Numerical analysis on effect of exit blade angle on cavitation in centrifu

International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 3, May - June (2013) © IAEME
359
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-ߝ 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-ߝ.
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
INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING
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ISSN 0976 – 6340 (Print)
ISSN 0976 – 6359 (Online)
Volume 4, Issue 3, May - June (2013), pp. 359-366
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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.

International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976
6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 3, May
2 EXPERIMENTATION
Experiment is carried out
having the backward bladed impeller.
and table 1 shows the specification of
Figure 2 Centrifugal pump test rig
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
Figure 3 Creo model of
Table 1 Centrifugal Test rig specification
Pump Total Head
Discharge
Speed
Motor
Measuring Tank
Sump tank
6359(Online) Volume 4, Issue 3, May - June (2013) © IAEME
361
Experiment is carried out at different operating conditions on the centrifugal pump
impeller. The figure 2 shows the test rig of the centrifugal pump
cification of the pump test rig.
Figure 2 Centrifugal pump test rig
SIMULATIONS
Impeller and the casing of the centrifugal pump respectively.
Creo model of impeller Figure 4 Creo model of casing
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
June (2013) © IAEME
on the centrifugal pump
shows the test rig of the centrifugal pump
Figure 4 Creo model of casing

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
WORKBENCH, which is shown in the figure 5 which
as the elements and the number of nodes and the elements generated are
respectively during the mesh. To predict the complex behavior of the flows
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
order to determine the formation of vapors inside the pump. At inlet a
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
After the completion of the solver part of the simulation, results like
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.
(a) 20 degree bladed impeller
(d) 60 degree bladed impeller
Figure 6 Water vapor contours at 1.5 lps
362
which is shown in the figure 5 which indicates that the tetrahedrons a
he 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
Figure 5 Meshing of the model
order to determine the formation of vapors inside the pump. At inlet and outlet pressure
order to match with the experimental.
AND DISCUSSIONS
After the completion of the solver part of the simulation, results like pressure contours
(b) 30 degree bladed impeller
(c) 40 degree bladed
impeller
(e) 70 degree bladed impeller
(f) 80 degree bladed
impeller
Figure 6 Water vapor contours at 1.5 lps
indicates that the tetrahedrons are used
77542 and 419333
inside the pump
nd outlet pressure
pressure contours
impeller
(f) 80 degree bladed
impeller

Figure 6 and 8 shows
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.
particular blade angle the volume occupied by the water vapor a
lps. As the density of the water is competitiv
are settled on the upper side of the casing and the impeller
Figure 7 and 9 shows the pressure variat
5 lps. On analyzing the pressure contours, higher pressure is observed at the discharge section
Figure 7 Pressure contours at 1.5 lps
Figure 8 Water vapor contour
363
phenomenon of water vapor formation at 1.5 lps
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
particular blade angle the volume occupied by the water vapor at 5 lps is more than the 1.5
s the density of the water is competitively higher than the water vapor, the
are settled on the upper side of the casing and the impeller.
shows the pressure variation across the centrifugal pump at 1.5 lps and
On analyzing the pressure contours, higher pressure is observed at the discharge section
(b) 30 degree bladed impeller (c) 40 degree bladed impeller
(e) 70 degree bladed impeller (f) 80 degree bladed impeller
Figure 7 Pressure contours at 1.5 lps
(b) 30 degree bladed
impeller
(e) 70 degree bladed
impeller
(f) 80 degree bladed impeller
Figure 8 Water vapor contours at 5 lps
ater vapor formation at 1.5 lps and 5 lps
that at higher exit blade angle, the volume occupied by the water
Also thing should be noted that for a
t 5 lps is more than the 1.5
ely higher than the water vapor, the water vapors
ion across the centrifugal pump at 1.5 lps and
On analyzing the pressure contours, higher pressure is observed at the discharge section
c) 40 degree bladed impeller
(f) 80 degree bladed impeller
(c) 40 degree bladed impeller
bladed impeller

