Week 8 - Simulating Cyclone separator with Discrete Phase Modelling
Aim:
To perform analysis on cyclone separator and calculate the separation efficiency and pressure drop.
Objective:
- To write a few words about any four empirical models used to calculate the cyclone separator efficiency.
- 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.]
- 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]
Introduction:
Cyclone seperator:
A cyclone separator is an economical device for removing particulate solids from a fluid system. The induced centrifugal force is tangentially imparted on the wall of the cyclone cylinder. This force, with the density difference between the fluid and solid, increases the relative settling velocity.
CYCLONE EFFICIENCY EMPIRICAL MODELS:
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 a flow resistance W. Core length zc , and core diameter d
The addition made by Iozia and Leith on the original Barthmodel are the core length zc and slope parameter β expression which is derived based on the statistical analysis ofexperimental data of cyclone with D = 0.25 m. The collection
efficiency ηi of particle diameter dpi
where the `d_(pc)` is cut by 50%
2. LI AND WANG MODEL:
The Li and Wang (1989) model includes particle bounce orre-entrainment and turbulent diffusion at the cyclone wall. A
two-dimensional analytical expression of particle distributionin the cyclone is obtained. Li and Wang model was 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
c=`c_0` at `theta = 0`
The resultant expression of collection efficiency of particle size
3.KOCH AND LICHT MODEL:
Koch and Licht (1977) collection theory recognized the inherently turbulent nature of cyclones and the distribution of
gas residence times within the cyclone. Koch and Lichtdescribe particle motion in the entry and collection regions
with the additional following assumptions:
• The tangential velocity of a particle is equal to the tangentialvelocity 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:
uRn = constant
A force balance and equation on particle collection yields the grade efficiency
G is a factor related to the configuration of the cyclone, n is related to the vortex and τ is the relaxation term.
4.LAPPLE MODEL:
Lapple (1951) 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 (1951) tocalculate a 50% cut diameter, dpc
The efficiency of collection of any particle size is
Geometry:
1.We use the extract volume to extract the fluid volume .
2. Untick the cyclone seperator model and the suppress physics.
Mesh:
We used body fitted cartesian mesh with an element size of 5 mm
and created name selections for inlet ,outlet 1 and outlet 2.
fluent setup:
We use steady state pressure based solver and gravity is turned negative value of 9.81 m/s is given in the y axis. Th k- epsilon RNG model is used.
Turn on the discrete phase model and create the injection setup and specify the particle size and the velocity.
Set the inlet outelet1 and outlet2 boundary conditions as Reflect ,escape and trap respectively.
Use the standard hybridization and compute from inlet.
Run 600 iterations and calculate.
Results:
Case 1:
Velocity = 3m/s , Particle size = 5e-6
Residuals:
Particle time:
Turbulence kinetic energy:
Seperation efficiency:
Particles.Trapped/Particles.Tracked = 252/378 = 66.7%
Case 2:
velocity 3m/s , particle size 1e-6:
Residuals:
Particle time:
Turbulence kinetic energy:
Seperator efficiency = 266/378 = 70.3%
Case 3:
velocity =3m/s , particle size =2e-6
Residuals:
Particle time:
Pressure:
Seperation efficiency = 266/378 = 70.37%
Case 4:
velocity = 3m/s , particle size =3e-6
Residuals:
Particle time:
Turbulence kinetic energy:
Seperation efficiency = 272/378 = 71.95%
case 5:
Velocity =3m/s, particle size = 4e-6
Residuals:
Particle time:
Seperator efficiency = 280/378 = 74.07%
Case 6:
velocity =1m/s, particle size = 5e-6
Residuals:
Particle time:
Turbulence kinetic energy:
Pressure:
Seperation efficiency = 301/378 = 79.01%
Pressure drop = 2.79 Pa
Case 7:
velocity =2m/s,particle size =5e-6
Residuals:
Particle time:
Turbulence kinetic energy:
Pressure:
Seperation efficiency = 278/378 =73.5%
Pressure drop = 11.84 Pa
Case 8:
velocity =3m/s ,particle size =5e-6
Residuals:
Particle time:
Pressure:
Turbulence kinetic energy:
Seperator efficiency = 260/378 = 68.7%
Pressure drop = 29.83Pa
case 9:
velocity =4 ,particle size =5e-6
Residuals:
Particle time:
Turbulence kinetic energy:
Pressure:
Seperator efficiency = 66.6%
Pressure drop = 50.1058
Case 10:
Velocity =5m/s , particle size =5e-6
Residuals:
Particle time:
Pressure:
Turbulence kinetic energy:
Seperator efficiency = 255/378 = 67.4%
Pressure drop = 74.5Pa
Swirl strength:
Swirl strength =0.05
Swirl strength =0.1
Swirl strength =0.01
Tables and inference
Varying the particle size with constant velocity of 3 m/s
| Particle size | Particle time (seconds) | seperator efficiency (%) |
| 1e-6 | 31.89 | 70.3 |
| 2e-6 | 10.21 | 70.37 |
| 3e-6 | 10.47 | 71.95 |
| 4e-6 | 2.406 | 74.07 |
| 5e-6 | 1.355 | 66.7 |
Varying the velocity with constant particle size of 5e-6:
| Velocity (m/s) | Particle time (seconds) | Seperator efficiency | Pressure drop |
| 1 | 9.684 | 79 | 2.79 |
| 2 | 3.335 | 73.5 | 11.84 |
| 3 | 5.497 | 68.7 | 29.83 |
| 4 | 2.59 | 66.6 | 30.108 |
| 5 | 1.68 | 67.4 | 74.5v |
Conclusion:
From the above pressure velocity plots , we can infer that the pressure is higher near the wall region from where the vortex forms.Due to high pressure region near the low pressure region is created at the center of the cyclone seperator from which the lighter particles escape through the top outlet and heavier particles are collected at bottom. But we can see some of the lighter particles are trapped in the bottom due to the interaction of inertial force sof lighter and heavier particles the kinetic energy is transferred from heavier to lighter particles.
The efficiency of cyclone seperator efficiency will decrease with increase in parameters like gas velocity,cuclone body diameter,gas exit diameter ,gas inlet duct , leakage of air into dust outlet
Pressure drop occurs because of:
1.Loss due to expansion of gas in chamber.
2.Loss due to rotational kinetic energy.
3.Loss due to wall frictional forces.