Experimental investigation on a centrifugal compressor by means of squealer tips.
Senthil, S. ; Mohan, N. Krishna
Introduction
The flow field in an impeller passage of a centrifugal compressor is highly complex, three-dimensional and unsteady. A designer in
designing a centrifugal compressor should have complete understanding
with the influencing parameters, there by they can design it for better
performance and efficiency. The performance of a compressor is
inherently deteriorated by different loses that occur at different
sections of the compressor stage. They are as follows: shock loss at an
inducer inlet, wall friction loss within impeller channels, tip leakage
loss, secondary flow losses, mixing loss at diffuser inlet, wall
friction loss within a diffuser, sudden expansion loss at a scroll
inlet, wall friction loss within a scroll.
Pampreen (1973) concluded that clearance effects have pronounced
influence on the performance of centrifugal and axial compressors
compared to Reynolds number effects. Ishida and Senoo(1981)[1] used two
entirely different types of centrifugal blowers one with a radial blade
impeller and the other with a backward blade impeller, measured the
pressure distribution along the shroud at five flow coefficient and
seven tip clearances. Senoo and Ishida (1987) observed the deterioration
of compressor performance due to tip clearance of centrifugal impeller.
They modified their theory on the tip clearance loss of centrifugal
impeller to include the variation of slip co-efficient of the impeller
due to the tip clearance, by deriving a rational relationship between
two empirical parameters in the theory. They have compared experimental
data in the literature with prediction, to select corresponding flow
rates of a compressor with different values of tip clearance loss.
Heyes et al (1992)[4] observed the effect of blade tip geometry on
the tip leakage flow in axial turbine cascades. The investigation
includes an examination of the performance of the cascades with a
variety of tip geometries. The effects of using plain tips, suction side
squealers, and pressure side squealers are reported. Ameriet al
(1998)[5] observed the effect of squealer tip on rotor heat transfer and
efficiency. Experimental investigations are performed to measure the
detailed heat transfer coefficient and static pressure distribution on
the squealer tip of a gas turbine blade in the five-blade stationary
linear cascade. Results show that the heat transfer coefficient on the
cavity surface and rim increases with an increase in tip clearance. The
heat transfer coefficient on the rim is higher than the cavity surface.
The cavity surface has a higher heat transfer coefficient near the
leading edge region than the trailing edge region. The squealer tip
blade provides a lower overall heat transfer coefficient when compared
to the flat tip blade. Azad et al. (2000)[6] compared the heat transfer
and flow on the squealer tip of a gas turbine blade with that on a flat
tip. Swamy (2001)[7] conducted tests with partial shrouds, with out
partial shrouds and turbulence generator on centrifugal compressor with
different values of tip clearance. The configuration with partial
shrouds shows higher efficiency and energy coefficients compared to
other configuration. Krain Hartmut (2002)[8] observed the effect of
unsteady diffuser flow in a transonic centrifugal compressor.
Comparative study of unsteady flows in a transonic centrifugal
compressor with vaneless and vaned diffusers a was observed by Curi
Michael (2005)[9].
Experimental Facility and Instrumentation
The present experimental investigations are carried on a low speed
centrifugal compressor set up available in the Thermal Turbomachines
Laboratory, Department of Mechanical Engineering, Indian Institute of
Technology Madras and Thermal Engineering Laboratory, Department of
Mechanical Engineering, Mailam Engineering College, Mailam. A schematic
layout of the experimental set up is shown in Figure 1. The experimental
set up consists of essentially a centrifugal impeller driven by a 5kW DC
motor with a rated speed of 2000 rpm. The DC motor is directly coupled
to the shaft carrying the impeller. The main components of the
compressor are suction duct, impeller, vaneless diffuser formed by the
front and rear walls of casing and volute casing of circular cross
section and delivery duct with a throttle outlet and nozzle at the
inlet.
[FIGURE 1 OMITTED]
The major geometrical details of the impeller are given below.
