Temperature effects on cavitation in an orifice plate 1

[sykhan88]

Cavitation is caused by changes in pressure. I want to know if temperature can have any effect on cavitation of fluid flowing through an orifice plate. The fluid in question is water. Most of the data available about cavitation is at room temperatures. Does increasing the temperature of water, particularly to superheated conditions, have any effect on how cavitation behaves in an orifice.
The orifice in question has a low L/D value and a rounded inlet edge.

 

[ione]

Cavitation is induced in a liquid when its pressure drops below its vapour pressure.
When water flows through an orifice plate it experiences a sudden pressure drop, and so being vapour pressure related to water temperature (more precisely vapour pressure increases as temperature increases) the higher the water temperature the higher the likelihood to have cavitation.
Now if you are dealing with superheated water, you are dealing with water in liquid state at a temperature above 100 °C (and below its critical temperature of 347 °C), and this obtained just applying an overpressure. So pressure drop in the orifice plate can induce cavitation.

If you want to go deeper on this matter, consider the Antoine equation which describes the relation between vapour pressure and pressure for pure liquid.

http://www.ddbst.com/en/online/Online_Calc_vap_Form.php

 

[sykhan88]

Thanks ione for the reply.
I realize thats the general behavior. You increase the temperature, you increase vapor pressure and so for the same liquid cavitation will occur quicker.
All materials change with temperature, but water shows changes which are much greater than would be expected from temperature considerations alone. Viscosity and surface tension of water drop with increasing temperature. You would not expect the same changes for water under 100 degree C. This would affect other parameters like Reynolds number etc
Now there are lots of mathematic formulas describing flow behavior through pressure differences. We can calculate discharge coefficients and mass flow rates etc. These formulas I believe are based on room temperatures.
Now, since properties change drastically at superheated conditions, are there any relations which take temperatures into condiserations when calculating discharge coefficients and mass flows. Or can we just input the new temperature dependant parameters(e.g density which was not highly-temperature dependant at normal conditions) in the same formulas.
I hope you understand what I am getting at?

 

[ione]

I agree that properties water change in superheated state, anyway superheated water is effectively a liquid in a stable state, and pressure is what ensures this. At least under vacuum conditions water is in a superheated state at temperatures well below the water boiling point at atmospheric pressure.
So why not apply formulas usually applied for liquids and orifice plates?

 

[sykhan88]

So what you are saying is that since water is under superheated state at temperatures well below the water boiling point at atmospheric pressure, we can use these formulas since they were formulated under temperatures below the water boiling point at atmospheric pressure.
I think the properties should still change drastically with temperature since its superheated regardless of wether its below or above the water boiling point at room temperature.
That aside,
my problem requires increase in temperatures above the boiling point, so the vacuum condition can not apply to it.
I have high inlet temperatures where water is superheated and then ambient outlet temperatures. So for such temperatures, do the same relations and formulas apply?

 

[Latexman]

This may be further complicated by what is called "frozen equilibrium". If the superheated water travels through the orifice quickly, equilibrium at the conditions that exist in the orifice is never really obtained at the plane of the orifice. Equilibrium is obtained further downstream, after sufficient time at fairly stable conditions, from the orifice, albeit at the conditions that exist at the downstream point. Some research on frozen equilibrium, two phase flow, and flashing flow may be beneficial.

Good luck,
Latexman

 

[ione]

Well I wanted just to emphasize that superheated water is in liquid state by definition and so why not apply consolidated formulas.

The discharge coefficient formulas account for fluid properties as Reynolds number enters there. Only if the formula for the discharge coefficient were valid in a specific range of Reynolds number, and your Reynolds number were out of this range, you could have doubts on the validity of the formula. But as far as I know there isn’t any restriction on Reynolds number to consider valid or invalid the formulas available.

 

[zdas04]

Guys, the definition of "cavitation" is "the formation and subsequent collapse of vapor bubbles in the flow". It is a phase change phenomena. Consequently, if the change in thermodynamic conditions going through the orifice drops the fluid below the boiling temperature for the new pressure, then bubbles will form. If the pressure recovery downstream of the orifice raises the pressure for the fluid temperature above the boiling point then the bubbles will collapse and cavitation can happen.

Of course temperature matters which was the original question.

David