PEMP
RMD 2501
Centrifugal Compressors
Session delivered by:
Prof Q.H.
Prof.
Q H Nagpurwala
07
© M.S. Ramaiah School of Advanced Studies
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Session Objectives
PEMP
RMD 2501
This session is intended to introduce the following:
• Basic aerothermodynamics of centrifugal compressors
• Types of impellers
• Euler turbine equation and velocity triangles
• Centrifugal compressor performance
• Stall and surge in centrifugal compressors
• Vaneless and vaned diffusers
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Centrifugal Compressor
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PEMP
RMD 2501
3
Introduction
PEMP
RMD 2501
‰
Turbomachines
T
b
hi
employing
l i centrifugal
t if
l effects
ff t for
f increasing
i
i fl
fluid
id
pressure have been in use for more than a century.
‰
The earliest machines using this method were hydraulic pumps followed
later by ventilating fans and blowers.
‰ A centrifugal
compressor was incorporated in the Whittle turbojet engine.
‰ Axial
A i l
fl
flow compressors are more suitable
it bl ffor llarger engines
i
iin tterms off
smaller frontal area (and drag) and 3-4% higher efficiency for the same
duty than centrifugal compressors.
‰
But for very small compressors with low flow rates, the efficiency of axial
compressors drops sharply, blading is small and difficult to make
accurately, and the centrifugal compressor is again preferable .
‰
Many applications are found in small gas turbines for road vehicles and
commercial helicopters as well as bigger applications, e.g., diesel engine
turbochargers,
g , chemical p
plant p
processes,, factory
y workshop
p air supplies,
pp
,
large-scale air-conditioning plant, etc.
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PEMP
RMD 2501
Applications of Centrifugal Compressors
Axial compressor
Centrifugal
compressor used by
Si F
Sir
Frank
k Whi
Whittle
l in
i
his gas turbine
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The Turbomeca
Centrifugal Compressor
fitt d to
fitted
t the
th RTM322
engine
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PEMP
RMD 2501
Applications of Centrifugal Compressors
Rolls Royce Goblin II engine using
centrifugal compressor
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Use of centrifugal compressor in
turbocharger
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Axial-Centrifugal Compressor
PEMP
RMD 2501
Honeywell TPE331-10U / 11U turboprop engine
employing axial and
centrifugal compressor
stages
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Automotive Gas Turbine Engine
PEMP
RMD 2501
(A) Accessory drive, (B) Compressor, (C)
Right regenerator rotor, (D) Variable nozzle
unit, (E) Power turbine, (F) Reduction gear,
(G) Left regenerator rotor,
rotor (H) Gas
generator turbine, (I) Burner, (J) Fuel
Nozzle, (K) Igniter, (L) Starter-generator,
(M) Regenerator drive shaft, (N) Ignition
unit
Gas turbine engine of a
Chrysler car
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Multistage Centrifugal Compressor
PEMP
RMD 2501
Dresser-Rand Company Multistage Centrifugal
Compressor
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Return Channels in Multistage
C t if l Compressor
Centrifugal
C
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PEMP
RMD 2501
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Nomenclature
C
n
N
r
U
Ue
W, V
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α
β
σ
ψ
ω
Absolute velocity
Number of vanes
Rotational speed
Radius
Impeller speed at tip
Impeller
p
speed
p
at mean radius of eye
y
Relative velocity
Absolute flow angle
Relative flow angle
g
Slip factor
Power input factor
Angular velocity
suffixes
a
r
w, θ, u
Axial component, ambient
Radial component
Whirl component
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PEMP
RMD 2501
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Single Stage Centrifugal Compressor
(Splitter blade)
Inducer
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PEMP
RMD 2501
(Vaned diffuser)
Impeller eye
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Centrifugal Compressor
PEMP
RMD 2501
Vaneless
space
Centrifugal compressor stage and velocity diagrams at impeller entry and exit
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PEMP
RMD 2501
Radial Impeller with Diffuser Vanes
Splitter Blade
Main Blade
Diffuser Vane
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Types of Impellers
•
•
•
•
•
•
•
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PEMP
RMD 2501
One sided
Two sided
Shrouded
Unshrouded
Radial vanes
Backward swept vanes
Forwardd swept vanes
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PEMP
RMD 2501
Types of Impellers
Back swept impeller
β 2 =< 900
Forward sweep
p
Radial exit
Radial exit impeller
β 2 => 900
β 2 = 900
Impeller with splitter blades
Shrouded impeller
Vθ < U2
Vθ = U2
Backward sweep
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Forward swept impeller
Vθ > U2
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Ideal Performance of Impellers
PEMP
RMD 2501
Head – flow characteristics for various
outlet blade angles
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PEMP
RMD 2501
Velocity Triangles
W2
β2
α2
U2
Outlet velocity triangle
α1= 0
U1
W1
Cw1= 0
β1
Inlet velocity triangle
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Inlet Prewhirl
PEMP
RMD 2501
‰
Prewhirl can be achieved by
fixing inlet guide vanes to the
compressor casing.
