INTERNATIONAL JOURNAL OF CIVIL ENGINEERING ......using commercial computational fluid dynamics (CFD)...

International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print),

ISSN 0976 – 6316(Online), Volume 5, Issue 5, May (2014), pp. 51-60 © IAEME

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SIMULATION MODEL TO PREDICT VELOCITY AND PRESSURE

DISTRIBUTION INSIDE THE HYDROCYLONE IN WATER TREATMENT

PLANT

Dr. AlaaHusaeenAl-Fatlawi

Head of Environmental Engineering Department, College of Engineering, University of Babylon,

Iraq

ABSTRACT

The objective of this research work is to predict the velocity and pressure distribution inside a

hydrocylone which used water as a liquid phase and inert/solid particles as a solid phase. Inside

diameter of this hydrocyclone is 85mm. The proportions of each dimension proposed by Bradley are

used in this work. In this study, turbulent and swirling flow within a hydrocyclone is simulated by

using commercial computational fluid dynamics (CFD) code 'FLUENT' v14.0, Gambit 2.4.6, Tecplot

360, CFD post computer software’s . The results clearly showed the contours and diagrams of

pressure and velocity inside the hydrocyclone. The pressure diagram indicates that pressure in center

of surface is less than the walls, while the velocity distribution is (7.173 m/s) which agreed with the

inlet theoretical velocity of (7.18 m/s).

Keywords: Hydrocyclone, Computational Fluid Dynamic, Fluent.

I. INTRODUCTION

One of the main purposes for which the hydrocyclone was created is to promote solid liquid

separation, particles separation, classification in different fields such as in environmental, mineral

and mining, power plants, and chemical processes. A general hydrocylone consist of conical section

connected to a cylindrical section. The hydrocylone is fitted with a tangential inlet and enclosed by

an end plate with an axially mounted overflow outlet. The concept of separation in hydrocylone

based on the principle of centrifugal force to separate, remove or to classify solids from bulk fluid,

the shape of particles, size and density have a direct effect on the separation efficiency.

Continuous researches and studies were carried out to increase the efficiency of

hydrocyclones, for that, it is very important to have a very good understanding of flow patterns, and

motion trajectories of particles inside the hydrocylone. In general, the swirling flow pattern inside the

INTERNATIONAL JOURNAL OF CIVIL ENGINEERING

AND TECHNOLOGY (IJCIET)

ISSN 0976 – 6308 (Print)

ISSN 0976 – 6316(Online)

Volume 5, Issue 5, May (2014), pp. 51-60

© IAEME: www.iaeme.com/ijciet.asp

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hydrocylone is the main flow feature of separation, in addition to several minor flow patterns

associated with the rotational flow and influence the trajectories of the particles. The tangential

velocity is a direct factor on which the centrifugal force depended on, so, its accurate determination

is crucial in prediction of the fractionation performance of hydrocyclones. In other words, the

operation conditions also affecting the hydrocylone performance not only the relationship between

the size of particle and hydrocylone centrifugation,Wen, (2003)

The purpose of this paper is to illustrate the value of computational fluid dynamic as a perfect

tool to investigate the flow pattern inside a hydrocylone, which is based on the design and operation

conditions. However, because of the complication of boundary layers and the separation which are

highly out of equilibrium, it has been very difficult to predict the flow inside a hydrocylone, but the

development of computational fluid dynamic has solved the challenge of strongly swirling. The CFD

technique is combined with the finite element algorithm and used to predict an initial design to be

able to understand the efficiencies of different hydrocyclone designs and modes of operation, which

then undergoes operational trials to confirm the effectiveness.

II. BACKGROUND AND PREVIOUS STUDIES

Dlamini, et al., (2005), studied a CFD simulation of a single phase hydrocyclone flow field.

In this study; the researchers investigated the hydrodynamics of a hydrocyclone which present a

complex internal flow structure as the numerical simulation of which remains a nontrivial task. They

reported on three-dimensional water-only computational fluid dynamics (CFD) hydrocyclone flow

field predictions and highlighted some of the issues concerned with the development of a CFD model

incorporating an air core. The potential for the application of CFD as a hydrocyclone design tool is

also discussed.

Shojaeefard, et al., (2006), have investigated the behavior of water flow and particles

trajectory inside a hydrocyclone by means of numerical and experimental methods and results have

been compared together. To have a numerical simulation, CFD software was used, andfor modeling

flow the RNG k–e model applied. Finally, the effect of particle size on hydrocyclone performance

has been studied. It was found that the grade efficiency and number of particle that exit from

underflow of the hydrocyclone is increased when bigger particles is used.A series of experiments has

been carried out in a laboratory with a hydrocyclone. Comparison shows that, there is a good

agreement between the CFD models and experimental result.

