Air cooling and humidity.

[Matthew Rogers]

Does humidity affect the amount of heat that can be removed from an engine with our air cooled engines? It certainly affects me, but we humans use evaporative cooling, not aluminum fins. However, does the extra water in the air allow it to absorb more heat per liter than dry air?

 

[chemgeek]

Removal of heat depends on thermal conductivity. Water vapor has a much lower thermal conductivity than air. (Low thermal conductivity is like a heat "blanket"). So humid air should be less efficient than dry air in heat transfer. But humid air doesn't contain that much water vapor by mole fraction so I suspect the change in thermal conductivity is fairly small compared to dry air.

 

[Hang 4]

Basically no difference up to 30C, then spread starts. @chemgeek has it right.

https://www.electronics-cooling.com/2003/11/the-thermal-conductivity-of-moist-air/

 

[Bill]

Steam, although very hot, is a very effective thermal insulator.

 

[sarangan]

Does thermal capacity have any effect here? Moist air does have a higher capacity.

 

[chemgeek]

Does thermal capacity have any effect here? Moist air does have a higher capacity.

Yes. Water has a much higher heat capacity than nitrogen or oxygen, mainly because of its having more vibrational modes than nitrogen or oxygen. That's the problem. It retains the heat like a blanket and doesn't transport it away.

Basically this is how thermal conductivity sensors work in gas chromatography. Stuff other than the carrier gas retains more heat over a hot filament, raising its temperature and resistance.

 

[MooneyDriver78]

Humidity reduces the horsepower produced, up to 10%.
Something to remember when you don’t seem to make book performance.

 

[Matthew Rogers]

Humidity reduces the horsepower produced, up to 10%.
Something to remember when you don’t seem to make book performance.

So if you are making 10% less horse power, you are also making 10% less heat, right? Could that actually cause it to seem like humid air cools better since the thermal conductivity appears to be similar at STP as shown by Hang 4?

 

[MooneyDriver78]

So if you are making 10% less horse power, you are also making 10% less heat, right? Could that actually cause it to seem like humid air cools better since the thermal conductivity appears to be similar at STP as shown by Hang 4?

Everything being equal, yes. Of course when it’s humid, it’s generally warmer.

 

[MauleSkinner]

Yes. Water has a much higher heat capacity than nitrogen or oxygen, mainly because of its having more vibrational modes that nitrogen or oxygen. That's the problem. It retains the heat like a blanket and doesn't transport it away.

But does it need to be “transported away”, since the air is flowing past the engine? (Or the engine is flowing past the air…or something like that.)

 

[chemgeek]

But does it need to be “transported away”, since the air is flowing past the engine? (Or the engine is flowing past the air…or something like that.)

Yes, the molecules have to move away from the object to remove the heat energy. While forced convection (airflow) helps, it doesn't solve the issue that water molecules retain more heat than oxygen or nitrogen in the air, so they are less efficient at removing heat from the object. The rms velocity of gas molecules is several times higher than that of your airplane through the air, and that's what gets molecules out of the boundary layer. They can only move so fast, dependent on the temperature, regardless of how fast you fly.