Kaveri & Aero-Engine: News & Discussion

ramana · Post by **ramana** » 22 Aug 2019 01:17

ldev,
Is blade frequency being called screech?
For only that would be eliminated by the modified M88 core.
Real screech is due to organ pipe resonance from jet exhaust.

ldev · Post by **ldev** » 22 Aug 2019 02:49

ramana wrote:ldev and prasad, I thought screech or jet engine noise is due to the mixing of turbulent flow from the the combustion and is like air flow in an organ pipe or flute.
In supersonic flow there is expansion and contraction similar to an accordion which adds to the phenomenon. So it needs nozzle exhaust tinkering.

Helmholtz resonator is if the casing vibrates. That's not screech but case vibration due to resonance. They can add stiffening ribs on case and make the frequency go higher and avoid acoustic range.

i think there is confusion in the reporting.

Ramana,

The reporting states that it is the turbulent mixing of the hot exhaust with the cooler by-pass air that is creating a shock wave with a high frequency which cannot exit because the nozzles are smaller. This high frequency shock wave is being called a screech. Again, according to the reporting the hot exhaust is not uniform in pressure across all injectors which may be the cause which in turn could be because of the placement/spacing of the injectors, or the spacing between individual injectors and the flameholder.

ramana · Post by **ramana** » 22 Aug 2019 07:18

Ok so the phenomena is the well known acoustic noise due to organ pipe effect. So how would a modified core change the screech effect?

ldev · Post by **ldev** » 22 Aug 2019 09:22

ramana wrote:Ok so the phenomena is the well known acoustic noise due to organ pipe effect. So how would a modified core change the screech effect?

Hi Ramana,

Again, according to the reporting, the screech in Kaveri could come from any of the following factors:

Fuel spray injector locations
Non-uniformity in the fuel-air fraction
Atomization process
Evaporation rates
Ignition process
Quality of the upstream flow
Blockages if any
Flameholder geometry

Quality of the upstream flow would include a modified core. Apparently Safran did CFD modeling on the engine to reach these conclusions. But where was it done and were folks from GTRE present is unknown.

ramana · Post by **ramana** » 22 Aug 2019 11:38

Dev, Can you point me to the reports?

agupta thanks

Prasad · Post by **Prasad** » 22 Aug 2019 11:44

ramana wrote:ldev and prasad, I thought screech or jet engine noise is due to the mixing of turbulent flow from the the combustion and is like air flow in an organ pipe or flute.
In supersonic flow there is expansion and contraction similar to an accordion which adds to the phenomenon. So it needs nozzle exhaust tinkering.

Helmholtz resonator is if the casing vibrates. That's not screech but case vibration due to resonance. They can add stiffening ribs on case and make the frequency go higher and avoid acoustic range.

i think there is confusion in the reporting.

There are many who have studied this in yinjineering, so please correct if i'm wrong. Screech is high frequency transverse and buzz is low frequency longitudinal combustion instability. Exhaust after LP turbine passes over afterburner assembly where more fuel is injected and ignited to generate more thrust. Jet pipe is obviously gonna get hot and hence you have a cheese grater type liner so that cooler bypass air enters the pipe to cool down the surface by not letting the afterburner exhaust touch the metal/composite surface and melt it.

We've heard that it has been fixed fwiw. There are a lot of papers on this with work ongoing. So who knows.

ldev · Post by **ldev** » 22 Aug 2019 19:18

ramana wrote:Dev, Can you point me to the reports?

agupta thanks

This is Part 1 of the issues Kaveri is facing aired on August 2, 2019 - all in Hindi
https://www.youtube.com/watch?v=6kpPugsUzew

This is Part 2 of the issues Kaveri is facing aired on August 7, 2019 - all in Hindi
https://www.youtube.com/watch?v=vuwkyz2ImNw

