rwessel From United States of America, joined Jan 2007, 1989 posts, RR: 2 Reply 1, posted (3 years 2 months 3 weeks 5 days 8 hours ago) and read 9047 times:
There have been plenty, dating back to Whittle. The RR Nene for a (very) historical example. The Williams FJ44 is a rather more modern example. Also many turboprops/shafts (PT-6 for example).
It's a feature more suited to smaller engines (including the very smallest - real* model airplane turbojets all, AFAIK, use centrifugal compressors). Axial compressor stages have a limited compression per stage (be comparison centrifugal compressors have much higher compression per stage), and small diameter compressors (of both types) suffer disproportionately from tip losses (where gas can escape past the gap between the blade tips and the case). The tip losses seriously degrade the already limited compression ratios for axial stages, with a much smaller impact on centrifugal compressors.
Many engines use a combination of centrifugal and axial compressors. Often one or two centrifugal compressors (in the case of two, they're often mounted back-to-back on the same disk), followed by one or more axial stages.
Engines with centrifugal compressors tend to be much shorter and fatter than ones with axial compressors (since you need fewer compressor stages, but they're bigger because of the then to run the airflow around the edge of the disk. Centrifugal compressors also have a significant structural advantage in some cases (since they're a solid block**), and have much higher FOD resistance. The also tend to fling the FOD off to the side, rather than pump it back into the engine. This can be significant if the front stages have to deal with FOD or ice.
*As opposed to piston driven the ducted fan often used to simulate jets for model airplanes.
**A number of centrifugal compressor stages that are a single ceramic composite piece have been built.
As always a very informative, albeit this time misinterpreted, response
To further clarify I was asking if there were any flying examples of engines that have both a centrifugal compressor AND a centrifugal turbine. I know there's plenty of engines with centrifugal compressors and regular axial flow turbines, but can't think of any used in airplanes with centrifugal turbines.
The only engines with that type of configuration seem to be either APUs or industrial engines, or those "junkyard" jet engines made from automotive turbochargers. But I don't know of any ever used in an airplane.
rwessel From United States of America, joined Jan 2007, 1989 posts, RR: 2 Reply 3, posted (3 years 2 months 3 weeks 5 days 7 hours ago) and read 8985 times:
Quoting FLY2HMO (Reply 2): To further clarify I was asking if there were any flying examples of engines that have both a centrifugal compressor AND a centrifugal turbine. I know there's plenty of engines with centrifugal compressors and regular axial flow turbines, but can't think of any used in airplanes with centrifugal turbines.
My mistake.
I can't think of any examples. It's also hard to think of any advantages a centrifugal turbine would have - the aerodynamics at the turbine end are much easier (just count the relative number of compressor and turbine stages in axial flow engines), and axial turbines work quite well in almost any size.
rwessel From United States of America, joined Jan 2007, 1989 posts, RR: 2 Reply 5, posted (3 years 2 months 3 weeks 5 days 6 hours ago) and read 8953 times:
Quoting FLY2HMO (Reply 4): What's interesting to note is that it has a single impeller where the front side acts as the compressor section and the back side as the turbine.
That's done on two stage centrifugal compressors commonly too. And in this case, it certainly has some structural advantages - it's hard to image a more direct transfer of power from the turbine to the compressor. What the application for this turbine? Emergency power generation?
kl671 From Canada, joined Jul 2005, 136 posts, RR: 0 Reply 7, posted (3 years 2 months 3 weeks 5 days 4 hours ago) and read 8908 times:
Quoting FLY2HMO (Thread starter): But was there any engine with this configuration that ever made it to an airplane?
The very first turbojet engine to take to the skies had a centrifugal turbine. The Heinkel 178 flew in 1939 was powereed by a Heinkel Hes 3B. The Smithsonian web site has a model of this engine and describes it as having a single stage radial turbine. To my knowledge, this was the only one.
Quoting FLY2HMO (Reply 4): So far the only (non-flying) example i've found is a Solar generator gas turbine set.
Capstone make small high speed industrial generator sets that have centrifugal turbines. These turbines also have air bearings. No oil system. The turbine and air bearings can be seen in the attached.
