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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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.

  • 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

Net Pressure Suction Head

NPSHNet Positive Suction Head[m]

hvvapor pressure head[m]

HAatmospheric pressure head[m]

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

NPSHNet Pressure Suction Head

Atmospheric pressure: HA = h4


4

z4

Suction Head

hs


Submergence of a turbine

NPSHNet Positive Suction Head[m]

hvvapor pressure head[m]

HAatmospheric pressure head[m]

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

NPSH available and NPSH required


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


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