle size with 50% collection efficiency (dp_50%) as the baseline needs to be determined, according to the following equation:

where μ is the viscosity of air, b is inlet width, N_e is the number of effective turns (number of turns the flow makes from the entrance to the midpoint of the core section), V_i is inlet velocity and ρ_p is particle density. The collection efficiency (the ratio of the collected fraction to the total at the inlet) for any other size, d_pj, can then be determined by

Leith-Licht Model: It is an example of this approach. It accounts for turbulence by assuming that at any height in the cyclone, uncollected dust is completely and uniformly mixed. Average residence time in the cyclone is determined from cyclone dimensions and gas throughput. The resultant expression for collection efficiency is

Iozia and Leith Model: It considers simple force balance on a particle in the cyclone. It assumes that centrifugal force and drag force act on particle and balance each other. The collection efficiency developed a logistic model based on Barth is given by

Barth Model: It is an example of the static particle approach. Barth calculated the terminal settling velocity for the static particle and the collection velocity of any particle size is determined from the ratio of its settling velocity to the terminal settling velocity. The model predicts a sharp increase in cyclone efficiency from zero for particles smaller than the critical diameter to unity for larger particles. In practice, a sharp separation is never observed because of fluctuations in radial and tangential.

Geometry:

The cyclone separator 3D geometry was taken from the given model. The volume inside of the cyclone separator was extracted and taken for analysis.

Meshing:

Element Size 10mm

Number of Elements 411139

Element Size 50mm

Number of Elements 19437

Element Size 20mm

Number of Elements 77163

FLUENT Setup:

For simulating the pollutant separation using the given model, a pressure-based steady-state solver is used with gravity enabled. A viscous model of k-epsilon RNG was selected with Swirl Dominated Flow and Standard Wall Functions. The Discrete Phase model was turned on and the injection was defined. Anthracite was selected as the inert material with a diameter of 5 micrometres and inlet velocity of 3m/s was provided. Inlet velocity of air = 3m/sec. SIMPLE scheme was selected as the solver and second-order upwind was chosen for both Turbulent kinetic energy and Turbulent Dissipation Energy.

The simulation was run for four particle behaviours i.e Trap, Reflect, Wall jet and Escape.

Case 1(Reflect):

Residuals obtained:

Number of particle tracked = 364, Escaped = 347, Incomplete = 17

Particle Track Time:

Velocity:

Vortex Region:

Case 2(trap):

Residuals Obtained

Number of particle tracked = 364, Escaped = 347, Incomplete = 17

Particle Track Time:

Velocity:

Vortex Region:

Case 3(Escape):

Residual Obtained

Number of particle tracked = 364, Escaped = 347, Incomplete = 17

Particle Track Time:

Velocity:

Vortex Region:

Case 4(Wall Jet):

Residuals Obtained

Number of particle tracked = 364, Escaped = 347, Incomplete = 17

Particle Track Time:

Velocity:

Vortex :

Performing the mesh dependancy test for the simulation:

Case 1(Element Size = 50mm):

Residuals Obtained

number tracked = 14, escaped = 13, aborted = 0, trapped = 0, evaporated = 0, incomplete = 1, incomplete_parallel = 0

Particle Time:

Velocity:

Vortex Region

Case 2(Element Size = 20mm):

Residuals Obtained

number tracked = 94, escaped = 78, aborted = 0, trapped = 0, evaporated = 0, incomplete = 16, incomplete_parallel = 0

Particle Time:

Velocity:

Vortex Region

Case 3(Element Size = 10mm):

Residuals obtained:

Number of particle tracked = 364, Escaped = 347, Incomplete = 17

Particle Track Time:

Velocity:

Vortex Region:

Conclusion:

This simulation helps us to calculate the efficiency with which the cyclone separator works. It helps us in constructing a better cyclone separator for separating particles of a specific diameter and also there is a wide variety of particles to select from the fluent database to perform the simulation. We also see that as we track a greater number of particles we get the solution that reaches almost practical value although not exact.