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DC-10 Engine Intake Vapour?  
User currently offlineMr Spaceman From Canada, joined Mar 2001, 2787 posts, RR: 9
Posted (12 years 9 months 2 weeks 5 days 21 hours ago) and read 3934 times:

Hi guys. I think that the photo below is pretty amazing. Can someone please explain what causes the vapour you can see in the engine intakes?

I understand that the air inside the engine's compressor stages [on some airline jets], heats up as much as 1,200 degress Celcius, and thus would cause condensation, however, the vapour in the photo is outside the engines, before entering the fan and first stage.

Why is this cool looking vapour on the outside?


Click for large version
Click here for full size photo!

Photo © Don Boyd


Chris
 Smile


"Just a minute while I re-invent myself"
12 replies: All unread, jump to last
 
User currently offlineJETPILOT From United States of America, joined May 1999, 3130 posts, RR: 29
Reply 1, posted (12 years 9 months 2 weeks 5 days 21 hours ago) and read 3859 times:

Warm humid air at ambient pressure enters the inlet. The inlet has a lower pressure than the ambient air and therefore has a lower temperature. The lower temperature causes the air to reach its dewpoint and condense into visible moisture.

Warming of air does not cause condensation. Cooling of it does. Just think of water condensing on the side of a cold soda can.

JET


User currently offlineVC-10 From United Kingdom, joined Oct 1999, 3695 posts, RR: 35
Reply 2, posted (12 years 9 months 2 weeks 5 days 21 hours ago) and read 3851 times:
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At high power and low airspeed there is pressure drop in the intake due to the suction. As a result of the low pressure the air cannot hold all the ambient pressure water vapour in solution, so the excess water condenses out.

User currently offlineMr Spaceman From Canada, joined Mar 2001, 2787 posts, RR: 9
Reply 3, posted (12 years 9 months 2 weeks 5 days 20 hours ago) and read 3839 times:

Thank You for the info gentlemen. Now a few more questions have entered my mind.

Is the lower pressure inside the air intake caused by the curvature of the intake's shape, thus acting like a venturi...or is it from the suction caused by the fan blades?

Do you think that in order for the humid air to reach it's dew point instantly, just inside the engine intakes, that the spread between the dew point and the ambient air temp outside must have already been within a degree or two?

Chris




"Just a minute while I re-invent myself"
User currently offlineMr Spaceman From Canada, joined Mar 2001, 2787 posts, RR: 9
Reply 4, posted (12 years 9 months 2 weeks 5 days 19 hours ago) and read 3830 times:

Hi JETPILOT. I just wanted to let you know that the condensation inside the jet engine that I was refering to, is the kind that is the result of the air being compressed and had absolutely nothing to do with the question that I was asking...sorry, at the moment my mind isn't fully in gear...I've got a wisdom tooth that's killing me!

Yes sir, I am aware that it is cold air that causes the moisture in warm air to condense and become visible.

Also, thanks to your explanation, I feel that I now know why you can sometimes see vapour forming over the wings of a airliner on a warm humid day...The colder air over the wing (caused by low pressure), is cooling the outside humid air down to it's dewpoint, hence the fog over the wing. Am I right?  Big thumbs up

Thanks,
Chris





"Just a minute while I re-invent myself"
User currently offlineJETPILOT From United States of America, joined May 1999, 3130 posts, RR: 29
Reply 5, posted (12 years 9 months 2 weeks 5 days 19 hours ago) and read 3830 times:

Bernoulis principle states that an increase in velocity coincides with a decrease in pressure, while a decrease in velocity is an increase in pressure.

Any time pressure is increased so is temperature as is when pressure is decreased temperature is decreased.

I would say that the air has to have abundant moisture like Miami has. But if we dont know the temperature decrease in the inlet we can't speculate as to the temperature dewpoint relationship. All we know is that the temperature is below the dewpoint in the inlet.

The inlet area is carefully designed to achieve velocities and pressures for efficiency and operation. What those characteristics are...I have no idea.

