Relative humidity and engine cooling

[BrettinSanAntonio]

I've come across a number of threads over the course of the summer and relative humidity is often considered when discussing the operating temperature of our engines.

My belief is this: I don't think humidity has anything affect with regard to cooling or operating temperature regulation for our trucks. Since radiators use heat transfer vs. evaporative cooling, relative humidity shouldn't pose any factor. There's no heat index for our rigs.

Thoughts?

[scottm]

I agree. As long as it's not raining.

[scottm]

Googled:

"Abstract  The convective heat transfer from a cylinder to a humid air stream flowing normal to the cylinder was investigated experimentally at atmospheric pressure over a range of variables which is relevant to the use of hot‐wire anemometry: air temperatures between 30 °C and 70 °C and velocities between 12 and 37 m/s. For molar fractions of water vapour up to 0.27, the heat transfer increased with increasing humidity. The ratio of heat transfer rates in humid air and dry air is a unique function of the molar fraction of water vapour, independent of the air temperature and flow velocity."

[BrettinSanAntonio]

I just read the source article or one similar to it and I believe they're refering to HVAC Chillers, not closed, pressurized systems for removing heat from an already hot engine.

Originally Posted by scottm

Googled:

"Abstract  The convective heat transfer from a cylinder to a humid air stream flowing normal to the cylinder was investigated experimentally at atmospheric pressure over a range of variables which is relevant to the use of hot‐wire anemometry: air temperatures between 30 °C and 70 °C and velocities between 12 and 37 m/s. For molar fractions of water vapour up to 0.27, the heat transfer increased with increasing humidity. The ratio of heat transfer rates in humid air and dry air is a unique function of the molar fraction of water vapour, independent of the air temperature and flow velocity."

[dgangle]

Wow, my head hurts. Been a while since I had to even remember what a mole fraction even is.

Engineer hat off, common sense hat on.....
30-70C is 86-158F. If it ever gets anywhere close to that upper end of this study then we are all going to have MUCH bigger problems that our vehicles overheating! The way I have always thought about this is simple. Innate objects like our car cooling systems are dumb. They do not know humidity, only heat or the absence of it. Our bodies' cooling systems are much different as they rely on evaporation as the principle method of cooling. The sweat evaporates slower because the air is saturated with water already.

Prolly opening a can-o-worms here, but good discussion nonetheless.

[BrettinSanAntonio]

Yep, I had to read it a few times to make the big words stick.

I know this: As a human, living in South Texas during the summer is uncomfortable. When fall, winter and spring are here, I'm happy because the air is cool and dry. Little humidity but greater temperature swings between high and low temperatures. In the summer, high humidity and small temperature swings due to the moisture content of the air.

[LukeZero]

Relative humidity is related in the fact that the higher the moisture content of the surrounding air, the more saturated it is with energy. [Latent vs. Sensible Load] The more saturated the air is, the harder it is to force more energy into it [out of the radiator]. [Delta T]

I earn a living in the HVAC industry. All of our calculations for air conditioning take into account the dry bulb temperature and the wet bulb temperature. Since the outdoor portion of the system is, in effect, a radiator, radiating heat that was picked up inside the dwelling, with no evaporative cooling- I believe it to be an accurate parallel to an automotive radiator.

This is partially offset by the fact that the moisture content of the air is actually carrying energy off of the radiating surface into the surrounding air. But that is negligible for the sake of this discussion.

Luke

[inkpot]

Howdy! Interesting question. I'm pretty sure that water transfers heat better than air. I think it's is a matter of how much humidity makes how much difference. The spray from a garden hose ( 100% humidity) will certainly cool off an overheated engine very quickly. What we really need for an answer is a link to someone's Master's Thesis on heat transfer at the molecular level. John

[LukeZero]

Originally Posted by inkpot

Howdy! Interesting question. I'm pretty sure that water transfers heat better than air. I think it's is a matter of how much humidity makes how much difference. The spray from a garden hose ( 100% humidity) will certainly cool off an overheated engine very quickly. What we really need for an answer is a link to someone's Master's Thesis on heat transfer at the molecular level. John

Would you settle for some of the calculations for said thesis, that at this point won't ever get written?

[landtank]

doesn't it have to do with the airs capacity to absorb heat. Very dry air has a low density and will heat saturate much quicker than that of humid more dense air. I would think that in hot dry conditions the air would saturate before it even made it completely through the radiator and produce a hot running engine.

And for those guys running pusher fans they are only increasing the total volume of air passing through in a given period of time essentially increasing the airs capacity to absorb heat.

[Tools R Us]

Originally Posted by landtank

doesn't it have to do with the airs capacity to absorb heat. Very dry air has a low density and will heat saturate much quicker than that of humid more dense air. I would think that in hot dry conditions the air would saturate before it even made it completely through the radiator and produce a hot running engine.

