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GE's Ceramic Matrix Turbine Blades


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This is a big deal.

 

Naturally, the journalists get the details wrong.  But what can you expect; they're journalists and they don't know anything.

 

The air that is used to circulate inside turbine blades and cool them does cause a loss to engine efficiency, but this is trivially small.  The big advantage is that the turbine inlet temperature (TIT) can be cranked up, and that allows better cycle efficiency.

 

The article correctly notes that this sort of material would have applications in powers station turbines as well, but if the cost can be brought down, the long-term implications are even greater than that.

 

In the 1970s, Toyota and a few other companies experimented with adiabatic, or thermally insulated piston engines.  They never quite got it to work, and the end of the 1970s oil crisis pretty well killed off development.  These CMCs could be the advance that makes the idea finally work.  Adabatic piston engines would be absurdly more efficient than current models, and would require much less cooling.

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...

Naturally, the journalists get the details wrong.  But what can you expect; they're journalists and they don't know anything.

...

 

   On side note i saw how journalist of main TV channel in Russia - "1st Channel" - was reading paper with wikipedia article about T-14, when he was doing quick report just in front of the new tank during parade rehearsal.

 

   Thanks for article, i need little bit time to read it and actually understand some parts  :D

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I've just remembered why this is a great idea, and I want to kick myself for forgetting it - they haven't made the ceramics ductile, they've made them resist crack growth which means you can put them in tension without major issues.

 

The stress field around a crack tip is rather complex, with the stress concentrated in a very small volume at the tip and peak values around 2 orders of magnitude higher than the average stress for a typical minute crack (i.e., ~1 atom tip radius). The pattern looks something like this:

7506-1.jpg

(from here)

The exact stress field is complex and can only be gathered from direct measurement or FEA simulations, and it has some rather useful properties as well as the awkward peak values. Generally measurements will only give the magnitude of stress at a point, however with FEA you can output a graph of stress in a single direction only. Stress perpendicular to the crack is the bulk of the stress, so the values are not much different to that of the magnitude graph, but the stress parallel to the crack (and perpendicular to the applied stress) is very different. For reasons I don't understand (must be something to do with poisson's ratio) there is an area of high stress parallel to the crack a small distance in front of the crack tip (the distance is about the same as the tip radius), with peak values of around 20% of the maximum stress perpendicular to the crack. This is very handy!

 

In fibre composite materials it's generally rather challenging to bond the fibres to the matrix, so the tensile strength parallel to the fibres will be much higher - this is why they tend to have fibres woven is several directions, or random spreading of fibres (like in some examples of fibreglass). This lower strength is very useful when a crack is introduced into the composite - if the bonding between the fibre and matrix is about 20% as strong as the fibres are, then the area of high stress parallel to the crack will create a void in the material as the crack approaches a fibre. When the crack meets this void it is effectively blunted - the single atom radius crack tip is now effectively infinitely large, so the crack is stopped at the fibre. This is known as the cook-gordon mechanism, and here's a diagram:

COOK_GORDON.JPG

From here

The new crack perpendicular to the old crack isn't an issue as long as the loading doesn't change, as cracks are not affected much by stress parallel to them. So in these turbine blades any small cracks that form will grow until they encounter a interface, and then get blunted. You'll probably need the fibres carefully aligned with the stress at that point, but with FEA it should be possible to create a map of the stress field throughout the blade. I also found a paper on crack mitigation in ceramics using this mechanism when googling for the pictures - I wonder if this would also work for multi-hit capability in ceramic armours?

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Further research has revealed that that journo really had no idea what they were talking about. They claimed it will be implemented on the LEAP engines in such a way as to suggest that LEAP will have Ceramic Matrix Composite (CMC) blades, when LEAP will only have a CMC high pressure turbine shroud per GE. Also according to that document there were CMC parts in the ADVENT variable bypass ratio engine (presumably only static parts like the high pressure shroud), and we should see CMC blades on the production equivalent of ADVENT. As a further point of interest, they claim that the fins lack of cooling channels means you can make them more aerodynamically efficient - so the lack of bleed air does have advantages for efficiency.

 

I also found this article, which makes for interesting reading. GE has invested over $100mil into making a new facility to produce CMC components, and they're far from the only ones interested in the technology.

 

ETA: this gets better - apparently the F136 was going to have CMC parts, until it got cancelled

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As a further point of interest, they claim that the fins lack of cooling channels means you can make them more aerodynamically efficient - so the lack of bleed air does have advantages for efficiency.

 

 

That is interesting.  You hear a lot of hand-wringing about the isentropic efficiency of the compressor, but comparatively little about the aerodynamic qualities of the turbine blades.

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Interesting question.  I don't know.  Since these are turbine blades, buried deep in the engine, they wouldn't contribute much to frontal RCS, but I would imagine that from the direct rear you would get some radar returns from the turbine blades, especially with conventional, circular nozzles.

 

Some of the early radar absorbing materials were composites that did have ceramics in them, but I think the ceramic was a carrier, and there was something else in the composite that did the radar-wave-gobbling.

 

If I had to guess, I would think ceramics are pretty strong radar reflectors.  As I understand it, a lot of RAM is based on little suspended metal particles that sort of act like antennae, except that instead of converting the radar wave into an electrical signal, they convert it into heat.

 

Since these blades are ceramic metal matrices though, they might absorb radar fairly well.

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