George and Tudor, (2007), studied a numerical study of liquid-solid separation process

inside the hydrocyclones with double cone sections. The major objective of this study was, using the

modern numerical techniques, to investigate particle transport processes within a hydrocyclone with

double cone sections, were the wastewater is depurated. This investigation consists of calculations of

the fluid flow inside the hydrocyclone, including particle trajectory, pressure losses and separation

efficiencies. The hydrocyclone has modeling with the proper geometrical relationship between the

cyclone diameter, inlet area, vortex finder, apex orifice, and sufficient length providing retention

time to properly separation particles. Obtained results of calculations were numerically verified as

well as compared with results published in the subject literature. The model predicted the velocity

particle and fractional recovery of solid particles requirements given the dimensions of the cyclone,

the physical properties of the fluid, and the volumetric flow rate.

Murthy and Udaya, (2012), studied parametric CFD studies on hydrocyclone, this research

article encompasses development of hydrocyclone simulation methodology through validation with

suitably designed experiments at a range of process conditions and further understanding on the

parametric design and operating conditions. The salient features of the methodology included

Eulerian primary phase flow field generation through steady state simulation using RSM turbulence

modeling, and evaluation of particle distribution behavior through discrete phase modeling using



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particle injection technique. The results are validated with water throughput, split and cyclone cut

size while classifying fly ash. The results have indicated a reasonable matching between the

simulated and the experimental values. The studies revealed that the cyclone cut size increases with

an increase in vortex finder diameter, a decrease in the spigot diameter, decrease in the inlet velocity

of the fluid, and decrease in the viscosity of the fluid.

Figure 1: Schematic diagram of hydrocylone,Murthy and Udaya, (2012)

III. OBJECTIVE OF STUDY

Proper hydrocyclone design is essential for achieving maximum performance and ensuring

the maximum and most reliable solids separation efficiency. However, there is still a lack of detailed

understanding of hydrocyclone flow behavior and separation mechanism that occur in hydrocyclone,

thus, more researches are needed in order to achieve these targets.

Up to date, the design of the solid liquid hydrocyclones has relied on empirical experience,

and more recently on CFD and numerical modeling, which has had some success owing to the

improvement of computing power. Still, CFD models require a large amount of computing power,

and simulations are time consuming and costly (Severino, 2007)

So, this work aims to use the latest computer programs such as AutoCAD 3D Mechanical,

Gambit 2.4.6, Ansys Fluent V.14, TecPlot 360 and CFD Post to predict the velocity and pressure

profile inside a hydrocylone.

IV. MODELING OF WATER FLOW IN HYDROCYCLONE

For a dilute fluid suspension, the incompressible Navier–Stokes equations supplemented by a

suitable turbulence model are appropriate for modeling the flow in a hydrocyclone. The most popular

turbulence model in use for engineering applications is the k–e model where the scalar variables k

and e represent the kinetic energy of turbulence and its dissipation rate, respectively. The standard k–

e model was used to represent the turbulence in the equipment. The model was used to predict the

water flow rates in the two outlet streams for different inlet velocities of water (Shojaeefard, et al.,

2006).



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For this model, both mass conversion equation and continuity conversion equation have been

solved:

a) Mass Conversation Equitation The continuity or mass conversion equation can be written as follows:

��

�� + ∇ (��) = Sm … (1)

This equation is the general form of the mass conversion equation; it is valid for the

incompressible and compressible flows as well. The (Sm) term represent the mass added to the

continuous phase from the dispersed second phase, i.e. solid particles added to the liquid phase.

For 2D axisymmetric geometries, the continuity equation is given by:-

��

�� +

�

�� (��) +

�

� (��) +

��

= Sm … (2)

Where � is the axial coordinate, is the radial coordinate, �� is the axial velocity, and � is the radial velocity

b) Momentum Conversation Equitation The following equations describe the transport of momentum in an inertial (non-accelerating)

reference frame:-

�

��ρv�� +∇. �ρv��v�� = −∇ρ+∇. �t�̿ + ρg� +F� … (3)

Where ρ is the static pressure, t ̿ is the stress tensor (described below), and ρg� is the gravitational body force. F� contains other source terms that may arise from resistances, sources, etc.

The stress tensor �̿ is given by:

t̿ = µ[�∇v�� +∇v��"� − #$∇. v��I] …(4)

Where µ is the molecular viscosity, I is the unit tensor, and the second term on the right-hand

side is the effect of volume dilation.

