Sweetenough
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Posts: 121
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Post by Sweetenough on Jan 31, 2022 10:40:40 GMT -5
Hi Guys, Finished and installed the new after burner this weekend and made a first test run yesterday. Worked quite well, easy to ignite. Had no load cell installed so I do not know the thrust, will be done next test. Almost had a blow out at approx 1:21 into the movie, I guess I reached the fuel limit. The shiny metal metall is gone once again Kind Regards Thomas www.youtube.com/watch?v=Kz0vtjiXmQU
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slittlewing
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Joined: November 2017
Posts: 458
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Post by slittlewing on Jan 31, 2022 13:18:24 GMT -5
Now that was HOT!! 😈😈😈
Awesome work mate, congratulations on another good test! I am surprised your camera didn’t melt haha. Would be really interesting to tie up the thrust vs P2 and fuel amount once this is fitted 👍👍👍
Cheers
Scott
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Post by andym on Jan 31, 2022 17:50:33 GMT -5
Real nice..... looking and sounding like a well running aftreburner.... flame looks like it was held well.... plenty of screaching.... still dont know why the power is in the sound.... but all the testing we did in the UK ..... when they screached you got more power..... listen to fire force.... you can even hear that screaching .... Wish i could make it to ice weekend
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jetric
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Post by jetric on Feb 1, 2022 3:30:51 GMT -5
The screeching is caused by unstable combustion setting up shock waves that reverberate forwards and backwards inside the reheat tube, this is not good and should be avoided if possable as the shock waves can cause damage to the turbine exducer blades by causing them to vibrate resulting in stress failure. This is why they use a screech liner in military afterburning jets in order to prevent the onset of screech. I have actually seen the converging cone on a reheat pipe that Andy Plezesco fitted to a RR Nimbus engine flex around by up to an inch when his reheat went into screech this eventually caused the converging cone to fail and I advised him to fit a screech liner. Richard S. Real nice..... looking and sounding like a well running aftreburner.... flame looks like it was held well.... plenty of screaching.... still dont know why the power is in the sound.... but all the testing we did in the UK ..... when they screached you got more power..... listen to fire force.... you can even hear that screaching .... Wish i could make it to ice weekend
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Sweetenough
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Posts: 121
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Post by Sweetenough on Feb 1, 2022 4:43:22 GMT -5
Thanks guys, Will log thrust next test to verify "optimal" settings, really looking forward to see if there is any improvement in thrust. If my calculations is correct the new after burner can take approximately the same amount of fuel as the old one before it goes beyond screeching to "flames out" Was hoping that the new design should allow more fuel with the new fuel distribution but I guess (as most of the work I do...) that more or less all available oxygen is combusted and therefore the limit is reached.
Kind Regards Thomas
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Post by finiteparts on Feb 1, 2022 7:36:01 GMT -5
The screeching is caused by unstable combustion setting up shock waves that reverberate forwards and backwards inside the reheat tube, this is not good and should be avoided if possable as the shock waves can cause damage to the turbine exducer blades by causing them to vibrate resulting in stress failure. This is why they use a screech liner in military afterburning jets in order to prevent the onset of screech. I have actually seen the converging cone on a reheat pipe that Andy Plezesco fitted to a RR Nimbus engine flex around by up to an inch when his reheat went into screech this eventually caused the converging cone to fail and I advised him to fit a screech liner. Richard S. Real nice..... looking and sounding like a well running afterburner.... flame looks like it was held well.... plenty of screeching.... still don't know why the power is in the sound.... but all the testing we did in the UK ..... when they screeched you got more power..... listen to fire force.... you can even hear that screeching .... Wish i could make it to ice weekend Richard, Just a clarification...they are not shock waves, they are just sound waves. The problem is when a natural frequency (longitudinal, tangential or radial) of the duct corresponds to the frequency of the fluctuating heat release. When these mechanisms couple, the heat release can amplify the pressure pulsations...since the heat release is an energy adder to the acoustic system. The hardware damage occurs when the fluctuating pressure strains the part enough to exceed it high cycle fatigue limit. The fuel system can also couple to the ducts acoustic response, especially if the fuel pressure across the injection orifice is relatively low. As the local gas pressure fluctuates, the pressure ratio on the orifice will fluctuate and drive a pulsation in the fuel flow. If this meets a certain coupling condition, it can drive a reinforcing heat release that amplifies the acoustic instability. I agree with you on the need for a screech liner or the addition of a Helmholtz resonator if the scrèech frequency is relatively "fixed". Without a mechanism to remove some of this acoustic energy, the potential for damage to hardware is elevated. Also, the fluctuating pressure can damage the thrust bearing. Since the turbine has less project area, it is less damaging that compressor surge, but it is still something to avoid. - Chris
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Post by finiteparts on Feb 1, 2022 7:43:49 GMT -5
A great resource for more information on this is Dr. Tim Lieuwen's (Georgia Tech) books on combustion instability.
