Week 8 - Simulating Cyclone separator with Discrete Phase Modelling

Simulating Cyclone separator with Discrete Phase Modelling

OBJECTIVES

• Explain the working of cyclone separator, methods to evaluate its efficiency and its practical importance and applications.
• Simulate the working of cyclone separator in Ansys Fluent using suitable models for the flow occurring within.
• To perform analysis on a given cyclone separator by varying flow parameters such as the particle diameter, particle velocity and evaluate the separation efficiency for each case.
• Discuss the results and inferences based on the results.

CYCLONE SEPARATOR

Cyclone separators provide a method of removing particulate matter from air or other gas streams at low cost and low maintenance. Cyclones are somewhat more complicated in design than simple gravity settling systems, and their removal efficiency is much better than that of settling chamber. Cyclones are basically centrifugal separators, consisting of an upper cylindrical part referred to as the barrel and a lower conical part referred to as cone.

They simply transform the inertia force of gas particle flows to a centrifugal force by means of a vortex generated in the cyclone body. The particle laden air stream enters tangentially at the top of the barrel and travels downward into the cone forming an outer vortex. The increasing air velocity in the outer vortex results in a centrifugal force on the particles separating them from the air stream. When the air reaches the bottom of the cone, it begins to flow radially inwards and out the top as clean air/gas while the particulates fall into the dust collection chamber attached to the bottom of the cyclone.

These types of cyclones have a distinctive and easily recognized form and are widely used in power and cement plants, feed mills and many other process industries.

The collection efficiency of cyclones varies as a function of density, particle size and cyclone design. Cyclone efficiency will generally increase with increases in particle size and/or density; inlet duct velocity; cyclone body length; number of gas revolutions in the cyclone; ratio of cyclone body diameter to gas exit diameter; inlet dust loading; smoothness of the cyclone inner wall.

Similarly, cyclone efficiency will decrease with increases in the parameters such as gas viscosity; cyclone body diameter; gas exit diameter; gas inlet duct area; gas density; leakage of air into the dust outlet. The efficiency of a cyclone collector is related to the pressure drop across the collector. This is an indirect measure of the energy required to move the gas through the system. The pressure drop is a function of the inlet velocity and cyclone diameter.

Collection efficiency

The collection or separation efficiency is most properly defined for a given particle size. As mentioned, fractional efficiency 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. Basically, it is the ratio of particles trapped to the particles tracked.

Several equations have been developed to predict the collection efficiency in cyclones through correlation equations.

1. Iozia and Leith Model

The model assumes that a particle carried by the vortex endures the influence of two forces: a centrifugal force and a flow resistance. The collection efficiency of particle diameter dpi can be calculated from

The cut diameter of the cyclone is defined as the size of the particles collected with 50% collection efficiency.

2. Li and Wang model

This model includes particle bounce or reattainment and turbulent diffusion at the cyclone wall.

The radial particle velocity and the radial concentration profile are variable for uncollected particles within the cyclone.

The tangential velocity is related to the radius of cyclone by uR^n = constant.

3. Koch and Licht model

The Kock and Licht collection theory recognized the inherently turbulent nature of the cyclones and the distribution of gas residence times within the cyclone. Following assumptions were used to describe the particle motion in the entry and collection regions.

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.

4. Lapple

This model was developed without considering the flow resistance. Lappe 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 50% cut diameter and the collection efficiency is given by, half width to the wall in the cyclone is collected with 50% efficiency. The 50% cut diameter and the collection efficiency are given by,

Project statement

1. With varying the particle diameter, at a constant velocity at the inlet the flow is simulated. separation efficiency is found out.
2. By varying the velocity at the inlet, at a constant diameter of the particles, the flow is simulated. separation efficiency and pressure drop in the cases are found out.

Geometry setup

1. Choose the Fluid flow (fluent) module and start with editing geometry.
2. Import the step file of the model into Space claim.
3. Fluid volume is extracted from the geometry by using the prepare option. so that we can analyze the fluid flow of the cyclone separator.
4. Hide the previous geometry and suppress physics since we are only interested in the fluid flow and not the external surface.

Figure: Pink geometry is the solid region (left) and the grey geometries are fluid regions prepared by using the volume extract tool in Ansys Space Claim.

Mesh setup

1.Main element size: 10mm

2. Method = Cartesian method

3. Cartesian element size= 10mm

Refined mesh

1. Element size: 6mm
2. Cartesian method size: 6mm
3. Inlet face meshing :2mm

Name Selections

The face selecting tool is selected. The inlets, outlet escape and outlet trap are named. The inlet is through which the particles enter the lighter impurities escape through the top(escape), and the heavier impurities are trapped in the bottom(trap).

Solver

1) General conditions

This analysis involves the orientation of the cyclone separator. As it is in the x-y plane, the acceleration due to gravity needs to be turned on. As acceleration due to gravity acts upwards, the g value is assigned as -9.81 on the positive y-axis. Pressure based solver is used. The steady-state analysis is done and the velocity formulation is normal to the boundary and absolute.

The K-epsilon (2 equation) model with RNG and swirl-dominated flow is selected. The wall modeling functions are standard wall functions.