This is where he comments on the termination of the Safran relationship and possible rumors
of a tie up with Rolls-Royce, could be pure speculation aired on August 17, 2019 - all in Hindi
https://www.youtube.com/watch?v=9bhbNdPtfvs

ramana · Post by **ramana** » 22 Aug 2019 23:19

Ldev, Will watch. Glad folks are following it in Hindi so more people can follow and not only English speaking folks.

ramana · Post by **ramana** » 22 Aug 2019 23:31

agutpa, I have seen those characteristic shock vortices/Mach waves in rocket exhaust also.
Thanks for the paper.
The sound intensity as a function of Mach Number aka eighth power law is first formulated by Dr. Lighthill. You might be surprised but IIT Madras had quite a few books in the library on jet engine noise long back.

See the last picture on that page the need to have a rounded edge for the exhaust nozzle to reduce separation. The radius as a ratio of the exhaust velocity is the key parameter to ensure reduce flow separation that generates the mixing flow.

JayS · Post by **JayS** » 23 Aug 2019 17:16

ramana wrote:agutpa, I have seen those characteristic shock vortices/Mach waves in rocket exhaust also.
Thanks for the paper.
The sound intensity as a function of Mach Number aka eighth power law is first formulated by Dr. Lighthill. You might be surprised but IIT Madras had quite a few books in the library on jet engine noise long back.

See the last picture on that page the need to have a rounded edge for the exhaust nozzle to reduce separation. The radius as a ratio of the exhaust velocity is the key parameter to ensure reduce flow separation that generates the mixing flow.

You are talking about the shock diamonds, I suppose. They are formed due to over expanded nozzle. Basically the nozzle is designed for altitude condition where ambient pressure is low, so at ground level, the nozzle ends up over expanding the flow.

Just one point, the screech in context of Kaveri is related to the combustion instability in the Afterburner (where flow is actually low subsonic, so no shocks involved). Screech is due to the acoustic waves generated by the thermo-fluidic instabilities which results in lower than expected thrust. The F135 engine which powers F-35 also had same issue, PW fixed it later.

JayS · Post by **JayS** » 23 Aug 2019 17:18

ramana wrote:Ldev, Will watch. Glad folks are following it in Hindi so more people can follow and not only English speaking folks.

once saw video of the same guy on LCA MK2. He is basically copy pasting stuff from all over the internet. Which is not bad in itself. But I don't think he understands all that he is talking about well enough. Half knowledge could be worse than no knowledge. just sayin.

ldev · Post by **ldev** » 23 Aug 2019 20:04

JayS wrote:
ramana wrote:Ldev, Will watch. Glad folks are following it in Hindi so more people can follow and not only English speaking folks.
once saw video of the same guy on LCA MK2. He is basically copy pasting stuff from all over the internet. Which is not bad in itself. But I don't think he understands all that he is talking about well enough. Half knowledge could be worse than no knowledge. just sayin.

He is copying visuals from other sites to help explain his narration. Listen to the narration which is accurate. In fact YT has warned him about copyright issues and he has now issued a disclaimer that he only uses visuals from other sites to help explain/educate his viewers on his explanations. In the absence of the visuals in his videos would anybody other than an jet engine designer be able to understand what the issues are?

In fact it was only on this site that I learnt that the final offer from Safran was supply of modified M-88 cores to meet their offset obligations under the 36 Rafale contract without any TOT. All other MSM sites and in fact even so called defence correspondents by omission made it sound as if there was a dispute over the amount i.e. Euro 250 + Euro 500 for supply of TOT which is not the case at all. And GTRE/DRDO is right to walk away from such an offer which is in fact a non-offer.

JayS · Post by **JayS** » 24 Aug 2019 17:47

agupta wrote:
JayS wrote:
Just one point, the screech in context of Kaveri is related to the combustion instability in the Afterburner (where flow is actually low subsonic, so no shocks involved). Screech is due to the acoustic waves generated by the thermo-fluidic instabilities which results in lower than expected thrust. The F135 engine which powers F-35 also had same issue, PW fixed it later.

