1. Irreversible Flow through A Turbine Stage
P M V Subbarao
Professor
Mechanical Engineering Department
Capcity is limited by the Performance……

2. Sequence of Events Leading to Generation of Entropy & Unavailable Enthalpy A stage
Steam
Thermal
Power
Residual Steam
Power
Stage Losses
Steam Kinetic &
Thermal Power
Nozzle Losses
Moving Blade
Losses
Blade Kinetic Power
Isentropic efficiency of
Nozzle
Blade Friction Factor

3. Definition of Isentropic/adiabatic Efficiency
Relative blade efficiency is calculated as:
Internal Relative Efficiency is calculated as:

4. Physical Locations for Occurrence of Losses
Isentropic flow :Station Suffixes
Irreversible flow
Station Suffixes
Nozzle annulus Losses
Nozzle Losses
Runner annulus Losses
Stage Losses
Runner Losses

5. Christening of STAGE LOSS - 1
The actual work done on the rotor blades is the true change in tangential momentum.
The overall integrated value can be calculated from the velocity conditions for the mass actually passing through the rotor blades.
The energy made available by the fluid is more than this!
The difference is turning into unavailable energy, but possessed by the fluid.
This is due to
the viscous turbulent flow past a selected blade profile, and loss in blade wakes;
the finite 3D blade having the walls at root and tip, and other end features;
the strange flow of fluid through wall cavities;

6. Christening of STAGE LOSS - 2
Not all the fluid passes past the rotor blades, because of leakage through
over the rotor blade tips;
diaphragm glands, and
balance holes,
The actual work per unit total mass flow is less than the ideal work.
Windage and bearing losses reduce the coupling power below that produced at the blades.
Losses resulting from partial admission lacing wire and wetness losses are also similar to windage loss.

7. Sub-division of Losses based on Non-interaction
Group I
Guide profile loss.
Runner profile loss.
Guide secondary loss.
Runner secondary loss.
Guide annulus loss
Runner annulus loss
Group 2
Guide gland leakage loss.
Balance hole loss.
Rotor tip leakage loss.
Lacing wire loss.
Wetness loss
Disc windage loss.
Losses due to partial admission.

8. Typical Distribution of Losses AStages

9. Revisiting of Design of Blade Profile
The local velocities are different at different positions on the blade.
Let the Mach number of the flow over our wing at a given point x be Mx.
The corresponding pressure coefficient can then be found using

10. Blade Profile & Losses
Historically, one essential feature sought during profile design for a given velocity triangle has been to reach minimum losses.
These include skin friction on the profile surface and trailing edge loss.
Owing to higher velocity levels and the occurrence of adverse pressure gradients, the skin friction loss on a typical subsonic profile is mainly determined by the flow behaviour on the suction side.
Typically more than 80 per cent of the skin friction loss occurs on the suction side.

11. The Flow on Suction Side of Blade
The flow on the suction side is characterized by:
the position of laminar–turbulent transition,
transition length and
diffusion rate.

12. Local Mach Number Distribution on A Blade

13. Load Distribution Vs Loss Occurrence
To reduce skin friction loss on the profile, the fraction of laminar surface length on the suction side has to be extended.
The friction loss varies with (velocity)2 for a laminar boundary layer.
The friction loss varies with(velocity)3 for turbulent boundary layer.
For a fixed profile loading and a given pitch–chord ratio, increase in the laminar length implies a shift in profile loading towards trailing edge.
This is known as Aft-loaded profile,(ALP).

14. Front Loaded Vs Aft Loaded Blades