# Vapor Pressure and Cavitation

1. Apr 4, 2016

### Lisciu

I straggle with getting full grasp of concept for Cavitation.

I was finding statement that : If The pressure of fluid is lower or equal to the Vapor Pressure then it will start to boiling. And I can't understood the reason behind this. Why actually it happens?

I understood where we came to the vapor pressure concept in closed system ( containter with lid). And this balance. And let say we have a vapor pressure equal 80kPa. Nothing is going to do in 20 o C. Then we reduce the pressure in container and it start to boiling. But why? It's hard to understand for me what is the reason for that? Because the pressure of liquid is higher then vapor?

And if we go further when we have opened lid it's even harded to imagine this. We have atmospheric pressure equal ~100 kPa. Then what is the value of vapor pressure? Is it equal then to atmospheric? If it's the partial pressure then it's a sum of some other pressure with atmospheric one? I don't know how to clear those things.

More to say that cavitation is if the liquid pressure is lower the vapor pressure or equal.

Additionally is the vapor pressure exist in the ocean? (under water)? If the cavitation occure on the pressure lower the vapor how it's possible? I was think about this cavitation as a pressure necesseary to be higher then pressure vapor to overcome it. But it's looks like it don't work like forces balance in mechanics. Is it like from higher pressure to lower and then this block the boiling?

2. Apr 4, 2016

### Staff: Mentor

If the pressure in a point inside a liquid is lower than the vapor pressure of the liquid, bubble can form in that place (vapor can "push" the liquid away). If the pressure of the vapor is lower, bubble won't form.

3. Apr 4, 2016

### Lisciu

I can't understand that.

First of all, what pressure of inside point? And then you mention vapor pressure of liquid (it's not a pressure of liquid but the pressure exert by vapor on surface of liquid. So we have two pressure of the same substance state?

Secondary why the pressure vapor can push something if it's acting on the top of the liquid phase and work against the liquid?

4. Apr 4, 2016

### Staff: Mentor

There is pressure in every point - if you were to put a manometer in the point it would tell what the pressure is there. When you jump into the lake, the deeper you go the higher the pressure is - and the pressure is property of the depth (so it can be assigned to any point below the surface).

Assuming liquid is in the equilibrium with its vapor (nothing unusual) there is a vapor pressure above the liquid surface, and the pressure inside of the liquid, the one mentioned above. These are two different things.

5. Apr 4, 2016

### Lisciu

Everything you mentioned above is correct and I know that. But still don't understand your previous post:

Here you wrote vapor pressure of the liquid...The vapor pressure of a pure substance is defined as the pressure exerted by its vapor in phase equilibrium with its liquid at a given temperature.
it's not the liquid pressure. It's just pressure exert on the liquid. And liquid have the pressure of it self (between molecules) like you mentioned above (changing with deep of the ocean).

I still see this like that: We have some vapor pressure acting on the surface of the liquid. This pressure push on the surface of the liquid. So if the pressure of the point inside of the liquid is lower it have not enough force to overcome the pressure force from vapour pressure so how it can be pushed away? It will be blocked by upper pressure (vapor acting on the surface) so it will can't move because there will be not enough force to overcome vapore pressure. That how I see this. And that why I don't undestand how it works. What actually mean for you pushed away?

6. Apr 4, 2016

### SteamKing

Staff Emeritus
Cavitation is a dynamic phenomenon: either the fluid is moving over something or something is moving thru the fluid, like a propeller blade.

When a propeller blade moves thru a fluid, zones of high and low pressure are created, as shown below:

The magnitude of the low pressure on the upper surface of the blade varies depending on the speed of the blade thru the fluid. If the blade turns faster, the pressure goes lower on the upper surface. If the low pressure drops below the vapor pressure of the fluid, then a cavitation bubble can form on the surface of the blade. The formation of the bubble is not the most serious problem, however. Because the low pressure created by the blade is subject to change, the cavitation bubble can collapse when the ambient pressure of the fluid increases momentarily above the pressure inside the bubble. This collapse can be strong enough to wear away part of the blade surface, causing pits and other imperfections in the blade, which imperfections reduce the amount of lift the blade generates and/or weaken the blade by removing material from it over time.

