International
Journal of Mechanical
Engineering
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ISSN 0976 – 6340(Print),
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0976 – 6359(Online), Volume 6, Issue 2, February (2015), pp. 01-09© IAEME
TECHNOLOGY (IJMET)
ISSN 0976 – 6340 (Print)
ISSN 0976 – 6359 (Online)
Volume 6, Issue 2, February (2015), pp. 01-09
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IJMET
©IAEME
COMPUTATIONAL ANALYSIS OF CENTRIFUGAL
COMPRESSOR WITH GROOVES ON CASING
P. Usha Sri*,
J. Deepthi Krishna**
*Professor, Department of Mechanical Engineering, University College of Engineering, Osmania
University, Hyderabad, Telangana – 500 007, India.
**Student, Department of Mechanical Engineering, University College of Engineering, Osmania
University, Hyderabad, Telangana – 500 007, India.
ABSTRACT
Computational results of flow field in a centrifugal impeller with grooves on casing are
presented in the paper. A low speed centrifugal compressor with 2% tip clearance is considered.
Analysis is carried out for three different cases, one without grooves on the casing, second with two
grooves on casing and third with three grooves on the casing. The three cases are studied at five
different flow coefficients φ=0.28, 0.34, 0.42 (design value), 0.48 and 0.52 using Ansys-CFX. Pressure
ratio improvement with two grooves on casing is observed. The leakage of flow over tip of the blade
through the grooves from pressure side to suction side of the blade is interacting with passage wake near
casing and reduction in passage wake region area is observed with two grooves on casing. Increase in
the velocity of the fluid in passage wake is also observed with two grooves on casing. Increase in
pressure ratio is observed for the casing with two grooves at all flow coefficients. For three grooves on
casing also, increase of velocity in passage wake is observed but reduction in pressure ratio is observed
due to more leakage flow. Reduction in pressure ratio for all flow coefficients is observed with three
grooves on casing.
Key Words: Centrifugal Compressor, Flow coefficient, Grooves on casing, passage wake.
1.
INTRODUCTION
In turbomachines, to desensitize tip clearance effects, squealer tips / partial shrouds, tip geometry
modifications, casing treatment etc. are suggested in the literature. P. Usha Sri and N. Sitaram observed
that the impeller with the chamfer on suction surface of the blade tip shows small improvement in
1
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN
0976 – 6359(Online), Volume 6, Issue 2, February (2015), pp. 01-09© IAEME
performance. With an increase in the chamfer dimension on suction surface of blade tip, performance
improvement is observed. Through experiments, N. Sitaram and S.M. Swamy have observed
performance improvement on centrifugal compressor with pressure side partial shrouds. S. Senthil and
N. Sitaram studied the performance of a centrifugal compressor by means of squealer tips. They
observed increase in energy coefficient and efficiency with squealer tips on pressure surface. S. Senthil
and N. Krishna Mohan found that sloping type squeeler tip has beneficial effects. Chi-Young Park et. al
have observed improvement of performance and surge margin with ring groove system on centrifugal
compressor. Fayez M. Wassef et. al have conducted experiments on centrifugal compressor and
observed improvement in limit of stability with addition of a ring and a groove in the casing in diffuser
region.
2. COMPUTATIONAL METHODOLOGY
The design details of the impeller which is used in the investigations are given below:
Inducer hub diameter, d1h
= 160 mm
Inducer tip diameter, d1t
= 300 mm
Impeller tip diameter, d2
= 500 mm
Blade height at the exit, b2
= 34.7 mm
o
No. of blades of impeller, Nb = 16
Blade angle at inducer hub, β1h= 53
o
o
Blade angle at inducer tip, β1t = 35
Blade angle at exit, β2
= 90
Thickness of the blade, t
= 3 mm
Rotor speed, N
= 2000 rpm
All angles are with respect to the tangential direction.
Centrifugal impeller with above specifications with 3 mm thickness throughout the blade, 2% tip
clearance is shown in Fig. 1. Assuming periodicity, single passage of centrifugal impeller is analysed. A
single passage of the impeller with inlet at 50 mm ahead of the impeller and outlet at a distance of 35
mm downstream of impeller is shown in Fig. 2. Casing is designed with a clearance of 0.7 mm
throughout the blade height. Total pressure is used for inlet boundary condition and mass flow rate at
outlet. Rotating frame of reference is given to the domain. ANSYS-CFX software is used for obtaining
the solution and standard k-ε turbulence model is used for the closure. The centrifugal compressor is
analysed at five different flow coefficients (0.28, 0.34, 0.42, 0.48 and 0.52), the design flow coefficient
being 0.42.