And for those guys running pusher fans they are only increasing the total volume of air passing through in a given period of time essentially increasing the airs capacity to absorb heat.

That's pretty much how it's been explained to me, moist air has more capacity to transfer, carry away heat than dry air.

The "how I feel when it humid thing" is not relevant. We cool by transpiring moisture, when it's humid the moisture takes longer to evaporate, so we feel hotter. Automotive cooling is dry surface, so it's all about the air's ability to transfer heat, when the air is dry, more airflow is needed to do the same job.

[clarkrw3]

Originally Posted by Tools R Us

That's pretty much how it's been explained to me, moist air has more capacity to transfer, carry away heat than dry air.

The "how I feel when it humid thing" is not relevant. We cool by transpiring moisture, when it's humid the moisture takes longer to evaporate, so we feel hotter. Automotive cooling is dry surface, so it's all about the air's ability to transfer heat, when the air is dry, more airflow is needed to do the same job.

Yes...that's what I have been trying to say over on the other thread

[Cattledog]

As the other have stated the heat capacity of air is dependent on the mole fraction of water vapor or other gasses in the mixture. Thankfully water vaporis the gas that changes most offen not the mixture of oxygen. Change the mixture's (e.g. water vapor) content and it's heat capacity changes.

I will see if I can find a table for the heat capacity of air at different mole fractions of water vapor. Let avoid including air pressure and temperature that will cause more changes too.

[BrettinSanAntonio]

Originally Posted by Tools R Us

That's pretty much how it's been explained to me, moist air has more capacity to transfer, carry away heat than dry air.

The "how I feel when it humid thing" is not relevant. We cool by transpiring moisture, when it's humid the moisture takes longer to evaporate, so we feel hotter. Automotive cooling is dry surface, so it's all about the air's ability to transfer heat, when the air is dry, more airflow is needed to do the same job.

Is the air's ability to absorb heat with more humid air that much of a factor as say, volume of air circulated? Are we talking real measurable numbers here?

I think I see where you're coming from with the humid air will absorb more heat than dry air in the context of heat transfer from a radiator but there was some confusion regarding the density and rate of absorption of heat based on the humidity level. I need to crack open a beer and let the concept percolate a bit.

[scottm]

Humid air is less dense than dry air, it seems like less density would mean less thermal mass to absorb heat. Opposite what I've read so far, so there must be some other mechanisms going on than simply thermal mass.

[NLXTACY]

Originally Posted by scottm

Humid air is less dense than dry air

Umm no. Humid air contains water molecules making its more dense then air with little to no water molecules.

[scottm]

Originally Posted by NLXTACY

Umm no. Humid air contains water molecules making its more dense then air with little to no water molecules.

? I'll have to check. In my early flying days humidity made air less dense. Density didn't seem important for jets, so it's been many years since I needed that info, maybe I have it backward.

[NLXTACY]

Altitude makes air less dense

[scottm]

Wiki:

"Humidity and air density
Humid air is less dense than dry air because a molecule of water is less dense than molecules of nitrogen and oxygen. Isaac Newton discovered this phenomenon and wrote about it in his book Opticks.[1] Avogadro's ideal gas law states that a fixed volume of gas at a given temperature and pressure always contains the same number of molecules regardless of what type of gas it is. Consider a cubic meter of dry air. About 78% of the molecules are nitrogen (N2), with a molecular weight of 28. Another 21% of the molecules are oxygen (O2), with a molecular weight of 32. The final 1% is a mixture of other gases. Combining these weights in the correct proportions gives an average molecular weight for air of about 29. If molecules of water vapor (H2O), of molecular weight 18, replace the diatomic nitrogen or oxygen molecules in this fixed volume then the weight of the air decreases, and hence the density decreases. Thus, humid air has a lower density than dry air at a specified temperature and pressure."

[NLXTACY]

What a minute.... Caveat Caveat!!!!!!

Temperature makes a HUGE roll in density. If the temperature rises, it expands the water molecules which expands its volume. That would make it less dense.

[scottm]

Originally Posted by NLXTACY

Altitude makes air less dense

The air inside my plane remains the same density at any altitude if I keep it pressurized the same.

[NLXTACY]

HAhahaha, I didn't take a minute to check my books but you got it from Wiki.

SCOTTM I humbly stand aside

Need to take temperature, altitude (pressure) and moisture all into context.

[NLXTACY]

Originally Posted by scottm

The air inside my plane remains the same density at any altitude if I keep it pressurized the same.

hahaha this is true!

[scottm]

Originally Posted by NLXTACY

What a minute.... Caveat Caveat!!!!!!

Temperature makes a HUGE roll in density. If the temperature rises, it expands the water molecules which expands its volume. That would make it less dense.

Would it matter if it had water molecules in it? Heating any gas makes the molecules more active and they need more room. Does humid air expand differently than dry? More or less? I have to get up in six hours, can't think this hard. G'nite.