Pressure rise : 300 mm of WG
Speed of rotation : 2000 rpm
Inducer tip dia : 300 mm
Impeller tip dia : 500mm
Balde angle at Inducer tip: 35[degrees]
Volume flow rate : 1.2[m.sup.3]/s
No. of blades of the impeller : 16
Inducer hub dia : 160 mm
Blade height at the exit :34.74 mm
Blade angle at inducer hub: 53[degrees]
Blade angle at exit a) at hub :75[degrees]b) at mean 90[degrees]c)
at tip : 105[degrees]
All the angles are measured with respected to tangential direction.
Spacers
Spacers are used to maintain the tip clearance accurately. Front
cover is moved axially to vary the tip clearance. There by the distance
between the front and rear cover walls would change. Three spacers are
used which are 90o apart at a radius of 270 mm. The spacers are away
from the probe, so that they would not interfere with the flow.
Squealers
Squealers are used to improve the performance of the compressor.
The Squealers are made of mild steel of 3 mm thickness. A squealer tip
with 6 mm recess is considered here. Squealers are welded on suction
side of the blade, pressure side of the blade and both suction and
pressure sides of the blade. Meridional view of impeller with the
squealer tips are shown in Figure 2.
[FIGURE 2 OMITTED]
Experimental Programme and Procedure
General
The Objective of the present investigations is to explore the
possibility of using squealer tips to improve the performance of
centrifugal compressor. The flow field in the tip region of centrifugal
compressor has been studied for decades. The flow is a three dimensional
phenomenon comprised of the complex interactions between the tip leakage
vortex, the turbulent end wall boundary layers, and often shock waves
distorted by rotational effects.
Experimental Programme
The present experiment programme consists of testing the
centrifugal compressors with the following tip configurations at three
flow coefficients
i. Suction side squealer
ii. Pressure side squealer
iii. Squealers on both suction and pressure sides
iv. Solid tip by filling up the space between squealers
Performance Characteristics
The performance study of centrifugal compressor is carried out at a
constant speed of 2000 rpm by varying throttle positions from high flow
rate (near choking ) to low rate (in surge region). Under steady state
conditions the inlet static pressure across the nozzle, the delivery
static pressure across the delivery duct, the input field and armature voltages and input field and armature currents are noted down. The speed
of the compressor is mainted at 2000 rpm indicated by a non-contacting
type tachomter. From these measurements, energy coefficient and
efficiency of the centrifugal compressor are computed and plotted
against the flow coefficient. The performance tests are conducted for
four configurations.
Shroud Wall static pressure measurements
Static pressure taps were provided on the shroud from the inducer
leading edge to the impeller exit. The shroud has its inner contour
shape in such a way to match the impeller blade tip from the inlet to
the exit of the impeller. The inlet flow gets into the impeller blade
passage through the annulus passage between the hub of the impeller and
stationary shroud. Hence the static pressure measurements on the shroud
give an indications of the static pressure developed in the shroud end
of the impeller. There are nine static pressure tappings on the shroud.
They are connected through tubes to the twenty channel scanning box.
Static pressure on the shroud is measured at the above mentioned flow
coefficients and tip clearances.
Exit Flow Measurements
A pre-calibrated five-hole probe is used to measure the flow
conditions at exit of the impeller. At the exit, the probe is traversed
axially from hub to shroud wall, by means of a traverse mechanism and
measurements were taken at 12 axial locations for three flow
coefficients ([phi] = 0.34, 0.28 and 0.18). The twelve axial locations
spans the axial distance along blade height, at intervals of 2mm near
the walls and 4mm at the center of the walls. For each axial station the
five hole probe is rotated in different angular positions. The
experiments are carried out for three flow coefficients namely, above
the design flow rate [[phi].sub.h] = 0.34 near design flow rate,
[[phi].sub.d] = 0.28 and below design flow rate [[phi].sub.l] = 0.18. At
each station, pressure readings of five-hole probe and wall static
pressure are recorded from digital micromanometer connected via the 20
channel scanning box.
