axial compressors

7/28/2019 Axial Compressors

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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



Power Consumed by an Ideal

Moving Blade

11f22f tanVtanVUmP


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Energy Analysis of An Axial Flow

Compressor StageInlet 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




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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

axial compressors

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