1

Flow Inside the Cyclone Separator

Kunal Kumar and Shreyas Karadahalli Nagesh

Aalto University

School of Engineering

Master’s program on mechanical engineering

Abstract:

The impact of inlet dimensions on the flow behavior inside

the cyclone separator has been examined using two

turbulence model approaches. Firstly, the single-phase flow

modelling of air inside the cyclone separator has been

analyzed in steady state conditions using 𝑺𝑺𝑻 𝒌 − 𝝎 model.

It has been seen that the tangential velocity is the most

important component for the flow inside cyclone separator.

It results in centrifugal force to separate the dust particles

with high densities from air. It has been obtained from the

results that decreasing the inlet dimensions e.g. inlet width

and inlet height increases the maximum tangential velocity

which in result magnifies the separation efficiency of the

cyclone separator. Secondly, multiphase modelling of air

and dust particles has been performed in unsteady

conditions using Reynolds Stress Turbulence model with

Lagrangian Multiphase model. The particle residence time,

velocity and position magnitudes were analyzed. The

𝑺𝑺𝑻 𝒌 − 𝝎 model and Reynolds Stress Turbulence model

performed great to predict the recirculating fluid flow

inside the cyclone separator in steady and unsteady

conditions respectively.

INTRODUCTION

Cyclone separators or cyclones belong to the group of air

pollution control devices. They are also known as pre-cleaners

as they are used to roughly remove larger pieces of particulate

matter [1]. They are easy to manufacture, contain no moving

parts, economical to operate and adaptable to the harsh

conditions; these advantages make them one of the most

important particle removal devices which are used in scientific

and engineering fields. The particle separation inside the

cyclone separators is managed by two swirling motions of the

fluid flow in vertically opposite directions. This is known as

double vortex phenomenon [11]. In cyclones, the strong

turbulence flow is used to separate the particles from the air or

the phases with different densities [2]. The air with the particles

enters tangentially in the cyclone separator. This generates

swirling motion of the air and pushes the particles towards the

outer wall. As the particles are having higher densities, they

spiral in the downward direction and are collected in the particle

collector dustbin which is attached to the downward outlet of

the cyclone separator. The clean air passes through the outlet

which is on the top of cyclone separator. Eventually, the swirl

and turbulence are two main phenomena in the phase separation

process in cyclone separators. The swirl produces the

centrifugal force in the solid or dust particles. This is the driving

force for the separation process. The turbulence disperses the

dust particles and magnifies the probabilities that the particles

are caught in the exit stream [2]. There are many geometrical

and operational parameters that influence the cyclone

performance. Elsayed and Lacor [2] studied numerically the

effect of inlet dimensions on the performance of the cyclone

separator. They found that the effect of inlet width is more

significant on the performance of the cyclone separator. The

geometry of the cyclone separator is simple but the fluid flow

is very complicated three-dimensional flow. The complexity of

the flow pattern has been discussed in various literatures e.g.

Hamed Safikhani and Pegah Mehrabian [10] and Elsayed and

Lacor [2]. Nowadays, laser Doppler anemometry (LDA) and

hot wire anemometry are frequently used to study

experimentally the flow behavior inside the cyclone separators

[10]. Computational Fluid Dynamics (CFD) turbulence models

have been proven very useful to analyze theoretically the flow

behavior inside the cyclone separators. In this study, we are

interested to scrutinize the impact of inlet dimensions on the

flow inside the cyclone separators. The CFD simulations for

single phase modelling were performed using 𝑆𝑆𝑇 𝑘 − 𝜔

turbulence model. The CFD simulations for multiphase

modelling were performed using Reynolds Stress Turbulence

model (RST) with Lagrangian Multiphase modelling. The

STAR-CCM+ was used to run the simulations.

MODELLLING APPROACH

Computer aided design (CAD) of Cyclone Separator

The cyclone separator was designed using the CAD package

named PTC Creo. It was designed on a pilot scale to perform

optimization studies and reduce computation time. The

geometry is explained by geometrical parameters such as inlet

height a, inlet width b, vortex finder diameter 𝐷𝑥, vortex finder

length S, cylinder height h, cone tip diameter 𝐵𝑐 and cyclone

total height 𝐻. In this study, we are using two cyclone

separators with different inlet dimensions. The following table

provides the detailed information about the geometrical

dimensions of cyclone separators. The units of dimensional

parameters are in the SI units.

