What fundamentally distinguishes cavitation and boiling as different phenomena?

Question

Cavitation and boiling are names for phenomenon that both involve sudden appearance of bubbles of vapor within a liquid, and in both cases they happen when the local hydrostatic pressure is lower than the vapor pressure of the fluid, but that doesn't necessarily mean they are the same thing.

In this video of an electrical heating element in water, between 01:00 and 02:00 the sound produced by rapid bubble collapse gets louder and louder but there are few visible bubbles. Is the process that produces this sound considered boiling or cavitation? What is the distinction?

I've left a provisional answer to the related question on another SE site: How (actually) do sub-cooled propellants reduce cavitation within turbo pumps and make feed easier? I couldn't bring myself accept the answer on that question that starts with the claim, "cavitation is boiling."

Although they are related, what fundamentally distinguishes cavitation and boiling as different phenomena?

Answer 1

Mechanical engineer here, former US Navy nuke. The textbook definition of cavitation is, from my nuclear training:

"The formation and subsequent collapse of vapor bubbles as the suction pressure falls below and then rises above saturation pressure."

This definition is referring to the suction pressure as in a pump, but I would say more generally and apparently against most of the other posters here that cavitation refers more to the formation and subsequent collapse of vapor bubbles than it does about how those vapor bubbles occur.

Now, I understand that the cavitation effect generally happens (or is most discussed when it happens) in pumps and propellers, but it also happens in boiling water.

When you bring water to a boil, initially it is quiet and there are no bubbles. At some transition point (nucleate boiling), bubbles form on the bottom of the pan, break away, but collapse before they reach the surface. This kind of boiling (referred to as a simmer in culinary terms) can correctly be referred to as cavitation. This is also a very noisy phase in the boiling process - this is the "noisy" period in OP's video.

After cavitation comes (for cooking, at least) the final boiling phase, in which the bulk fluid boils and bubbles reach the surface of the water (departure from nucleate boiling). Despite boiling appearing to be more vigorous, this is actually much quieter because cavitation is no longer occurring.

Cavitation is the pinging sound a pot of water makes before a full boil. Once a full boil is achieved, steam bubbles reach the surface and the quality of the sound changes from a pinging to more of a gurgle.

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All of that said, there has been a lot of talk on other posts about boiling being the application of heat and cavitation being about the reduction of pressure. Again, the reduction of pressure (below saturation pressure) is a cause of cavitation, but reducing pressure is not the definition of cavitation.

The term for creating vapor bubbles by reducing pressure is called flash distillation or flash evaporation. The term for creating vapor bubbles by increasing heat is called boiling.

The term cavitation refers to the formation and subsequent collapse of the vapor bubbles. Cavitation happens in pumps, in a pot of spaghetti water, in a submarine propeller, etc. It is not restricted to either mode of creation (pressure or heat). The video in OP's post shows cavitation during a boiling process.

:EDIT:

I felt challenged by Air's comment to produce a source for the definition of cavitation that I provided here. The line I quoted above is as-memorized from about 15 years ago now. I have (on a bookshelf at home) a condensed technical handout of non-classified information we were given at the conclusion of the nuclear training courses for personal reference. In trying to find this manual online, I found a technical publications website that appears to reproduce some of the content we were taught in the nuclear power training program.

The first mechanical science volume has a section on cavitation that states,

If the pressure drop is large enough, or if the temperature is high enough, the pressure drop may be sufficient to cause the liquid to flash to vapor when the local pressure falls below the saturation pressure for the fluid being pumped. Any vapor bubbles formed by the pressure drop at the eye of the impeller are swept along the impeller vanes by the flow of the fluid. When the bubbles enter a region where local pressure is greater than saturation pressure farther out the impeller vane, the vapor bubbles abruptly collapse. This process of the formation and subsequent collapse of vapor bubbles in a pump is called cavitation.

(Emphasis added) The definition we were instructed to memorize (as I quoted at the top) is the condensed version of this statement for reproduction on exams.

Now, there is no source on this particular website, where the reference volumes are broken out by section, as to where this material originates, but at the top of the page is given the DOE document "DOE-HDBK-1018/1".

You can look that number up and find the document posted in full on the Department of Energy's website, where that passage can be found on page 12.

Further, regarding the comment about "industry not toeing the line of the US Navy," the copy hosted on the DOE website includes a foreword and an overview that state the material was prepared with input from the nuclear industry and is intended for use in training nuclear operators. So, maybe some industries do not use the definition of cavitation I have provided, but the nuclear industry does, and it seems like (from Bryon Wall's comment) that the chemical industry does as well.

Answer 2

I think this is more about language than physics. The basic physical phenomenon - the phase change from liquid to gas when the vapor pressure equals the hydrostatic pressure in the fluid - is the same for boiling and cavitation.

In common (non-scientific) usage, "boiling" means heating the liquid until its vapor pressure is equal to the internal pressure of the fluid. In most "non-scientific" cases, the heating is done at (approximately) constant pressure with an interface between the liquid and a gas (e.g. water and air), and the vaporized liquid (steam) leaves the liquid and mixes with the gas, transferring heat from the liquid to the gas.

On the other hand "cavitation" is a local reduction of the pressure in the liquid, at (approximately) constant temperature. As with boiling, some of the liquid vaporizes when the liquid pressure equals the vapor pressure, but the vapor can't escape anywhere because the surrounding liquid is at a higher pressure. If a bubble of vapor starts to move through the fluid, it soon reaches a point where the fluid pressure is higher, and it collapses back into a liquid.

