NUMERICAL ANALYSIS ON EFFECT OF EXIT BLADE ANGLE ON CAVITATION IN CENTRIFU

					INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING
  International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
  6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 3, May - June (2013) © IAEME
                         AND TECHNOLOGY (IJMET)

ISSN 0976 – 6340 (Print)
ISSN 0976 – 6359 (Online)                                                     IJMET
Volume 4, Issue 3, May - June (2013), pp. 359-366
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    NUMERICAL ANALYSIS ON EFFECT OF EXIT BLADE ANGLE ON
             CAVITATION IN CENTRIFUGAL PUMP

              Shalin Marathe                                       Rishi Saxena
             M.E. (CAD/CAM)                                     Assistant Professor
      Mechanical Engineering Department                  Mechanical Engineering Department
       Sardar Vallabhbhai Patel Institute                 Sardar Vallabhbhai Patel Institute
          of Technology, VASAD                                of Technology, VASAD



   ABSTRACT

           This paper presents the effect of outlet blade angle on cavitation in centrifugal pump.
   The experiment is performed on a centrifugal pump test rig consisting of backward bladed
   impeller at different operating conditions and characteristics of the pump are predicted.
   Modeling of the centrifugal pump along with the different configuration of the impeller
   having different exit blade angles is carried out using Creo Parametric. Numerical simulation
   is carried out using ANSYS CFX and standard k- turbulence model is implemented for the
   analysis purpose. Cavitation is clearly predicted in the form of water vapor formation inside
   the centrifugal pump from the simulation results. Analytical analysis is carried out in order to
   find out NPSHr of the pump and Cavitation number (σc) which indicates the cavitation
   phenomenon in the centrifugal pump. From the results it has been found that the pump having
   low value of the blade exit angle will have less chances of getting affected by the cavitation
   phenomenon.

   KEY WORDS: ANSYS CFX, Cavitation, Cavitation number, Centrifugal pump, NPSHr,
   Numerical Simulation, Turbulence Model k-

   1 INTRODUCTION

          In centrifugal pump, an increase in the fluid pressure from the pump inlet to its outlet
   occurs during operation. This pressure difference developed in the pump drives the fluid
   through the system. The centrifugal pump creates an increase in pressure by transferring

                                                 359
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 3, May - June (2013) © IAEME

mechanical energy from the motor to the fluid through the rotating impeller as shown in
figure 1. Centrifugal pump faces a problem of cavitation. In general, cavitation occurs when
the liquid pressure at a given location is reduced to the vapor pressure of the liquid.
Cavitation begins when the absolute pressure at the inlet falls below the vapor pressure of the
water. This phenomenon may occur at the inlet to a pump and on the impeller blades,
particularly if the pump is mounted above the level in the suction reservoir. Under this
condition, vapor bubbles form at the impeller inlet and when these bubbles are carried into a
zone of higher pressure, they collapse abruptly and hit the vanes of the impeller, near the tips
of the impeller vanes causing damage to the pump impeller, violet vibrations and noise,
reduce pump capacity, reduce pump efficiency.




                                  Figure 1 Centrifugal pump

         W.G.Li [1] stated that the blade discharge angle has a strong but equal influence on the
head, shaft power and efficiency of the centrifugal oil pump for various viscosities of liquids
pumped. The rapid reduction in the hydraulic and mechanical efficiencies is responsible for
the pump performance degradation with increasing viscosity of liquids. E.C.Bacharoudis et al
[2]
    found that as the outlet blade angle increases the performance curve becomes smoother and
flatter. But M.H.S.Fard et al [3] stated that pump performance goes down when the pump
handles high viscosity working fluids because high viscosity results in disc friction losses
over outside of the impeller. SHI Weidong et al[4] investigated that the oversize impeller
outlet width leads to poor pump performances and increasing shaft power. Cavitation also
affect the performance of the pump and it must be avoided. D.Somashekar et al [5] suggested
that in order to avoid the cavitation available NPSH of the system must be equal to or greater
than the NPSH required by the centrifugal pump and similar recommendations were given by
the M.K.Abbas [6]. A.Stuparu et al [7] performed numerical investigation of the multiphase
flow inside the storage pump which underlines the fact that the pumping head drops due to
the development of cavitation phenomena. A.Goto et al [8] found that at the high flow rate
cavitation bubbles appear at the leading edge on pressure side incipiently and the head drops
gradually. J.B.Jonker et al [9] suggested that cavitation inception for the forward swept
impellers occurs at half-span of the leading-edge, while it occurs close to the shroud for the
backward-swept impeller. Also It has been found that the backward bladed impeller gives the
highest efficiency to the centrifugal pump in compare to the forward and radial bladed
impeller. But the energy transfer is less for the backward bladed impeller in comparisons of
radial and the forward bladed impeller.