He has some great lectures on Youtube that are highly recommended.
- Chris
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jetric
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Joined: December 2014
Posts: 132
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Post by jetric on Feb 1, 2022 11:33:50 GMT -5
Are these pressure waves simular to the pressure waves that are utilised in a two stroke engine expansion chamber exhaust (tuned exhaust) because in the two stroke exhaust you are utilising the pressure wave from the exhaust port opening and resultant release of the blow down pressure and with the use of specific cone angles and length of the exhaust pipe to actually reflect the pressure wave back to the exhaust port at just the right time before the port closes to push the small amount of escaped unburnt mixture back into the cylinder thus supercharging it. Are our cones that we are using on our D.I.Y. engine reheat pipes not causing any pressure waves to reverberate back and forth simular to the two stroke expansion chamber exhausts? Richard S. The screeching is caused by unstable combustion setting up shock waves that reverberate forwards and backwards inside the reheat tube, this is not good and should be avoided if possable as the shock waves can cause damage to the turbine exducer blades by causing them to vibrate resulting in stress failure. This is why they use a screech liner in military afterburning jets in order to prevent the onset of screech. I have actually seen the converging cone on a reheat pipe that Andy Plezesco fitted to a RR Nimbus engine flex around by up to an inch when his reheat went into screech this eventually caused the converging cone to fail and I advised him to fit a screech liner. Richard S. Richard, Just a clarification...they are not shock waves, they are just sound waves. The problem is when a natural frequency (longitudinal, tangential or radial) of the duct corresponds to the frequency of the fluctuating heat release. When these mechanisms couple, the heat release can amplify the pressure pulsations...since the heat release is an energy adder to the acoustic system. The hardware damage occurs when the fluctuating pressure strains the part enough to exceed it high cycle fatigue limit. The fuel system can also couple to the ducts acoustic response, especially if the fuel pressure across the injection orifice is relatively low. As the local gas pressure fluctuates, the pressure ratio on the orifice will fluctuate and drive a pulsation in the fuel flow. If this meets a certain coupling condition, it can drive a reinforcing heat release that amplifies the acoustic instability. I agree with you on the need for a screech liner or the addition of a Helmholtz resonator if the scrèech frequency is relatively "fixed". Without a mechanism to remove some of this acoustic energy, the potential for damage to hardware is elevated. Also, the fluctuating pressure can damage the thrust bearing. Since the turbine has less project area, it is less damaging that compressor surge, but it is still something to avoid. - Chris
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Post by racket on Feb 1, 2022 18:25:24 GMT -5
Hi Andy
Maybe the screeching aids combustion by "shattering??" the larger fuel particles produced by our relatively low A/B fuel pressures , the resulting "improved" combustion is the reason for the "best power" in our fairly short A/Bs , just a thought ....I know nuthin' about A/Bs :-)
Cheers John
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jetric
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Post by jetric on Feb 2, 2022 1:47:36 GMT -5
Hi John, As I have stated before I don't think the fuel stands up much to the 750oC gas stream, it is fully vaporised within a couple of inches of travel in the gas stream. As for extra power when the reheat screeches it could be down to the pulsation it causes in the gas stream making the gas stream that is exiting the end of the reheat simular to a pulse jet engine. Personally I have not noticed any improvement in thrust with screeching. Andy, have you actually measured the difference in thrust output with and without screeching or did it just sound more powerful? Richard S. Hi Andy Maybe the screeching aids combustion by "shattering??" the larger fuel particles produced by our relatively low A/B fuel pressures , the resulting "improved" combustion is the reason for the "best power" in our fairly short A/Bs , just a thought ....I know nuthin' about A/Bs :-) Cheers John