i
‰ This
changes the inlet velocity
triangle.
triangle
‰ The
work capacity of the
compressor decreases.
Wsp = U2Cw2 - U1Cw1
‰
It is
i required
i d to
t properly
l design
d i
the inlet guide vanes in order to
minimise pressure loss.
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Pre-Swirl Guide Vanes
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PEMP
RMD 2501
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Impeller Intake Mach Numbers
PEMP
RMD 2501
• Air enters axially towards the inducer and then flows through the
radial channel of the impeller.
• Flow separation might occur on the suction surface (convex face)
off the
th curvedd partt off the
th impeller
i
ll vane.
• There is possibility of shock waves in the inducer tip region for
high pressure ratio compressors.
• At higher altitudes M will increase
as there is a decrease in T.
T
M =
V1
γ RT1
• To reduce M, V must reduce.
• Thi
This can be
b done
d
by
b introducing
i t d i
prewhirl at inducer inlet.
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Specific Work
PEMP
RMD 2501
Specific work is given by Euler turbine equation
W&c = (Wc m& ) = (U 2Cw 2 − U1Cw1 ) = (h02 − h01 )
If the flow enters the impeller axially,
axially then α1= 0 and Cw1= 0,
0 and
the specific work is given by
W&c = (Wc m& ) = (U 2Cw 2 ) = (h02 − h01 )
For impeller with radial exit, β2= 90° and Cw2= U2, hence specific
work
W&c = (Wc m& ) = U 22 = (h02 − h01 )
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PEMP
RMD 2501
Compression Process on T-s Diagram
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Specific Work
PEMP
RMD 2501
Absolute velocity is given by,
by
Rothalpy,
Adding and subtracting ½ U 2, this becomes
From the velocity triangle
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Specific Work
PEMP
RMD 2501
Across the impeller
impeller, I1 = I2, hence
This expression provides the reason why the static enthalpy rise in a centrifugal
compressor is so large compared with a single-stage axial compressor.
The term
and the term
is the contribution from the diffusion of relative velocity
is the contribution from the centrifugal action caused by
th change
the
h
in
i radius.
di
For axial entry to the compressor,
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, and specific work
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Power Input Factor
PEMP
RMD 2501
Due to friction between the casing and the air carried around by
the vanes, and other losses which have a braking effect such as
di friction
disc
f i ti or windage,
i d
the
th applied
li d torque
t
andd therefore
th f
the
th
actual work input is greater than the theoretical value. To
account for this, a power input factor ψ can be introduced, and
the specific work is given by
W&c = (Wc m& ) = ψ (U 2Cw 2 − U1Cw1 ) = ψ (h02 − h01 )
ψ >1
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PEMP
RMD 2501
Slip
Even under ideal (frictionless) conditions the
relative flow leaving the impeller does not
receive perfect guidance from the vanes and the
flow is said to slip. Hence, β2 > β2’.
average
g relative
flow angle
The slip velocity is defined as
vane angle
Velocity triangle at
impeller exit
Cθ ≡ C w
Cws = Cw' 2 − Cw 2
Cw2
σ = ' <1
andd Sli
Slip Factor,
F t
Cw2
Cw2
For radial impeller, σ =
U2
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Slip Factor Correlations
PEMP
RMD 2501
Stanitz correlation
Wiesner correlation
cos β 2
σ s = 1−
Z 0.7
Stodola correlation
and β’2 is measured from radial direction.
