1. 1 1 1 Cavitation in hydraulic machinery

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3. 3 3 3 The collapse of the bobble close to a surface will be asymmetric. A jet stream will be formed in the center and hits the surface with large impulse. It has been measured pressure pulses up to 1000 bar and velocities around 200 m/s in a collapsing bubble. The collapse creates local pressure oscillation with a large amplitude. It is not known if it is the jet stream, pressure pulse or both that causes the damage to the surface.

4. 4 4 4 Cavitation over a ving profile Ref. Morten Kjeldsen

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7. 7 7 7 Saturated water vapor pressure versus temperature Stages of cavitation Types of cavitation in hydraulic machines Ref. Hydraulic Machines, Turbines and Pumps G.I. Krivchenko

8. 8 8 8 NPSHNet Positive Suction Head[m] h v vapor pressure head[m] H A atmospheric pressure head[m] z 2 Height above ref. line at location 2[m] z 4 Height above ref. line at location 4[m] c 2 mean velocity at location 2[m/s]  s loss coefficient[ - ] NPSH Net Pressure Suction Head

9. 9 9 9 Losses

10. 10 Let us introduce the vapor pressure, h v :

11. 11 NPSH Net Pressure Suction Head Atmospheric pressure: H A = h 4

12. 12 Suction Head hshs

13. 13 Submergence of a turbine NPSHNet Positive Suction Head[m] h v vapor pressure head[m] H A atmospheric pressure head[m] H S Submergence[m] c 2 mean velocity at location 2[m/s]  s loss coefficient[ - ]

14. 14 NPSH available and NPSH required NPSH available –This is the NPSH that is given by the site where the turbine is installed NPSH required –This is the NPSH that the turbine required for non-cavitating operation

15. 15 Law of Thoma Provided that similar hydraulic cavitating flow remain unchanged relative to the flow canals, the relations of hydraulic similar flow, are valid also for flow including cavitation.

16. 16 Thoma’s Cavitation Coefficient Speed number  Thoma’s Cavitation Coefficient 

17. 17 Critical Cavitation Coefficient Efficiency,  [ - ] Thoma’s Cavitation Coefficient,   Critical  = 3 %