TABLE 1: Detailed description of cyclone separator

dimssensions

Cyclone A B C

Parameters (in mm)

Inlet height, a 150 95 150

Inlet width, b 50 65 50

Vortex finder diameter, 𝐷𝑥 140 140 140

Vortex finder length, S 450 450 450

Cylinder height, h 250 250 250

Cone tip diameter, 𝐵𝑐 100 100 100

Cyclone total height, H 1000 1000 1000

The Cyclone Separator C was chosen to perform multiphase

modelling.

2

Inlet 1

Vortex finder

(Overflow)

2

Cylinder 3

Conical

shape

4

Cone tip and

Underflow

5

Figure 1: Cyclone Separator CAD model with dimensional

description

Description of the numerical model

RANS (Reynolds Average Navier Stokes Equation)

For an incompressible fluid flow, the continuity equation and

momentum equations are given as:

𝜕��𝑖

𝜕𝑥𝑖= 0

(1)

𝜕��𝑖

𝜕𝑡+ 𝑢��

𝜕��𝑖

𝜕𝑥𝑗= −

1

𝜌 𝜕��

𝜕𝑥𝑖+ 𝜐

𝜕2��𝑖

𝜕𝑥𝑖𝜕𝑥𝑗−

𝜕

𝜕𝑥𝑗𝑅𝑖𝑗

(2)

Where ��𝑖 is the mean velocity, 𝑥𝑖 is the position, �� is the mean

pressure, 𝜌 is the constant gas density, 𝜐 is the kinematic

viscosity and 𝑅𝑖𝑗 = 𝑢𝑖′𝑢𝑗

′ is the Reynolds stress tensor. Here,

𝑢𝑖′ = 𝑢𝑖 − ��𝑖 is the 𝑖𝑡ℎ fluctuating velocity component.

Selection of the turbulence model

Due to the strong swirling flow inside the cyclone separator, it

is very essential to describe the turbulent behavior of the flow

accurately. The development in the computational technology

have enabled us to use several turbulence models for predicting

the flow field inside the cyclone separator. They range from

standard 𝑘 − 휀, RNG 𝑘 − 휀, 𝑘 − 𝜔 model, Reynolds Stress

Turbulence model (RST), Large Eddy Simulation (LES) and

multiphase model such as Lagrangian Multiphase (LMP) model

[4]. The Large Eddy Simulation (LES) is an alternative to the

Reynolds averaged Navier- Stokes (RANS) approach.

According to [2], the standard 𝑘 − 휀, RNG 𝑘 − 휀 models are

not fully optimized for strongly swirling flows in cyclone

separators. The 𝑘 − 휀 model adopts the assumption of isotropic

turbulence so its not suitable for the flow in a cyclone separator

which has anisotropic turbulence [12]. According to many

researchers, the Reynolds Stress Turbulence model is more

capable of predicting the complex flow behavior inside the

cyclone separator as it forgoes the assumption of isotropic

turbulence and solves the transport equation for each

component of Reynolds stress [12]. It is predicted the most

suitable turbulence model for the cyclonic flow even though it

is computationally more expensive. According to [5], the 𝑘 −

𝜔 model with curvature correction is sufficient for simpler

cases because this model is specially designed to capture the

curvature dominated flow and it is computationally efficient

than RSM. In our study, we are first using 𝑘 − 𝜔 model with

curvature correction and then switching to RST with LMP to

analyze air and dust particle behavior in cyclone separator.

The 𝑘 − 휀 Model

The 𝑘 − 휀 Model is the most widely used turbulence model. In

this model, the turbulent kinetic energy (k) and dissipation rate

(휀) are calculated from the transport equations which are known

as model equations of 𝑘 𝑎𝑛𝑑 휀. These are used to compute the

eddy viscosity as

𝑣𝑡 =𝑘2

휀

(3)

Where, 𝑣𝑡 is the turbulent eddy viscosity.