7. Apr 4, 2016

### Staff: Mentor

Wrong way around. It is vapor pressure that needs to push the liquid away to create a bubble.

8. Apr 4, 2016

### Lisciu

Okay, They other question why it need to be low to do it? It should be high to push with more energy the liquid? And also why we have vapor pressure in water? (vapor is not liquid and it confused me )

9. Apr 4, 2016

### SteamKing

Staff Emeritus
If you take a cup of liquid water and put it in a vacuum chamber, the liquid will turn to vapor and boil away. You don't need to boil the liquid with heat; lowering the ambient pressure below the vapor pressure of the liquid is sufficient.

10. Apr 5, 2016

### Lisciu

I know that. But I don't understand why it can be possible. What mechanism step by step is responsible for that. Why we talking even about vapor pressure inside the liquid. In most book it's write that vapor pressure is the pressure exert on the surface of the liquid.

11. Apr 5, 2016

### Staff: Mentor

And once the bubble is created that's exactly what happens.

The difference between gas and liquid is mainly in the distance between molecules (at least below critical temperature). Molecules inside of the liquid fluctuate, the faster the fluctuations, the higher the temperature is. If at any particular moment, because of these fluctuations, single molecule happens to be in a small cavity, it is like if a bubble was already created. Then, if the the vapor pressure is lower than the pressure in this particular point, the bubble will close, crushed by the liquid. If the pressure of the vapor is higher, the bubble will start to grow.

12. Apr 5, 2016

### Staff: Mentor

At an interface between a liquid and a vapor, the pressure of the vapor is equal to the equilibrium vapor pressure of the substance at the temperature of the interface and the pressure of the liquid is also equal to the equilibrium vapor pressure at the temperature of the interface. At the interface, both temperature and pressure are continuous across the interface.

Regarding cavitation, when cavitation occurs, it isn't that the pressure of the vapor gets high. It is that the pressure of the liquid gets low; low enough for vapor to form at the existing temperature.

13. Apr 5, 2016

### Lisciu

Okay seems like I understand more.
1. But few question rises. If the vapor pressure will be going infiniti high we will receive large bubbles that can't explode?

2. What is going on in case of the open situation ( I mean the container without lid). We have this atmospheric pressure that surrnding us. And the Vapor pressure that it's smaller then atmosperic one (in ambient temperature 20 oC. But isn't true that vapor pressure exist already in atmospheric one? I mean that the water vapor all the time, so we have some parial pressure from vapor in atmospehre already. The issue with this for me is that how then it can be that partial pressure will be bigger then total one?

3. Underwater we have the same situation like in closed lid? Because there are mixtured molecules of water and gas phase that can form those bubbles in proper condition(fish breathe somehow).

14. Apr 5, 2016

### SteamKing

Staff Emeritus
No one said anything about the vapor pressure being "inifiniti" high, whatever that means. The vapor pressure just has to be greater than the ambient pressure in the fluid.
Cavitation doesn't necessarily need a container to occur. The cavitation which occurs behind a ship's propeller is not confined within any container. The low pressure behind the propeller must be lower than the vapor pressure + atmospheric pressure for cavitation to occur. This means that cavitation is more likely to occur when a propeller blade is closer to the surface of the water than when it is deeper.
I think you are confusing dissolved oxygen and other gases here with water vapor. Fish breathe the oxygen which is dissolved in the water. No oxygen dissolved in the water, fish suffocate.

15. Apr 5, 2016

### Staff: Mentor

[QUOTE="SteamKing, post: 5433356, member: 301881" The low pressure behind the propeller must be lower than the vapor pressure + atmospheric pressure for cavitation to occur. [/QUOTE]
I don't think that this is correct. The absolute pressure behind the propeller must be lower than the vapor pressure for cavitiation to occur. The gauge pressure must be lower than the vapor pressure minus atmospheric pressure.

Chet

16. Apr 5, 2016

### SteamKing

Staff Emeritus
I don't think that this is correct. The absolute pressure behind the propeller must be lower than the vapor pressure for cavitiation to occur. The gauge pressure must be lower than the vapor pressure minus atmospheric pressure.

Chet[/QUOTE]
Yes, I got them mixed up. Thanks for the correction.