Out of comfort zone, a study of turbine and jet engines.
Out of comfort zone, a study of turbine and jet engines.
(OP)
As far as jet engines go they have really grown in size, especially from aircraft manufactures not wanting more than 2 on the plane. I was wondering about how size affects a turbo fan engine? Since the speeds that the turbines and compressors run at would generate huge centrifugal loads, and since more mass needs to be added to such things as the wheels the blades are mounted on, at what point is the limit of material strength reached do to size? Lately there have been some engine failures that are considered extremely rare, and is why I thought of this.
I would say these sort of failures are analogous to rod failures due to over speed and high reciprocating weight in piston engines.
I would say these sort of failures are analogous to rod failures due to over speed and high reciprocating weight in piston engines.





RE: Out of comfort zone, a study of turbine and jet engines.
There are many great references out there on modern high bypass turbofans, Aviation Week & Space Technology is fantastic, and online Leehamnews.com provides great insight into engines, air-frames, and the business of flying.
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P.E. Metallurgy, Plymouth Tube
RE: Out of comfort zone, a study of turbine and jet engines.
The move to multi-shaft and geared designs is the result of the bypass increases hitting the top of the efficiency curve, resulting in a new search for inefficiencies to eliminate.
RE: Out of comfort zone, a study of turbine and jet engines.
There are lots of very interesting design, materials, and fabrication issues with modern aviation.
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P.E. Metallurgy, Plymouth Tube
RE: Out of comfort zone, a study of turbine and jet engines.
RE: Out of comfort zone, a study of turbine and jet engines.
RE: Out of comfort zone, a study of turbine and jet engines.
Very small turbines (hold in your hand) will run 500,000 rpm.
The combination of flow, heat transfer, and stress at the blade root combine to create optimum speed ranges.
The HP turbine section of a modern high bypass turbine is not that large, the blades are still only 2"-3" long.
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P.E. Metallurgy, Plymouth Tube
RE: Out of comfort zone, a study of turbine and jet engines.
RE: Out of comfort zone, a study of turbine and jet engines.
Did you read his post?
RE: Out of comfort zone, a study of turbine and jet engines.
Abbreviations should not be used with out definitions at first writing. Everyone knows HP as Horse Power.
RE: Out of comfort zone, a study of turbine and jet engines.
That's all I'm saying.
RE: Out of comfort zone, a study of turbine and jet engines.
Thank you for that great professionalism. And the brilliant post in the Billet piston thread as well.
RE: Out of comfort zone, a study of turbine and jet engines.
RE: Out of comfort zone, a study of turbine and jet engines.
Blade centrifugal stresses (regardless of blade size) are roughly a function of tip speed squared. You may want to look into the Buckingham Pi theorem (dimensional analysis) that you can find in most any turbomachinery book. Shepherd is my favorite. Even though it is old, the basic principles don't change.
https://www.amazon.com/Principles-Turbomachinery-D...
In a similar manner, the Euler equation as applied to turbomachinery (Leonhard Euler - circa 1755) may lead one to conclude that pressure ratio in a turbomachine is also roughly a function of tip speed squared. Pressure ratio is one of the defining thermodynamic characteristic of any heat engine (usually derived from compression ratio (volume ratio) in a closed volume machine like a recip engine), so you may conclude that everything scales against rotor tip speed.
I hope that I am being helpful - I don't necessarily expect you to go digging into the old turbomachinery textbooks. But please consider that the compressor wheel stress in a 60 mm turbocharger compressor wheel spinning at 3000 rev/sec (180,000 rpm) is roughly the same as the large compressor rig hub stresses in a 6 meter diameter compressor wheel spinning at 30 rev/sec (1800 rpm). Both have a tip speed of 565 meter/sec. And, ignoring the Reynolds number effects, both turbomachines will run about the same pressure ratio.
I think the thought process as applied to model scaling for turbomachinery can be considered analogous to IC engines. An large cathedral engine powering an ocean going freighter may have very similar BMEP and mean piston speed as the much smaller Cummins engine powering a Dodge pick-up truck. (At least they would be similar if both were four strokes.) Both operate considering similar principles. See link to presentation that I find interesting.
http://www.prof-ges.com/lectures/HTL%202012%20recc...
D. Vincent