Effect of tip clearance on a centrifugal compressor

1. INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online), Volume 5, Issue 9, September (2014), pp. 379-384 © IAEME
AND TECHNOLOGY (IJMET)
ISSN 0976 – 6340 (Print)
ISSN 0976 – 6359 (Online)
Volume 5, Issue 9, September (2014), pp. 379-384
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IJMET
© I A E M E
EFFECT OF TIP CLEARANCE ON A CENTRIFUGAL COMPRESSOR
P. Usha Sri*, J. Deepti 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.
379
ABSTRACT
Computational analysis of low speed centrifugal compressor is carried out with finite volume
method using ANSYS-CFX software. Centrifugal compressor impeller with three values of
clearances i.e., 0%, 2% and 5% of blade height at trailing edge are examined at five flow coefficients
f=0.28, 0.34, 0.42 (design value), 0.48 and 0.52. The effect of tip clearance on static pressure from
inlet to outlet of the compressor is analyzed. The drop in static pressure with increase in tip clearance
is found to be high at the tip of the blade due to high pressure fluid leakage at the tip of the blade.
Performance reduction with tip clearance is observed. Total pressure and velocity at outlet are
analysed for five flow coefficients.
Keywords: Centrifugal Compressor, Flow Coefficient, Tip Clearance.
1. INTRODUCTION
Tip clearance in centrifugal compressor causes the leakage of high pressure fluid from
pressure surface to suction surface of the impeller blade, making the flow field highly complex and
affecting the performance. The required tip clearance can be obtained by shifting the casing in radial
or axial or combined radial and axial directions. Hayami (1997) has found from his experiments that
axial movement of the casing has better efficiency over the movement of casing in radial and axial
directions. Radial movement of casing increases clearance at inducer, which reduces the operating
range. The tip clearance studies are conducted to understand the flow behavior in order to minimise
the effect of tip clearance. Swamy and Pandurangadu(2013), Pampreen (1973), Mashimo et al.
(1979), Sitaram and Pandey (1990) have conducted experimental studies and suggested that by
reducing the tip clearance gap size, the tip clearance effect can be minimised. The effect of tip
leakage on flow behavior in rotating impeller passage was computationally carried out by Usha Sri
and Sitaram (2004), Hark-Jin Eum et al. (2004), Hathaway et al. (1993).

2. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online), Volume 5, Issue 9, September (2014), pp. 379-384 © IAEME
The design details of the impeller which is used in the present investigation 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
No. of blades of impeller, Nb = 16 Blade angle at inducer hub, b1h = 53
Fig. 2: Computational domain of single passage
380
2. COMPUTATIONAL METHODOLOGY
Fig. 1: Centrifugal compressor
o
Blade angle at inducer tip, b1t = 35
o Blade angle at exit, b2 = 90
o
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 3mm thickness through out the blade is
shown in Fig. 1. A single passage of the impeller with inlet at 50mm ahead of the impeller and outlet
at a distance of 35mm downstream of impeller is shown in Fig. 2. Casing is designed with a
clearance of 0.7mm throughout the blade height. Three tip clearances of 0%, 2% and 5% of trailing
edge blade height are obtained by moving the casing axially. The 0% clearance model, which is not
practicable, is generated for reference. 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-e turbulence model is used for the closure. The centrifugal
compressors of three clearances (0%, 2% and 5%) were analysed at five different flow coefficients
(0.28, 0.34, 0.42, 0.48 and 0.52). The design flow coefficient is 0.42.
3. RESULTS AND DISCUSSIONS
blade Periodic
Boundaries
outlet
inlet
Centrifugal compressor with three tip clearances 0%, 2% and 5% at five flow coefficients
0.28, 0.34, 0.42, 0.48 and 0.52 were analysed. Static pressure variation from inlet to outlet of the
domain and static pressure variation with flow coefficient for three tip clearances were plotted. Total
pressure graphs, blade loading charts, pressure contours and velocity vectors are analysed.
3.1 Static pressure distribution from inlet to outlet: Static pressure variation along meridional
distance for three tip clearances at five flow coefficients is shown in Fig. 3. Static pressure is
constant before the impeller passage. Static pressure drop at impeller leading edge is observed which
causes the fluid to accelerate in to the compressor. Pressure is increasing steadily in the impeller
passage for all tip clearances due to the energy transfer taking place the impeller. The drop in static

3. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online), Volume 5, Issue 9, September (2014), pp. 379-384 © IAEME
pressure with increase in tip clearance is found to be high at the tip of the blade due to high pressure
fluid leakage at the tip of the blade.
3.2 Static pressure variation with flow coefficient: Static pressure at outlet versus flow coefficient
graph for three different tip clearances are shown in fig. 4. The static pressure maximum is found at
flow coefficient of 0.34 while reduced pressure rise at flow coefficient 0.28 is observed. At flow
coefficient 0.28, for 0% clearance pressure rise is very less as separation of flow is observed. With
increase in tip clearance, the static pressure is also reducing for all flow coefficients. Static pressure
is reducing with increase in flow coefficient.
Fig. 3: Static Pressure from inlet to outlet Fig. 4: Static Pressure Vs Flow coefficient
3.3 Total pressure variation with flow coefficient: Total pressure at outlet versus flow coefficient
graph for three different tip clearances is shown in fig. 5. Total pressure rise is reduced at flow
coefficient 0.28 is observed. At flow coefficient 0.28, for 0% clearance, total pressure rise is very
less as separation of flow is observed. With increase in tip clearance, the total pressure is reducing
for all flow coefficients. Total pressure is reducing with increase in flow coefficient after design flow
coefficient.
3.4 Blade loading chart: Blade loading charts at design flow coefficient 0.42 for three tip clearances
were shown in fig. 6 to 8. Low static pressure on suction and high pressure on pressure side of the
blade is observed. With increase in tip clearance, static pressure on both pressure side and suction
side are reducing.
Fig. 5: Static Pressure Vs Flow coefficient Fig. 6: Blade loading for 0% clearance at
381
f=0.42

4. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online), Volume 5, Issue 9, September (2014), pp. 379-384 © IAEME
3.5 Static pressure contours at mid span: Static pressure contours at mid span of the blade for
f=0.42 are shown in fig. 9 to 11. Gradual increase of static pressure from inlet to outlet is clearly
observed at all tip clearances. High pressure on pressure side of the blade and low pressure on
suction side of the blade are observed at all tip clearances. With increase in tip clearance, reduction
in pressure on both pressure side and suction side is found.
382
Fig. 7: Blade loading for 2% clearance
at f=0.42
Fig. 8: Blade loading for 5% clearance
at f=0.42
Fig. 9: pressure contours for 0% clearance with f=0.42 at mid span
Fig. 10: pressure contours for 2% clearance with f=0.42 at mid span

5. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online), Volume 5, Issue 9, September (2014), pp. 379-384 © IAEME
Fig. 11: pressure contours for 5% clearance with f=0.42 at mid span
3.6 Velocity Vectors at Outlet: Velocity vectors at the exit of the impeller is shown in fig.12 to14.
The velocities are high on suction surface than pressure surface because of blade curvature. With
increase in clearance, the velocity on both pressure side and suction side is decreasing. For 2% and
5% clearance, high velocity of the fluid above the blade from pressure side to suction side through
tip clearance is clearly seen.
Fig. 12: Velocity vectors at exit of the blade
Fig. 14: Velocity vectors at exit of the blade for 5% clearance with f=0.42
383
for 0% clearance with f=0.42
4. CONCLUSIONS
Fig. 13: Velocity vectors at exit of the blade
for 2% clearance with f=0.42
Tip clearance effects in a low speed centrifugal compressor impeller with three different
values of clearances i.e., 0%, 2% and 5% are examined at five flow coefficients 0.28, 0.34, 0.42, 0.48
and 0.52. The static pressure distribution from inlet to outlet of the compressor and total pressure,
static pressure graphs at the outlet of the compressor show that with increase in tip clearance the

6. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online), Volume 5, Issue 9, September (2014), pp. 379-384 © IAEME
losses increase. The drop in static pressure with increase in tip clearance is found to be high at the tip
of the blade due to high pressure fluid leakage at the tip of the blade. With increase in clearance, the
velocity on both pressure side and suction side is decreasing. For 2% and 5% clearance, high velocity
of the fluid above the blade from pressure side to suction side through tip clearance is clearly seen.
384
5. ACKNOWLEDGEMENTS
The authors acknowledges All India Council for Technical Education (AICTE) for the
financial assistance provided for the project under RD scheme.
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