Project

CFD analysis on Cyclone Separator

JJagaveerapandiyan Selvaraj20 Mar 2020

Cyclone Separator

Cyclone Separator is a means of separating different liquid phases (different liquid densities), or, separating particles from a gas stream. Cyclone separators often form part of a pre-cleaning stage prior to a gas or liquid being discharged. This article focuses on gas cyclone separator.

gas cyclone and hydro cyclone

Gas cyclones are used to remove entrained particles from a gas stream.

Hydrocyclones are used for separating fluids of different densities.

Cyclone separators can be installed as single units, or in multiples, known as multi-cyclones. It is also possible to install cyclones in series, or, in parallel.

Separators can be installed with a horizontal or vertical orientation

Gas Cyclone Separators

Gas cyclone separators are grouped into two main categories, reverse-flow and axial-flow.

Reverse flow cyclone separators are cone-shaped. Gas enters the top of the separator body, flows downwards, then flows back upwards and is discharged

Various reverse flow cyclone separator designs exist. Below is another variation of the reverse flow cyclone.

For axial flow (a.k.a. straight through) cyclone separators, gas enters at one end and is discharged at the opposite end. Axial flow separators are not as common as reverse flow separators.

Cut Point

The volumetric flow rate and geometry of the cyclone separator define the cut point. The cut point is the point at which particles are removed from the gas stream at 50% efficiency. This measurement is an industry-standard measurement and can usually be obtained from the original equipment manufacturer.

Efficiency

cyclone’s ability to separate particles from the flowing gas stream. Because particles have different sizes, the efficiency rating is usually given for varying particle sizes.

Empirical models used for cyclone separator efficiency

Lapple Mode

The basic model that assumes laminar flow. appel’s model first calculates the particle diameter (assuming spherical particles) `d_(p50%)` that will have 50% collection efficiency under specific conditions.

`d_(p50%)=sqrt((9mub)/(2piN_eV_irho_p)`,

where `mu` is the viscosity of the gas,

b is the inlet width,

`N_e` denotes the number of effective turns the gas makes within the cyclone (usually empirically measured).

`V_i` is the inlet velocity of the gas

`rho_p` is the particle density

`eta=(1/(1+(d_(p50%)/d_(pj))^2))`

represents the collection efficiency of a specific-sized particle.

Iozia and Leith Model

Assumes turbulent flow.

`d_(p50%)=(0.693/A)^(n+1)`

n is the vortex exponent

`A=2[KQrho_p(n+1)/(18muD_c^3)]^(1/(2(n+1))`

where Q is the gas flow rate

K is the configuration factor

`D_C` is the cyclone diameter

collection efﬁciency for particle

`eta=1-exp[-0.693(d_(pj)/d_(p50%))^(1/(n+1))]`

Li and Wang Model

the model includes sparticle bounce or re-entrainment and turbulent diffusion at the cyclone wall

Assumption

The radial particle velocity and the radial concentration proﬁle are not constant for uncollected particles within the cyclone.`uR^n=constant`

Boundary conditions with the consideration of turbulent diffusion coefﬁcient and particle bounce re-entrainment on the cyclone wall

`c_0=c` at `theta=0`

collection efﬁciency for particle

`eta=1-exp(-lamdatheta)`

`theta=(2pi(S+L))/a`

`lamda=((1-alpha)Komega)/(D_rr^n)`

`k=((1-n)(rho_p-rho_g)d^2Q)/(18mub(r_w^(1-n)-r_n^(1-n))`

Koch and Licht Mode

turbulent nature of cyclones and the distribution of gas residence times within the cyclone

The tangential velocity of a particle is equal to the tangential velocity of the gas ﬂow,

there is no slip in the

Assumption

the tangential direction between the particle and the gas.

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

`eta=1-exp(-2[(GtauQ)/D^3(n+1)]^(0.5/(n+1)))`

factor related to the conﬁguration

`G=(8K_c)/(K_a^2+K_B^2)`

related to the vortex

`n=1-(1-(12D)^(0.14)/2.5)((T+460)/530)^0.3`

relaxation term

`tau=(rho_pd_p^2)/(18mu)`

Boundary condition Injection

Inlet

reflect

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

The normal coefficient of restitution defines the amount of momentum in the direction normal to the wall that is retained by the particle after the collision with the boundary

where is the particle velocity normal to the wall and the subscripts 1 and 2 refer to before and after the collision, respectively. Similarly, the tangential coefficient of restitution defines the amount of momentum in the direction tangential to the wall that is retained by the particle.

