Characterization of Losses in Turbine Cascades P M V Subbarao Professor Mechanical Engineering Department Convert the problems into Opportunities……

Steps in Analysis of Blade Cascades

Remarks on Turbine Performance Predictions TURBINPEER FORMANCE can only be satisfactorily determined by tests on full scale machines. Such tests, however, reflect the aggregate effect of a large number of features influencing total losses. For a basic understanding of turbine performance it is necessary to analyze the behavior of individual features. Development and testing of linear cascades is essential in understanding the losses generated by individual features.

The difference - 1 A linear cascade differs from blading in a real turbine in two ways. First, differences occur when the cascade tests are carried out: (1) with a different working fluid; (2) with a different Reynolds number; (3) with a different scale of blade; (4) with a different surface roughness; (5) with a different Mach number. Differences of this sort, if they occur, are capable of being corrected, with the major proviso that the information is available as to how the correction should be made.

The difference - 2 Second difference is more fundamental and exist as: Cascade flow: Uniform inlet conditions & Linear flow. Turbine flow: Inlet flow containing wakes, disturbances due to preceding flows & Annular flow. Cascade flow: Walls stationary relative to blades. Turbine flow: Walls may be moving relative to blades. Differences of this type are an inherent limitation in the use of stationary linear cascade data. It is very essential to compare the results of carefully interpreted cascade data with actual turbine performance, and to deduce from the overall result the magnitude and importance of the errors involved

Cascades, Stage & Turbine Cascades are classified into: Stationary cascades : Nozzle Cascades Moving cascades : Rotor Cascades. A stage is a combination of a nozzle cascade and rotor cascade with a minimum gap between them. Number of such Stages together make a turbine

Sequence of Energy Losses in A stage Steam Thermal Power Steam Kinetic & Thermal Power Blade kinetic Power Nozzle Losses Moving Blade Losses Stage Losses Isentropic efficiency of Nozzle Blade Friction Factor

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

Losses in nozzle & moving blades Losses in kinetic energy of steam while flowing through cascades are classified into: –Energy losses of steam before entering the nozzles/moving blades, –Frictional resistance of the nozzles walls/moving blade walls, –Viscous friction between steam molecules, –Deflection of the flow, –Growth of boundary layer, –Turbulence in the Wake and –Losses at the roof and floor of the blades.

Losses in A Turbine Stage Loss in nozzle blades. Loss in moving blades. Loss due to exit velocity. Loss due to friction of the disc and blade banding Loss due to steam leakages through clearances.

Christening of STAGE LOSS - 1 The work done on the rotor blades is indicated by the 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 given up by the gas is more than this! The difference is due to the friction on the blade profiles, and loss in blade wakes (profile loss); the friction on the walls at root and tip, and other end effects (secondary loss); the sudden enlargements in the fluid path, or wall cavities (annulus loss).

Christening of STAGE LOSS - 2 Not all the fluid passes through the rotor blades, because of leakage through –diaphragm glands, –balance holes, and –over the rotor blade tips; The actual work per unit total mass flow is less than the work done on the blades per unit blade mass flow as evaluated in the last slide. 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