Fig. 1 Centrifugal compressor
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International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN
0976 – 6359(Online), Volume 6, Issue 2, February (2015), pp. 01-09© IAEME
Fig. 2 Computational domain of single passage
On casing of the impeller, circular grooves of width 3 mm and depth 4 mm are made. Three
cases of centrifugal compressor, one without grooves,
grooves, second with two grooves on casing at streamwise
locations of 0.515, 0.555 and third with three grooves on casing at streamwise locations of 0.515, 0.555,
0.595 are modeled as shown in figure 3 to 5. Grid with boundary conditions
conditions for three different cases is
shown in figures 6-8.
Fig. 3 Compressor without
grooves on casing
Fig. 6 Compressor without
grooves on casing
Fig. 4 Compressor with two
grooves on casing
Fig. 7 Compressor with two
grooves on casing
3
Fig. 5 Compressor with three
grooves on casing
Fig. 8 Compressor with three
grooves on casing
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN
0976 – 6359(Online), Volume 6, Issue 2, February (2015), pp. 01-09© IAEME
3. RESULTS AND DISCUSSIONS
A low speed centrifugal compressor of radial blades with grooves
grooves on casing is analysed. The
pressure contours, velocity contours on the casing of the compressor are plotted for flow coefficient of
0.34. Velocity vectors, total pressure contours and static pressure contours
urs are plotted on a meridional
plane at a streamwise location of 0.62. Pressure variation along streamwise direction, pressure ratio at
all flow coefficients and percentage deviation in velocity along streamwise direction with grooved
casing are presented.
3.1 Static Pressure Contours
ontours on Casing:
Pressure contours on casing for three cases, without grooves on casing, with two grooves on
casing and with three grooves on casing is shown in figures 9-11.
9 11. The contours show gradual pressure
pres
rise from inlet to outlet of the compressor due to dynamic action of the rotating impeller. Pressure
gradient above the blade is observed due to the high pressure on pressure side and low pressure on
suction side of the blade. For the casing with two grooves, no significant change in pressure is observed.
For the casing
ing with three grooves,
grooves, low pressures on both pressure and suction side of the blade is
observed due to more leakage of flow from the grooves.
Fig. 9 Pressure contours on
casing without grooves
Fig. 10 Pressure contours on
casing with two grooves
Fig. 11 Pressure contours on
casing with three grooves
3.2 Velocity Contours on Casing
asing
Velocity contours on casing
asing for tthree different casess are shown in figures 12
12-14. The contours
show gradual increase of velocity from inlet to outlet of the impeller. Fluid
luid flow with low velocity from
pressure side of the blade to suction side is observed through grooves.
Fig. 12 Velocity contours on
casing without grooves
Fig. 13 Velocity contours on
casing with two grooves
4
Fig. 14 Velocity contours on
casing with three grooves
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN
0976 – 6359(Online), Volume 6, Issue 2, February (2015), pp. 01-09© IAEME
3.3 Static pressure
ressure Contours at Meridional Plane
P
Pressure contours at a meridional plane just after the grooves for three cases is shown in figures
15-17.
17. The contours show high pressure on pressure side (PS) of the blade, low pressure on suction side
(SS) of the blade. With two grooves on casing, a slight pressure
pressure drop is observed on suction side. With
three grooves on casing, significant reduction in pressure on suction side is observed.
observed
PS SS
Fig. 15 Pressure contours on
meridional plane for without
grooves on casing
PS SS
Fig. 16 Pressure contours on
meridional plane for impeller
with two grooves on casing
PS SS
Fig. 17 Pressure contours on
meridional plane for impeller
with three grooves on casing
3.4 Total Pressure Contours at Meridional
M
Plane
Total pressure contours on meridional plane just after the grooves for three cases are shown in
figures 18-20. On suction side near casing, low total pressure area caused due to passage wake is
observed. With two grooves on casing, low total pressure area of passage wake is reduced. The leakage
flow from the grooves
ooves is interacting with passage wake. With three grooves on casing, the passage wake
area is further reduced, but total pressure in this area is much lower due to more leakage of flow from
three grooves.
