1. DESIGN OF CASCADE for AXIAL FLOW COMPRESSORS
P M V Subbarao
Professor
Mechanical Engineering Department
Arrangement of aerofoils for best Group Performance…

2. 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.

3. Flow Angles &Velocities
Inlet Velocity Triangle

4. INLET CONDITIONS
Va1
Va1
Vr1
Vr1

5. 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.

6. Rothalpy Based Design
Define the rothalpy which is constant throughout the rotor.
The rothalpy is useful for calculating the outlet conditions of the rotor.
Further in to the calculations the relative Mach number and the axial Mach number will be used.

7. 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.

8. 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

9. Irreversible Flow through Cascades

10. Performance of Aerofoil
Camber angle, q
Naca 65 : bin-l
Circula rarc : bin-l

11. Selection of Design Parameters
A high pressure rise per stage will decrease the number of stages for a given overall pressure rise.
A high pressure rise per stage is obtained using:
High blade speed.
High inlet flow velocity.
High fluid deflection in rotor blades.

12. Fluid Deflection

13. Outlet flow Angle
Loss coefficient
Deflection

15. Cascade Testing

16. Cascade Testing

17. Blade Speed
For a given rotor speed the velocity of the blade at the tip will be maximum.
The centrifugal stress in the rotor blades depends on the rotational speed, the blade material and length of the blade.
The maximum centrifugal stress is given by,
b, hub-tip diameter ratio.
K varies in the range 0.55 – 0.65.

18. Multi Stage Compression
Loss in capacity due to variation of velocity is defined as work done factor.
Work done factor, l, decrease with number of stages.

20. Radial Variation of Blade Angles

21. Thermodynamics of Irreversible Compressor

22. Multi Stage Compressor

24. Compressor Maps