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AXIAL FLOW COMPRESSORS
P M V Subbarao
Professor
Mechanical Engineering Department
An Efficient Way to Ingest Life in Large amount of Fluids !!!
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An Option for High Specific Speed
In aero applications, the specific speed is defined as:
43
hQNs
and the flow coefficient as
2
22Dr
Q
4
3
p
mn
Ns
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Schematic representation of an axial flow
compressor
It is easy to design a turbine that will work. It requires a
considerable skill to design a compressor that will work
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Antonov An-225 Mriya
The Antonov An-225 Mriyais a strategic airlift cargo aircraft,
designed by the Antonov Design Bureau in the 1980s.
Payload: 250,000 kg (550,000 lb) !!!
Cruise speed: 800 km/h. Altitude: 11,000 m (36,100 ft).
Thrust Required: 1350 kN
Power plant: 6 ZMKB Progress D-18 turbofans.
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The Progress D-18T ( Lotarev D-18T)
General characteristics
Type: Three-spool high bypass turbofan engine with a single-stage fan.
Fan diameter: 2.33 m (91.73 in)
Dry weight: 4,100 kg (9,039 lb)
Components
Compressor: Seven-stage IP compressor, seven-stage axial HP
compressor Combustors: Annular combustion system
Turbine: Single-stage HP turbine, single-stage IP turbine, four-stage LP
turbine
Performance
Maximum thrust: 229.77 kN
Overall pressure ratio: 27.5
Bypass ratio: 5.7
Turbine inlet temperature: 1,600K
Thrust-to-weight ratio: Approx 5.7:1
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Stages of an Axial-flow Compressor
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Selection of Pressure Ratio per Stage
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The first step in a design of Axial Flow
Compressor....
The Aerofoil
A Cascade of Aerofoils..
Invention of high population element
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Aerofoil Geometry
1: zero lift line
2: leading edge
3: nose circle
4: camber5: thickness
6: upper surface
7: trailing edge
8: main camber line
9: lower surface
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Geometrical Description of NACA 65
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NACA 65 Series of Aerofoils
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Cascade of Aerofoils
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Viscous flow through Cascade
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Cascade Geometry
= stagger angle ( positive
for a compressor cascade)
a1 = blade inlet anglea
1= blade outlet angle
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Lift & Drag of a cascade
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Selection of Inlet flow angle
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Cycling of Kinetic Energy in Axial Flow
Compressor
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Macro Geometric Specification of An Axial
Compressor
The geometry of a compressor can be
categorised into 3 main designs types,
A Constant Outer Diameter (COD),A Constant Mean Diameter (CMD) or
A Constant Hub Diameter (CID),
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Specifications of An Axial
Compressor There are several different parameters that can
specify a particular compressor.
The first set of input parameters are based on the
running conditions for the machine.
These involve mass flow, pressure ratio , rotational
speed and the number of stages.
Stage degree of reaction : For controlling the
distribution of the load between the rotor and the
stator.
If this is not of importance, the outlet flow angle
for the each stage must be set instead.
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Va12/cp
p1T1
p01T
01
p03 = p02 T03 = T02
Va22/cp
p2
Va32/cp
p3
s
T
1w12w2 rVrVmTP
Thermodynamics of An Axial flow
Compressor Stage
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Kinematics of An Axial Flow
Compressor Stage
Inlet Velocity Triangle
Outlet Velocity Triangle
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Kinetics of An Axial Flow
Compressor StageRate of Change of Momentum:
11f22fw1w2 tanVtanVmVVmF
Inlet Velocity Triangle
Outlet Velocity Triangle
Power Consumed by an Ideal
Moving Blade
11f22f tanVtanVUmP
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Energy Analysis of An Axial Flow
Compressor StageInlet Velocity Triangle
Outlet Velocity Triangle
Change in Enthalpy of fluid in
moving blades :
0102p0102 TTcmhhmP
p
2
a11
p
2
a22p
2cVT
2cVTcm
p
2
a1
p
2
a212p
2c
V
2c
VTTcm
2
V
2
Vhhm
2
a1
2
a212
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2
V
2
Vhh
2
r2
2
r112
p
2
r2
p
2
r112
2c
V
2c
VTT
Isentropic compression in Rotor Blade
1TTppp1
2112
1
Degree of Reaction of A Stage, R :
0102
12
hh
hhR
2
a1
2
a2
2
r2
2
r1
2
r2
2
r1
VVVV
VVR
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Compressible Flow Machines
Owing to compressibility of gas in a compressor
The degree of reaction for equal pressure rise in
stator and rotor will be greater than 0.5.
The stage total pressure rise will be higher in
order to get equal static pressure rise in stator
and rotor.
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Power input to the compressor :
0102p0103p0103act TTcmTTcmhhmP
Current Practice:
21 fff VVV
2211 tantantantan aa fV
U
Theoretical Power input to the compressor:
12fw1w2th tantanUVmVVUmP
21fth tantanUVmP
Inlet Velocity Triangle
Outlet Velocity Triangle
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For an isentropic compressor:
21f0103pth tantanUVmTTcmP
0Sp0103p0103pth TcmTTcmTTcmP
1
01
0S
01
iso03,
T
T1
p
p
p
21f0s
ctantanUVT
1
01
0Sstage
01
act03,
T
T1
p
p
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1
01
p
21f
stage
01
act03,
T
c
tantanUV
1
p
p
2a1
2a2
2r2
2r1
2r2
2r1
VVVV
VV
R
2211
f
tantantantanV
U
2
211fr2
cos
tantantanVV