Cyclone Seperator CFD simulation
Objective
To analyse the given Cyclone Separator model at various boundary conditions at the inlet using discrete phase modelling and also perform a grid dependency test.
Introduction
A Cyclone Separator is used to remove particulate matter from a stream of air or water by means of vortex separation. Rotational effects and the effect of gravity is used to separate the particles from the fluids.
1. Empirical Models for Cyclone Separator
Lapple Model(1951)
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.
Iozia and Leith Model(1990)
Iozia and Leith logistic model is a modified version of the Barth(1956) model which is developed based on the force balance. The model assumes that a particle carried by the vortex involves the influence of two forces. This model changed the Barth model by changing the length and slope parameters.
Koch and Licht Model(1977)
Koch and Licht theory recognized the turbulent nature of the cyclone and the distribution of gas residence times within the cyclone. They described the particle motion at the entry and the collection regions.
The following assumptions were made,
- No-slip in the tangential direction between the particles and the gas.
- The tangential velocity is related to the radius of the cyclone by uR=constant. Where u=gas viscosity and R=radius.
Li Enilang and Wang Yingmin Model(1989)
The Li Enilang and Wang Yingmin model includes particle bounce entrainment and turbulent diffusion at the cyclone wall. A two-dimensional analytical expression of particle distribution in the cyclone is obtained.
The model was based on the following assumptions,
- The radial particle velocity and the radial concentration profile are not constant for uncollected particles within the cyclones.
- The tangential velocity is related to the radius of cyclone by uR=constant.
2. Various Boundary Conditions at the inlet
The simulation consisted of a single mesh of 10mm base size with allowance up to 20mm and all the other settings as default. Linear meshing was done with 79583 nodes and 407774 elements.
Fig 2.1
Fig 2.1 shows the meshed model. The same mesh is used to carry out the analysis under various boundary conditions at the inlet.
Fig 2.2
Fig 2.2 represents the mesh metrics of element quality of the above mesh. The average element quality was found to be 83%. It is also evident that the mesh consisted of only Tetrahedral elements.
The turbulent model used is the 2-equation k-epsilon RNG model.
Inlet velocity= 3m/s
Number of particles tracked= 364
Size of the particles= 5e-3mm
- Reflect
In discrete phase modelling, the first inlet boundary condition is Reflect.
Fig 2.3
Fig 2.3 shows the Anthracite particle time track and the time taken by the particles.
Particles Tracked= 364, Particles Escaped= 294, Incomplete= 70
Time taken by the particles= 49.72 seconds
Mass flow at Inlet= 0.0569625 kg/s
Mass flow at Upper outlet= -0.0217648 kg/s
Mass flow at Lower outlet= -0.0351713 kg/s
Fig 2.4
Fig 2.4 is the Vortex core at a certain swirling velocity
- Escape
Fig 2.5
Fig 2.5 shows the Anthracite particle time track and the time taken by the particles.
Particles Tracked= 364, Particles Escaped= 291, Incomplete= 73
Time taken by the particles= 49.94 seconds
Mass flow at Inlet= 0.0569625 kg/s
Mass flow at Upper outlet= -0.0217614 kg/s
Mass flow at Lower outlet= -0.0351795 kg/s
Fig 2.6
Fig 2.6 is the Vortex Core region
- Trap
Fig 2.7
Fig 2.7 shows the Anthracite particle time track and the time taken by the particles.
Particles Tracked= 364, Particles Escaped= 291, Incomplete= 73
Time taken by the particles= 49.94 seconds
Mass flow at Inlet= 0.0569625 kg/s
Mass flow at Upper outlet= -0.0217614 kg/s
Mass flow at Lower outlet= -0.0351795 kg/s
Fig 2.8
Fig 2.8 is the Vortex Core region
- Wall-jet
Fig 2.9
Fig 2.9 shows the Anthracite particle time track and the time taken by the particles.
