Out of comfort zone, a study of turbine and jet engines. 2

[enginesrus]

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.

 

[EdStainless]

The size, and therefore the speed at the tips is why they have gone to three shaft and geared designs. This lets them rotate each section at a more suitable speed. You really have many related, but separate sections, (inlet duct), bypass fan, low pressure compressor, HP comp, (combustion), HP turbine, LP turbine, (exhaust).
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

 

[SwinnyGG]

The size of high-temp rotating assemblies hasn't increased as much as you probably think- as time goes on, higher and higher levels of bypass have been incorporated into engine designs. This causes the fan to grow and the size of the case to grow, but the turbine to not grow nearly as much.

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.

 

[EdStainless]

The very large fan blades, and even the larger compressor blades are very light weight. They are either composite or hollow Al, or some combination. The compressor blades are often hollow Ti.
There are lots of very interesting design, materials, and fabrication issues with modern aviation.

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P.E. Metallurgy, Plymouth Tube

 

[SwinnyGG]

Definitely agree with that. The metallurgy and casting techniques that go into hot-section compressor blades is pretty incredible stuff.

 

[enginesrus]

There is still the centrifugal force and heat stress as well as any vibration caused by the blades. How large of diameter are those turbine disks? And I was under the impression that they turn at over 8K rpms? Also for Non aircraft there are some very large IC turbines, curious of what sort of rpm they run at?

 

[EdStainless]

The rotational speed goes up as the size goes down. Some of the very large land and marine turbines have the output shaft running 3,000rpm (LP comp and LP turbine are on this shaft). The spool carrying the HP comp and HP turbine may be running 2-3x this speed.
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

 

[enginesrus]

3000 rpms? So that would be the power turbine then. So again what does the basic engine run at? You do know that the power turbine is not the basic engine but the device that the basic engine "Blows" into that creates the shaft HP.

 

[SwinnyGG]

The spool carrying the HP comp and HP turbine may be running 2-3x this speed.

Did you read his post?

 

[enginesrus]

Sorry it didn't register, again I'm not a turbine person, I'm an old IC recip person. HP ? It just didn't click.
Abbreviations should not be used with out definitions at first writing. Everyone knows HP as Horse Power.

 

[SwinnyGG]

If you're going to be condescending, you should make sure that what you think someone is saying, is what they are actually saying.j

That's all I'm saying.

 

[enginesrus]

jgKRI, All I have to say to your post is just wow!
Thank you for that great professionalism. And the brilliant post in the Billet piston thread as well.

 

[moon161]

Hey, you're saying you don't know the field, ask for answers and then correct the answers of the people who answer your questions. Take a moment and reflect on that.

 

[turbomotor]

This is an interesting subject, and I think we may want to go back to basics and think about the discussion of turbomachinery size scaling from first principles.

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-Dennis-G-Shepherd/dp/0024096601

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 2012 reccords in the engine world 260212.pdf

D. Vincent