# Cavitation in hydraulic machinery - PowerPoint PPT Presentation

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Cavitation in hydraulic machinery. Cavitation in hydraulic machinery. The collapse of the bobble close to a surface will be asymmetric.

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Cavitation in hydraulic machinery

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### Cavitation in hydraulic machinery

• 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.

### Cavitation over a ving profile

Ref. Morten Kjeldsen

Types of cavitation in hydraulic machines

Saturated water vapor pressure versus temperature

Stages of cavitation

Ref. Hydraulic Machines, Turbines and Pumps

G.I. Krivchenko

NPSH

z2Height above ref. line at location 2[m]

z4Height above ref. line at location 4[m]

c2mean velocity at location 2[m/s]

sloss coefficient[ - ]

4

z4

Losses

4

z4

Let us introduce the vapor pressure, hv :

4

z4

Atmospheric pressure: HA = h4

4

z4

hs

### Submergence of a turbine

HSSubmergence[m]

c2mean velocity at location 2[m/s]

sloss coefficient[ - ]

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

### 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.

Thoma’s Cavitation Coefficient

Thoma’s Cavitation Coefficient s

Speed number W

### Critical Cavitation Coefficient

Dh= 3 %

Efficiency, h [ - ]

sCritical

Thoma’s Cavitation Coefficient, s