on varying the blade angle from 20 degree to 80 degree in both the condition of 1.5 lps and 5
lps. Due to the increased discharge there is no pressure difference in 2
impeller from inlet section to discharge section. T
lps is less than the 1.5 lps for pump
pressure development inside the centrifugal pump as we
lead to the low pressure regions and also to the
in pressure from inlet section to outlet section for all
Since in the impeller section there is no change
recirculation and back flow will continue until the desired pressure is attainable. A low
pressure region is observed near the
(a) 20 degree bladed impeller (b) 30 degree bladed impeller
(d) 60 degree bladed impeller (g)70 degree bladed impeller
Figure 9 Pressure Contour
Figure 10 Result comparison
Figure 10 shows the good
results. Figure 11 shows the effect
Suction Head required). It indicates
0
0.5
1
1.5
2
2.5
0 2 4 6 8
DISCHARGE(lps)
EXPERIMENT NUMBER
EXPERIMENTATION
SIMULATION
364
gle from 20 degree to 80 degree in both the condition of 1.5 lps and 5
lps. Due to the increased discharge there is no pressure difference in 20 degree bladed
impeller from inlet section to discharge section. The pressure developed inside the
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 there is no change in pressure, it will lead to the phenomenon of
pressure region is observed near the tongue section of the casing due to formation of eddies.
(b) 30 degree bladed impeller (c) 40 degree bladed impeller
(g)70 degree bladed impeller (h) 80 degree bladed impeller
Figure 9 Pressure Contours at 5 lps
comparison
Figure 11NPSHr vs. Discharge
shows the good agreement of the simulation results with the
effect of discharge on to the value of NPSHr
ed). It indicates that as the value of the discharge increases the NPSH
10 12
EXPERIMENT NUMBER
EXPERIMENTATION
0
1
2
3
4
5
6
7
8
0 0.002 0.004
NPSHr
Q (DISCHARGE) (m3/sec)
gle from 20 degree to 80 degree in both the condition of 1.5 lps and 5
0 degree bladed
he pressure developed inside the pump at 5
. So there is a decrease in
increases the discharge. So that may
There is gradual rise
configuration of the blade angles.
in pressure, it will lead to the phenomenon of
section of the casing due to formation of eddies.
(c) 40 degree bladed impeller
(h) 80 degree bladed impeller
Figure 11NPSHr vs. Discharge
experimental
(Net Positive
he discharge increases the NPSHr
0.006

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
discharge conditions on to the cavitation
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
along with the performance. For all the configurations of the blade exit angles similar
behavior is observed.
Figure 12 Cavitation number vs. Discharge For Different blade
CONCLUSIONS
It can be concluded from the results that the use of impeller
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
obtained from the simulation. Since the cavitation phenomenon
will lead to the erosion of the impeller material, that will reduce
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
REFERENCES
[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.
[2] E.C. Bacharoudis, A.E. Filios, M.D. Mentzos And D.P. Margaris
Of A Centrifugal Pump Impeller By Varying The Outlet Blade Angle” In The Open
Mechanical Engineering Journal, 2008, 2, 75
[3] M.H.S.Fard And F.A.Bhoyaghchi
Outlet Angles In A Centrifugal Pump When Handling Viscous Fluid" In American
Journal Of Applied Science 2007.
-5
0
5
10
15
20
25
30
35
0 2
CAVITATIONNUMBER
365
l 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
cavitation number which shows that as the discharge increases
Also with increase in the cavitation number the head of the pump will decrease
e performance. For all the configurations of the blade exit angles similar
configuration
It can be concluded from the results that the use of impeller having low exit blade
phenomenon of the cavitation is identified for higher exit blade angles in the
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,
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.
Guang LI “Blade Exit Angle Effects on Performance of a Standard Industrial
Department Of Civil And Environmental Engineering Cvng
1001: Mechanics Of Fluids.
, A.E. Filios, M.D. Mentzos And D.P. Margaris- " Parametric Study
Mechanical Engineering Journal, 2008, 2, 75-83.
M.H.S.Fard And F.A.Bhoyaghchi –” Studies On The Influence Of The Various Blade
Journal Of Applied Science 2007.
4 6 8 10 12
DISCHARGE (LPS)
20 DEGREE
30 DEGREE
40 DEGREE
60 DEGREE
70 DEGREE
80 DEGREE
l lead to the cavitation phenomenon. So all the pump should be
effect of different
number which shows that as the discharge increases
Also with increase in the cavitation number the head of the pump will decrease
e performance. For all the configurations of the blade exit angles similar
having low exit blade
phenomenon of the cavitation is identified for higher exit blade angles in the contours
is undesirable for the pump it
the efficiency, discharge,
in a drastic manner. Hence, if we use the blade angle in a range of 20 degree to 30
Guang LI “Blade Exit Angle Effects on Performance of a Standard Industrial
Department Of Civil And Environmental Engineering Cvng
" Parametric Study
f The Various Blade
20 DEGREE
30 DEGREE
40 DEGREE
60 DEGREE
70 DEGREE
80 DEGREE

366
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Numerical analysis on effect of exit blade angle on cavitation in centrifu