The sudden pressure waves in the liquid, which are created when the bubbles collapse, can cause damage to metal components like propellors, water turbines, etc.

Answer 3

The short answer is that cavitation and boiling both refer to a phase change from liquid to gas that causes bubbles to form, where cavitation is driven by a drop in pressure and boiling is driven by an increase in temperature. For a citation, see Cavitation and Bubble Dynamics, page 1:

A rough but useful way of distinguishing these two processes is to define cavitation as the process of nucleation in a liquid when the pressure falls below the vapor pressure, while boiling is the process of nucleation that ocurs [sic] when the temperature is raised above the saturated vapor/liquid temperature. Of course, from a basic physical point of view, there is little difference between the two processes... The differences in the two processes occur because of the different complicating factors that occur in a cavitating flow on the one hand and in the temperature gradients and wall effects that occur in boiling on the other hand.

If you want to know how this distinction can be useful, the full text of an older edition of the book is available via the Caltech library web site. Finding a more recent edition at the library shouldn't be difficult, considering the work has been cited nearly 3,000 times according to Google Scholar.

The long answer starts by noting that this quote doesn't pretend to give the only definitions of cavitation and boiling; it explicitly proposes one way of defining them as two processes that is "rough but useful." I expect Dr. Brennen would agree that there exist contexts in which other definitions are more useful.

In a very general sense, "cavitation" can mean the spontaneous appearance of cavities (also known as voids or bubbles) within a liquid. If you're researching how different materials or surface geometries promote or suppress nucleation, this might be a more useful definition to you than one that excludes heating.

In a more restrictive sense, "cavitation" can mean only that subset of the former that occurs at relatively constant temperature, in the presence of a solid interface, which later implodes and contributes to wear on mechanical components. If you're building a propulsion system for a submarine, this might be a more useful definition than either of the previous two.

The word "boiling" pre-dates modern thermodynamics so we shouldn't be surprised if it's hard to pin down. We typically think of boiling as a process involving bubbles but film boiling is an exception—clearly the folks researching what happens when you apply a ton of heat at a solid/liquid interface thought it was useful to put this phenomenon in the same category as nucleate boiling.

On the other hand, liquids are also said to "boil" in a vacuum (and here's a video of that, if you're curious—try to make out where the nucleation occurs!). Do you think the folks at NASA care whether boiling requires heat when they're working to mitigate risks associated with explosive decompression? I don't.

You gain very little by expecting, or giving the expectation of, objectively correct terminology. If you're doing some technical writing on the subject and intend to distinguish between cavitation and boiling, just make your definitions explicit. Do your due diligence to ensure that your definitions aren't a significant departure from consensus, or else build a very strong argument to support them.

Answer 4

I see, like me, you want a simple answer. In the kettle, the water is heated to boiling point around the element, but the surrounding water is not. Steam cannot exist at less than 100° C. at atmospheric pressure, so when the steam contacts the cooler water, it immediately condenses, leaving no cushioning effect, so it's like metal against metal.

Cavitation doesn't have to involve vaporization. A liquid like hydraulic fluid, if the pump inlet is restricted, vacuum bubbles form. Because there is no air in them to cushion the impact, again, it's like metal against metal. The pump sounds like it's crunching metal shavings. Even though it's a liquid, it impacts like metal & will fatigue metal parts. It also occurs if the flow is over a surface like the side of a ball & there is not enough pressure to keep it following the surface, or it flows off the edge of a surface like a propeller. The liquid is thrown off the surface and vacuum bubbles form, then collapse with no cushioning, giving a metallic crackling sound and eroding the propeller edges. It is even worse for submarines. It says "Here I am!" to the enemy. You can demonstrate the effect with a garden hose & a bucket of water. Take it up a flight of stairs. remove the fittings, and drape the hose over the handrail or hold it deep in the bucket and start syphoning the water, then slam your finger over the inlet. You'll here a faint metallic crack from inside the highest part of the hose as the water keeps going, then slams back against itself.

Answer 5

I'm a little confused, if you ask in relation about pump cavitation then my ans : cavitation emphasized on reverse boiling (when vapor back to liquid, the collapsing bubble). That's when destructive action done, surface impingement. Thats why impeller parts get damaged is near exit.

Answer 6

From a corrosion science standpoint, cavitation refers to what happens when a vapor bubble collapses against the surface upon which it was formed and thereby damages it.

The vapor bubble can be formed in the separated flow or "vacuum pocket" behind an overspeeding propeller blade or as a local vapor explosion on the surface of a hot object.

In the case of a hot object, the bubble collapse can occur either because the top of the vapor bubble moves far enough off the hot surface to encounter cooler fluid, which triggers its condensation and collapse (this is exactly what makes the kettle or heating element "roar" right before the bulk of the water gets hot enough that the whole of it begins to boil) or if the heat source is abruptly shut off immediately after the vapor explosion occurs. In either of these two cases, the bubble very quickly shrinks down to a point and the inrush of water behind it then strikes that point with pressures of order ~hundreds of atmospheres, on areas of order ~1 to 10 square microns.

The accumulation of cavitation damage is the leading cause of wearout mode failures in the heating elements of a thermal inkjet printhead, in which the cavitation is vigorous enough to beat holes down through the tantalum metal and/or silicon nitride or carbide which form a protective layer on top of the heaters.