DESIGN OF AXIAL FLOW COMPRESSORS

Proper Integration of Mild Compression Stages !!!

P M V SubbaraoProfessor

Mechanical Engineering Department

Design Specifications

• The different input parameters, used in design Process are:– Main specification– Detailed specification– Inlet specification

Specifications of Axial Flow Compressor

• Main specification• Type of compressor• Mass flow• Number of stages• Pressure ratio of

each stage• Rotational speed• Stage reaction

• Inlet specification• Inlet flow angle, • Stage flow coefficient

• Hub tip ratio, rhub/rtip

Parameter variations throughout the compressor

• Certain parameters in the compressor will vary in the compressor, namely:

• Tip clearance, e/c• Aspect ratio, h/c• Thickness chord ratio, t/c• Axial velocity ratio, AVR• Blockage factor, BLK• Diffusion factor, DF• Stage Loading distribution• A simple linear distribution for the parameters may,

for simplicity, be used except for the stage loading.

The stage load distribution throughout the compressor

Mean stream line analysis

• The calculations are based on mean line stream analysis i.e. one dimension.

• The mean radius is used in the calculations to determine the blade speed.

• Normally when calculating with the mean line stream method, the mean radius will not change.

• But by changing the mean radius throughout one stage will give a more accurate design.

• The mean radius will be kept constant in the space between rotor and stator as well for the space between each row.

• A change in radius in the space between each blade row won’t make a big difference in the end result.

• It is more crucial to have a change in radius in the blade them self since this will have a more noticeable effect.

Design Calculation process

• Module 0, Inlet geometry• To be able to solve the inlet geometry the inlet

flow velocity, Vf, must be known.

• If this velocity is unknown, an iterative process must be used.

• By approximating the value of Vf, the density can be found.

• With help of mass continuity a new inlet flow velocity can be calculated.

• This value is then used to start over the calculation until converged.

• The first step is to get hold off the thermodynamic properties in the inlet of the compressor.

• The inlet pressure and temperature is known and from these the enthalpy and entropy can be found.

Algorithm: Inlet Geometry

• Inlet Parameters: M,p,T …..

• Specify inlet flow angle, i

• Calculate flow area:.BLKV

mArea

f

1/2

2

tip

hub

tip

rr

1

Arear

π

1/22tip

2hub

rms 2

rrr

60

Nr2U rms

mean

π

Blockage FactorThe blockage factor is here denoted as, BLK. The geometry is the same for the rotor inlet as for the stator-outlet in the previous stage. A result of this is that the blockage factor should be the same for the rotor-inlet and the stator-outlet at the previous stage.

From the definition of the cross section area and the mean radius, the hub radius, the mean radius or the tip radius can be calculated depending if the compressor is of the type CID, CMD or COD.

.BLK.ρV

mArea

1f11

Stage load coefficient2

w1w22 U

VV

U

Δh ψ

Stage flow coefficientU

Vfφ

Stage reaction0103

12

hh

hh

φ

de Haller number

• Compressor stages both the rotors and the stators are designed to diffuse the fluid.• Transfer and transform kinetic energy into an increase in static enthalpy and static pressure of the fluid. •The more the fluid is decelerated, the bigger pressure rise, but boundary layer growth and wall stall is limiting the process. •To avoid this, de Haller proposed that the overall deceleration ratio, i.e. Vr2 / Vr1 and Va3 / Va2 in a rotor and stator respectively, should not be less than 0.72 (historic limit) in any row.

Module 1: Rotor-inlet Triangle

• When starting the calculation, the geometry from the inlet calculations is used.

• The calculation for the entire stage is repetative.• Conside the rotor-inlet conditions, i.e. station 1, will have

the same velocity and radius as the stator-outlet, i.e. station 3, for the previous stage.

1)3(i1

1)f,3(if,1

1)rms,3(irms,1

αα

VV

rr

Flow Angles &Velocities

Inlet Velocity Triangle

Va1

Va1

Vr1

Vr1

INLET CONDITIONS

Static Properties

Static properties:

Now that the velocity is known, the static enthalpy can be calculated. With help from the entropy other fluid dynamic properties like pressure, temperature, density etc. can be found.

To be able to move from the rotor-inlet towards the outlet of the rotor a relationship between these must be used.

Rothalpy Based Design

Define the rothalpy which is constant throughout the rotor.

The rothalpy is useful for calculating the outlet conditions of the rotor.

2

U

2

VhI

22r

Further in to the calculations the relative Mach number and the axial Mach number will be used.

Module 2, Rotor-outlet/stator-inlet

•There are two separate modules in module 2. •The first, 2.1, is for the calculation of the entropy rise in the rotor. •The second, 2.2, calculates the mean radius of rotor-outlet. •Both of these are iteration processes where an approximated value is first guessed and then a new value is calculated to adjust the approximated first value.

Iteration Loop:Flow angles and velocities : The mean radius at rotor-outlet in unknown so a value for this must be approximates to be able to find out the blade speed. A new value for this will be calculated further on in the calculation.

Since a change in radius throughout the rotor is occurring a modification to the definition of the stage load coefficient must be made. A modification is made based on the blade velocity at the rotor-outlet.

Outlet Velocity Triangle