FLY2HMO From , joined Dec 1969, posts, RR: Reply 8, posted (3 years 2 months 3 weeks 5 days 4 hours ago) and read 8899 times:
Quoting kl671 (Reply 7): To my knowledge, this was the only one.
Sweet! Good catch. It almost looks more like a turbocharger than a jet engine. Which is understandable as both technologies were being developed roughly at the same time.
Quoting kl671 (Reply 7): These turbines also have air bearings. No oil system.
I''ve heard lots of talk of the big 3 engine makers using GTF, ceramics, and a bunch of other witchcraft in their engines in development but nobody seems to be seriously looking into air bearing technology.
Quoting kl671 (Reply 7): The turbine and air bearings can be seen in the attached
tdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80 Reply 9, posted (3 years 2 months 3 weeks 5 days 3 hours ago) and read 8860 times:
Quoting FLY2HMO (Reply 8): I''ve heard lots of talk of the big 3 engine makers using GTF, ceramics, and a bunch of other witchcraft in their engines in development but nobody seems to be seriously looking into air bearing technology.
I'm not sure that air bearings can handle the kinds of loads that a large jet's bearings will see. There's also the issue that roller bearings are "free" where an air bearing takes bleed air away, which impacts fuel economy.
kl671 From Canada, joined Jul 2005, 136 posts, RR: 0 Reply 10, posted (3 years 2 months 3 weeks 5 days 1 hour ago) and read 8824 times:
Quoting tdscanuck (Reply 9): I'm not sure that air bearings can handle the kinds of loads that a large jet's bearings will see.
You are absolutley correct that air bearings are impractical in an aero application today. I would however take a bet that in 20 or 30 years we shall see air bearings in service on large aero engines. Unfortunately I don't think I will be around in 30 years to collect my winnings.
Quoting tdscanuck (Reply 9): There's also the issue that roller bearings are "free" where an air bearing takes bleed air away, which impacts fuel economy.
Roller bearings are not "free". Rotating element bearings require copious amounts of oil for cooling and lubrication. To provide this oil the engine needs pressure and scavange pumps with the associated drive gearboxes and gears, pressure control valves, oil filters, oil coolers, reservoirs, and the oil inself. The weight savings by removing the oil system would be significant. Oil system maintence and logistics costs go away with air bearings. The advantages more than outway the use of a little bit of extra bleed air.
All we need is someone to invent a large aero engine that does not use oil.
Feel free to criticise me in 20 or 30 years if my predictions do not come true. Air bearings are the way of the future.
miller22 From United States of America, joined Nov 2000, 707 posts, RR: 5 Reply 11, posted (3 years 2 months 3 weeks 5 days 1 hour ago) and read 8814 times:
Pratt JT-15D has axial and centrifugal compressors.
tdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80 Reply 12, posted (3 years 2 months 3 weeks 5 days 1 hour ago) and read 8801 times:
Quoting kl671 (Reply 10): Roller bearings are not "free". Rotating element bearings require copious amounts of oil for cooling and lubrication. To provide this oil the engine needs pressure and scavange pumps with the associated drive gearboxes and gears, pressure control valves, oil filters, oil coolers, reservoirs, and the oil inself. The weight savings by removing the oil system would be significant. Oil system maintence and logistics costs go away with air bearings. The advantages more than outway the use of a little bit of extra bleed air.
I should have said "free in terms of bleed air"...obviously, it takes energy to drive and haul around the oil system. However, the power demands of the oil system are pretty small compared to the losses you get from even small quantities of bleed air. Plus, I don't think you could get rid of the oil system...you still need something to keep the rotors spinning freely before they generate enough pressure to get the air bearings working, and when the engine is just sitting there windmilling on the ramp.
FLY2HMO From , joined Dec 1969, posts, RR: Reply 13, posted (3 years 2 months 3 weeks 5 days ago) and read 8785 times:
Quoting kl671 (Reply 10): You are absolutley correct that air bearings are impractical in an aero application today
I wonder how many psi it would take to start a theoretical air-bearing-equipped GE90. Surely the plane would need to carry around a hefty APU or some sort of high pressure air reservoir, or some sort of compressor system.
