Axial flow turbine
- 1. 1 Lect- 20
- 2. Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay 2 Lect-20 In this lecture... • Axial flow turbine • Impulse and reaction turbine stages • Work and stage dynamics • Turbine blade cascade
- 3. Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay 3 Lect-20 Axial flow turbines • Axial turbines like axial compressors usually consists of one or more stages. • The flow is accelerated in a nozzle/stator and then passes through a rotor. • In the rotor, the working fluid imparts its momentum on to the rotor, that converts the kinetic energy to power output. • Depending upon the power requirement, this process is repeated in multiple stages.
- 4. Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay 4 Lect-20 Axial flow turbines • Due to motion of the rotor blades two distinct velocity components: absolute and relative velocities in the rotor. • This is very much the case in axial compressors that was discussed earlier. • Since turbines operate with a favourable pressure gradient, it is possible to have much higher pressure drop per stage as compared with compressors. • Therefore, a single turbine stage can drive several stages of an axial compressor.
- 5. Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay 5 Lect-20 Axial flow turbines • Turbines can be either axial, radial or mixed. • Axial turbines can handle large mass flow rates and are more efficient. • Axial turbine have same frontal area as that of the compressor. • They can also be used with a centrifugal compressor. • Efficiency of turbines higher than that of compressors. • Turbines are in general aerodynamically “easier” to design.
- 6. Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay 6 Lect-20 Axial flow turbines 1 2 3 Hot gas Exhaust Nozzle/stator Rotor Disc An axial turbine stage
- 7. Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay 7 Lect-20 Velocity triangles • Elementary analysis of axial turbines too begins with velocity triangles. • The analysis will be carried out at the mean height of the blade, where the peripheral velocity or the blade speed is, U. • The absolute component of velocity will be denoted by, C and the relative component by, V. • The axial velocity (absolute) will be denoted by Ca and the tangential components will be denoted by subscript w (for eg, Cw or Vw) • α denotes the angle between the absolute velocity with the axial direction and β the corresponding angle for the relative velocity.
- 8. Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay 8 Lect-20 Velocity triangles U C1 V3 V2 C2 Rotor Stator/Nozzle 1 2 3 β3 β2 α1 α3 α2 U C3
- 9. Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay 9 Lect-20 Types of axial turbines • There are two types of axial turbine configurations: Impulse and reaction • Impulse turbine • Entire pressure drop takes place in the nozzle. • Rotor blades simply deflect the flow and hence have symmetrical shape. • Reaction turbine • Pressure drop shared by the rotor and the stator • The amount of pressure drop shared is given by the degree of reaction.
- 10. Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay 10 Lect-20 Work and stage dynamics 01 3 2 01 0 03 02 03 01 0 03 01 3 2 3 3 2 T c ) C C ( U T T , is ratio work stage The T T T T T Let ) T T ( c w or ) C C ( U w is mass unit per work the Therefore, . U U U turbine, axial an In ) C U C (U m P equation, momentum angular the Applying p w w p t w w t 3 2 w w 2 − = − = − = − = − = = ≈ − = Δ Δ
- 11. Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay 11 Lect-20 Work and stage dynamics U C C U h U w w w 2 2 t 3 2 0 − = = Δ • Turbine work per stage is limited by – Available pressure ratio – Allowable blade stresses and turning • Unlike compressors, boundary layers are generally well behaved, except for local pockets of separation • The turbine work ratio is also often defined in the following way:
- 12. Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay 12 Lect-20 Impulse turbine stage U V2 V3 V3 C2 Rotor Stator/Nozzle 1 2 3 β3 β2 α2 α3 C3 C2 α2 V2 β2 U β3 Ca Cw2 Cw3 Vw3 Vw2
- 13. Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay 13 Lect-20 Impulse turbine stage − = − = − = = − − = ⇒ − = 1 2 1 2 2 2 2 2 2 3 2 2 3 2 3 α Δ α β β tan U C U U h is ratio work turbine the , Or tan U C U ) U C ( V C C and V V , Therefore flow. the deflects simply rotor the turbine, impulse an In a 2 0 a w w w w w w
- 14. Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay 14 Lect-20 50% Reaction turbine stage Rotor Stator/Nozzle 1 2 3 U V2 V3 V3 C2 β3 C3 C2 α2 V2 β2 U
- 15. Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay 15 Lect-20 Impulse turbine stage − = − − = 1 2 2 2 3 α Δ α tan U C U h becomes ratio work turbine the And ) U tan C ( C velocity, axial constant for , Therefore l. symmetrica are triangles velocity the turbine, reaction 50% a In a 2 0 a w
- 16. Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay 16 Lect-20 Turbine Cascade • A cascade is a stationary array of blades. • Cascade is constructed for measurement of performance similar to that used in axial turbines. • Cascade usually has porous end-walls to remove boundary layer for a two-dimensional flow. • Radial variations in the velocity field can therefore be excluded. • Cascade analysis relates the fluid turning angles to blading geometry and measure losses in the stagnation pressure.
