c5load From United States of America, joined Sep 2008, 917 posts, RR: 0 Posted (5 years 7 months 4 days 5 hours ago) and read 5706 times:
I've noticed many times on very cold days, even when everything from my car exhaust to my breath is steaming, jet engines don't. Why is this? I have noticed, at least on the C-5, a little steam coming out of the pylon-engine connection, so I know it is cold enough to steam.
"But this airplane has 4 engines, it's an entirely different kind of flying! Altogether"
Maverick623 From United States of America, joined Nov 2006, 6017 posts, RR: 7
Reply 1, posted (5 years 7 months 4 days 4 hours ago) and read 5700 times:
Quoting c5load (Thread starter): I've noticed many times on very cold days, even when everything from my car exhaust to my breath is steaming, jet engines don't. Why is this? I have noticed, at least on the C-5, a little steam coming out of the pylon-engine connection, so I know it is cold enough to steam.
What you are referring to is not steam, but merely condensation due to a low temperature/dew point spread..... same exact principle as fog.
Your car exhaust condenses readily because of all the impurities in it.
Your breath condenses because there's a LOT of water vapor in it.
Jet engine exhaust doesn't condense because it's too hot. The steam you see coming out of connections is just that.... evaporated water.
tdscanuck From Canada, joined Jan 2006, 12710 posts, RR: 80
Reply 4, posted (5 years 7 months 4 days 1 hour ago) and read 5534 times:
Quoting c5load (Thread starter): I've noticed many times on very cold days, even when everything from my car exhaust to my breath is steaming, jet engines don't. Why is this?
1) It's too hot (as Maverick623 said in Reply 1)
2) They do, just not in a form you can see
3) Jet exhaust has a very low fuel/air ratio, so the relative humidity in the exhaust is really low
prebennorholm From Denmark, joined Mar 2000, 6892 posts, RR: 54
Reply 5, posted (5 years 7 months 3 days 5 hours ago) and read 5061 times:
A jet engine always produced roughly one m3 steam for every liter fuel burned. It converts all the fuel into steam and CO2 (except for a tiny part not completely burned fuel which generates black smoke).
You never see steam. Steam is invisible.
What you see on a cold day at your car exhaust is the steam condensing into water droplets. What you see is not steam, but liquid water. Air can only contain a certain amount of steam, and that amount is very dependent upon air temperature. When that amount is exceeded, then the remaining steam condenses into water droplets. Normally you see the water droplets from your car exhaust only for a short while after starting a cold car. In that situation the steam condenses to water droplets already inside the still rather cold exhaust pipe.
The contrails behind a high cruising is steam which have condensed into water droplets which immediately froze into ice particles due to the low ambient air temperature. At those very low temperatures air can contain very little steam, therefore it condenses.
Normally we do not see exhaust from a jet engine on the ground. That is because the steam is less concentrated since only a part of the air in the engine core is actually used for combustion, and because the exhaust is immediately mixed with the ambient air and the fan air, and because the exhaust is very hot.
If we imagine that we connected a several hundred feet long steel exhaust pipe behind the engine core (core alone, not the fan), then we might on a cold and damp day see condensation from the end of that pipe, until the pipe itself got hot. Such a test would be similar to your car.
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