# Heat loss due to Evaporation Cools remaining liquid, so why not model it that way?

1. Aug 20, 2009

### rkguy

I am curious. Maybe I don't understand something, but many articles I come across perform an energy balance on a solid and on a liquid.

**I assume evaporation acts on the remaining liquid and not directly on the substrate. Correct?**

They always include heat loss due to evaporation as energy loss from the solid but not the liquid. Would it be more proper to perform a balance on the liquid wherein evaporation removes heat (watts per square meter of surface area? simply KJ?) from the water surface, which would then increase convection from the solid substrate to the water itself?

THere must be a good reason I haven't once seen it described that way (other than laziness).

2. Aug 20, 2009

### Mapes

Re: Heat loss due to Evaporation Cools remaining liquid, so why not model it that way

Hi rkguy, welcome to PF. I'm a little confused by your comment about "evaporation as energy loss from the solid." Can you give a couple examples of the articles you're looking at?

3. Aug 20, 2009

### rkguy

Re: Heat loss due to Evaporation Cools remaining liquid, so why not model it that way

Mostly heat loss from pavement surfaces into water runoff.
For example, an article says that during a rain event

the balance on the pavement is change in energy = Rsolar net + Rlong net + Convection to water + Evaporation + Sensible heat + ground conduction

generally, R solar and ground conduction are positive (heating pavement), and Convection, Evaporation, Sensible heat, and Conduction are negative. Rlong varies. (all in W/m2)

4. Aug 20, 2009

### Mapes

Re: Heat loss due to Evaporation Cools remaining liquid, so why not model it that way

I agree that this seems confusing. The convection and evaporation terms seem to be double counting energy losses, as liquid evaporation would effectively cool the liquid and thus increase convective heat loss from the pavement. I concur that adding an energy balance on the liquid would make for a more complete (though also more complex) analysis.

5. Aug 20, 2009

### Topher925

Re: Heat loss due to Evaporation Cools remaining liquid, so why not model it that way

I think your confusing heat transfer with heat transport. In your parking lot analogy, when water is evaporating from the puddle the water vapor takes heat energy with it. Heat is not transferred from the liquid water to the water vapor.

Under your average parking lot conditions water exists in the liquid or vapor form. It is not phase changing from liquid to gas form.

6. Aug 20, 2009

### Staff: Mentor

Re: Heat loss due to Evaporation Cools remaining liquid, so why not model it that way

Also, unless you are talking about actual puddles - and decent depth ones - the energy content of the water is probably a whole lot less than the energy content of the parking lot.