AXIAL FLOW COMPRESSORS

P M V Subbarao

Professor

Mechanical Engineering Department

An Efficient Way to Ingest Life in Large amount of Fluids !!!

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

&

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�

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.

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

Stages of an Axial-flow Compressor

Selection of Pressure Ratio per Stage

The first step in a design of Axial Flow

Compressor�..�..

The Aerofoil�

A Cascade of Aerofoils�..

Invention of high population element ��

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

Geometrical Description of NACA 65

NACA 65 Series of Aerofoils

Cascade of Aerofoils

Viscous flow through Cascade

Cascade Geometry

λ = stagger angle ( positive

for a compressor cascade)

α’1 = blade inlet angle

α’1 = blade outlet angle

Lift & Drag of a cascade

Selection of Inlet flow angle

Cycling of Kinetic Energy in Axial Flow

Compressor

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),

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.

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

Kinematics of An Axial Flow

Compressor Stage

Inlet Velocity Triangle

Outlet Velocity Triangle

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 −=•

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

( )

−=−

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

−+−

−=

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.

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

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

( )

−

−

+=

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

−+=