Err...not quite, JayS. Yes - Absolutely correct its in the afterburner fuel injectors - and the region where the combustion instability sets up ("the driver") may be subsonic, but does not change the fact that "screech" is associated with supersonic jet flows. Remember usually the AB injectors are upstream of (variable geometry) throats of (usually for military) supersonic nozzles.

See here: reference to "when" (full afterburner)
https://www.military.com/dodbuzz/2011/0 ... -fix-found

When you get screech, you cannot throttle / power/ augment the engine as aggressively as you want to, hence reduced thrust. The instability , or the acoustic waves by themselves, do not result in lower thrust...but maybe here I am just being too literal with your words vs. your intent ! In which case sorry in advance.

What I meant is, the Jet screech references you guys were discussing were of cases where screech originated due to the jet itself, without any upstream influence from the flow. So one can have screech even when entire engine is working OK, but if there is some issue with the nozzle exit geometry itself perhaps which gives rise to feedback of acoustic waves to the nozzle exit setting a cyclic response with high frequency. But in case of Kaveri the problem is one of "Screech in Afterburner", as far as my understanding goes. And due to improper combustion there, the thrust augmentation does not happen to the full extent, hence the loss of thrust.

Here is a PhD thesis from IISC. The work might have been associated with Kaveri itself.

http://cgpl.iisc.ernet.in/site/Portals/ ... Thesis.pdf

Two types of instabilities are encountered in afterburners. They are buzz and
screech. Reheat buzz is a low frequency, self-excited oscillation that can occur
above a certain fuel-air ratio. Screech is accompanied by high frequency pressure
oscillations that may be of such magnitude as to cause rapid deterioration of the
burner. Screech might be, or closely related to, some form of resonant oscillation
or also known as flame-driven resonant oscillations or combustion instability.
The afterburner-inlet conditions at which screech occur differs widely for various
afterburner designs. Combustion-driven flow oscillations that arise in combustors
and the afterburners are difficult to predict

Gyan · Post by **Gyan** » 24 Aug 2019 19:56

Can our Resident Experts comment on HTFE & HTSE also?

What's the comparison between say, HTSE & Airdiden-3 Turbomeca engine?

Similarly, Al-55 & HTFE?

Are the specifications realistic, achievable? Are the engines future ready?

JayS · Post by **JayS** » 25 Aug 2019 20:16

HAL has a tender to outsource Single Crystal Blade casting for LPT and HPT blades. My best guess, this is for Adour 871 engine. Because this doesn't look like its for either Al31FP or HTSE/HTFE. Looks like this is one of the Desikaran initiative and word reverse engineering is used in it...!!

https://hal-india.co.in/Tender_Details. ... ivkey=OQ==

Indranil · Post by **Indranil** » 25 Aug 2019 20:51

The tenders says that the HPT blade is for a turboshaft engine while the LPT blade is for a turbofan engine.

fanne · Post by **fanne** » 25 Aug 2019 20:57

in English please

JayS · Post by **JayS** » 25 Aug 2019 21:13

Indranil wrote:The tenders says that the HPT blade is for a turboshaft engine while the LPT blade is for a turbofan engine.

Thanks for pointing that out. I did not catch that one.

So HPT SCB could be either Shakti or HTSE-1200, later being the obviously likely one.

LPT blades are for Adour 871, its LPT has 94 blades and the material is a RR developed alloy.

Gyan · Post by **Gyan** » 25 Aug 2019 21:35

Should not HAL be doing this Single Crystal blade casting. After all who else in India would have such technology lying around? And it's odd that we have still not indigenised Adour Engine after 40 years

fanne · Post by **fanne** » 25 Aug 2019 23:12

Adour 871 (for Hawk, not Jags) - is the current LPT - SC or DS (from RR that is?)? Are we upping it to SC blade? (Maybe to get experience). Same Q as Gyan, why the tender. It is not that Abdul has this technology in his Garage, most likely 1-2 shops (Karaput? or some DRDO lab - MDNL?) has that technology, why this charade? to be above board and not anyone accused of hanky panky?

