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helius:
Ultrasonic power can produce "steam" but it is rather cold. I don't fully understand the kinetics but it causes fine droplets to fly out through the water reservoir into the air. When playing with a device like the one pictured below, with the tank removed, a shallow pool of water remains above the transducer. If you defeat the safety switch you can see it operating and feel the cavitation with your finger. It is hot immediately around the transducer when operating but the rest of the pool remains cold.

coppice:

--- Quote from: helius on October 29, 2023, 06:41:18 pm ---Ultrasonic power can produce "steam" but it is rather cold. I don't fully understand the kinetics but it causes fine droplets to fly out through the water reservoir into the air. When playing with a device like the one pictured below, with the tank removed, a shallow pool of water remains above the transducer. If you defeat the safety switch you can see it operating and feel the cavitation with your finger. It is hot immediately around the transducer when operating but the rest of the pool remains cold.

--- End quote ---
I think the difference between boiling the water and using ultrasonic cavitation is the cavitation approach produces a much bigger spread of kinetic energies in the molecules of the vapour. Molecules leaving the surface of boiling water are most just energetic enough to leave, and will quickly be below the threshold again. A lot of molecules leaving the ultrasonic device are way above that threshold. and will stay that way for a while. Certainly the resulting vapour is very different. Not only does it not give you the immediate sense of heat that putting your hand in the cloud above a kettle gives, as the vapour condenses on your skin, and pumps all its remaining energy into you. It also condenses more slowly, and only leaves puddles around the machine if you seriously constrict the air flow.

Nominal Animal:

--- Quote from: helius on October 29, 2023, 06:41:18 pm ---Ultrasonic power can produce "steam" but it is rather cold.
--- End quote ---
Better call it "mist" or "fog", as it is very small liquid droplets suspended in air, and not traditional steam (containing water vapour, water in gaseous phase).

Ultrasonic aerosolizers/nebulizers/atomizers/etc. do not convert the liquid to vapor (gas): there is no phase transition.  They only mechanically chop the liquid up into tiny little droplets.  There is basically no temperature change, and only very little energy spent in the mechanical chopping action.

helius:

--- Quote from: Nominal Animal on October 29, 2023, 09:34:43 pm ---Ultrasonic aerosolizers/nebulizers/atomizers/etc. do not convert the liquid to vapor (gas): there is no phase transition.  They only mechanically chop the liquid up into tiny little droplets.  There is basically no temperature change, and only very little energy spent in the mechanical chopping action.

--- End quote ---
Ultimately there must be a phase transition, because it increases the humidity of the air in the environment. Humidity is a function of the amount of vapor in the air. (Phenomena like fog or mist are created from water vapor as it condenses in proximity to the cool earth.)

Even the steam rising from a kettle is a mixture of water vapor and condensing droplets, because otherwise you wouldn't be able to see it; water vapor is as transparent as air.

I suppose you could correctly say that the thermal energy is provided by the ambient air molecules into which the droplets are emitted, as they share their kinetic energy with vapor molecules that come out of the droplets. This is the reverse from droplets condensing from steam, in which the hot water vapor gives up its heat to the cool air and recondenses.

TimFox:
Although dictionaries may differ on this, I was taught that "steam" was the visible material that included water vapor and droplets, as seen in a teakettle or steam locomotive, but that "water vapor" was invisible.
Humidity, however, can contain both water vapor and water droplets.

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