Computational study of cyclone separator with different particle size and boundaries condition

Introduction

Cyclones are devices that employ a centrifugal force generated by a spinning gas stream to separate particles from the carrier gas. In general, cyclone designs are of two types: the straight-through cyclone and the more common conventional reverse flow cyclone. The cyclone usually consists of both conical and cylindrical parts, which together form the body of the cyclone, the inlet pipe, which is mounted tangentially onto the side wall of the cylindrical part near the top of the cyclone, the separation chamber, the dust chamber, and the vortex finder also called vortex tube, which is fixed on the top of the cyclone and protrudes some distance into the body of the cyclone. The separation process in cyclone separators is caused by centrifugal forces. Rotational gas flow is produced by the tangential inlet. Particles contained in the gas flow are forced radially outward to the wall by centrifugal force due to its swirling motion. The particles move downwards along the cylindrical and conical wall towards the lower part of the cyclone, called the apex, and into the dust outlet, where they are collected. The clean gas leaves the cyclone upwards through the centrally arranged vortex tube.

Empirical Models Used for Cyclone Separator Efficiency

1. Iozia and Leith Model
2. Li and Wang Model
3. Koch and Licht Model
4. Lapple Model

1.Iozia and Leith Model

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, Z, and flow resistance, W. Core length, z_c, and core diameter, d_c, are given as

The addition made by Iozia and Leith on the original Barth (1956) model is the core length z_c 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 η_i of particle diameter dpi can be calculated from

2.Li and Wang Model

The Li and Wang (1989) 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 diffusion coefficient and particle bounce re-entrainment on the cyclone wall are:

The concentration distribution in a cyclone is given as:

3.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 the 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 to cyclone by:

A force balance and an equation on the particles collection yields the grade efficiency n_i.

4.Lapple Model

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. 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, d_pc is

Computational boundary condition

The simulation to determine the variation in the output parameters is found by carrying out the simulations with the same conditions except for the DPM settings.

The settings used for the whole simulation is as follows:

The model used: RNG K-e model for swirl dominated the flow

Discrete Phase modeling: Continuous model no. of steps for particle track 50000.

Injections: Anthracite (5 microns diameter)

Inlet conditions: 3 m/s (Discrete phase and continuous phase)

Outlet: Pressure outlet conditions with gauge pressure 0 Pa.to the

Comparison between Reflect, Escape, Trap, and Wall-Jet

Element Size – 1 mm

No of Element – 998069

Particle Size 5e-06

Boundary Condition 1.Reflect

Boundary Condition 2.Escape

Boundary Condition 3.Trap

Boundary Condition 4.Wall Jet

Comparison b/w boundary condition for same particle size
Boundary Cond	Reflect	Escape	Wall-Jet	Trap
Number Tracked	1300	1300	1300	1300
Escaped	543	533	533	543
Aborted	0	0	0	0
Trapped	0	0	0	0
Evaporated	0	0	0	0
Incomplete	757	766	766	757
Incomplete Parallel	0	1	1	0
Mass Flow Inlet Kg/s	0.05696	0.05696	0.05696	0.05696
Mass Flow Outlet 1 Kg/s	-0.01716	-0.01709	-0.01709	-0.01716
Mass Flow Outlet 2 Kg/s	-0.03976	-0.03984	-0.03984	-0.03976

Comparison between different particle size in reflect boundary

Case 1 Particle Size 5e-06

Case 2 - Particle Size 2.5e-06

Case 3 Particle Size 1e-06

Comparison in reflect boundary for different particle size
Boundary Cond	Reflect - 1	Reflect – 2	Reflect – 3
Particle Size	1e-06	2.5e-06	5e-06
Number Tracked	1300	1300	1300
Escaped	711	741	543
Aborted	0	0	0
Trapped	0	0	0
Evaporated	0	0	0
Incomplete	589	558	757
Incomplete Parallel	0	1	0
Mass Flow Inlet Kg/s	0.05696	0.05696	0.05696
Mass Flow Outlet 1 Kg/s	-0.01716	-0.01716	-0.01543
Mass Flow Outlet 2 Kg/s	-0.03976	-0.03976	-0.04151

Conclusion

The plot outcome was the same for all the 4 boundaries (Reflect, Escape, Wall-jet, and Trap) and the different DPI input conditions produced no variations in results at all.

If the particle size is larger and the sectional area of entry into a cyclone is smaller, the separation efficiency of the cyclone is higher.

When the particle diameter exceeds a critical size, the particles will deposit on the wall of the conical chamber.

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.