Cavitation in hydraulic machinery
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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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  • 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
Cavitation over a ving profile asymmetric.

Ref. Morten Kjeldsen


Types of cavitation in hydraulic machines asymmetric.

Saturated water vapor pressure versus temperature

Stages of cavitation

Ref. Hydraulic Machines, Turbines and Pumps

G.I. Krivchenko


NPSH asymmetric.

Net Pressure Suction Head

NPSH Net Positive Suction Head [m]

hv vapor pressure head [m]

HA atmospheric pressure head [m]

z2 Height above ref. line at location 2 [m]

z4 Height above ref. line at location 4 [m]

c2 mean velocity at location 2 [m/s]

s loss coefficient [ - ]


4 asymmetric.

z4

Losses


4 asymmetric.

z4

Let us introduce the vapor pressure, hv :


4 asymmetric.

z4

NPSHNet Pressure Suction Head

Atmospheric pressure: HA = h4


Suction head

4 asymmetric.

z4

Suction Head

hs


Submergence of a turbine
Submergence of a turbine asymmetric.

NPSH Net Positive Suction Head [m]

hv vapor pressure head [m]

HA atmospheric pressure head [m]

HS Submergence [m]

c2 mean velocity at location 2 [m/s]

s loss coefficient [ - ]


Npsh available and npsh required

NPSH available asymmetric.

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

NPSH available and NPSH required


Law of thoma
Law of Thoma asymmetric.

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 asymmetric.

Thoma’s Cavitation Coefficient s

Speed number W


Critical cavitation coefficient
Critical Cavitation Coefficient asymmetric.

Dh= 3 %

Efficiency, h [ - ]

sCritical

Thoma’s Cavitation Coefficient, s


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