I've had the privilege of standing inside a GE-90-94B once and remember rotating the fan around with my hand, it was easy to move but at the same time HEAVY, kinda like moving an elephant on ice skates
I'd imagine it would take a huge amount of pressure to get the shaft in a GE90 to float.
Actually, it just occurred to me, what about electromagnetic bearings? Surely more convenient than air since they're either on or off, but I guess you'd need heavy stators and electrical wiring and generators...
kl671 From Canada, joined Jul 2005, 136 posts, RR: 0 Reply 14, posted (3 years 2 months 3 weeks 3 days 22 hours ago) and read 8557 times:
Quoting tdscanuck (Reply 12): However, the power demands of the oil system are pretty small compared to the losses you get from even small quantities of bleed air.
I suspect the opposite but I cannot come up with hard numbers. Do you have any figures on the difference on engine efficiency between an equivalent power being extracted via bleed air verses a power take off from the compressor shaft?.
Quoting tdscanuck (Reply 12): I don't think you could get rid of the oil system...you still need something to keep the rotors spinning freely before they generate enough pressure to get the air bearings working,
Capstone have a turbine that has no oil system. They have solved the starting problem although they are keeping quiet on how they do it. The Capstone is an industrial turbine, but it demononstrates the technology.
Quoting FLY2HMO (Reply 13): Actually, it just occurred to me, what about electromagnetic bearings? Surely more convenient than air since they're either on or off, but I guess you'd need heavy stators and electrical wiring
and generators...
Trans Canada Pipe Lines (TCPL) are a great fan of magnetic bearings.They use them in a large number of their centrifugal natural gas compressors. TCPL are among the largest operators of industrial gas turbines in the world. Wonderful as it is, their magnetic bearing system is too heavy to fly.
Drfix2fly From United States of America, joined Oct 2009, 27 posts, RR: 0 Reply 15, posted (3 years 2 months 3 weeks 1 day 13 hours ago) and read 8279 times:
it is a turbo prop / shaft but the t700 / ct7 is the one that comes to my mind, the first several stages are axial the last one is centrifugal
you will find this on the Apache, black hawk, for the t700, and Saab 340 , cassa cn235, let 610 and the latest big Sikorsky model eludes me .
jetlife2 From United States of America, joined Jul 2006, 214 posts, RR: 25 Reply 16, posted (3 years 2 months 3 weeks 1 day 2 hours ago) and read 8195 times:
Air bearings and magnetic bearings are very much under study, and demonstrations have been made with "modern" aero and aero derivative gas turbines. They are not at all far fetched. The architecture of gas turbine engines is heavily driven by the needs of the oil system...bearing sumps, oil supply system (tank, pumps, tubes, bearing support design, fuel-oil HX, nozzles...) and scavenge system (same plus filters), shaft venting, on and on. Losing all of this is very attractive. Work continues.
MD11Engineer From Germany, joined Oct 2003, 13337 posts, RR: 64 Reply 17, posted (3 years 2 months 3 weeks 20 hours ago) and read 8135 times:
Quoting Drfix2fly (Reply 15): it is a turbo prop / shaft but the t700 / ct7 is the one that comes to my mind, the first several stages are axial the last one is centrifugal
you will find this on the Apache, black hawk, for the t700, and Saab 340 , cassa cn235, let 610 and the latest big Sikorsky model eludes me .
The threadstarter asked about centrifugal turbines not centrifugal compressors
Drfix2fly From United States of America, joined Oct 2009, 27 posts, RR: 0 Reply 19, posted (3 years 2 months 2 weeks 1 day 12 hours ago) and read 7712 times:
L-188 From United States of America, joined Jul 1999, 29350 posts, RR: 62 Reply 20, posted (3 years 2 months 2 weeks 1 day 2 hours ago) and read 7650 times:
I believe that the same is true of the superior Garrett 731 of Lear 35 fame.
As noted centrifical compressors are generalli stronger and less likely to be damaged then axial blades, but as the engine gets larger, the axial begins to have a weight advantage.
I admit that is a generality, I have seen fodded out TPE-331's before.
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