The 𝑘 − 𝜔 Model

The 𝑘 − 𝜔 model is also most commonly used turbulence

model. This model includes two transport equations for

turbulence kinematic energy (k) and the specific dissipation rate

(𝜔). This model has various commonly used variations

including Wilcox’s 𝑘 − 𝜔 model, Wilcox's modified 𝑘 − 𝜔

model and shear stress transport SST 𝑘 − 𝜔 model [6]. The SST

𝑘 − 𝜔 model is very good option for our case as it combines

the 𝑘 − 𝜔 model and 𝑘 − 휀 model in such a way that 𝑘 − 𝜔 is

used in the inner region of the boundary layer and switches to

the 𝑘 − 휀 model in the free shear flow. The SST turbulence

model is [4] [6]

Kinematic Eddy Viscosity

𝑣𝑇 =𝑎1𝑘

max(𝑎1𝜔, 𝑆𝐹2)

(4)

Turbulence Kinetic Energy

𝜕𝑘

𝜕𝑡+

𝑈𝐽𝜕𝑘

𝜕𝑥𝑗= 𝑃𝑘 − 𝛽∗𝑘𝜔 +

𝜕

𝜕𝑥𝑗[(𝑣 + 𝜎𝑘𝑣𝑇) (

𝜕𝑘

𝜕𝑥𝑗) ]

(5)

Specific Dissipation Rate

𝜕𝜔

𝜕𝑡+ 𝑈𝑗 (

𝜕𝜔

𝜕𝑥𝑗

) = 𝛼𝑆2 − 𝛽𝜔2 +𝜕

𝜕𝑥𝑗

[(𝑣 + 𝜎𝜔𝑣𝑇) (𝜕𝜔

𝜕𝑥𝑗

) ]

+ 2(1 − 𝐹1)𝜎𝜔2 (1

𝜔) (

𝜕𝑘

𝜕𝑥𝑖

) (𝜕𝜔

𝜕𝑥𝑖

)

(6)

The Reynolds Stress Turbulence model (RST)

In RSM, the transport equation is written as

𝜕(𝜌𝑢𝑖′𝑢𝑗

′ )

𝜕𝑡+

𝜕

𝜕𝑥𝑘

(𝜌𝑢𝑘𝑢𝑖′𝑢𝑗

′ ) = 𝐷𝑖𝑗 + 𝑃𝑖𝑗 + Π𝑖𝑗 + 휀𝑖𝑗 + 𝑆

(7)

Where the left two terms are local time derivative of stress and

convective transport term respectively. The right-hand side five

terms are:

The stress diffusion term:

𝐷𝑖𝑗 = −𝜕

𝜕𝑥𝑘

[𝜌𝑢𝑖′𝑢𝑗

′𝑢𝑘′ + (𝑝′𝑢𝑗

′ )𝛿𝑗𝑘 − 𝜇 (𝜕

𝜕𝑥𝑘

𝑢𝑖′𝑢𝑗

′ )]

(8)

The shear production term:

𝑃𝑖𝑗 = −𝜌 [𝑢𝑖′𝑢𝑘

′ 𝜕𝑢𝑗

𝜕𝑥𝑘+ 𝑢𝑗

′𝑢𝑘′ 𝜕𝑢𝑖

𝜕𝑥𝑘 ]

(9)

3

The pressure strain term:

Π𝑖𝑗 = 𝑝 (𝜕𝑢𝑖

′

𝜕𝑥𝑗

+

𝜕𝑢𝑗′

𝜕𝑥𝑖

)

(10)

The dissipation term:

휀𝑖𝑗 = −2𝜇𝜕𝑢𝑖

′

𝜕𝑥𝑘

𝜕𝑢𝑗′

𝜕𝑥𝑘

(11)

The source term: S (12)

Velocity distribution in cyclone separator

The flow velocities in cyclone separators are investigated under

there components which are axial velocity (𝑉𝑥), radial velocity

(𝑉𝑟) and tangential velocity (𝑉𝜃) [4]. The axial velocity and

tangential velocity have the biggest contributions for the flow

behavior of the cyclone separators. The axial velocity

determines that how much amount of the mass flow rates should

go from overflow and underflow. However, the radial velocity

has less contribution, it is important in such a way that it

provides solid particles with larger residence time so that they

can be separated. The tangential velocity component is the most

important component in a cyclonic flow and it has the highest

magnitude [4]. Because of the tangential velocity, the

suspended particles in the fluid flow are subjected to the

tangential flow and will be separated [4] [7].

