Irreversible Flow through A Turbine Stage

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

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