Jump to content
Sturgeon's House

GE's Ceramic Matrix Turbine Blades


Recommended Posts

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.

Link to comment
Share on other sites


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

Link to comment
Share on other sites

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:


(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:


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?

Link to comment
Share on other sites

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

Link to comment
Share on other sites

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.

Link to comment
Share on other sites

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.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


  • Similar Content

    • By Collimatrix
      At the behest of @Lord_James, this shall be the thread for general discussion of conventional passive metallic armor.  Whether it's steel, titanium, magnesium, exotic laminates of all three, this is the thread for it.
      In answer to your earlier question, Lord_James, relatively small amounts of boron, in steels that have the appropriate levels of carbon, form intergranular barriers that dramatically slow the diffusion of carbon out of the austenite crystals during quenching.  Long story short, this means that the depth of material that can be effectively hardened is much greater.
    • By Collimatrix
      I've had an idea for a post like this rattling in my head for a while.  I thought we should have some sort of resource and discussion on jet engines, how they work, and what the various figures of merit are.
      For simplicity, I'll talk about thrust turbines in aircraft, although there are a whole host of important gas turbine applications, like the power gas turbines used in helicopters, tanks and ships, to say nothing of steam turbines.

      This is a representative schematic of a turbojet.  For some reason, all schematic diagrams of turbojet engines show axial compressors, even though centrifugal compressors are still common.
      Air flows in through the inlet. Rotating compressor blades, attached to a central shaft or spool and stationary stator blades compress air.  This compressed air is fed to the combustors, where fuel is burned to heat the air (in principle heat energy could be added by means other than combustion, as in a nuclear gas turbine).  This hot air is then fed to the turbine section where it spins turbine blades, which are mounted to the same shaft as the compressor blades.  Some of the energy from the hot gas is used to turn the turbines, but the rest rushes out the back where a nozzle extracts maximum thrust from it.
      For best performance, the pressure ratio, that is, the ratio of the pressure of air in the final stage of the compressor to free atmospheric air, should be as high as possible.  Pressure ratio is related both to specific power (how much thrust you can get per kilogram of engine) and efficiency (how much thrust you can get per kilogram of fuel):

      This chart shows the idealized relationship between efficiency (Y axis) and pressure ratio (X axis).  Higher is better, but after a certain point there are somewhat diminishing returns.
      Pressure ratio has risen dramatically since the development of the first gas turbines.  The Jumo 004 in the ME-262 had a pressure ratio of 4:1 or so, while the Trent 900 in the Airbus A380 manages nearly 40:1!  Civil turbine engines can support somewhat higher compression ratios, since airliners cruise for long periods of time at a given altitude and speed.  Military aircraft have fighter jocks shoving the throttles this way and that and operate over a wide range of altitudes and speeds, so there needs to be some more wiggle room.  Most contemporary fighter aircraft engines have pressure ratios between 20:1 and 30:1.
      Additionally, contrary to what the diagram above shows, in most thrust jet engines, not all of the air from the compressor goes through the core:

      A portion of the air bypasses the core, and simply joins the air that went through the core in the nozzle.  The bypass ratio is the ratio (in mass per second) of air that goes around the core relative to the air going through the core.  An engine with any amount of bypass air is called a turbofan, while one without is a turbojet.
      This bypass air serves two purposes.  The first is that because it did not go through the core and past the combustor, it is relatively cool and therefore helps to cool the engine nozzle.  The variable geometry nozzles (overlapping metal flower looking thingies) used on modern fighter aircraft require some amount of bypass air to maintain structural integrity:

      The second purpose of bypass air is to generate thrust.  At low altitudes and airspeeds, bypass air is a more efficient way to generate thrust than core air.
      Airliner engines were initially turbojets, but these quickly sprouted bypass ducts.  Airliner engines currently have much higher bypass ratios than fighter aircraft engines, as again, fighter aircraft are expected to operate at least occasionally at speeds and altitudes where high bypass is not efficient.  Additionally, higher bypass ratios make engines wider, and therefore draggier, as well as less responsive to throttle input.  The Trent 900 has a bypass ratio of 8.5:1, while the RB199 has the highest bypass ratio of any fighter engine at 1.1:1.  The F119 in the F-22 raptor has an unusually low bypass ratio of .2:1, making it practically a pure turbojet with just enough bypass air for nozzle cooling only.  It is also possible to make variants of engines that incorporate existing core designs, but have a different bypass ratios.  The F135 in the JSF, for example, has essentially the same core as the F119, but has a bypass ratio of .55:1.  Because the JSF does not supercruise, it is worth trading off some performance at supersonic speeds in favor of efficiency in subsonic.
      The turbine section of the engine is the most demanding in terms of materials science.  The higher the temperature at which the turbine operates, the more efficient and power-dense the engine can be.  The demands of higher and higher performance have long since pushed temperatures past the operating range of steels, so now turbine blades in top of the line engines are made of magical nickel alloys with magical microstructure, cooling air actively circulated inside of them, and magical ceramic coatings:

      The expertise to make high performance gas turbine blades is kept secret and restricted to a handful of companies in the world's richest nations.
      Finally, the hot air from the turbine and the bypass air (if any) are mixed and expelled out of a nozzle.  Because they operate within a narrow band of altitudes and speeds, simple, fixed-geometry nozzles are acceptable for airliners.  Combat aircraft typically need complex, actively cooled variable-geometry nozzles.  The most recent development in this area has been the so-called 2D, or rectangular nozzle, as seen in the F119 engine:

      These nozzles are heavier, and sacrifice some efficiency (not that it really matters; the F119 is a mighty beast with thrust to spare) in exchange for lower radar and infra-red cross section.  Additionally, the F119 can point the thrust up or down by twenty degrees, which enhances the agility of the F-22, especially at high altitudes.
      Most combat aircraft engines feature afterburners.  An afterburner dumps additional fuel into the hot air aft of the turbine stage.  Afterburners provide a dramatic increase in thrust, usually doubling it, but have a disproportionate increase in fuel consumption, usually increasing it by a factor of seven or so.
    • By Collimatrix
      Definitely a must-see.
  • Create New...