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					 International Journal of Civil
                                 JOURNAL OF CIVIL ISSN
                                              and Technology (IJCIET),
INTERNATIONALEngineering May (2014), pp. 51-60 © IAEME 0976 – 6308 (Print),
 ISSN 0976 – 6316(Online), Volume 5, Issue 5,
                                                                       ENGINEERING
                      AND TECHNOLOGY (IJCIET)
ISSN 0976 – 6308 (Print)
ISSN 0976 – 6316(Online)                                                           IJCIET
Volume 5, Issue 5, May (2014), pp. 51-60
© IAEME: www.iaeme.com/ijciet.asp
Journal Impact Factor (2014): 7.9290 (Calculated by GISI)
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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

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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

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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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

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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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

            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:-

                    ρ˜                 ρ˜˜ =           ρ       –    ρ‰                                    … (3)

   Where ρ is the static pressure, – is the stress tensor (described below), and ρ‰ is the
gravitational body force. contains other source terms that may arise from resistances, sources, etc.
   The stress tensor is given by:

–       µ           ˜          ˜              ˜                                                           …(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 (                  ]+     [rµ (        ] + Fx

                                                                                                          …(5)

And

    (       )+             (            )+             (      )=-        +        [rµ (         ]+        [rµ (               ] -
                µ
2µ +                                         Fr
                                                                                                          …(6)

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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

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)
                                                               3
                     Density                             kg/m         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
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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

        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

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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

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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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




              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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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




                          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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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

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
      Engineering & Technology (IJCIET), Volume 3, Issue 1, pp. 45 - 61, ISSN Print:
      0976 – 6308, ISSN Online: 0976 – 6316.




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