In this lecture... Radial flow turbines Types of radial flow turbines Thermodynamics and aerodynamics Losses in radial flow turbines

Transcription

1 Lect- 35 1

2 In this lecture... Radial flow turbines Types of radial flow turbines Thermodynamics and aerodynamics Losses in radial flow turbines

3 Radial turbines Lect-35 Development of radial flow turbines dates back to 1830 s by Fourneyron, who developed the radial outflow type turbine. Later on Francis and Boyden developed the radial inflow type turbine. The inward-flow radial (IFR) turbine covers tremendous ranges of power, rates of mass flow and rotational speeds. IFR turbines are used in a variety of applications ranging from hydroelectric power plants to small gas turbines. 3

4 Radial turbines There are two types of inward flow radial turbines Cantilever turbine 90 o IFR turbine Cantilever turbine Similar to the impulse type turbine Little change in relative velocity across the rotor Aerodynamically very similar to the axial impulse turbine Can be designed in a similar manner as axial turbines. 4

5 Radial turbines Lect Nozzle blades Rotor blades Flow C V C a3 V 3 U U 3 Cantilever turbine arrangement and velocity triangles 5

6 Radial turbines Lect o IFR turbine This turbine has a striking similarity with a centrifugal compressor. The flow direction and blade motion are reversed. The flow enters the turbine radially and exits the turbine axially. Straight radial blades are generally preferred as curved blades would incur additional stresses. The rotor or impeller ends with an exducer. Usually the flow exiting the rotor passes through a diffuser to recover KE, which would otherwise be wasted. 6

7 Radial turbines Lect-35 Volute/Scroll 1 Nozzle blades Rotor blades 3 4 Flow Diffuser C α β 3 V C a3 V 3 U U 3 90 o IFR turbine arrangement and velocity triangles 7

8 Thermodynamics of radial turbines We shall consider a 90 o IFR turbine. Components include: nozzle, radial bladed rotor and diffuser. We shall assume complete adiabatic expansion in the turbine. Frictional processes cause the entropy to increase in all the components. There is no change in stagnation enthalpy/temperature across the nozzle and the diffuser. 8

9 Thermodynamics of radial turbines T 01 T 01 =T P 01 P 0 P 1 P T 03 =T 04 U 0 rel ss 3s 4 P 03 P 04 P 4 P 3 V c p C 3 c p T-s diagram for an IFR turbine s 9

10 Radial turbines Lect-35 Volute/Scroll 1 Nozzle blades Rotor blades 3 4 Flow Diffuser C α β 3 V C a3 V 3 U U 3 90 o IFR turbine arrangement and velocity triangles 10

11 Thermodynamics of radial turbines 11

12 Thermodynamics of radial turbines 1

13 Thermodynamics of radial turbines 13

14 Thermodynamics of radial turbines Nominal design Defined by a relative flow of zero incidence at the rotor inlet (V =C r ). An absolute flow at the rotor exit that is axial (C 3 =C a3 ). Therefore, with C w3 =0 and C w =U, the specific work for nominal design is W=U. 14

15 Thermodynamics of radial turbines 15

16 Thermodynamics of radial turbines T 01 T 01 =T P 01 P 0 P 1 P T 03 =T 04 U 0 rel 03 03ss 3 3ss 3s 4 P 03 P 04 P 4 P 3 V c p C 3 c p T-s diagram for an IFR turbine s 16

17 Thermodynamics of radial turbines 17

18 Thermodynamics of radial turbines 18

19 Thermodynamics of radial turbines 19

20 Losses in radial turbines 0

21 Losses in radial turbines 1

22 Losses in radial turbines

23 Losses in radial turbines In general, losses in a IFR turbine can be classified as: nozzle blade row boundary layers, rotor passage boundary layers, rotor blade tip clearance, disc windage (on the back surface of the rotor), kinetic energy loss at exit. The above sources of losses are of significance for determining the optimum design geometry. 3

24 Losses in radial turbines Incidence losses At off-design conditions, the fluid is likely to enter the rotor at a relative flow angle different from the optimum angle. This leads to an additional loss component due to incidence angles. Often defined as equal to the kinetic energy corresponding to the component of velocity normal to the rotor vane at inlet. There is an increase in entropy and hence a corresponding loss in enthalpy due to incidence. 4

25 1: before entry to rotor V Lect-35 Losses in radial turbines β β, opt V : after entry to rotor T U ss 3s P 01 0 rel P 0 P 1 P P 03 P 04 P 4 P 3 V c p C 3 c p T-s diagram for an IFR turbine s 5

26 In this lecture... Radial flow turbines Types of radial flow turbines Thermodynamics and aerodynamics Losses in radial flow turbines 6