                                              360
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 3, May - June (2013) © IAEME

2 EXPERIMENTATION

       Experiment is carried out at different operating conditions on the centrifugal pump
having the backward bladed impeller. The figure 2 shows the test rig of the centrifugal pump
                         cification
and table 1 shows the specification of the pump test rig.




                           Figure 2 Centrifugal pump test rig


                       Table 1 Centrifugal Test rig specification
                    Pump Total Head                       12 m
                       Discharge                         1.5 lps
                         Speed                         2900 rpm
                         Motor                            1 HP
                     Measuring Tank           400 400 450 mm Height
                        Sump tank                 600 900 600 mm Height


               SIMULATIONS
3 MODELING AND SIMULATION

        Creo Parametric 1.0 version is used for geometric modeling of the impeller having
different blade angles and the casing. The figure 3 and figure 4 shows the Creo model of the
Impeller and the casing of the centrifugal pump respectively.




        Figure 3 Creo model of impeller           Figure 4 Creo model of casing


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International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 3, May - June (2013) © IAEME

        ANSYS CFX (14.0) is the simulation tool which is used for the numerical analysis of
the centrifugal pump. Meshing is carried out using the Auto Mesh feature of the ANSYS
                                                                                   are
WORKBENCH, which is shown in the figure 5 which indicates that the tetrahedrons a used
                      he
as the elements and the number of nodes and the elements generated are 77542 and 419333
respectively during the mesh. To predict the complex behavior of the flows inside the pump
standard k- model is adopted.




                                Figure 5 Meshing of the model

        One of the major advantages of the ANSYS is that the user can give the boundary
conditions close to the actual operating conditions. Analysis is carried out in a Steady state
condition taking 1 atmospheric as the reference pressure. Rotating velocity is provided to the
impeller along the Z direction. Two fluids namely as Water and Water vapor are selected in
                                                                          and
order to determine the formation of vapors inside the pump. At inlet and outlet pressure
conditions are provided and the discharge of the pump is determined from the simulation in
order to match with the experimental.

5 NUMERICAL RESULTS AND DISCUSSIONS

       After the completion of the solver part of the simulation, results like pressure contours
and the water vapor formation contours are generated as shown in the figure for the different
discharge conditions like 1.5, 5 and 10 lps, for all the configurations of the impellers.




                                                                      (c) 40 degree bladed
(a) 20 degree bladed impeller    (b) 30 degree bladed impeller
                                                                            impeller




(d) 60 degree bladed impeller                                         (f) 80 degree bladed
                                 (e) 70 degree bladed impeller
                                                                            impeller
                         Figure 6 Water vapor contours at 1.5 lps
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International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 3, May - June (2013) © IAEME

                                                      ater
        Figure 6 and 8 shows phenomenon of water vapor formation at 1.5 lps and 5 lps
respectively It indicates that at higher exit blade angle, the volume occupied by the water
vapor is more as compare to lower exit blade angle. Also thing should be noted that for a
                                                                  at
particular blade angle the volume occupied by the water vapor at 5 lps is more than the 1.5
      s                             competitively
lps. As the density of the water is competitively higher than the water vapor, the water vapors
                                                    impeller
are settled on the upper side of the casing and the impeller.




(a) 20 degree bladed impeller    (b) 30 degree bladed impeller     c)
                                                                  (c) 40 degree bladed impeller




(d) 60 degree bladed impeller    (e) 70 degree bladed impeller    (f) 80 degree bladed impeller

                           Figure 7 Pressure contours at 1.5 lps




 (a) 20 degree bladed impeller   (b) 30      degree     bladed (c) 40 degree bladed impeller
                                 impeller




(d) 60 degree bladed impeller        (e) 70 degree bladed
                                                                 (f) 80 degree bladed impeller
                                           impeller

                                               contours
                          Figure 8 Water vapor contour at 5 lps

                                           variation
        Figure 7 and 9 shows the pressure variation across the centrifugal pump at 1.5 lps and
5 lps. On analyzing the pressure contours, higher pressure is observed at the discharge section

                                             363
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 3, May - June (2013) © IAEME

                        gle
on varying the blade angle from 20 degree to 80 degree in both the condition of 1.5 lps and 5
                                                                               20
lps. Due to the increased discharge there is no pressure difference in 20 degree bladed
                                                   The
impeller from inlet section to discharge section. The pressure developed inside the pump at 5
                                                                     .
lps is less than the 1.5 lps for pump having 20 degree impeller. So there is a decrease in
pressure development inside the centrifugal pump as we increases the discharge. So that may
lead to the low pressure regions and also to the cavitation phenomenon. There is gradual rise
in pressure from inlet section to outlet section for all other configuration of the blade angles.
                          ion
Since in the impeller section there is no change in pressure, it will lead to the phenomenon of
recirculation and back flow will continue until the desired pressure is attainable. A low
pressure region is observed near the tongue section of the casing due to formation of eddies.




