Does cavitation cause pressure increase?

[mahony]

Hi Guys,

 

If water is in a sealed unit and some of the water is pumped out until it reached a certain negative pressure say-100kPa. At this point the water cavitates and air comes out of the water. what will happen the pressure as the air comes out?

 

 

Thanks a million for any help

[Wilmot McCutchen]

Please clarify "as the air comes out." Are you continuing to pump out the air? Stripping the dissolved noncondensible gases from the water and from the sealed unit will make the cavitation implosions stronger because the cushion of noncondensibles is gone. Shear, e.g. from shaking the sealed unit, will give a crackling sound.

[alpha2cen]

The cause of a cavitation is liquid to gas phase transform. At the low pressure caused by high fraction loss, liquid phase is changed to gas phase. This is an important problem for designing a good pump for liquid transport.

[John Cuthber]

100 KPa isn't a negative pressure.

Anyway, shortly after the system cavitates, the water will evaporate and raise the pressure to the vapour pressure of water at that temperature.

[mahony]

Sorry for the delay in getting back and thanks for all the help its fantastic help!

 

I apologise I am not a scientist but an Engineer thats why I am asking you guys,

 

The temperature is controlled at 20 degrees Celcius. The pressure applied as rightly pointed out is -80 to -100kPa which is about 20 to 1kPa absolute pressure (I think I'm right there) The water wouldnt boil and turn to vapour at this surely but it is cavitating( air bubbles forming). The water is stored in sand in a sealed container (the reason for all this is I am a Geotehnical Engineer and I'm trying to understand a certain ground condition). Air bubbles appear to be created and rise and form an air pocket at the top. With an air pocket created now at the top of the container would you expect a change in pressure.

 

Another question is does anyone know where I could find information on the effect of Time. I mean if negative pressure is created slowly or fast does it make a difference?

 

Thanks for any help

[John Cuthber]

The water will have a vapour pressure of about 2.6 KPa and will boil if you try to drop the pressure below that.

The effect of time should be small. It will boil quickly, then settle down. However as it boils it will get cold. If you then seal the system and let it warm up the pressure will rise.