Discrete Phase Model

The model is enabled for interaction with the continuous phase. DPM interval level is 10. For injection, the particle type is inert. From the fluent database, anthracite is selected. It is released from the inlet, with velocity in the x-direction being 3m/s. This velocity may be manipulated further. The particle diameter chosen for the first analysis is one micrometer. The particle diameter varies from one micrometer to five micrometers to calculate the separation efficiencies.

DPM Settings

The boundary conditions are provided to three named selections, inlet, outlet (1) top and outlet (2) bottom.

For the inlet, the momentum conditions are velocity is 3m/s in the x-direction, being absolute and normal to the boundary. The DPM condition is set to reflect. This signifies that the particles are not allowed to escape or get trapped in the inlet.

For the top outlet, the Momentum condition is the pressure condition, The DPM condition is set to escape. Particles can escape from this Boundary

The bottom outlet is also a pressure outlet, with the DPM condition being trapped.

Here above all boundary conditions, we have given different DPM conditions like Reflect, Escape and Trap.

Reflect: It is the Particle Rebound off the boundary in question with a change in its momentum as defined by the coefficient of restitution.

Escape: The particle is reported as having “escaped”, when it encounters the boundary in question. Trajectories calculated are terminated.

Trap: The Trajectory calculated is 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. IN the case of combusting particles, the remaining volatile mass is passed into the vapor phase.

• Solution methods: The pressure velocity coupling is chosen as COUPLED scheme. The gradient is least-square cells based. Pressure, Momentum, Turbulent kinetic energy and turbulent dissipation rate are second-order upwind.
• Initialization: Standard initialization is done with flow from inlets.

• Geometry runs for 500 iterations

Results

Case 1 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.]

• 1μm 3m/s (mesh length 6mm inlet face 2mm)

• 2μm 3m/s (mesh length =6mm inlet face mesh =2mm)

• 3μm 3m/s (mesh length 8mm)

• 4μm 3m/s (mesh length =6mm inlet face mesh =2mm)

• 5μm 3m/s (mesh length 10mm)

Case 2 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].

• 1m/s 5μm (mesh length 10mm)

• 2m/s 5μm (mesh length 10mm)

• 3m/s 5μm (mesh length =6mm inlet face mesh =2mm)

• 4m/s 5μm (mesh length =8mm)

• 5m/s 5μm (mesh length =6mm inlet face mesh =2mm)

Google drive link https://drive.google.com/drive/folders/1j-FhRvNm9UQy1RMYvhe-bQMik6uHJqk1?usp=sharing

Conclusion

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 particle 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 = Total inlet pressure - Total Outlet Pressure

As the “escape” DPM condition is given to the outlet-top in this case, so outlet-top is considered in the pressure drop calculation.

S.no	Velocity and diameter	Tracked	Escaped	Trapped	Incomplete	Collector Efficiency	pressure drop (Pa)
Case 1 1	3 m/s and 1micron	2567	-	2567	-	100%	1.045
2	3 m/s and 2 microns	2567	-	2567	-	100%	5.584
3	3 m/s and 3 microns	91	11	80	-	87%	8.821
4	3 m/s and 4 microns	2567	3	2564	-	99.8%	10.125
5	3 m/s and 5 microns	91	26	65	-	71.4%	13.156
Case2 1	1 m/s and 5 microns	91	11	80	-	87.8%	17.178
2	2 m/s and 5 microns	91	12	79	-	86.8%	5.584
3	3 m/s and 5 microns	2567	46	2521	-	98.3%	8.821
4	4 m/s and 5 microns	284	6	278	-	97.8%	10.125
5	5 m/s and 5 microns	2567	69	2498	-	97.8%	13.156

*In the above table the larger tracked and trapped elements are from the refined mesh

• From the above pressure velocity plots, we can infer that the pressure is higher near the wall region which forms the outer vortex.
• Due to higher pressure near the wall, a low-pressure region is created at the center of the cyclone separator because of which the lighter particles escape from the top outlet and heavier particles are collected at the bottom outlet.
• Simulation is done for particle size varying from 1μm to 5μm with air and particle velocity as 3m/s.
• From the results, it is observed that heavier particles are trapped at the bottom outlet and lighter particles are escaped at outlet 1. But we can also see that lighter particles also get trapped at the bottom outlet because of interaction between lighter and heavier particles and there is an exchange of inertial forces between them as a result there is a transfer of Kinetic energy from heavier particle to the lighter particle, as a result, we can observe a certain number of heavier particles escapes through the top outlet.
• The collection efficiency of cyclones varies as a function of particle size, density, and cyclone design. Cyclone efficiency will generally increase with the increase in particle size, density, inlet duct velocity, cyclone body length, number of gas revolutions in the cyclone, the ratio of cyclone body diameter to gas exit diameter, inlet dust loading, smoothness of the cyclone inner wall.
• The efficiency of the cyclone will decrease with an increase in the parameters such as gas viscosity, cyclone body diameter, gas exit diameter, gas inlet duct area, gas density, and leakage of air into the dust outlet. With the increase in inlet velocity or particle size the efficiency increases.
• The collection efficiency of the cyclone separator is related to the pressure drop across the cyclone. Here pressure drop is the indirect measure of the energy required for the gas to move the particles. Pressure drop is related to the inlet velocity of air and particle size.