(a) 20 degree bladed impeller                              (b) 30 degree bladed impeller              (c) 40 degree bladed impeller




(d) 60 degree bladed impeller                                (g)70 degree bladed impeller             (h) 80 degree bladed impeller

                                                                     Contours
                                                   Figure 9 Pressure Contour at 5 lps


             2.5                                                                          8
                                                EXPERIMENTATION
                    2                           SIMULATION
                                                                                          7
  DISCHARGE (lps)




                                                                                          6
             1.5                                                                          5
                                                                                  NPSHr




                                                                                          4
                    1
                                                                                          3
             0.5                                                                          2
                                                                                          1
                    0                                                                     0
                        0       2     4     6          8          10   12                     0       0.002             0.004   0.006
                                    EXPERIMENT NUMBER
                                                                                                       Q (DISCHARGE) (m3/sec)



                                                                                              Figure 11NPSHr vs. Discharge
                            Figure 10 Result comparison

        Figure 10 shows the good agreement of the simulation results with the experimental
results. Figure 11 shows the effect of discharge on to the value of NPSHr (Net Positive
                    ed).                                   he
Suction Head required). It indicates that as the value of the discharge increases the NPSHr

                                                                            364
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 3, May - June (2013) © IAEME

                               l
will also increase and that will lead to the cavitation phenomenon. So all the pump should be
operated with in its range of design flow rate. Figure 12 shows the effect of different
discharge conditions on to the cavitation number which shows that as the discharge increases
the cavitation number increases and that is nothing but the indication of cavitation
phenomenon. Also with increase in the cavitation number the head of the pump will decrease
               e
along with the performance. For all the configurations of the blade exit angles similar
behavior is observed.



                                 35
                                                                                         20 DEGREE
                                 30
                                                                                         30 DEGREE
                                 25
           CAVITATION NUMBER




                                 20                                                      40 DEGREE


                                 15                                                      60 DEGREE

                                 10
                                                                                         70 DEGREE
                                  5
                                                                                         80 DEGREE
                                  0
                                       0    2       4          6          8   10    12
                                  -5
                                                        DISCHARGE (LPS)

                               Figure 12 Cavitation number vs. Discharge For Different blade
                                                      configuration


CONCLUSIONS

        It can be concluded from the results that the use of impeller having low exit blade
angle is much efficient than the impeller having higher exit blade angle because the
phenomenon of the cavitation is identified for higher exit blade angles in the contours
obtained from the simulation. Since the cavitation phenomenon is undesirable for the pump it
will lead to the erosion of the impeller material, that will reduce the efficiency, discharge,
head in a drastic manner. Hence, if we use the blade angle in a range of 20 degree to 30
degree it provides efficient conditions for operating the centrifugal pump.

REFERENCES

          Guang
 [1] Wen-Guang LI “Blade Exit Angle Effects on Performance of a Standard Industrial
     Centrifugal Oil Pump”- Department Of Civil And Environmental Engineering Cvng
     1001: Mechanics Of Fluids.
                      ,                                   Margaris
 [2] E.C. Bacharoudis, A.E. Filios, M.D. Mentzos And D.P. Margaris- " Parametric Study
     Of A Centrifugal Pump Impeller By Varying The Outlet Blade Angle” In The Open
                                              75
     Mechanical Engineering Journal, 2008, 2, 75-83.
                                                                   f
 [3] M.H.S.Fard And F.A.Bhoyaghchi –” Studies On The Influence Of The Various Blade
     Outlet Angles In A Centrifugal Pump When Handling Viscous Fluid" In American
     Journal Of Applied Science 2007.

                                                             365
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 3, May - June (2013) © IAEME

 [4] SHI Weidong, ZHOU Ling*, LU Weigang, PEI Bing, and LANG Tao-" Numerical
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 [5] Mr. D. Somashekar1, Dr. H. R. Purushothama –”Numerical Simulation Of Cavitation
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