A normal or tangential coefficient of restitution equal to 1.0 implies that the particle retains all of its normal or tangential momentium after the rebound (an elastic collision). A normal or tangential coefficient of restitution equal to 0 implies that the particle retains none of its normal or tangential momentum after the rebound.

Nonconstant coefficients of restitution can be specified for wall zones with the reflect type boundary condition. The coefficients are set as a function of the impact angle.

trap

The trajectory calculations are terminated and the fate of the particle is recorded as \"trapped\'\'. In the case of evaporating droplets, their entire mass instantaneously passes into the vapour phase and enters the cell adjacent to the boundary. In the case of combusting particles, the remaining volatile mass is passed into the vapour phase.

escape

The particle is reported as having \"escaped\'\' when it encounters the boundary in question. Trajectory calculations are terminated.

wall-jet

that the direction and velocity of the droplet particles are given by the resulting momentum flux, which is a function of the impingement angle, $\"$\\phi$\"$ , and Weber number.

The wall-jet boundary condition assumes an analogy with an inviscid jet impacting a solid wall. The equation shows the analytical solution for an axisymmetric impingement assuming an empirical function for the sheet height ( $\"$H$\"$ ) as a function of the angle that the drop leaves the impingement

`H(Psi)=H_pie^(beta(1-Psi/pi)`

`Psi=-pi/betaln(1-P(1-e^-beta))`

`sin(phi)=(e^beta+1)/((e^beta-1)(1+(pi/beta)^2))`

where $\"$H_{\\pi}$\"$ is the sheet height at $\"$\\Psi=\\pi$\"$ and $\"$\\beta$\"$ is a constant determined from conservation of mass and momentium. The probability that a drop leaves the impingement point at an angle between $\"$\\Psi$\"$ and $\"$\\Psi+\\delta\\Psi$\"$ is given by integrating the expression for

The wall-jet boundary condition is appropriate for high-temperature walls where no significant liquid film is formed, and in high-Weber-number impacts where the spray acts as a jet. The model is not appropriate for regimes where film is important (e.g., port fuel injection in SI engines, rainwater runoff, etc.).
``interior\'\' means that the particles will pass through the internal boundary. This option is available only for internal boundary zones, such as a radiator or a porous jump.

It is also possible to use a user-defined function to compute the behaviour of the particles at a physical boundary. See the separate UDF manual for information about user-defined functions.

Because you can stipulate any of these conditions at flow boundaries, it is possible to incorporate mixed discrete phase boundary conditions in your FLUENT model.

CFD Analysis

Geometry extraction

base model

Extracted volume

MESH

case 1a

coarse mesh

Element size- 45, elements- 20727

Element Quality

case 1b

medium mesh

Element size-30, elements- 31219

Element Quality

case1, case2, case3, case4

fine mesh

Element size-10 elements- 407468

Element Quality

setup

Gravity enables

viscous

turbulent

K-epsilon-RNG- swirl generated,

Discrete phase-Model-injection

Surface-inlet

material-anthracite

x-velocity=3m/s

Diameter-5e-6

Boundary condition

velocity inlet-3m/s

outlet top & bottom

case 1a coarse mesh reflect

case 1b medium-mesh reflect

case 1c fine mesh reflect

case 2 fine mesh trap

case 3 fine mesh escaped

case 4 fine mesh wall Jet

Initialize

standard initialize-inlet

Solution

cases	number tracked	escaped	Aborted, trapped, Evaporated, Incomplete parallel	incomplete
Element size- 45, elements- 20727
Case1 a reflect	14	14	0	0
Element size-30, elements- 31219
Case1 b reflect	40	40	0	0
Element size-10 elements- 407468
Case 1 reflect	364	333	0	31
Case 2 trap	364	333	0	31
Case 3 escaped	364	333	0	31
Case 4 Wall Jet	364	333	0	31