For 2D axisymmetric geometries, the axial and radial momentum conversion equations are

given by:

�

��(��') +

(

�

��(��) +

(

�

�(��) = -

��

�� +

(

�

�� [rµ (2 �*

�*−#

$�∇. ��] + (

�

� [rµ (

�*��+ ��

�*�] + Fx

…(5)

And

�

��(��) +

(

�

��(��) +

(

�

�(��) = -

��

�� +

(

�

�� [rµ (

��*−�*

��] + (

�

� [rµ (2 ��

��−#

$�∇. ��] -

2µ

�+

+ #

$

µ

�∇. �� + � *

+

+Fr

…(6)



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Where:

∇. �� = �*�*

+ ��

�*� +

�

…(7)

1- BOUNDARY CONDITIONS It was necessary to specify boundary conditions at the inlet, outlet and at the walls of

hydrocyclones. Inlet velocity was used as a boundary condition, which means that the value of the

velocity is specified. A uniform velocity profile was specified by introducing the inlet velocity and

this gave the required mass flow rate. To determine the influence of the flow rate on the velocity

field and to improve the predicted axial and tangential velocity profile, a pressure boundary was used

to model the outlet conditions. At the walls, the default of no slip condition was applied, i.e. the

velocity equals to zero at the wall. The normal logarithmic wall function was used to specify the flow

conditions in the cells adjacent to the wall. The fluid properties at the inlet used in this study are

specified in Table 1 below.

Table 1: Physical properties of water and inert particles

a. Water -liquid (fluid) Property Units Value(s)

Density kg/m3 998.2

Cp (Specific Heat) J/kg.k 4182

Thermal Conductivity w/m.k 0.6

Viscosity kg/m.s 0.001

Molecular Weight kg/kmol 18.015

b. Inert-particles Property Units Value(s)

Density kg/m3 1920

Cp (Specific Heat) J /kg.k 1680

Thermal Conductivity w/m.k 0.045

The hydrocyclone in this study has a 85 mm diameter of cylindrical section as shown in Figure 2.

Figure 2: Hydrocyclone geometry

All dimensions are in mm



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By using GAMBIT, pre-processing software, an unstructured triangular mesh with 1,260,881

elements has been used for the main body of hydrocyclone. The mesh is shown in Figures 3 and 4

uses unstructured triangular mesh for the main body of the hydrocyclone. In this model the tangential

inlet shown is meshed for simplicity using triangular elements.

Figure 3: Unstructured triangular mesh of hydrocyclone with 100% active elements

Figure 4: Grid elements in the (xy) axis

2- SOLUTION STEPS In addition to solving transport equations for the continuous phase, CFD allows to simulate a

discrete second phase in a Lagrangian frame of reference. This second phase consists of spherical

particles dispersed in the continuous phase. CFD computes the trajectories of these discrete phase

entities, as well as heat and mass transfer to/from them. The coupling between the phases and its

impact on both the discrete phase trajectories and the continuous phase flow can be included. We can

include a discrete phase in our CFD model by defining the initial position, velocity, size of individual

particles. These initial conditions, along with our inputs defining the physical properties of the

discrete phase, are used to initiate trajectory and mass transfer calculations.

For this model, a discrete phase model with a tolerance of 10-5

has been used. For the

operation conditions, we define gravitational acceleration in direction y (-9.86m/s2). After defining



57

materials, boundary conditions and operating conditions, the next step is to solve for CFD. A

SIMPLE scheme pressure velocity coupling has been used for the solution method. A (10,000)

iterations needed to get the peak tangential velocity in the simulation. Running of this model on a

dual core computer processor toke (60 hrs), with minimum accuracy of (1e-6

).

V. RESULTS AND DISCUSSION

Despite the simplicity of its construction of hydrocyclone, it displays a quite complex internal

behavior, including features as high preservation of vorticity, vortex breakdown and flow diagram.

For the stated geometry, boundary conditions, and operation conditions, the pressure distribution

inside the hydrocyloneis presented in Figures 5 and 6. These figure show a half cross section of the

effects of pressure on the separation and planner view for pressure distribution inside the

hydrocyclone. TheseFigures clearly indicate that pressure in center of surface is less than the walls.

While Figure 7 shows the path lines of particles colored by time inside the hydrocyclone.

Figure 5: Vertical section for pressure distribution inside the hydrocyclone

Figure 6: Planner view for pressure distribution inside the hydrocyclone



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Figure 7: Path lines of particles colored by time inside the hydrocyclone

It is notable that particles needs maximum (0.99 sec) to reach the underflow and (3.31 sec) to

rise to the overflow. This is due to the high velocity near the wall and slow velocity in the core of

hydrocyclone.