PS SS
Fig. 18 Total pressure
contours on meridional plane
for without grooves on casing
PS SS
Fig. 19 Total pressure
contours on meridional plane
for impeller with two grooves
on casing
PS SS
Fig. 20 Total pressure
contours on impeller with
three grooves on casing
3.5 Velocity Contours at Meridional Plane
Velocity contours on meridional plane for three cases are shown in figure 21
21-23. The contours
show improved velocities on suction side with grooves. The low velocity passage wake area on suction
side of the blade is reducing with two grooves and three
three grooves on casing. However, the velocity in
passage wake region is much lower with three grooves on casing as more leakage flow is interacting
with the main flow.
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International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN
0976 – 6359(Online), Volume 6, Issue 2, February (2015), pp. 01-09© IAEME
PS SS
Fig. 21 Velocity contours on
meridional plane for without
grooves on casing
PS SS
Fig. 22 Velocity contours on
meridional plane for impeller
with two grooves on casing
PS SS
Fig. 23 Velocity contours on
meridional plane for impeller
with three grooves on casing
3.6 Velocity Vectors at Meridional
ional Plane
P
Velocity vectors on meridional
meridional plane for three cases are shown in figure
figures 24-26. Passage wake
with low velocity is observed on suction side near casing of the blade. With grooves on casing,
reduction in passage wake area is observed.
PS SS
Fig. 24 Velocity vectors on
meridional plane for without
grooves on casing
PS SS
Fig. 25 Velocity vectors on
meridional plane for impeller
with two grooves on casing
PS SS
Fig. 26 Velocity vectors on
meridional plane for impeller
with three grooves on casing
3.7 Static Pressure Contours
ontours at Span 0.7
Pressure contours in blade to blade view, at span 0.7 is shown in figures 27-29.
27
The contours
show gradual pressure rise from inlet to outlet of the compressor due to dynamic action of the rotating
impeller. With two grooves on casing, no pressure change
hange is observed. But with three groves on casing,
the pressure at outlet is reduced.
Fig. 27 Pressure contours for
without grooves on casing
Fig. 28 Pressure contours for
impeller with two grooves on
casing
6
Fig. 29 Pressure contours for
impeller
im
with three grooves
on casing
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN
0976 – 6359(Online), Volume 6, Issue 2, February (2015), pp. 01-09© IAEME
3.8 Velocity Contours at Span
pan 0.7
Velocity contours in blade to blade view, at span 0.7 is shown in figures 30-32.
30
The contours
show low velocity region on suction side of the blade. With grooves on casing, the llow velocity region
is reducing and also velocity improvement is observed.
Fig. 30 Velocity contours for
without grooves on casing
Fig. 31 Velocity contours for
impeller with two grooves on
casing
Fig. 32 Velocity contours for
impeller with three grooves
on casing
3.9 Static Pressure Distribution
Static pressure distribution along streamwise direction from inlet to outlet is shown in figure 33.
This plot elucidate that the pressure from inlet to the outlet of the compressor is increasing
increa
gradually
along the stream wise direction due to the dynamic head developed by the rotating impeller. A drop in
static pressure near streamwise direction of 0.2 is observed for all cases due to the acceleration of the
flow in to the eye of the impeller.
r. Static pressure reduction is observed with the grooves on casing near
the location of grooves. But after the grooves location, static pressure is almost equal to the casing
without grooves. With two grooves on casing, static pressure at outlet is more tthan the static pressure of
casing without grooves.
istribution
3.10 Total Pressure Distribution
Total pressure distribution along streamwise direction around the grooves is shown in figure 34.
Gradual increase of pressure along streamwise direction because of dynamic action of the impeller is
observed. Total pressure improvement with grooves is observed. With
ith grooves on casing, the fluid flow
through grooves from pressure side to suction side of the blade is interacting with passage wake on the
suction side of the blade. Though pressure change is not significant, substantial velocity improvement is
the cause for total pressure rise around the grooves location.
102400
102500
no
groove
102000
Total Pressure
Static Pressure
103000
101500
101000
0
0.5
1
1.5
no groove
2 grooves
3 grooves
102200
102000
101800
101600
0.2
Stream wise location
0.4
0.6
Stream wise location
Fig. 33 Static Pressure
re from inlet to outlet
Fig. 34 Total Pressure distribution
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International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN
0976 – 6359(Online), Volume 6, Issue 2, February (2015), pp. 01-09© IAEME
Percentage Deviation in Velocity
3.11 Percentage Deviation of Velocity with Grooves
The velocity from inlet to outlet of the compressor passage of grooved casing is compared with
the casing without grooves. Percentage deviation in velocity with two grooves is shown in figure 35.
Near the streamwise location where grooves are present, a significant velocity improvement of 35% is
observed.