- 17. Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay 17 Lect-20 Turbine Cascade • Turbine cascades are tested in wind tunnels similar to what was discussed for compressors. • However, turbines operate in an accelerating flow and therefore, the wind tunnel flow driver needs to develop sufficient pressure to cause this acceleration. • Turbine blades have much higher camber and are set at a negative stagger unlike compressor blades. • Cascade analysis provides the blade loading from the surface static pressure distribution and the total pressure loss across the cascade.
- 18. Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay 18 Lect-20 Turbine Cascade
- 19. Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay 19 Lect-20 Turbine Cascade • From elementary analysis of the flow through a cascade, we can determine the lift and drag forces acting on the blades. • This analysis could be done using inviscid or potential flow assumption or considering viscous effects (in a simple manner). • Let us consider Vm as the mean velocity that makes and angle αm with the axial direction. • We shall determine the circulation developed on the blade and subsequently the lift force. • In the inviscid analysis, lift is the only force.
- 20. Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay 20 Lect-20 Turbine Cascade Inviscid flow through a turbine cascade
- 21. Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay 21 Lect-20 Turbine Cascade m m w w m m L w w m m w w cos ) tan (tan C S C V ) V V ( S V C V L C t, coefficien Lift form, l dimensiona - non a in lift Expressing ) V V ( S V V L , lift and ) V V ( S , n Circulatio α α α ρ ρ ρ ρ Γ ρ Γ 1 2 2 2 1 1 2 2 2 1 1 2 1 2 2 − = − = = − = = − =
- 22. Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay 22 Lect-20 Turbine Cascade • Viscous effects manifest themselves in the form to total pressure losses. • Wakes from the blade trailing edge lead to non-uniform velocity leaving the blades. • In addition to lift, drag is another force that will be considered in the analysis. • The component of drag actually contributes to the effective lift. • We define total pressure loss coefficient as: 2 2 2 1 02 01 V P P ρ ω − =
- 23. Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay 23 Lect-20 Turbine Cascade Viscous flow through a turbine cascade
- 24. Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay 24 Lect-20 Turbine Cascade m D m L m m m m tan C cos ) tan (tan C S C , t coefficien lift the , Therefore cos S V cos S L lift effective The cos S D , by given is Drag α α α α α ω Γ ρ α ω α ω + − = + = + = 1 2 2
- 25. Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay 25 Lect-20 Turbine Cascade • Based on the calculation of the lift and drag coefficients, it is possible to determine the blade efficiency. • Blade efficiency is defined as the ratio of ideal static pressure drop to obtain a certain change in KE to the actual static pressure drop to produce the same change in KE. m L D b D m C C m C C b sin C C , definition lift the in term C the neglect we If cot tan L D L D α η α α η 2 2 1 1 1 1 + = + − =
- 26. Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay 26 Lect-20 In this lecture... • Axial flow turbine • Impulse and reaction turbine stages • Work and stage dynamics • Turbine blade cascade
- 27. Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay 27 Lect-20 In the next lecture... • Axial flow turbine • Degree of Reaction, Losses and Efficiency