Week 8 - Simulating Cyclone separator with Discrete Phase Modelling

 

CYCLONE SEPARATOR:

Cyclone sepatator or simply cyclone are separation devices that use the principle of inertia to remove particulate matter from flue gases. cyclone separators is one of many air pollution removers known as precleaners since they generally remove large pieces of particular matter. this present finer filtration methods from having to deal with large more abrsive particles later on . in additional cyclone separators can operate in parallel and this system is known as amulticyclone.

Cyclone seperator work much like a centrifugal but with a countinous feed of dirty air. in a cyclone separator dirty flue gas is fed into a chamber . this inside of the chamber creates a spiral vortex similar to a tornado. this spiral formation and the separation is shown in fig. the lighter components of this gas have less inertia so it is easier for them to be influenced by the vortex and travel up it. contrarily latge components of particulate of matter have more inertial and are not as easily influenced by the vortex.

since these large particles have difficulty following the high speed spiral motion of the gas and the vortex the particles hit the inside wall of the container and drop down into a collection hopper. these chamber are shapped like an upside down cone to promote the collection of these particles at the bottom of the container. the cleaned flue gas escapes out the top of the chamber.

FOUR EMPIRIAL MODELS :

To used to calculate the cyclone separator efficiency:

* IOZIA AND LEITH MODEL:

Iozia and leith logistic model is a modified version of barth model which is developed based on force balance. the model assumed that a partcle carried by the vortex endurance the influence of two forces a centrifigual force, z and a flow resistance, w. core length ,zc, and core diameter, dc are given as:

B is an expression for slope parameter derived based on the statistical analysis of experimental data of a cyclone with D=170

0.25 m given as:

and dpc is the 50% cut size given by barth:

where core length ,zc,and core diameter,dc are given as

 

* LI AND WANG MODEL:

The li and wang model includes particle bounce or reentrainment and turbulent diffusion at the cyclone wall. a twodimensional analytical expression of particle distribution in the cyclone is obtained li and wang model was developed based 180 on the following assumption:

The radial particle velocity and the radial concentration profile are not constant for uncollected particles within the cyclone.

Boundary condition with the consideration of turbul185 lent diffusion coefficient and particle bounce reentrainment on the cyclone wall are:

 

* KOCH AND LICHT MODEL:

Koch and licht collection theory recognized the inherently turbulent nature of cyclone and the distribution of gas residence time within the cyclone Koch and licht described particle motion in the entry and collection region with the ad-200 aditional following assumption:

- the tangential velocity of a particle is equal to the tangential velocity of the gas flow there is no slip in the tangential direction between the particle and the gas:

- the tangential velocity is related to the radius of cy-205 clone by: u Rn=constant.

a force balance and an equation on the particles collection yields the grade efficiency ni:

 

* LAPPLE MODEL:

Lapple model was developed based on force balance without considering the flow resistance lapple assumed that a par215 ticle entering the cyclone is evenly distribute 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 emperical relationship developed by lapple to calculate a 50% cut diameter dpc is

The collection efficiency of cyclone varies as a function of density , particle size and cyclone design.Cyclone efficiency will generally increase with increase in particle size and or density,inlet duct velocity , cyclone body length.

number of gas revolution 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 increses 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 from the above discussion it is clear that samll cyclone are more efficient than large cyclones. small cyclones however have a higher pressure drop and are limited with respect to volumetric flow rates. another option is arrange smaller cyclone in series and in parallel to substantially increase efficency at lower pressure drops. these gains are somewhat compensated however by the increased cost and maintenance problems. also these type of arrangement tend to plug more easily, when common happers are used in such arrangement different flows through cyclone can lead to reentrainment problems.

 

GEOMETRY:

 

Gravity is enabled in the -ve y-direction.

the swirl dominated RNG K-eplision model is used here to capture the flow more accurately.

The discrete phase modeling is used to track the flow of the particles.

No of step for particle tracking=50000

injection material=anthracite(5 micron in diameter)

velocity inlet=3m/s

outlet pressure=0pa

DPM setting are varied between reflect,escape,trap& wall jet.

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

escape= particle escapes out when encountered by the boundary.

trap= the trajectory calculation is terminated and fate of the aprticle is recorded as a trap.

wall jet= the wall jet type boundary condition is applicable for high temperature walls where no significant liquid film is found & high weber no impacts when the spray acts as a jet. this model is not applicable to regimes where the film is important.

 

SOLUTION:

 

 

RESULTS:

CASE 1 : In this we are making the velocity constant and we are visulaizing the difference in result for different particle size.

results for the particle size of 1 micron with a velocity of 3 m/s:

 residual:

pressure at inlet:

pressure at outlet:

 52/98*100=53.06%

 

 

 

results for the particle size of 2 micron with a velocity of 3 m/s:

residual:

 residual:

 

 pressure at inlet:

 

pressure at oulet:

 

 

 

=75/98*100=76.53%

 

 results for the particle size of 5 micron with a velocity of 3 m/s:

residual:

 

 

  pressure at inlet:

 

 pressure at oulet:

 98/98*100=100%

 

 CASE :2

RESULTS FOR THE VARYING AND KEEPING THE PARTICLE SIZE CONSTANT 5 MICRON:

Inlet velocity of the particle and the discreate phase is same 1m/s:

residual:

 

 pressure at inlet:

 

  pressure at oulet:

=7/98*100=7.14%

 

Inlet velocity of the particle and the discreate phase is same 3m/s:

residual:

pressure at inlet:

  pressure at oulet:

=89/98*100=90.81%

 

Inlet velocity of the particle and the discreate phase is same 5m/s:

residual:

pressure at inlet:

  pressure at oulet:

 

=98/98*100=100%

 

 

CONCLUSION:

1. Simulation is done for particle size varying from 1m to 5m with air and particle velocity as 3m/s .from this it is observed that heavier particle is trapped at the bottom outlet and lighter particles are escaped at the outlet1 . but we can aslo see that lighter particle also get trapped at the bottom outlet because of interaction 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 cyclone varies as a function of particle size , density,and cyclone design.

2. cyclone efficiency will generally increase with the increase in particle size,density ,inlet duct velocity, cyclone body length , number of gas revolution in the cyclone, the ratio of cyclone body diameter to gas exit diameter, inlet dust loading , smoothness of the cyclone inner wall.

3. the efficiency of the cyclone will decrease with an increase in the parameter such as gas viscosity cyclone body diamter, gas exit diameter, gas inlet duct area, gas density, leakage of air into dust outlet. with increase in inlet velocity or particle size the efficiency increases.