Heat transfer from an air current across a body of water

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Discussion Overview

The discussion centers on calculating the time-dependent change of temperature of air moving across a water/air boundary, specifically in a natural convection setup. Participants explore the variables and conditions affecting heat transfer between the air and water, including humidity and flow characteristics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant seeks guidance on variables needed to calculate the temperature of air after it moves parallel to a water boundary, specifying conditions such as air temperature and flow rate.
  • Another participant questions whether the flow is ducted or open, indicating the importance of flow type in heat transfer calculations.
  • A participant describes the setup as somewhat open, involving natural convection and a venturi section, which may affect the heat transfer dynamics.
  • Concerns are raised about the scale of the device, suggesting that thin layers of air may cool more effectively than thicker layers, and proposing the use of a heat exchanger for better heat transfer efficiency.
  • One participant notes that if the air is at 100% relative humidity (RH) upon entry, cooling over the water could lead to precipitation, while air at 0% RH would experience evaporation, affecting the humidity and energy exchange.
  • A suggestion is made to refer to a resource that outlines calculations for adiabatic saturation temperature, emphasizing that the length of the duct and flow rate will influence whether the air achieves 100% humidity.
  • It is mentioned that the water pool's mass flow rate is relevant, and the temperature of the water may change if there is no liquid water flow.

Areas of Agreement / Disagreement

Participants express various viewpoints on the factors influencing heat transfer, including flow characteristics, humidity, and device scale. No consensus is reached on a specific approach or solution.

Contextual Notes

Limitations include the dependence on the specific setup and conditions not fully detailed, such as whether the water pool is being replenished and the effects of varying humidity levels on heat transfer.

Infinitybyzero
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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.
 
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Is this ducted flow or open air flow ?
 
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.

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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 .
 
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.
 
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.
 

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