Relationship of air dissipating heat from water in a heat exchanger

[knight92]

I carried out an experiment in a wind tunnel and it shows that as I increase the air velocity the water temperature drop is bigger but the rise in air temperature going through the radiator becomes lesser. Am I wrong to understand that its due to the fact that the air mass flow rate has increased therefore the air stays in "one" place for less time so it doesn't have the time to reach the higher temperature ? So the faster your car goes the less difference there is in between the air temperature going in and coming through the radiator ? I keep contradicting myself thinking the air temperature rise should increase with air velocity as it is dissipating more heat. Can anyone explain this to me please ? Thank you. For ease of comparison I have attached a graph too.

 

[mfb]

Can you explain your setup and the measurements a bit more?

The graph says "drop in / rise in [...] temperature" but the scale gives values close to room temperature, which suggests your values are absolute temperatures and not differences.
This would also mean your water gets hotter with more air flow - if the idea is to cool water with air flow, this is counterintuitive.

If you know the temperature difference of the air, it could be interesting to look at the product of temperature difference and velocity.

 

[cjl]

Given the values, I wonder if the measurements were taken as temperature deltas in degrees C, and then converted to Kelvin by adding 273.15 (even though that's not the correct procedure for a temperature delta). That would explain the large values close to room temp.

 

[sanka]

I haven't looked at your values in any great detail but the characteristics you are seeing in your measurements are to be expected. You are on the right track when you say that "its due to the fact that the air mass flow rate has increased therefore the air stays in "one" place for less time so it doesn't have the time to reach the higher temperature".
Essentially as the air is moving faster through the heat exchanger, there is less time for the energy to diffuse through the adjacent layers of moving air and thus the terminal temperature of air will be lower. However, the actual heat transfer will obviously increase due to the additional thermal mass that is flowing through the heat exchanger which can absorb the thermal energy. Also, the increase in mass flow rate will lead to greater advection which is heat transfer by the bulk motion of the fluid.

The characteristic you are seeing is termed the "heat exchanger effectiveness". Have a quick look through a heat transfer book and you will see that this quantity decreases with increased air velocity.

Hope this helps.

 

[knight92]

Yes I think I wrongly converted the values to kelvin, I turned the difference between temperatures (degree celcius) to kelvins instead of converting both inlet and outlet temperature values to kelvins first. Also can you recommend any good books for heat transfer ? Something that easier to understand before going into detail? Thank you.

 

[knight92]

Can you explain your setup and the measurements a bit more?

The graph says "drop in / rise in [...] temperature" but the scale gives values close to room temperature, which suggests your values are absolute temperatures and not differences.
This would also mean your water gets hotter with more air flow - if the idea is to cool water with air flow, this is counterintuitive.

If you know the temperature difference of the air, it could be interesting to look at the product of temperature difference and velocity.

The setup is basically a hot water bath pumping water at 90 degrees Celsius to the radiators and thermocouples are inserted on both inlet and outlet of the radiators to measure the temperature difference. The radiators are inside a venturi shaped wind tunnel, so the fan on the other end sucks air through it and the air passes through the radiators.

 

[knight92]

Also I apologise for my late reply I did not have access to a computer

 

[russ_watters]

Sorry I didn't see this thread sooner; your understanding of what is happening is fine. Increasing airflow gives a lower air delta-T and higher heat transfer because more air spend less time in contact with it.

 

[knight92]

Sorry I didn't see this thread sooner; your understanding of what is happening is fine. Increasing airflow gives a lower air delta-T and higher heat transfer because more air spend less time in contact with it.

Can you refer me to any good books I can read about this ? I also want to read a book to reference this theory and prove that this is actually happening rather than say this is what I think. Thank you.

 

[sanka]

Yes I think I wrongly converted the values to kelvin, I turned the difference between temperatures (degree celcius) to kelvins instead of converting both inlet and outlet temperature values to kelvins first. Also can you recommend any good books for heat transfer ? Something that easier to understand before going into detail? Thank you.

There are a lot of very good heat transfer books out there. Opinions tend to differ on which is best, all down to a matter of preference I suppose. If your looking for a sort of introduction to the topic I would suggest "Heat Transfer" by Holamn. Good explanations of the various physical phenomena in this book which helps with understanding of a problem.

 

[russ_watters]

Agreed/expand: pretty much any college textbook in "Heat Transfer" will do.