c5load
Topic Author
Posts: 344
Joined: Tue Sep 16, 2008 9:40 pm

Why Don't Jet Engines Produce Steam?

Sun Mar 14, 2010 2:10 am

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
Posts: 4632
Joined: Thu Nov 30, 2006 9:13 am

RE: Why Don't Jet Engines Produce Steam?

Sun Mar 14, 2010 2:20 am

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.
"PHX is Phoenix, PDX is the other city" -777Way
 
Fly2HMO
Posts: 7207
Joined: Sat Jan 24, 2004 12:14 pm

RE: Why Don't Jet Engines Produce Steam?

Sun Mar 14, 2010 3:45 am

Quoting c5load (Thread starter):
Why Don't Jet Engines Produce Steam?

Oh yes they do (but not technically steam as Maverick said):

http://www.youtube.com/watch?v=UtISR1_OzSQ

Skip to 5:00

Basically a contrail starting from the ground since it's cold enough.

[Edited 2010-03-13 19:47:47]
 
Maverick623
Posts: 4632
Joined: Thu Nov 30, 2006 9:13 am

RE: Why Don't Jet Engines Produce Steam?

Sun Mar 14, 2010 5:44 am

Quoting Fly2HMO (Reply 2):
Basically a contrail

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.
"PHX is Phoenix, PDX is the other city" -777Way
 
tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

RE: Why Don't Jet Engines Produce Steam?

Sun Mar 14, 2010 5:51 am

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

Tom.
 
prebennorholm
Posts: 6409
Joined: Tue Mar 21, 2000 6:25 am

RE: Why Don't Jet Engines Produce Steam?

Mon Mar 15, 2010 1:21 am

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
 
LTC8K6
Posts: 1444
Joined: Fri Jun 05, 2009 8:36 pm

RE: Why Don't Jet Engines Produce Steam?

Mon Mar 15, 2010 3:07 am

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
Posts: 1444
Joined: Fri Jun 05, 2009 8:36 pm

RE: Why Don't Jet Engines Produce Steam?

Mon Mar 15, 2010 3:09 am

Under special circumstances, you can get ice crystal contrails at ground level if it is very cold out and conditions are right.
 
musang
Posts: 788
Joined: Sun Apr 08, 2001 4:11 am

RE: Why Don't Jet Engines Produce Steam?

Tue Mar 16, 2010 2:19 pm

Quoting c5load (Thread starter):
a little steam coming out of the pylon-engine connection
Quoting Maverick623 (Reply 1):
The steam you see coming out of connections is just that.... evaporated water.

Isn't this oil venting?

musang

Who is online

Users browsing this forum: Google Adsense [Bot], Starlionblue and 27 guests