Figure 2: Vortex flow in Cyclone Separator [4]

The figure 2 helps to understand the double vortex phenomenon

of the cyclonic flow. The figure shows the axial velocity

distribution in radial direction inside the cyclone separator. As

we see that between the cyclone axis and the wall, axial velocity

has the turning point. It describes that fluid motion is in

downward direction for the outer vortex and it is upward for the

inner vortex [4].

Figure:3 Axial velocity distribution in radial direction [4]

Figure 4- Tangential velocity distribution in a cyclone [4]

CASE SETUP

The case setup includes:

1. Preprocessing; to create the regions of interest, mesh

generation

2. Physical model setup

3. Post processing

Mesh generation

The flow domain of the cyclone separator was meshed using

surface remesher, polyhedral mesh, surface wrapper and prism

layer mesher. The surface remesher increased the overall

quality of the cyclone separator geometry. It has also optimized

the cyclone geometry for the volumetric mesh by triangulating

the surface. The polyhedral mesher was used to mesh entire

volume of the cyclone separator because it is very efficient to

handle the recirculating flows. There are some other advantages

of polyhedral meshes such as they provide fast convergence

with less number of iterations and consumes less memory.

Apart from this, polyhedral meshes provide balanced solution

for complex mesh generation. Additionally, the wall friction in

the cyclone separator causes the pressure drop. This was the

main reason to use prism layer model because it provides good

resolution of the turbulent boundary layer. Moreover, the

extruder was also used to extrude the inlet, overflow and

underflow in such a way that inlet and outlets remain at a

reasonable distance to reduce the possibilities of reverse flows.

After deciding the appropriate mesh, the base size was chosen

5 mm for all the cases. In this study, the cyclone separator A

was the main geometry to perform grid independence study,

optimization studies and multiphase modelling.

4

Figure 5: The mesh geometry of the Cyclone Separator A

Boundary Conditions:

In our case, we have one inlet and two outlets named overflow

and underflow. The overflow is used for the clean air and the

underflow is used for the dust particles. The inlet boundary

condition was velocity inlet. The flow split outlet boundary

conditions were used at outlets. The wall boundary condition

was used for the other boundaries. The mass flow rate was

chosen 0.045 kg/sec and air density 1.2 𝑘𝑔/𝑚3 was used for all

the cyclone separators. The turbulent intensity was 5% [2] and

turbulent length scale was kept at default level.

The following table provides the description of boundary

conditions with their respective values

TABLE 2: Description of Boundary Conditions

Boundary

conditions

Cyclone

A

Cyclone

B

Cyclone

C

Velocity inlet 5.0 m/s 6.07 m/s 5.0 m/s

Flow split outlet

(Overflow)

Split

ratio, 0.5

Split

ratio, 0.5

Split

ratio, 0.5

Flow split outlet

(Underflow)

Split

ratio, 0.5

Split

ratio, 0.5

Split

ratio, 0.5

Physical model setup

We have performed CFD simulations with two approaches. One

is steady state, three- dimensional and incompressible single-

phase modelling using air. For the single-phase modelling, we

used the SST- 𝑘 − 𝜔 turbulence model. The reason is that this

model provides the curvature correction to capture the

curvature dominated flows. The second approach is implicit

unsteady, three dimensional and incompressible multiphase

modelling. The Reynolds Stress Turbulence model with

Lagrangian Multiphase model was used to predict the flow

behavior of air and dust particles inside the cyclone separator.

Based on literature review, the volume fraction of the dust

particles was taken as 10% of the total volume flow rate and the

particle diameter was chosen 2 × 10−6𝑚. The time step for the

unsteady simulation was kept at default level of 𝑡 ≈ 0.001 𝑠 in

STAR-CCM+.

Furthermore, the segregated solver was used to solve the

velocity and pressure terms in uncoupled way. The segregated

solver uses less memory and it is most suitable for our case, as

the density was kept constant. The second order discretization

scheme was used to simulate the flow inside the cyclone

separator as the flow behaves like the swirling flow.

