Aerodynamic Design of Centrifugal Compressor

Dr. Firman Hartono

Aerodynamic Design of Centrifugal Compressor

Setelah mengikuti kuliah ini mahasiswa dapat:

- Melakukan proses perancangan aerodinamika kompresor sentrifugal

Referensi:

- Hill, P. and Peterson, Mechanic and Thermodynamic of Propulsion

- Boyce, M.P., Gas Turbine Engineering Handbook

- Rolls Royce, The Jet Engine

Centrifugal CompressorsPrinciple of Operation

The impeller is rotated at high speed by the turbine and air is continuously induced into the centre of the impeller. Centrifugal action causes it to flow radially outwards along the vanes to the impeller tip, thus accelerating the air and also causing a rise in pressure to occur.

The air, on leaving the impeller, passes into the diffuser section where the passages form divergent nozzles that convert most of the kinetic energy into pressure energy. In practice, it is usual to design the compressor so that about half of the pressure rise occurs in the impeller and half in the diffuser.

To maximize the airflow and pressure rise through the compressor requires the impeller to be rotated at high speed, therefore impellers are designed to operate at tip speeds of up to 1,600 ft per sec. By operating at such high tip speeds the air velocity from the impeller is increased so that greater energy is available for conversion to pressure.

To maintain the efficiency of the compressor, it is necessary to prevent excessive air leakage between the impeller and the casing; this is achieved by keeping their clearances as small as possible.

The Airflow

Impeller Types

2’ = 90o 2’ < 90o 2’ > 90o

Velocity Triangles

Centrifugal Compressor Parts

A centrifugal compressor usually consist of:

Inlet guide vanes

Impeller

inducer

shrouded / un shrouded

splitter

Diffuser

Volute / Scroll

Inlet Guide VanesThe IGV give circumferential velocity to the fluid at the inducer inlet (pre-whirl).

since

Mrel

Mrel

Pre-whirl

Pre-whirl Types: o Free vortex pre-whirl

o Forced vortex pre-whirl

o Control vortex pre-whirl

ImpellerAn impeller in a centrifugal compressor imparts energy to the fluid.Consist of: Inducer Radial blades

Two D surfaces for flow analysis

ImpellerImpeller and Meridional Plane

ImpellerVelocity Profile

ImpellerSlip Factor

Slip, is caused by:

Coriolis force

Boundary layer development

Leakage

Number of vanes

Vane thickness

2‘ angle between relative flow and tangential direction‘

‘

‘

‘

DiffuserDiffusing passages have a vital role in obtaining good performance from turbo-

machines.Their role is to recover the maximum possible kinetic energy leaving the

impeller with a minimum loss in total pressure.To avoid flow separation, the opening angle should be maximum 7o.

Losses

Losses

Rotor Losses

Stator Losses

Shock losses

Incidence loss

Disc friction loss

Diffusion blading loss

Clearance loss

Skin friction loss

Recirculating loss

Wake-mixing loss

Friction loss

Centrifugal Compressor DesignKnown variable from thermodynamic design: c, c, m

1. Calculate temperature rise using eq.

2. Determine blade number Z, 2 and Wr2/U2.

3. Calculate U2 from eq.

In case no pre-whirl (IGV) exist, (UC)1 = 0, then

whereNote: = , Wm2 = Wr2

stress limited tip speed for current material technology is about 650 m/s !

‘

Centrifugal Compressor Design

4. Calculate Wr2 from known value Wr2/U2 and U2.

6. Determine impeller eye area and impeller eye tip diameter D1t using eq.

5. Calculate M2 from eq.

A

AM

21

12

where

01

02

2

2

2

2

22

2

2

01

2

tan1

TT

UW

UW

a

UA

rr

limited tip speed for high efficiency (>85%) is about M2 = 1.5 !

M1rel < 1.4 to avoid strong shock wave

Centrifugal Compressor Design

7. Calculate rpm from eq.

8. Calculate D2 from eq.

9. Calculate axial depth, b2 from eq.

where

Centrifugal Compressor Design

10. Blade Design• To avoid blade separation, it is suggested to design a constant deceleration blade, i.e.:

• For constant deceleration blade, several approaches can be used, for example:

> constant meridional velocity

> constant breadth

> linear breadth (straight sloping)

Centrifugal Compressor Design

Inducer

• Inducer angle distribution depend on the inlet flow condition

• For axial inlet air flow:

Note: e (r) + 1 (r) = 90o.

Centrifugal Compressor Design

Radial Clearance

• To avoid shock in diffuser passage, the high impeller outlet velocity should be reduced by means of an enough radial clearance

Clearance calculation• Calculate * and r* from known M2 and .

• Determine M inlet diffuser ( < 1 to avoid shock)

• By using the known *, r* and M, calculate r inlet diffuser

1

1

2

2

11

1

2

tan

tan

M

2

1

2

21

1

21

sin

sin

MM

r

r

TJE 500 N Compressor Design

TJE 500 N is a turbojet engine designed to produce static thrust of about 500 N at sea level standard condition

The compressor’s design point: c = 4.06, m = 0.88 kg/s, c = 78%

Questions?