Cooling system thoughts

[LeeG]

for the experimentaly minded, here is a way to prove that wind chill only affects something that is wet:

Take 2 apples, peel one.  Go for drive and hold both out your window.  The peeled one will get colder until the surface is dried out. the other wont change temperature much at all.

[jtanguay]

There is also different factors that decrease the rad temp.  First off, its metal, so it conducts heat quite well.  The fast moving air flowing through the rad will be somewhat pressurized (at about 100km/h), and allows for greater heat removal, on the molecular scale.  If its cold + humid, it can cool down even more.  Of course the pressure I'm talking about is pretty small, but does make a difference.

To explain, the molecules slamming into your rad are being somewhat compressed to eachother, allowing them to absorb more heat than they would going at a slower speed.  

Of course the temp differential is nowhere near as great as the 'wind chill'

[avocado_tom]

If its cold + humid, it can cool down even more.  Of course the pressure I'm talking about is pretty small, but does make a difference.

Actually, humid air is less dense than normal air, so it doesn't cool quite as well as dry air, which is fairly counter-intuitive.

[jtanguay]

If its cold + humid, it can cool down even more.  Of course the pressure I'm talking about is pretty small, but does make a difference.

Actually, humid air is less dense than normal air, so it doesn't cool quite as well as dry air, which is fairly counter-intuitive.

I wonder how that works?  Water can hold 4 times the amount of heat that air can hold.  The moisture in the air will be as cold as the air, and as such will be able to absorb 4 times the amount of heat.   Isn't that how water injection works?

Yes air below 0C can be humid too...  Us poor Canadians know all about that!  The cold won't kill ya, its the humidity!

[avocado_tom]

If its cold + humid, it can cool down even more.  Of course the pressure I'm talking about is pretty small, but does make a difference.

Actually, humid air is less dense than normal air, so it doesn't cool quite as well as dry air, which is fairly counter-intuitive.

I wonder how that works?  Water can hold 4 times the amount of heat that air can hold.  The moisture in the air will be as cold as the air, and as such will be able to absorb 4 times the amount of heat.   Isn't that how water injection works?

Actually, we're both wrong.   (or both right )

Humid air is less dense than dry air (h20 has a lower molecular weight than n2; gas density at atmospheric temperatures - i.e. where the Ideal Gas Law applies - is based on molecular weight), therefore, for a given speed you'll get a more MASS flowing through your radiator which helps your cooling.

However, humid air has a higher heat capacity than dry air.  It's not four times as much - that's LIQUID water vs. air - but more like 1.5 to 2 times as much on a mass basis.  But again, that's on a MASS basis, not a molar basis - which is what we care about for gasses.  

On a *molar* basis, water vapor has a slightly higher heat capacity than N2, but not much.   (I don't have time to do the exact calcs)  

So: I think it's probably a wash and they cool your radiator just about the same.  

Wikipedia has some of the heat capacity data here: http://en.wikipedia.org/wiki/Specific_heat_capacity but they're measuring the Cp of H20 vapor at 100C and N2 at 25C, so you can't compare them as heat capacity is dependant on temperature.  Same for the Air Cp's listed: one is dry air at 0C and the other is humid air at 25C.

[jtanguay]

Well my explanation only applies to cold air (with is more dense).  In hot weather, dry is definitely better.  I also stated that the moisture in the air will absorb 4 times the heat.  The air won't

This post has gone really scientific