BRF Project: India's Kaveri Engine Saga

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BR Mainsite Crew
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Re: BRF Project: India's Kaveri Engine Saga

Postby maitya » 04 Jul 2019 20:09

This very old (circa 2014) post of mine should have been "preserved" in this sticky thread ... but for some reason I didn't - so doing now:
maitya wrote:
NRao wrote:A long article:

An 'engine' for India's growth
Given the specifications of the AMCA a much higher thrust engine than the designed output of the Kaveri will be required for the AMCA even though it is envisioned as a twin engine aircraft. Accordingly, tender documents show that GTRE's next turbofan is in the 110 KN wet and 75 KN dry thrust category. An engine of this capability will certainly require GTRE to master single crystal blade(SCB) technology, integrated rotor disk and blades and super alloys of nickel and cobalt. The Kaveri currently uses directionally solidified blade technology and neither that nor even first generation SCBs which can now be fashioned in India will suffice for the new engine.

Good article and quite a few data points - covers quite a bit of ground from a lay-man turbofan technology etc also. Also confirmation of 75KN dry thrust Kaveri-II is in the works.

However, some important points need further detailing (as it's too high-level):

1) Wrt SCB - yes, 3rd gen SCB (where Ru gets introduced and Re % gets capped at around 5-6%) is the need of the hour, no doubt. But that's not what will automatically get the required high TeT numbers (of say 1650-1700deg C), that one would expect from an contemporary military turbofan.

Pls note Thrust can be up-scaled (to a certain degree) by increasing mass flow.
Problem with that kind of a solution is that the Thermal efficiency (and Propulsive efficiency as well) will go for a toss. And to maintain balance between an increased Thrust and the thermal (and propulsive) efficiencies, both TeT and OPR needs enhancement to a large degree.

And that's exactly where-in lies the catch.

A 3rd Gen SCB, on it's own will give a max 70-90deg C upscaling of TeT (pls note 2nd Gen ones gives around 30-40deg C max upscaling). From a DS-based Kabini/Kaveri TeT of ~1455deg C that takes one to not even to 1550deg C TeT, well short of 1650-1700 deg C that one would desire from an contemporary turbofan.
And it needs to be further noted here that those fantastic figures like 30-40% increase in Creep-rupture strength etc were for polycrystalline to directional strengthening graduations - for DS to SC graduations, the improvements were far more modest (max 10% or so) and sometimes can even be none between generations as well - as the following schematic shows:

And the answer to that catch is BOTH,
a) Multipath (serpentine) Impingement Cooling air-passage
b) Thermal Barrier Coating

The contemporary turbine TeT improvement is less thru the material technology (including casting techniques like Single Crytal or DS etc) front and more thru the complicated multipath impingement cooling fabrication tech and TBC. That's the frontier of turbojet/turbofan technical research and dev (and not so much on SCBs etc) where an experienced partner is an absolute must.

As just by mastering SCB (from Shivji's AI13 pics and few other such open-source revealing, it seems we are almost there), we will not be able to reach that level of TeT etc. So even if we meet the thrust levels via sledge-hammer techniques like mass-flow increase etc, it will not be efficient enough (read high-SFC "fuel guzzling" and heavier etc).

Oh betw, this above points doesn't take away anything from the need of mastering 2nd-3rd gen SCB etc - as making the turbine blades hollow to allow these multi-path incipient cooling passages etc will require thin-walled blade-material technology while retaining the higher-order of creep-rupture-resistance and Thermal-Mechanical fatigue strengths. And that's exactly where SCB gens become vital (as SC processing normally ensures that the thin section properties are optimised i.e. as section thickness is reduced a standard superalloy ruptures in less time than a simple bar - and that PC > DS > SC is the order of thin section property reduction).

So long-story short (will bring these points in some more detail in the Kaveri saga thread), if we can master the required raw-material-forming and casting technology for 3rd Gen SCBs etc well and good - but if we can't, we can still make do with DS ones (and achieve maybe somewhat conservative but still acceptable performance parameters) PROVIDED we have considerably progressed on the blade-cooling and the TBC technology.

2) Wrt Compressor Blisk technology - the temperature regime we have already aimed for in the current Kaveri precludes Ti usage in the last couple of stages of the HPC and we are firmly in the Polycrystalline Superalloy regime there-in. Manufacturing blades and disks with such complex metallurgical superalloys is a huge challenge - which other contemporary turbofan makers overcomes by the using alternate machining paths like Linear Frictional Welding (LFW) etc.

The theory ofcourse is to manufacture the complex-aerofoil-shaped blades and the disks separately (with disks having "slots" to fit-in the blade-roots) and join them by LFW. Problem is standard welding or other joining methods will mean considerable weakness in those joints constraining the rotational/mechanical stress it can withstand. Not so with LFW, where it’s possible (not uncommon actually) to have the joint strength more than that of the blade and disk themselves.

Now we need to beg-borrow this technology (for superalloys and not for some Ti/Steel LFW etc) to achieve relevant blisks (and the associated SPRs without compromising on the weight front) - until then need to live with standard programming of the heavier bolted-blades-on-disk compressor stages.

But again if we have mastered/understood the rotor-dynamics of the HPC, overcome the myriad inter-stage resonant-vibration impacts etc, controlled the interstage shock-waves in a 1.6M or so tip-speed compressor stages etc etc etc, absence of blisks can be lived with. Yes it will not be super-duper 30 OPR achieving stuff, but maybe a SDRE 27 OPR regime, enough to achieve those thrust figures etc.

3) There are other seemingly small things like Compressor blade surface finish – now in the complex Rotor-CFD world, laminar flow drag (a major source of skin-friction drag losses) is directly proportional to blade surface smoothness. Nowadays in contemporary turbofans, 6-8micron surface smoothness in the HPC stages have been achieved while rest of the world struggles around 30-40micron smoothness of these stages. Another very jealously and closely guarded technology who nobody will ever part with.

And so many such things …

Point is beyond F404 tech, thrust and efficiency improvements in military turbofans are sum-total of such incremental technological (and sometimes even pure engineering and manufacturing) improvements in almost all aspects of CFD, Mechanical, Thermal and Material tech.
And with Kaveri-I we are trying to baseline all these aspects at F404 level. Once that’s done, these other improvements (say in Kaveri-II) will need taking up, some concurrently and a few others, sequentially – a long and hard road ahead, just that we shouldn’t stop trudging along.

So let's keep trucking ...

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