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

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

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

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

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;

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.

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.

Typical Distribution of Losses AStages

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

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.

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.

Local Mach Number Distribution on A Blade

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).

Front Loaded Vs Aft Loaded Blades