The grid independence study

The grid independence study was also performed on Cyclone

Separator A. The following table shows the computational

results for three grid types

TABLE 3: The details of grid independence study for Cyclone

Separator A

Number of cells Pressure drop

(Pa), Overflow

Pressure drop

(Pa), Underflow

N1= 422148 83 99.5

N2= 952130 91.5 110

N3=1797747 95 118

% Difference ≈ 3.8 ≈ 7.2

Note= The percentage difference is between N3 and N2.

As we see that the maximum difference between the results is

around 7% which is the pressure drop at underflow. This

difference is due to lack of experimental data for the CFD

simulations of single- phase flow inside the cyclone separator.

The chosen meshing size is N2=951189 for the post processing,

optimization study and multiphase flow modelling.

RESULTS AND DISCUSSIONS

The axial velocity, tangential velocity and radial velocity

profiles were analyzed for all the cyclone separators e.g.

Cyclone Separators A, B and C. The three plane sections with

line probes were used to plot different velocity profiles and

absolute total pressure profiles. These sections are named as

- Mid plane

- Overflow and

- Underflow

5

Convergence

The convergence has been achieved for all the cyclone separators including A, B and C. The strategies to achieve the convergence is

based on many literature reviews. To achieve the convergence, we considered two major aspects. Firstly, the residuals should below

1E-4. Secondly, the quantities such as velocity magnitude and pressure drop should be measured until they become constant. For steady

state single phase flow modelling of cyclone separators, A and B using 𝑆𝑆𝑇 𝑘 − 𝜔 𝑚𝑜𝑑𝑒𝑙, both first order and second order

discretization scheme was applied to achieve the convergence. For unsteady multiphase flow modelling using RST with LPM, second

order discretization scheme was used to achieve convergence.

The following figures show the convergence achieved with both turbulence models.

Figure 6: Convergence achieved for cyclone separator A with N2= 951189 cells using 𝑆𝑆𝑇 𝑘 − 𝜔 𝑚𝑜𝑑𝑒𝑙.

Figure 7: Convergence achieved for cyclone separator C with N2= 951189 cells using 𝑅𝑆𝑇 𝑤𝑖𝑡ℎ 𝐿𝑃𝑀

6

Axial Velocity

As seen in figures 11, 12 and 13, the variation in the axial

velocity profile is limited close to the wall for all cyclone

separators with changing the inlet dimensions. The cyclone

separators A and C has same inlet dimensions whereas cyclone

separator has different inlet dimensions. The axial velocity

profile for all cyclone separators is almost similar except the

inner region. It means that the change in inlet dimensions

affect the axial velocity of the inner region more than the outer

region.

Figure 8: Axial velocity profile for cyclone separator A

Figure 9: Axial velocity profile for cyclone separator B

Figure 10: Axial velocity profile for cyclone separator C

Figure 11: Axial velocity profiles vs radial position for

cyclone separator A at different cross sections

Figure 12: Axial velocity profiles vs radial position for

cyclone separator B at different cross sections

7

Figure 13: Axial velocity profiles vs radial position for

cyclone separator C at different cross sections

Tangential velocity

The tangential velocity is the most important component of the

fluid flow inside the cyclone separators. It results in centrifugal

force for particle separations. As seen in figures 17,18 and 19,

the tangential velocity distribution is almost similar in the

inner region at different sections for the same cyclone. The

tangential velocity changes in the outer region; this may be due

to reduction in the velocity magnitude in outer region.

Moreover, the change in inlet dimensions such as inlet width

and inlet height affects the tangential velocity. Increasing the

inlet height and width (Cyclone A) decreases the maximum

tangential velocity. In our case, cyclone separators A and C

has large inlet dimensions than cyclone B. The effect of inlet

dimensions helps to understand that decreasing the inlet

dimensions will help to increase the separation efficiency of

the cyclone separators. Moreover, it can also be concluded that

for a particular application of cyclone separator, an appropriate

ratio of inlet width to height should be defined. The inlet width

to inlet height ratio for cyclones A and C is 0.3 whereas it is

0.7 in case of cyclone B.