You will also see this phenomena over the wings of an airplane on takeoff. At high angles of attack the low pressure area over the wing causes condensation to form.

JET


User currently offlineJETPILOT From United States of America, joined May 1999, 3130 posts, RR: 29
Reply 6, posted (12 years 9 months 2 weeks 5 days 19 hours ago) and read 3823 times:

Compression does not form condensation. Expansion does. There certainly is no expansion going on in a jet engines compressor.

Yes your conceptual understanding of fog forming over the wings is correct.

JET


User currently offlineJETPILOT From United States of America, joined May 1999, 3130 posts, RR: 29
Reply 7, posted (12 years 9 months 2 weeks 5 days 19 hours ago) and read 3822 times:

Heres a pic of both forming....


Click for large version
Click here for full size photo!

Photo © PixAir



JET


User currently offlineEGGD From United Kingdom, joined Feb 2001, 12443 posts, RR: 36
Reply 8, posted (12 years 9 months 2 weeks 5 days 18 hours ago) and read 3814 times:

ok guys, i got a question for you.

you know Vapour Trails, well can they form on takeoff on a really cold day (like -30 celcius)?

Cheers, Dan


User currently offlineMr Spaceman From Canada, joined Mar 2001, 2787 posts, RR: 9
Reply 9, posted (12 years 9 months 2 weeks 5 days 18 hours ago) and read 3814 times:

Like always JETPILOT, thanks for sharing your knowledge.

With regards to me thinking that compressing air through a jet engine would cause condensation to occur...I was assuming this would happen. I assumed that if you squeeze air to over 400 p.s.i. that the moisture in it would condense.

I should know better than to assume something. I know that the PIC of an airliner should never just "assume" that the co-pilot has put the gear down.

Another reason why I felt that a jet engine would cause condensation to occur is because of what I do for a living. I drive tractor trailers. All trucks that use "Air Brakes" have an Air Compressor which supplies air to tanks at 120 p.s.i. At the end of my shift, I have to drain water from these air tanks. Aparently a result of compressing air.

Also, between the air compressor and the supply tanks, there is an "Air Dryer" which removes as much water from the system as possible, after being compressed, before it reaches the supply tanks.

Well JETPILOT, you sure made things crystal clear for me regarding the reason for the vapour in the engine intakes and over the wings. When it comes to compressing air and condensation though...I'm sure I'm confused!

Thanks
Chris  Smile



"Just a minute while I re-invent myself"
User currently offlineJETPILOT From United States of America, joined May 1999, 3130 posts, RR: 29
Reply 10, posted (12 years 9 months 2 weeks 5 days 17 hours ago) and read 3809 times:

The condensation that forms in the compressed air tanks forms when the tank pressure drops. Again a drop in pressure is a drop in temperature. Like those cans of compressed air used to clean electronics....squirt the air...the pressure inside the can drops and it gets too cold to hold.

Same with the tanks for the airbrakes. When the brakes are applied the pressure inside the tank drops the air in the tank epands and cools condensing the vapor in the air.

Hope that clears it up for you.

JET


User currently offlineMr Spaceman From Canada, joined Mar 2001, 2787 posts, RR: 9
Reply 11, posted (12 years 9 months 2 weeks 4 days 20 hours ago) and read 3775 times:

Thanks JETPILOT. Your explanation about condensation forming during air expanding with lowering pressure makes perfect sense to me now.

All I had to do was think about how a cloud forms because of warm moist air rising and expanding into cooler, lower pressure air...and bingo, condensation occurs!

Thanks again for taking the time to help me. Now I can relax.  Smile

Chris



"Just a minute while I re-invent myself"
User currently offlineJETPILOT From United States of America, joined May 1999, 3130 posts, RR: 29
Reply 12, posted (12 years 9 months 2 weeks 4 days 17 hours ago) and read 3758 times:

Glad to help....Your Welcome

JET


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