c5load From United States of America, joined Sep 2008, 917 posts, RR: 0 Posted (3 years 2 months 1 week 2 days 23 hours ago) and read 3610 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, 4744 posts, RR: 6 Reply 1, posted (3 years 2 months 1 week 2 days 23 hours ago) and read 3604 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.
Maverick623 From United States of America, joined Nov 2006, 4744 posts, RR: 6 Reply 3, posted (3 years 2 months 1 week 2 days 19 hours ago) and read 3453 times:
Which I forgot to add in..... Contrails are the same exact thing as the other scenarios I described, except they stand out because the water deposes into ice.
tdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80 Reply 4, posted (3 years 2 months 1 week 2 days 19 hours ago) and read 3438 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?
Three things:
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, 6018 posts, RR: 55 Reply 5, posted (3 years 2 months 1 week 2 days ago) and read 2965 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.
Always keep your number of landings equal to your number of take-offs, Preben Norholm
LTC8K6 From United States of America, joined Jun 2009, 415 posts, RR: 0 Reply 6, posted (3 years 2 months 1 week 1 day 22 hours ago) and read 2903 times:
Water vapor is produced by jet engines and it is not visible. The wv sublimes into ice crystals at high altitudes, which you see as a contrail. Low altitude contrails are made of tiny water droplets.
LTC8K6 From United States of America, joined Jun 2009, 415 posts, RR: 0 Reply 7, posted (3 years 2 months 1 week 1 day 22 hours ago) and read 2903 times:
Under special circumstances, you can get ice crystal contrails at ground level if it is very cold out and conditions are right.
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