Energy requirements for creating water mist

[gpowell]

The basis of the question is: in creating a mist that you sprayed on a surface could you get an energy
transfer to the surface equivalent to an optical emission in the infrared? (Far Infrared).

How much energy is imparted to room temp water in creating a spray of particles around 5 microns or greater? I realize that the energy level will relate to particle size and creation rate.
If you sprayed a surface with these particles it seems like the physical reactions taking place would be quite complicated several competing processes occurring: the energy of the particles hitting the surface, evaporation etc?
Would appreciate hearing your thoughts on this.

 

[trurle]

For 5um water droplets coalescing into film, you theoretically can get enough surface energy release for temperature rise of 21 C. The problem is loss for evaporation (~800 W/m2 at +5C) and conduction though - about 400 W/m2 at 20 C temperature rise, to make surface energy dominant you need roughly water film growth speed at least 30 um/s. Normally that fine spray&film energy balance would be dominated by evaporation, not by surface energy.

 

[russ_watters]

The basis of the question is: in creating a mist that you sprayed on a surface could you get an energy
transfer to the surface equivalent to an optical emission in the infrared? (Far Infrared).

Welcome to PF!

Could you please elaborate on what you are trying to do. Infrared radiation heat transfer depends on the temperature of the emitter and target, which you didn't specify. And water mists tend to cool, not heat the surface they are sprayed on. So what exactly is this idea supposed to accomplish?

 

[gpowell]

Welcome to PF!

Could you please elaborate on what you are trying to do. Infrared radiation heat transfer depends on the temperature of the emitter and target, which you didn't specify. And water mists tend to cool, not heat the surface they are sprayed on. So what exactly is this idea supposed to accomplish?

I think the question could be reduced to whether in converting water to a spray there is a net heat gain when the spray hits a surface? In this case the surface is human skin and all temperatures (except the skin) or at room temperature. I would think the relationship would change if the water contained a long chain organic compound in dilution. Does that make sense?

 

[trurle]

I think the question could be reduced to whether in converting water to a spray there is a net heat gain when the spray hits a surface? In this case the surface is human skin and all temperatures (except the skin) or at room temperature. I would think the relationship would change if the water contained a long chain organic compound in dilution. Does that make sense?

Yes, net gain of energy when spray hit the water-coated surface do exist. Fine water spray create the suffocation feeling in moment when inhaled exactly for this reason - water film in lungs heat up after absorbing spray, resulting in water evaporation and oxygen displacement. Also, buildup of water create oxygen diffusion barrier, making suffocation worse with time. The droplet range you are looking for is typical of alveolar inhalator.

Regarding soluble molecules, they will not have much effect. To have significant effect, you need surface-sticking molecules (detergents or such).

 

[gpowell]

Yes, net gain of energy when spray hit the water-coated surface do exist. Fine water spray create the suffocation feeling in moment when inhaled exactly for this reason - water film in lungs heat up after absorbing spray, resulting in water evaporation and oxygen displacement. Also, buildup of water create oxygen diffusion barrier, making suffocation worse with time. The droplet range you are looking for is typical of alveolar inhalator.

Regarding soluble molecules, they will not have much effect. To have significant effect, you need surface-sticking molecules (detergents or such).

 

[gpowell]

Thanks for your help. I have learned a lot. I encountered this problem in a conversation and thought it was interesting. As a side question, if there is a net heat gain under these conditions would you (conceptually) expect to see infrared emissions? Or would those require a cooling molecule to emit?

 

[trurle]

Thanks for your help. I have learned a lot. I encountered this problem in a conversation and thought it was interesting. As a side question, if there is a net heat gain under these conditions would you (conceptually) expect to see infrared emissions? Or would those require a cooling molecule to emit?

I think you can observe some of emission at 200um (hydrogen bond bending) and 6.08um (water molecule bending) in addition to dominant thermal continuum. It would be an interesting experiment to see.