Non Dimensionalization of flow parameters
The different parameters of performance used in the analysis of
experimental results are defined and determined in the following
sections. The following formulae are used to calculate the flow
parameters
Flow Coefficients, [phi]
The actual volume flow through the compressor is expressed as a
nondimensional parameter flow coefficient [phi], defined as the ratio of
radial velocity to the peripheral velocity at the impeller exit.
[phi] = V/[[pi]d.sub.2][b.sub.2][U.sub.2] =
[[pi]d.sub.2][b.sub.2][cr.sub.2/[[pi]d.sub.2][b.sub.2][U.sub.2] =
[cr.sub.2]/[U.sub.2]
Where V-volume flow rate
[d.sub.2]--impeller tip diameter
[b.sub.2]--width at the impeller exit
[Cr.sub.2]--radial velocity at the impeller exit
[U.sub.2]--peripheral velocity at the impeller exit
The actual volume flow rate is obtained from the calibration curve of the inlet nozzle corresponding to the static pressure differential at
the nozzle section. In the present investigation, the flow coefficients
selected are: [[phi].sub.d] = 0.28 (near design) [[phi].sub.l] = 0.18
(below design) and [[phi].sub.h] = 0.34 (above design)
Energy Coefficient, [psi]
The specific work W for the centrifugal compressor is expressed
nondimensionally as the energy coefficient, [psi]
[psi] = 2W/[U.sup.2.sub.2]
where the specific work W is obtained as the difference of specific
total energy between inlet and exit of the impeller. For incompressible
flow, the specific work is given by
W = [P.sub.D] - [P.sub.s]/[rho] + [C.sup.2.sub.D] -
[C.sup.2.sub.s]/2 + g[DELTA]z
where [P.sub.d] and [P.sub.s] are the average static pressure
measured at the exit and inlet sections and [C.sub.d] and [C.sub.s] are
air velocities at the exit and inlet sections respectively. [DELTA]Z is
the geometric level difference between delivery and suction flanges and
[rho] is the density of air. The density of the air is calculated from
atmospheric pressure and temperature.
Efficiency, [eta]
The efficiency is the ratio of fluid power output to the coupling
power input of the compressor.
[eta] = [N.sub.eff]/[N.sub.c] = [rho]VW/[N.sub.c]
[N.sub.eff] = Fluid power, Watts
[rho] = Density of air, kg/[m.sup.3]
V = Volume flow rate, [m.sup.3]/s
W = Specific work, [m.sup.2]/[s.sup.2]
[N.sub.c] = Coupling power, watts = [[eta].sub.m]
[[V.sub.a][I.sub.a] + [V.sub.f][I.sub.f]]
[[eta].sub.c] = Efficiency of the motor
[V.sub.a] = Armature voltage, Volt
[I.sub.a] = Armature current, Ampere
[V.sub.f] = Field Voltage, Volt
[I.sub.f] = Field current, Ampere
Wall Static pressure Coefficient, [[psi].sub.ws]
Wall static coefficient is defined as
[psi]ws = [P.sub.ws] - [P.sub.atm]/1/2 [rho][U.sup.2.sub.2]
where Pws = Static pressure on the diffuser hub or shroud wall
Total pressure coefficient
Total pressure coefficient is define as,
[[psi].sub.TP] = [P.sub.o] - [P.sub.atm]/1/2 [rho][U.sup.2.sub.2]
where [P.sub.o] = Total pressure
Static pressure coefficient, [[psi].sub.s]
Static pressure coefficient is defined as
[[psi].sub.SP] = [P.sub.s] - [P.sub.atm]/1/2 [rho][U.sup.2.sub.2]
Where [P.sub.s] = Static pressure
Non Dimensional Velocities
They are defined as follows
Non-dimensional total velocity = C / [U.sub.2]
Non-dimensional radial velocity = [C.sub.r] / [U.sub.2]
Non-dimensional axial velocity = [C.sub.x] / [U.sub.2]
Non-dimensional tangential velocity = [C.sub.u] / [U.sub.2]
Results and Discussion
Centrifugal compressor performance, in terms of energy coefficient
across the impeller, derived from inlet nozzle static and delivery
static pressures measured for all four configurations is presented. The
four configurations are
i. Squealers on suction surface
ii. Squealers on pressure surface
