Simulation of Cyclone Separator in Ansys Fluent

OBJECTIVE

      To perform an analysis on the cyclone separator model using four different boundary conditions & also vary the number of particles through the inlet.

Cyclone Separator 

It is a method of removing particulates from an air, gas or liquid stream, without the use of filters, through vortex separation. When removing particulate matter from liquid, a hydro cyclone is used; while from gas, a gas cyclone is used. Rotational effects and gravity are used to separate mixtures of solids and fluids. The method can also be used to separate fine droplets of liquid from a gaseous stream.

A high speed rotating flow is established within a cylindrical or conical container called a cyclone. Air flows in a helical pattern, beginning at the top (wide end) of the cyclone and ending at the bottom (narrow) end before exiting the cyclone in a straight stream through the center 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. In a conical system, as the rotating flow moves towards the narrow end of the cyclone, the rotational radius of the stream is reduced, thus separating smaller and smaller particles. The cyclone geometry, together with the volumetric flow rate, defines the cut point of the cyclone. This is the size of the particle that will be removed from the stream with a 50% efficiency. Particles larger than the cut point will be removed with greater efficiency and smaller particles with a lower efficiency as they separate with more difficulty or can be subject to re-entrainment when the air vortex reverses direction to move in direction of the outlet.

Four empirical models used for cyclone separator efficiency

IOZIA AND LEITH MODEL
        Iozia and Leith\'s logistic model is a modified version of the Barth 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 model are 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

LI AND WANG MODEL
The Li and Wang 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\'s model were developed based on the following assumptions:
• The radial particle velocity and the radial concentration profile are not constant, for uncollected particles within the cyclones.
• Boundary conditions with the consideration of turbulent the diffusion coefficient and particle bounce re-entrainment on the cyclone wall are:

The tangential velocity is related to the radius of cyclone by:
uR^n = constant.
The concentration distribution in a cyclone is given as

where

and

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

where

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 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:
uR^n = constant.
A force balance and an equation on the particles collection yields the grade efficiency ηi

where

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. He 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

where Ne is the number of revolutions

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

PROCEDURE

•  Open Space claim & import the cyclone separator file & use prepare --> volume extract --> select inlet & two outlet edges to create a fluid volume.

                     a) Geometry                                              b) Volume extract

• Open the mesh create three named selection i) Inlet ii) Outlet-1 iii) Outlet-2 & generate baseline mesh.

             c) Mesh                                        d) No of elements

• Open fluent i) Solver - Pressure based type & steady-state                                                          ii) Gravity = -9.81m/s^2 @ y                                                                                   iii) Viscous model - k-epsilon.                                                                                      RNG - Enhance the accuracy of swirling flows. Select swirl dominated flow & standard wall function.   

                             e) Viscous model                                                                                                

DPM - Discrete Phase Model is used to represent the solid particles in a gas or liquid droplets in a gas. It consists of two phases i) Continuous phase - fluid flowing in a control volume.    ii) Discrete phase - Smaller particles that interact with the continuous phase. It can exchange mass, momentum & energy with the continuous phase. 

Interaction contains parameters used for performing coupled calculations of the continuous and discrete phase flow. 

Interaction with Continuous Phase enables a coupled calculation of the discrete phase and the continuous phase.

Update DPM Sources Every Flow Iteration enables calculation of particle source terms at every DPM Iteration. It is recommended for unsteady simulations.

Number of Continuous Phase Iterations per DPM Iteration allows you to control the frequency at which the particles are tracked and the DPM sources are updated.

Particle Treatment contains options for choosing to treat the particles in an unsteady or a steady fashion.

Unsteady Particle Tracking enables unsteady tracking of particles.

• Track with Fluid Flow Time Step enables the use of fluid flow time steps to inject the particles.

• Inject Particles at contains parameters to decide when to inject the particles for a new time step.

• Particle Time Step enables the injection of particles for every particle time step.

  Fluid Flow Time Step enables the injection of particles for every fluid flow time step. In any case, the particles will always be tracked in such a way that they coincide with the flow time of the continuous flow solver.

  Particle Time Step Size specifies particle time step size for the calculation.

Number of Time Steps allows you to specify the number of time steps for the calculation.

                           f) Discrete phase model

             Injectors --> create i) Particle type - Inert ii) Material - Anthracite                           iii) Injection type - surface & inlet, X - velocity - 3m/s, dia - 5e-6 m.

                                  g) Injector model

          Boundary conditions - Inlet - Velocity = 3m/s & DPM - Reflect

                                            h) Inlet B.C

         Select standard method & choose to compute from - inlet & initialize the solver & run it for 600 iterations.

                                             i) Scale Residuals

         File --> export --> particle history data --> injector_0 --> browse --> write - this option is used to save the file.

                                             j) DPM Iteration

         Open CFD Post, import the fluent particle track file.

REFLECT

The particle rebounds off the boundary in question with a change in its momentum as defined by the coefficient of restitution.

The element size of the mesh is set to 20 mm & Boundary conditions - Inlet - Velocity= 3m/s & DPM - Reflect.

            k) Anthracite particle time                                    l) Vortex core region

DPM Iterations

Mass flow rate 

          m) Inlet                                 n) Outlet-1                              o) Outlet-2

TRAP

The trajectory calculations are terminated and the fate of the particle is recorded as trapped. In the case of evaporating droplets, their entire mass instantaneously passes into the vapor phase and enters the cell adjacent to the boundary.

Boundary conditions - Inlet - Velocity= 3m/s & DPM - Trap.

DPM Iterations

               p) Anthracite particle time                                    q) Vortex core region

ESCAPE

The particle is reported as having \"escaped\'\' when it encounters the boundary in question. Trajectory calculations are terminated.

Boundary conditions - Inlet - Velocity= 3m/s & DPM - Escape.

DPM Iterations

             r) Anthracite particle time                                    s) Vortex core region

WALL Jet

The wall-jet type boundary condition is appropriate for high-temperature walls where no significant liquid film is formed, and in high-Weber-number impacts where the spray acts as a jet. The model is not appropriate for regimes where the film is important.

Boundary conditions - Inlet - Velocity= 3m/s & DPM - Wall jet.

DPM Iterations

               u) Anthracite particle time                                    v) Vortex core region

Case 2:

The element size is reduced to 10 mm & simulation again ran for 600 iterations by keeping the other parameters the same.

Case 3:

The number of cells at the inlet determines the trap of the flow so the inlet mesh size alone refined to 2.5 mm & simulation again ran for 600 iterations by keeping the other parameters the same

Results.

Case 1:

              w) Anthracite particle time                                    x) Vortex core region

Case 2:

                y) Anthracite particle time                                  z) Vortex core region

INFERENCE

• The reflect, trap, escape & wall jet all produced the same results for the different discrete phase boundary conditions.                                                                         

  	Reflect	Trap	Escape	Wall jet
  Tracked	94	94	94	94
  Escaped	93	93	93	93
  Aborted	0	0	0	0
  Trapped	0	0	0	0
  Evaporated	0	0	0	0
  Incomplete	1	1	1	1

   
• The DPM phase depends on the type of the fluid & simulation type is used to decide the boundary condition of it.
• The mass flow rate is also identical irrespective of the element size & type of DPM phase. The increase in element size is used to capture the properties like velocity, pressure for the flow with more accuracy.