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Slip and Specific Work
PEMP
RMD 2501
Taking into account slip factor and power input factor,
W&c = (Wc m& ) = σψ (U 2Cw 2 − U1Cw1 ) = ψ (h02 − h01 )
and for radial impeller with axial inlet
W& = ψ σ U 22
Temperature rise, T03 − T01 =
p03 ⎛ T ⎞
= ⎜⎜ ⎟⎟
p01 ⎝ T01 ⎠
'
03
γ (γ −1)
ψ σ U 22
cp
⎡ ηc (T03 − T01 ) ⎤
= ⎢1 +
⎥
T
01
⎣
⎦
⎡ ηc ψ σ U
= ⎢1 +
c pT01
⎢⎣
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2
2
⎤
⎥
⎥⎦
γ (γ −1)
γ (γ −1)
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PEMP
RMD 2501
Diffuser
‰
In a centrifugal compressor, the flow leaving the impeller, passes through diffuser.
‰ The
diffuser can be vaneless space, vaned or a combination of both.
‰ The
function of the diffuser is to convert the exit kinetic energy
gy into pressure.
p
‰
Diffuser being a static part,
the total conditions (pressure
and temperature)
p
) do not
change across it. But the
static pressure and
temperature increase with
consequent decrease in
absolute velocity.
Vaneless
diffuser
Vaned diffuser
‰
In high
g stage
g pressure
p
ratio
compressors, the diffuser
leading edge region is critical
because of high Mach
numbers giving rise to
shocks and shock losses.
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Types of Diffusers
Uniform thickness
curved vanes
Aerofoil shape
vanes
Uniform thickness
straight vanes
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RMD 2501
Wedge shape
vanes
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Straight Wedge-Shaped Diffuser
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PEMP
RMD 2501
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Pipe Diffuser
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PEMP
RMD 2501
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Diffuser Performance
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RMD 2501
Isentropic change in static enthalpy
Diffuser efficiency =
Actual change in static enthalpy
(
(
h2 s − h1 1 / 2 C12 − C22s
=
ηD =
h2 − h1 1 / 2 C12 − C22
)
)
T2 s T1 − T1
=
T2 T1 − T1
For the isentropic process 1-2s
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Diffuser Performance
PEMP
RMD 2501
F constant
For
t t temperature
t
t
process 01-02
01 02
From the gas law
U
Upon
iintegrating
t
ti for
f the
th whole
h l process we gett
For the constant pressure process, 2s - 2,
Therefore
Equating these expressions for the entropy increase and using
Therefore
and
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PEMP
RMD 2501
Volute System
Vaneless
V
l
space
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Compressor Performance
PEMP
RMD 2501
Performance
P
f
characteristics
h
t i ti are plotted
l tt d as variation
i ti off totall pressure ratio
i and
d
isentropic efficiency versus corrected mass flow rate, m& T01 p01 for various
corrected speeds N T01 .
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Stall and Surge
Stall /
surge line
Stable operating
point
Any stable operating point lies at the
intersection of the compressor and
throttle characteristics
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PEMP
RMD 2501
Instability (stall) occurs when the
slope of the throttle characteristic
becomes larger than that of the
compressor characteristic
h
t i ti
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PEMP
RMD 2501
Stall, Surge and Choke
Stall
ω
Pressure
surface
Suction
surface
Design
point
Surge
Choke
Deflected flow
+i
Stall and Surge
At low flow rates at a given speed,
the reduction in axial velocity
causes the flow to enter the inducer
at large positive incipience resulting
in flow separation on the suction
surface leading to the phenomenon
of rotating stall.
Stall can also initiate at the diffuser
d to large
due
l
positive
i i incidence
i id
at
reduced flow rates.
07
Choking
At hi
highh flow
fl rates
t att a given
i
speed,
d the
th pressure
and density reduce, causing an increase in radial
velocity (continuity equation). The relative
velocityy also become high
g with negative
g
incidence at inducer and diffuser leading ends.
Finally, choking may occur owing to large flow
blockage due to separation on the pressure
surface or due to formation of shocks in the
inducer / diffuser passages.
© M.S. Ramaiah School of Advanced Studies
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Session Summary
•
•
•
•
•
07
PEMP
RMD 2501
The construction and components of centrifugal compressors
are explained.
l i d
Types of impellers and diffusers are introduced.
Concept of slip and prewhirl
prewhirl, and their effects on the specific
work output are discussed.
Performance characteristics, including the phenomenon of
choking and stall / surge, are discussed.
Performance analysis of diffusers is explained in detail.
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PEMP
RMD 2501
Thank you
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