An important analysis comes from the velocity profiles. The liquid axial velocity component

is an indication of the magnitude of the two spirals depicted in Figure (8) and therefore determines

the volumetric distribution of the product between the overflow and underflow streams. A locus or

envelope of zero axial velocity is a significant feature of this velocity component and divides the

outer downward flowing and the inner upward flowing fluid layers. The axial velocities increase

with distance from the envelope, with the inner spiral having a considerably higher maximum

velocity.

Figure 8: Axial velocity vs radial position

The tangential velocity (Figure 9) increases traversing towards the core of the hydrocyclone,

before decreasing rapidly at the interface with the air core. The associated velocity gradients are

steepest in the region below the vortex finder. The tangential velocity profiles assume a compound

vortex structure, known as a Rankine vortex, which constitutes free and forced vortices near the

hydrocyclone wall and the central vertical axis, respectively. A parabolic peak, intermediate between

the two vortex regions, marks a gradual transition between the two distinct and uniquely defined

vortex structures.



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Figure 9: Tangential velocity vs radial position

Figure 10 shows the radial velocity inside the hydrocylone, the magnitude of radial velocity

is much smaller than that of the tangential or axial velocity which agree with what (Kelsal, 1952)

proposed. However, very little information is available about this velocity component. In practice,

the tangential and axial velocities are usually measured (Leeuwner and Eksteen, 2008).

Figure 10: Radial velocity vs radial position

The model also gives the contour of pressure as shown in Figure 11, The pressure is high in the

upper wall of the hydrocyclone, meanwhile inside the air-core is the lower pressure. Those results

are agreed with theory.

Figure 11: Pressure vs radial position



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VI. CONCLUSIONS

It can be concluded that the velocity, pressure and flow pattern within a hydrocyclone

chamber can be modeled using CFD. This will easily allow researchers studying how changes in the

shape of hydrocyclone will influence its operating performance. The ability of modern

supercomputers allows the approximation of three-dimensional flow pattern in hydrocyclones to be

investigated. That will give in a near future a better understanding of hydrocyclone performance.

REFERENCES

[1] Rama Murthya, UdayaBhaskarb, (2012), “Parametric CFD studies on hydrocyclone”, Research Development and Technology, Tata Steel Ltd, Jamshedpur, 831007, India &

ArcelorMittal Global R & D, 3001 E. Columbus Drive, East Chicago, IN 46312, USA.

[2] Severino, G. J., (2007), "Mechanistic Modeling of Solid-Liquid Separation in Small Diameter Hydrocyclones", The Graduate School, University of Tulsa, USA.

[3] Wen-Ching Yang, (2003), "Handbook of Fluidization and Fluid-Particle Systems", Published March 19th 2003 by CRC Press.

[4] Kelsal, D.F., 1952,"A study of the motion of solid particles in a hydraulic cyclone", Transactions of the Institution of Chemical Engineers. 30, 87– 108.

[5] Leeuwner M.J and Eksteen J.J., (2008), “Computational fluid dynamic modelling of two phase flow in a hydrocyclone”, Department of Process Engineering, University of

Stellenbosch.

[6] Dlamini M.F. , Powell M.S., and Meyer C.J., (2005),“ A CFD Simulation Of A Single Phase Hydrocyclone Flow Field”, Department of Chemical Engineering, UCT, Rondebosch,

Cape Town, South Africa.

[7] Shojaeefard M. H., Noorpoor A.R., Yarjiabadi H., Habibian M., (2006), “Particle Size Effects on Hydrocyclone Performance”, Automotive Engineering Department, Iran

University of Science and Technology. Islamic Republic of Iran.

[8] George Ipate, Tudor Căsăndroiu, (2007), “Numerical Study of Liquid-Solid Separation Process inside the Hydrocyclones with Double Cone Sections”, Department of Biotechnical

Systems, University “Politehnica”, Bucharest, Romania.

[9] A. Rizk, A. Aldeberky and N. Guirguis, (2014), “Comparison Between Natural Cross and Hybrid Ventilation for Hot Climate by using CFD”, International Journal of Civil

Engineering & Technology (IJCIET), Volume 5, Issue 2, pp. 71 - 80, ISSN Print:

0976 – 6308, ISSN Online: 0976 – 6316.

[10] R Radhakrishanan and A Praveen, (2012), “Sustainability Perceptions on Wastewater Treatment Operations in Urban Areas of Developing World”, International Journal of Civil

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0976 – 6308, ISSN Online: 0976 – 6316.

INTERNATIONAL JOURNAL OF CIVIL ENGINEERING ......using commercial computational fluid dynamics (CFD)...

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