Answering my own question it does
https://watermark.silverchair.com/v002t ... o8bmpHXESc

JayS · Post by **JayS** » 25 Aug 2019 23:32

^^ Link doesn't work.

Adour 871 uses SCB for LPT.

unless we know who will end up making the SCB, lot of questions will remain unanswered. But given this is an indigenization efforts, it must be some desi entity. Would it be a MNC owned pvt subsidiary or shuddh desi company..? Who knows.

chetak · Post by **chetak** » 25 Aug 2019 23:35

fanne wrote:Adour 871 (for Hawk, not Jags) - is the current LPT - SC or DS (from RR that is?)? Are we upping it to SC blade? (Maybe to get experience). Same Q as Gyan, why the tender. It is not that Abdul has this technology in his Garage, most likely 1-2 shops (Karaput? or some DRDO lab - MDNL?) has that technology, why this charade? to be above board and not anyone accused of hanky panky?

Answering my own question it does
https://watermark.silverchair.com/v002t ... o8bmpHXESc

tech available only in govt/public sector in India unless some Indian player is able to outsource it abroad.

Gyan · Post by **Gyan** » 25 Aug 2019 23:41

If HAL/DRDO are looking at Reaper and Avenger Class UCAVs then we will need both Turboprop engine and turbofan engine
Turbofan Engine requirement will met by HTFE and Kaveri.
I wonder if HTSE can have a turboprop variant of 1500hp class for a Reaper Class UCAV

Cain Marko · Post by **Cain Marko** » 26 Aug 2019 01:41

Is Hal trying to upgrade the Jag engine in-house? That would be something...

Indranil · Post by **Indranil** » 26 Aug 2019 04:21

What is most interesting to me is that HAL does not consider its crown jewels to protect. Or, it really wants to become the systems' integrator. Both of which are great signs.

Prasad · Post by **Prasad** » 26 Aug 2019 09:33

Cain Marko wrote:Is Hal trying to upgrade the Jag engine in-house? That would be something...

Adour engine project has been ongoing for a while now. compressor side appears to be done and turbine side is being done now. Blades were machined by a private company in bangalore earlier. Not sure if they're being done now also.

JayS · Post by **JayS** » 26 Aug 2019 10:15

Do you mean to say, screech cannot be generated by the instabilities in the jet itself..?

Anyway this is off topic for Kaveri.

ramana · Post by **ramana** » 27 Aug 2019 05:11

Err, if it helps explain the issues of screech on Kaveri its quite germane right here.

abhik · Post by **abhik** » 27 Aug 2019 09:24

Kaveri is dead, there is hardly any news on what progress has been made in last 10+ years. Same vague info about about snecma core, screech issue, spreadsheet pressure ratios etc. are recycled every few years for time pass discussion here. I don't think this is really informing anybody (you have a poster who had put 2+2 together to to come to the conclusion that "screech" issue was solved by replacing the core with the French one).

sum · Post by **sum** » 27 Aug 2019 12:21

^^ Remembering reading about a comment on some Kaveri related article by some poster about how the tons of stuff happening under wraps will be revealed at the right time and there will be good news.

Not sure if just some random kite flying or something ongoing under a gag order inside GTRE

maitya · Post by **maitya** » 27 Aug 2019 12:40

Prasad wrote:
Cain Marko wrote:Is Hal trying to upgrade the Jag engine in-house? That would be something...
Adour engine project has been ongoing for a while now. compressor side appears to be done and turbine side is being done now. Blades were machined by a private company in bangalore earlier. Not sure if they're being done now also.

