# Heat transfer from an air current across a body of water

1. Aug 7, 2015

### Infinitybyzero

Hello, I am looking for the general direction that I need to go to calculate the following:

The time-dependent change of temperature of a finite, but continuous, mass of air moving across a water/air boundary.

Let's say you have 150 degree F air moving at 0.007m^3/s over a relatively infinite source of water which is at 80 degrees. What sort of variables will I need to calculate the temperature of the air after it's moved parallel to the water boundary?

Thanks.

2. Aug 7, 2015

### Nidum

Is this ducted flow or open air flow ?

3. Aug 7, 2015

### Infinitybyzero

Somewhat open; I'm using a natural convection setup with an infinite air mass moving through a venturi section (which has the water), into a semi enclosed container, where it will no longer be in contact with the water.

4. Aug 7, 2015

### Nidum

The scale of the device will be relevant . Thin layers of air will cool much better and be more predictable than thick layers .

Also direct flat on flat contact of air/water may not be best arrangement for effective heat transfer . At least consider a simple heat exchanger with very large surface areas exposed to air and to water .

In any case ideally need to have movement of water as well as air . This may occur naturally or could be induced .

5. Aug 7, 2015

### Baluncore

If the air is 100% RH at entry, then flows over the colder water, precipitation may occur as the air cools.
If the air is 0% RH at entry, evaporation of water over an infinite path will increase the RH to 100%.
Whatever happens, the air that exits the chamber will be saturated, 100% RH.
You must therefore consider the energy exchange due to the possible phase changes of water.

6. Aug 8, 2015

### 256bits

Have a look at this,
https://www.ohio.edu/mechanical/thermo/Applied/Chapt.7_11/Chapter10a.html
which shows how to calculate for the adiabatic saturation temperature of a mass of air flowing over water. the duct is assumed to be very long so that the air exits at 100% humidity.

Note that the liquid pool of water has a mass flow rate m(dot)f, but you do not say if your water pool is being replenished or not.
Not to say whether your air achieves 100% humidity as that will depend upon your length of duct and flow rate.
If you have no liquid water flow, the temperature of the water will change also.

You can use that as a guide. Most surely you will have to adapt to your particular situation.