Particles Tracked= 364, Particles Escaped= 292, Incomplete= 72
Time taken by the particles= 49.94 seconds
Mass flow at Inlet= 0.0569625 kg/s
Mass flow at Upper outlet= -0.0216961 kg/s
Mass flow at Lower outlet= -0.0352789 kg/s
Fig 2.11
Fig 2.11 is the Vortex Core region at a certain swirling velocity.
3. Grid dependency test using Reflect boundary condition.
Grid dependency test is nothing but varying the mesh size and checking for the convergence in the result. In this case, due to the academic limit, the mesh could be made finer only up to a certain limit. Three cases with mesh sizes 15mm, 12mm and 10mm are simulated.
The Turbulent model used in the simulation is the 2-equation k-epsilon with swirl dominated flow. The inlet velocity is 3m/s and the surface injection of particles is the set-up. The particle type is set to Anthracite and the size of 5 micrometres. The calculation is done from the inlet using Standard Initialization.
- 15mm mesh size.
Fig 3.1
Fig 3.1 is the meshed model with a size of 15mm, having an allowance up to 30mm. The element order is linear and all the other settings are left as default. The mesh consists only of Tetrahedral elements. The mesh has 31397 nodes and 157313 elements.
Fig 3.2
The element quality mesh metric is shown in Fig 3.2. The average mesh quality is found to be 83.84%.
Fig 3.3
Fig 3.3 shows the Anthracite particle time track and the time taken by the particles.
Particles Tracked= 132, Particles Escaped= 78, Incomplete= 54
Time taken by the particles= 9.272 seconds
Mass flow at Inlet= 0.0569625 kg/s
Mass flow at Upper outlet= -0.170604 kg/s
Mass flow at Lower outlet= -0.0398978 kg/s
Fig 3.4
Fig 3.4 is the Vortex Core region at a certain swirling velocity.
https://drive.google.com/open?id=1eOBEU077B4TH9O0GggUpiaAmaJ33elfO
- 12mm mesh size
Fig 3.5
Fig 3.5 is the meshed model with a size of 15mm, having an allowance up to 30mm. The element order is linear and all the other settings are left as default. The mesh consists only of Tetrahedral elements. The mesh has 31397 nodes and 157313 elements.
Fig 3.6
The element quality mesh metric is shown in Fig 3.6. The average mesh quality is found to be 83.84%.
Fig 3.7
Fig 3.7 shows the Anthracite particle time track and the time taken by the particles.
Particles Tracked= 226, Particles Escaped= 189, Incomplete= 37
Time taken by the particles= 11.35 seconds
Mass flow at Inlet= 0.0569625 kg/s
Mass flow at Upper outlet= -0.0211616 kg/s
Mass flow at Lower outlet= -0.0357844 kg/s
Fig 3.8
Fig 3.8 is the Vortex Core region at a certain swirling velocity.
The animation of the simulation can be viwed using
https://drive.google.com/open?id=17vgv6eQFm-VWsmg0v6lGeE4f8Zq6CeQm
- 10 mm mesh size
Fig 3.9
Fig 3.9 is the meshed model with a size of 15mm, having an allowance up to 30mm. The element order is linear and all the other settings are left as default. The mesh consists only of Tetrahedral elements. The mesh has 31397 nodes and 157313 elements.
Fig 3.11
The element quality mesh metric is shown in Fig 3.11. The average mesh quality is found to be 83.84%.
Fig 3.12
Fig 3.12 shows the Anthracite particle time track and the time taken by the particles.
Particles Tracked= 364, Particles Escaped= 305, Incomplete= 59
Time taken by the particles= 11.97 seconds
Mass flow at Inlet= 0.0569625 kg/s
Mass flow at Upper outlet= -0.0217277 kg/s
Mass flow at Lower outlet= -0.0352268 kg/s
Fig 3.13
Fig 3.13 is the Vortex Core region at a certain swirling velocity.
The animation of the simulation can be viwed using
https://drive.google.com/open?id=1d5dVstQe6ZErEjXEJGuegNzllYDF3rf-
Conclusion
- Smaller the mesh size, larger will be the number of particles.
- Reflect boundary condition yields better results compared to other inlet boundary conditions.