Figure 14: Tangential velocity profile for cyclone separator A

Figure 15: Tangential velocity profile for cyclone separator B

Figure 16: Tangential velocity profile for cyclone separator C

8

Figure 17: Tangential velocity profiles vs radial position for

cyclone separator A at different cross sections

Figure 18: Tangential velocity profiles vs radial position for

cyclone separator B at different cross sections

Figure 19: Tangential velocity profiles vs radial position for

cyclone separator C at different cross sections

Radial velocity

Radial velocity has less contribution for the flow field inside

the cyclone separator. But, it will provide the larger residence

time to the dust particle (Figure 29) so that they can separate

from the air. The figures 23, 24 and 25 show the radial velocity

profiles from cyclone A, B and C. According to the figures,

the positive radial velocity indicates the fluid flow far from the

central axis (near the wall or outer vortex) and the negative

radial energy indicates the fluid flow towards the center axis

of cyclone separator (inner vortex). Moreover, cyclones A and

C have same inlet dimensions the turbulence models were

different. The radial profiles for these cyclones seem very

much similar to each other.

Figure 20: Radial velocity profile for cyclone separator A

Figure 21: Radial velocity profile for cyclone separator B

9

Figure 22: Radial velocity profile for cyclone separator C

Figure 23: Radial velocity profiles vs radial position for

cyclone separator A at different cross sections

Figure 24: Radial velocity profiles vs radial position for

cyclone separator B at different cross sections

Figure 25: Radial velocity profiles vs radial position for

cyclone separator C at different cross sections

Absolute total pressure

The absolute total pressure profiles at almost similar for the

inner regions of the cyclone separators. There is slight change

in the profile near to the wall but that is negligible. The

availability of the experimental data would provide better

possibilities to analyze the absolute total pressure profiles.

Figure 26: Absolute total pressure profiles vs radial position

for cyclone separator A at different cross sections

Figure 27: Absolute total pressure profiles vs radial position

for cyclone separator B at different cross sections

10

Figure 28: Absolute total pressure profiles vs radial position

for cyclone separator C at different cross sections

Dust particle behavior inside the cyclone separator C

The multiphase modelling of air and dust particles was

performed using Reynolds Stress Turbulence model with

Lagrangian multiphase modelling. The flow behavior of dust

particles inside the cyclone separator has been examined using

particle residence time, velocity magnitude and position

magnitude. From figures 30 and 31, it is seen that the dust

particles and the air are having almost same velocity

magnitude profile along the cyclone axis.

Figure 29: Dust particles residence time

Figure 30: Dust particles velocity magnitude

Figure 31: Velocity magnitude of Air

Figure 32: Dust particles position magnitude

CONCLUSION

To conclude, the fluid flow inside the cyclone separator has

been analyzed using two turbulence modelling approaches,

steady state 𝑆𝑆𝑇 𝑘 − 𝜔 𝑚𝑜𝑑𝑒𝑙 and unsteady Reynolds Stress

Turbulent Model with Lagrangian Multiphase modelling. The

optimization studies were also performed to find out the

impact of inlet dimensions on flow behavior inside the cyclone

separator. After performing the CFD simulations and

discussing the results, the following conclusions have been

obtained:

1. The maximum tangential velocity in the cyclone

separator increases with decreasing the inlet

dimensions. This provides first hand estimation to

increase the efficiency of cyclone separator.

2. The change in inlet dimensions affects the axial

velocity in inner region than the outer region.

11

3. It has also been analyzed that in case of steady state

single phase modelling with air, 𝑆𝑆𝑇 𝑘 − 𝜔 model

provides very good convergence and predicts the

recirculating fluid flow inside the cyclone separator.

4. For multiphase modelling of air and dust particles

inside the cyclone separator, Reynolds Stress

Turbulence Model has been proven great to predict

the unsteady, swirling and three- dimensional

turbulent flow inside the cyclone separator as well

as Lagrangian multiphase model also predicted the

dust particles behavior in an expected manner.

5. The lack of experimental data for single-phase flow

modelling inside the cyclone separator

circumvented us for validation of our results.

REFERENCE

1. http://energyeducation.ca/encyclopedia/Cyclone_se

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cyclone inlet dimensions on the flow pattern and

performance

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Zhengchao Wang, Analysis of Flow Field in

Optimal Cyclone Separators with Hexagonal

Structure Using Mathematical Models and

Computational Fluid Dynamics Simulation

4. Erdem Kucukal, Experimental and CFD

investigations of the fluid flow inside a

hydrocyclone separator without an air core

5. The STAR-CCM+ documentation

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