40
35
30
25
20
15
10
5
0
-5
0
0.2
0.4
0.6
0.8
1
Sreamwise Location
Fig. 35 Percentage deviation in velocity from inlet to outlet
3.12 Pressure Ratio at Off Design Conditions
Outlet to inlet pressure ratio at different flow coefficients is shown in fig. 36. Increase in
pressure ratio with two grooves on casing is observed at all flow coefficients. But with three grooves on
casing, reduction in pressure ratio is observed at all flow coefficients because of more fluid leakage
through the three grooves.
Static Pressure Ratio
1.0154
1.0152
1.015
1.0148
1.0146
No Groove
1.0144
2 Grooves
1.0142
3 Grooves
1.014
0.25
0.3
0.35
0.4
0.45
0.5
Flow Coefficient
Fig. 36 Pressure ratio for different flow coefficients
8
0.55
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN
0976 – 6359(Online), Volume 6, Issue 2, February (2015), pp. 01-09© IAEME
4. CONCLUSIONS
A low speed centrifugal compressor with 2% tip clearance with two grooves on casing and three
grooves on casing are analysed at five different flow coefficients. The results are compared with the
casing without grooves. With three grooves on casing, though velocity at the meridional section around
the grooves is increased, the pressure at outlet is reduced because of more leakage of flow over the
blades from grooves. With two grooves on casing, due to the interaction of the leakage flow from
grooves with passage wake, the velocity and total pressure improvement is observed. Also with two
grooves on casing, static pressure rise at outlet is observed for all coefficients.
5. ACKNOWLEDGEMENTS
The authors acknowledges All India Council for Technical Education (AICTE) for the financial
assistance provided for the project under R&D scheme.
REFERENCES
1.
2.
3.
4.
5.
6.
7.
8.
9.
P. Usha Sri and N. Sitaram, Computational Investigation of Flow in a Centrifugal Impeller with
Chamfered Blade Tips: Effect of Tip Clearance, National Conference on CFD Applications in
Power &b Industry Sectors, organised by BHEL R&D, Hyderabad during Nov 17-18, 2006
S. Senthil and N. Sitaram, 2002, “Performance Improvement of a Centrifugal Compressor by
means of Squealer Tips”, Proc. of the 4th ICPF Beijing, China, 26-29.
S. Senthil and N. Krishna Mohan, Experimental Investigation on a centrifugal compressor by
means of rectangular, elliptical and sloping squeeler tips, Indian Journal of Science and
Technology, Vol.2, No. 7, July 2009, pp 30-34.
Chi-Young Park, Young-Seok Choi, Kyoung-Yong Lee and Joon-Yong Yoon, Numerical Study
on the Range Enhancement of a centrifugal compressor with a ring groove system, Springer,
Journal of Mechanical Science and Technology 26 (5)(2012) 1371-1378.
Fayez M. Wassef, Ahmed S. Hassan, Hany A. Mohamed and Mohamed A. Zaki, Stability and
Performance of a Low Speed Centrifugal Compressor with Modified Casing, Journal of
Engineering Sciences, JES, Assiut University, Vol. 32, No. 5, pp. 2025-2047, 2004
N. Sitaram and S.M. Swamy, Performance Improvement of a Centrifugal Compressor by Passive
Means, International Journal of Rotating Machinery, Volume 2012, Article ID 727259, 9 pages.
P. Usha Sri and J. Deepti Krishna, “Effect of Tip Clearance on A Centrifugal Compressor”
International Journal of Mechanical Engineering & Technology (IJMET), Volume 5, Issue 9,
2014, pp. 379 - 384, ISSN Print: 0976 – 6340, ISSN Online: 0976 – 6359.
Jyothi P.N, A. Shailesh Rao, M.C. Jagath, and K. Channakeshavalu, “Understanding The Melt
Flow Behaviour of Za Alloys Processed Through Centrifugal Casting” International Journal of
Mechanical Engineering & Technology (IJMET), Volume 4, Issue 1, 2013, pp. 163 - 172, ISSN
Print: 0976 – 6340, ISSN Online: 0976 – 6359.
Shalin Marathe and Rishi Saxena, “Numerical Analysis on Effect of Exit Blade Angle on
Cavitation In Centrifugal Pump” International Journal of Mechanical Engineering & Technology
(IJMET), Volume 4, Issue 3, 2013, pp. 359 - 366, ISSN Print: 0976 – 6340, ISSN Online: 0976 –
6359.
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