smartt1982 From United Kingdom, joined Nov 2007, 225 posts, RR: 0 Posted (5 years 7 months 1 week 3 days 17 hours ago) and read 9205 times:
Hoping someone can help me with this. My understanding of bypass engines is that mass of cold air goes around the hot core with the minority and smaller hot air coming out the middle section. What I am confused about and what I cannot seem to find an answer to is the purpose behind the long thin tube that extends out from the middle of what I would call the hot exhaust section.
MD11Engineer From Germany, joined Oct 2003, 14718 posts, RR: 62
Reply 3, posted (5 years 7 months 1 week 3 days 13 hours ago) and read 9050 times:
All gas turbine engines use pressurised seals for their shaft bearing compartments to keep the oil in. The result is that the scavenge oil returning to the oil tank via the scavenge pump, chip detectors and scavenge filter contains a lot of air, it is more a foam than a liquid.
Engines use a centrifugal oil / air separator in the circuit to remove the air from the oil and to vent the system to the ambient to avoid overpressure. PW and RR use centrifuges attached to their main gear box, which vent overboard through a hole in the cowling. GE and CFM use the hollow N1 shaft as a centrifuge. The heavier oil sticks to the inside of the shaft and gets returned into the circuit via holes, while the lighter air stays in the center and gets blown out through the pipe in the back. The air blown out is not completely free of oil, so when the engine has just been shut down, you´ll see some oil vapour coming out from this pipe.
MadDogJT8D From United States of America, joined Oct 2009, 415 posts, RR: 1
Reply 5, posted (5 years 7 months 1 week 3 days 10 hours ago) and read 8880 times:
Great post. I've seen a similar center shaft with the whole in the center like the CFM56 on the CF6-80A and C series, never knew what the difference was about (just through observation, not working on the engines).
What about PW? The PW4000 has what looks like a closed compressor exhaust cone similar to the back of the RR. How do PW engines vent the centrifugal air/oil separator?
How clever! I never knew that. Very smart way of doing things!
Quoting smartt1982 (Thread starter): purpose behind the long thin tube that extends out from the middle of what I would call the hot exhaust section.
As others have noted, the tube dumps sealing air overboard. If you look inside this tube, you can often see a thin layer of coked up oil on the inside surface. More info on bearing seals and air / oil separators can be found here.