Indigenisation of Adour HPT DS blades is almost 8 year old hand - Read here

Quote from the 2011 Tech Focus article ...

DMRL also took up the extremely challenging assignment of developing technology for production of high-pressure turbine (HPT) rotor blades of Jaguar aircraft's ADOUR MK811 engine.The blades had complex aerofoil geometry with tight dimensional tolerances and stringent metallurgical specifications. Processing technology for this critical and complex directionally solidified (DS) aerofoil casting was fully developed by DMRL and the blades were delivered to HAL, Koraput.

Normally LPTs are either at a similar, or may be one level lower, in tech level than the HPTs blades/vanes - though there is no such hard and fast rule. So this SC casted LPT blades for Adour is quite interesting.

maitya · Post by **maitya** » 27 Aug 2019 12:54

^^ Betw from the same TF 2011 doc ...

The process is currently being used for making silica cores for Kaveri aerofoil castings that include HPT turbine blades and vanes and LPT blades. It is being adapted for making alumina cores that are required for producing single crystal aerofoil casting

Producing SC casted blades/vanes are much more technology intensive than simply being able to Vacuum Investment Casting of blades/vanes (one such intricacy is vanes are much more challenging to cast, SC or DS, than blades etc etc) ... someday I'll able to finish the 4th part of the series (from the prev page).

uskumar · Post by **uskumar** » 27 Aug 2019 12:55

India cancels Jaguar upgrade over Honeywell’s $2.4B price tag
sorry if it is already posted

Pratyush · Post by **Pratyush** » 27 Aug 2019 13:02

Guys, I have some queries regarding screech issue with Kaveri.

I ask as I have no clue what so ever regarding a jet engine.

1) If it is related to after burner then non after burning application should still have a flyable engine? Correct?

2) the screech, is it is physical issue that will adversely effect the safety of the aircraft?

Gyan · Post by **Gyan** » 27 Aug 2019 16:23

GE 414 Engine is only USD 5 Million, how come Jaguar upgrade engineering be is USD 13 Million each?

JayS · Post by **JayS** » 27 Aug 2019 16:35

agupta wrote:
JayS wrote:Do you mean to say, screech cannot be generated by the instabilities in the jet itself..?

Anyway this is off topic for Kaveri.

No that's not what I meant. Just saying screech is defined as a sub-set of (and happens only in) supersonic jet phenomena. Some instability (wether driven by a sharp lip or a afterburner flame/combustion dynamics) creates the disturbances that then become part of feedback loops.

See here: http://people.ece.umn.edu/users/mihailo ... reech.html or even on wiki https://en.wikipedia.org/wiki/Jet_noise
"Screech is produced by a feedback mechanism in which a disturbance convecting in the shear layer generates sound as it traverses the standing system of shock waves in the jet" ..... the disturbance triggers can be different

Screech is a phenomenon characterized by high frequency tonal acoustic waves. Again, what you are talking about is for Supersonic Jets - jet screech. And there is screech in context of After burner induced by combustion instability which is separate from the jet screech, that you have been referring to. These acoustic waves generated by the combustion instability in the A/B are trapped inside the jet pipe (they cannot travel downstream of the nozzle beyond the throat as flow becomes supersonic there and are reflected. They cannot travel upstream as well, due to presence of turbines, so they keep getting reflected back and forth in the jet pipe. ) and they can damage the HW inside. To attenuate them, acoustic liner with holes is used inside the jet pipe. If the frequency is low like 100Hz, they are called Rumble or buzz ( there is another buzz phenomenon for supersonic inlets but that's different one) and if the frequency is high like 500Hz they are call Screech.

These screech waves produced by A/B may or may not induce Jet screech, depending on whether they can induce flow instabilities in the flow at the throat of the nozzle strong enough that would travel thru the diffuser and then in the outside jet shock structure. I am not too sure about this part whether it will at all kick start screech outside in the jet. But the acoustic waves remain inside as they cannot pass thru the throat (its a physical boundary for them) and are reflected back.

