axial flow compressorslibvolume6.xyz/mechanical/btech/semester5/turbomachines/...• stage degree of...
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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
h
QNs
∆=
&ω
and the flow coefficient as
2
22Dr
Q
ωφ
&
=
4
3
∆
=
ρ
ρ
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 Mriya is 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: 1,600°K
• 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 line2: leading edge
3: nose circle4: camber
5: thickness
6: upper surface7: trailing edge
8: main camber line9: 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
λ = stagger angle ( positive
for a compressor cascade)
α’1 = blade inlet angle
α’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
p01T01
p03 = p02 T03 = T02
Va22/cp
p2
Va32/cp
p3
s
T
( )1w12w2 rVrVωmTωP −== &
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 Stage
Rate of Change of Momentum:
( ) ( )11f22fw1w2 tan αVtan αVmVVmF −=−=••
Inlet Velocity Triangle
Outlet Velocity Triangle
Power Consumed by an Ideal Moving Blade
( )11f22f tanαVtan αVUmP −=•
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Energy Analysis of An Axial Flow
Compressor Stage
Inlet Velocity Triangle
Outlet Velocity Triangle
Change in Enthalpy of fluid in moving blades :
( ) ( )0102p0102 TTcmhhmP −=−=••
+−+=
•
p
2
a11
p
2
a22p
2c
VT
2c
VTcm
−+−=
•
p
2a1
p
2a2
12p2c
V
2c
VTTcm
−+−=
•
2
V
2
Vhhm
2a1
2a2
12
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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
−
=−−
1T
Tppp
1
2112
1γ
γ
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 βαβα +=+=fV
U
Theoretical Power input to the compressor:
( ) ( )12fw1w2th tan αtan αUVmVVUmP −=−=••
( )21fth tan βtan βUVmP −=•
Inlet Velocity Triangle
Outlet Velocity Triangle
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For an isentropic compressor:
( ) ( )21f0103pth
tanβtanβUVmTTcmP −=−=••
( ) ( ) 0Sp0103p0103pth ∆TcmTTcmTTcmP•••
=−=−=
−
+=
1γ
γ
01
0S
01
iso03,
T
∆T1
p
p
( )p
21f0s
c
tan βtan βUV∆T
−=
−
+=
1γ
γ
01
0Sstage
01
act03,
T
∆Tη1
p
p
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( )
−
−
+=
1γ
γ
01
p
21fstage
01
act03,
T
c
tanβtanβUVη
1p
p
2a1
2a2
2r2
2r1
2
r2
2
r1
VVVV
VVR
−+−
−=
2211
f
tan βtan αtan βtan αV
U+=+=
[ ]2
211fr2
cos β
tan αtan βtan αVV
−+=