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Post by racket on Mar 20, 2018 23:25:40 GMT -5
Hi Mike
A 210 mm first stage axial comp wheel would potentially flow massive amounts of air , like >10 lbs/second and consume/burn 2 US gallons of kero per minute.
Even centrif comp wheel needs a decent sized combustor , our 100 mm induced ,~150 mm exducer comp wheels need a can of at least ~250 mm dia simply to allow sufficient cross sectional area within the annular flametube for reasonable/reliable combustion..............we need approximate 3 inducer areas for our flametube cross section to get air velocities low enough .
We can't scale from a large high pressure ratio axially equipped engine, they can enjoy "smallish' combustors due to the high air densities entering them , with our "low compression" engines we need more volume .
Yep , the larger the engine rotative the less rpm , generally speaking ,...... a generously sized/diametered turbine wheel also helps to lower sustain speeds , at least the same OD as the comp for those "lowish" sustains.
Cheers John
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Post by smithy1 on Mar 28, 2018 21:04:03 GMT -5
Hi Smithy Pretty close :-) .....I think the C20's 6 axial stage only produces ~2.5-3:1 at most , the centrif stage with its very backswept blading multiplies that by maybe ~2.5:1 Cheers John Hi John, This from the Rolls-Royce C20B training manual: At 100% N, (50,970) RPM, the compressor rotor pumps approximately 44.5 cubic feet of air per second. Thus, 44.5 cubic feet per second times .07647 pound per cubic foot equals approximately 3.4 pounds per second air flow through the engine on a standard NACA day at sea level conditions and 100% NT RPM. With NACA standard day static sea level conditions and 100% N1 RPM, the temperature rise across the compressor (inlet to outlet) is approximately 500F and the pressure rise is approximately 7:l. The compressor rotor requires a considerable amount of shaft horsepower to pump air and give this air a pressure and temperature rise. On a standard day, the compressor rotor requires approximately 600 SHP at 100% RPM. Smithy
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Post by racket on Mar 29, 2018 16:55:19 GMT -5
Hi Smithy
Interesting :-)
~7:1 with ~500 F rise comes out at ~77.5% comp efficiency , not too bad for a relatively small axial/centrif combo at that sorta PR , especially considering the relative age of the design .
Thanks for the numbers :-)
Cheers John
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Post by turboron on Mar 29, 2018 20:07:25 GMT -5
John, the efficiency is especially good considering the centrifugal impeller has a big ring on the inducer. It is in the flowpath as I recall. The ring ties the inducer blades together to solve a frequency problem.
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Post by smithy1 on Apr 2, 2018 22:41:28 GMT -5
Hi Smithy Interesting :-) ~7:1 with ~500 F rise comes out at ~77.5% comp efficiency , not too bad for a relatively small axial/centrif combo at that sorta PR , especially considering the relative age of the design . Thanks for the numbers :-) Cheers John I believe the numbers are marginally better these days as the "numbers" I quoted are from a very old training manual...late 1970's or early 1980's I believe...manufacturing techniques are better and the axial part blade paths are now made of plastic, which means they can run them really tight and the blades cut their own clearances..! And Ron is quite correct, the centrifugal impeller does indeed have a structural ring which ties all the inducer vanes/blades together...and we now use a "custom contour" on the impeller blade path, they now CNC the impeller shroud so it follows the impeller perfectly. Smithy.
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Post by turboron on Apr 3, 2018 7:11:28 GMT -5
Smithy, excellent news. I always thought there was at least 1 or 2 % improvement in the centrifugal stage efficiency by improving the flowpath.
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