Mr Spaceman
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DC-10 Engine Intake Vapour?

Thu Jul 12, 2001 3:00 am

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
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"Just a minute while I re-invent myself"
 
JETPILOT
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RE: DC-10 Engine Intake Vapour?

Thu Jul 12, 2001 3:21 am

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
 
VC-10
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RE: DC-10 Engine Intake Vapour?

Thu Jul 12, 2001 3:22 am

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.
 
Mr Spaceman
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RE: DC-10 Engine Intake Vapour?

Thu Jul 12, 2001 4:05 am

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"
 
Mr Spaceman
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RE: Jetpilot

Thu Jul 12, 2001 4:48 am

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"
 
JETPILOT
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RE: DC-10 Engine Intake Vapour?

Thu Jul 12, 2001 4:52 am

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
 
JETPILOT
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RE: DC-10 Engine Intake Vapour?

Thu Jul 12, 2001 4:55 am

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
 
JETPILOT
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RE: DC-10 Engine Intake Vapour?

Thu Jul 12, 2001 5:04 am

Heres a pic of both forming....


Click for large version
Click here for full size photo!

Photo © PixAir



JET
 
EGGD
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RE: DC-10 Engine Intake Vapour?

Thu Jul 12, 2001 6:16 am

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
 
Mr Spaceman
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RE: Jetpilot

Thu Jul 12, 2001 6:42 am

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"
 
JETPILOT
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RE: DC-10 Engine Intake Vapour?

Thu Jul 12, 2001 7:09 am

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
 
Mr Spaceman
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RE: Jetpilot

Fri Jul 13, 2001 4:01 am

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"
 
JETPILOT
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RE: DC-10 Engine Intake Vapour?

Fri Jul 13, 2001 7:05 am

Glad to help....Your Welcome

JET

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