iii. Squealers on both suction and pressure surface
iv. Solid tip by filling up the space between squealers
Performance Characteristics
The effect of configuration on the performance of the centrifugal
compressor is shown in Figures 3 and 4. Figure 3 shows the performance
characteristics in terms of energy coefficients, [psi] vs. flow
coefficients, [phi]. Figure 4 shows the performance characteristics in
terms of efficiency, [eta] vs. flow coefficient, [phi]. From the
performance curves; it is found that pressure surface squealer tip
configuration show increased energy coefficient and efficiency across
the compressor compared to other configurations. Also it is observed
that the effect of suction surface squealer tip configuration is
comparable to that of pressure surface squealer tip configuration.
[FIGURE 3 OMITTED]
[FIGURE 4 OMITTED]
Static pressure distribution on the shroud wall
The static pressure distributions on the front shroud for three
configuration at three flow coefficients are shown in Figures 5,6 and 7.
On the front shroud, static pressure taps are provided from the inducer
leading edge to the impeller exit. The shroud has its inner contour
shaped to match the impeller blade tip from the inlet to the exit of the
impeller.
The static pressure measurements on the shroud give an indication
of variation of the static pressure developed in the tip region of the
impeller. From the figure it can be seen that more deceleration pronounced at higher flow coefficients. The pressure initially decreases
due to suction and then uniformly increases, indicating that there is no
dead zone or eddies near the shroud region inside impeller and of the
energy is transferred smoothly to the fluid near the shroud.
[FIGURE 5 OMITTED]
[FIGURE 6 OMITTED]
[FIGURE 7 OMITTED]
Mass Averaged flow performance of Impeller
Mass averaged performance of compressor (Figure.8) shows the
variation of mass averaged values of total pressure coefficient
([[psi].sub.o2]) and static pressure coefficient ([[psi].sub.s2]) with
flow coefficient. These values are obtained by mass averaging the flow
parameters measured with the five hole probe at three flow coefficient.
The mass averaged values of total pressure coefficient ([[psi].sub.o2])
and static pressure coefficient ([[psi].sub.s2]) at the impeller exit
for three configurations at three flow coefficients clearly indicate
increase in total and static pressure for pressure surface squealer tip
configuration compared to other configurations.
[FIGURE 8 OMITTED]
Conclusions
The following conclusions are drawn from the present investigations
1. Pressure surface squealer tip has benefical effects in
increasing energy coefficient and efficiency of the tested compressor.
2. Mass averaged total and static pressures obtained from the probe
traverses are higher for configuration with pressure surface squealer
3. Suction surface squealer tip has reasonable beneficial effect on
the energy coefficient and efficiency.
Acknowledgments
The research described herein was supported by Thermal Turbo
machinery Laboratory, Indian Institute of Technology, Madras, India.
Special thanks is given to Dr. N. Sitaram, Professor, Department of
Mechanical Engineering, IIT, Madras for his contributed and support in
creating this research work.
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S. Senthil (1) * and N. Krishna Mohan (2)
(1) Assistant Professor, Department of Mechanical Engineering,
Mailam Engineering College, Mailam, Villupuram District--604 304, Tamil
Nadu, India. Email: senthil_iit@yahoo.com
(2) Professor, Department of Mechanical Engineering, Annamalai
University, Annamalai Nagar, Chidambaram, Tamil Nadu, India
Corresponding Author: senthil_iit@yahoo.com