ArjunPandit · Post by **ArjunPandit** » 27 Aug 2019 16:40

Gyan wrote:GE 414 Engine is only USD 5 Million, how come Jaguar upgrade engineering be is USD 13 Million each?

should be because of amortization over 1000 engines and no design/ip inputs
PS: 1500 engine was delivered in 2015

JayS · Post by **JayS** » 27 Aug 2019 16:48

maitya wrote:
Prasad wrote: Adour engine project has been ongoing for a while now. compressor side appears to be done and turbine side is being done now. Blades were machined by a private company in bangalore earlier. Not sure if they're being done now also.
Indigenisation of Adour HPT DS blades is almost 8 year old hand - Read here

Quote from the 2011 Tech Focus article ...
DMRL also took up the extremely challenging assignment of developing technology for production of high-pressure turbine (HPT) rotor blades of Jaguar aircraft's ADOUR MK811 engine.The blades had complex aerofoil geometry with tight dimensional tolerances and stringent metallurgical specifications. Processing technology for this critical and complex directionally solidified (DS) aerofoil casting was fully developed by DMRL and the blades were delivered to HAL, Koraput.
Normally LPTs are either at a similar, or may be one level lower, in tech level than the HPTs blades/vanes - though there is no such hard and fast rule. So this SC casted LPT blades for Adour is quite interesting.

Adour 871 (AJT Hawk) has SCB LPT blades.

JayS · Post by **JayS** » 27 Aug 2019 16:51

Pratyush wrote:Guys, I have some queries regarding screech issue with Kaveri.

I ask as I have no clue what so ever regarding a jet engine.

1) If it is related to after burner then non after burning application should still have a flyable engine? Correct?

2) the screech, is it is physical issue that will adversely effect the safety of the aircraft?

1. That's why Ghatak is going ahead with dray Kaveri.
2. Screech or rumble related to the A/B can damage the engine from inside. That's a safety issue. Loss of thrust is another issue, but it need not be a safety issue.

JayS · Post by **JayS** » 27 Aug 2019 23:21

Overview of Gas Turbine Augmentor Design, operation
And Combustion Oscillation
https://engineering.purdue.edu/AAECours ... r%20Review

The phenomenon referred to as “screech” (see Section 6.2) is a type of combustion instability
characterized by high frequency, high amplitude pressure oscillations. The high pressures and
temperatures accompanying low-altitude, high-speed flight is conducive to screech. The onset
of screech during the combustion process contributes to an increased rate of flame propagation
and improved augmentor combustion efficiency. However, screech also results in higher
operating temperatures in the augmentor. Moderate to severe screeching conditions can cause
rapid deterioration and failure of the augmentor liner, flameholder, and the fuel injection
system. Numerous documented hardware failures have been encountered affecting the
augmentor liner, flameholders and fuel system components after only a few minutes of
operation during screeching conditions (Refs. 14, 17-26, 34). To counteract this, practical
augmentors are designed with a screech liner that consists of a hot (core) side and cold (fan
duct) side. This allows for film cooling and some amount of screech suppression. The screech
liner is generally designed to address high frequency combustion oscillations (> 1000 Hz).
Due to hardware damage of the augmentor during screech operating conditions, it is essential
that an understanding of the screech phenomenon is gained and methods for eliminating
screech are devised. Several investigators have studied augmentor screech phenomena (Refs.
20, 26, 34). From these studies, several theories have emerged that attempt to describe the
screech phenomenon. Thus far, the capability of these analyses to predict screech and the low
pressure acoustic coupling referred to as “rumble” has been limited. Rumble is a phenomenon
similar to screech caused by low frequency oscillations in the augmentor. Rumble is a concern
because it causes extreme structural vibrations in the engine, which can cause the engine to
break apart, resulting in mission failure and, in extreme cases, loss of the entire aircraft.