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Post by racket on Feb 2, 2022 3:32:45 GMT -5
Hi Richard
Totally agree , we need to measure thrust outputs to quantify any improvements , thats why I have my 200 lb spring balance hooked up , it doesn't care about the sound the engine is making ............LOL , once past 50 lbs of thrust , noise is just noise , impossible to judge anything :-)
With regards evaporation rates of our fuels, my Gas Turbine Combustion text by Lefebvre gives some quite "lengthy" time frames , a 100 micron - 4 thou of inch droplet of diesel takes ~10 milliseconds and was the reason I changed to a 2:1 petrol/kero mix for my 12/118 engine with evaporation rates about half that of diesel .
Some of the Papers on Afterburners I've read where they vary the length of the afterburner (residence time) , the thrust outputs are greatly affected by reducing the residence time.
I might need to read up a bit more before firing up my length of stainless pipe so I can justify its underperforming :-)
Cheers John
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jetric
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Posts: 132
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Post by jetric on Feb 2, 2022 6:09:19 GMT -5
John, With regards to residence time does the flame that is out past the end of the reheat nozzle actually add anything at all to the net thrust of the reheat, would it be best to try and keep all the flame inside the reheat tube? Richard S. Hi Richard Totally agree , we need to measure thrust outputs to quantify any improvements , thats why I have my 200 lb spring balance hooked up , it doesn't care about the sound the engine is making ............LOL , once past 50 lbs of thrust , noise is just noise , impossible to judge anything :-) With regards evaporation rates of our fuels, my Gas Turbine Combustion text by Lefebvre gives some quite "lengthy" time frames , a 100 micron - 4 thou of inch droplet of diesel takes ~10 milliseconds and was the reason I changed to a 2:1 petrol/kero mix for my 12/118 engine with evaporation rates about half that of diesel . Some of the Papers on Afterburners I've read where they vary the length of the afterburner (residence time) , the thrust outputs are greatly affected by reducing the residence time. I might need to read up a bit more before firing up my length of stainless pipe so I can justify its underperforming :-) Cheers John
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Sweetenough
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Posts: 121
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Post by Sweetenough on Feb 2, 2022 8:14:59 GMT -5
Hi Guys, Will use a load cell connected to the same data logging system that the engine data goes to during next test. Hopefully that can give clarity in what happens with thrust during screeching. My very personal guess is that screetch itself does not affect thrust much, it is just an indication that you are getting close to fuel max limit. And since you are at maximum fuel you get the highest thrust with a little bit of screetch. But that is just a "farmers-guess" as we say in Sweden :-) , hope to present logged data after next test run that is planed for next weekend. Will also try to meassure the screech frequency with an FFT phone app, just for fun.
The gasturbines at work with with dry low emission (DLE) combustion systems all have combustion pulsation supervison system. To enable as much pre-mixed fuel as possible in order to get lowest possible NOX emissions witout dangerous "screetch" The main fear is frequencies close to any natrual frequency of parts/structures, that will lead to pre-mature failures really fast.
Kind Regards Thomas
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Post by racket on Feb 2, 2022 16:43:55 GMT -5
Hi Richard
I would imagine there are a number of variables with this answer , idealy from a SFC point of view we'd like all fuel consumed inside the A/B , but ..........LOL, theres always buts , we might end up with more thrust if there are some external flames if the dwell time is insufficient for full combustion of an ideal fuel quantity and extra is added in an attempt to burn all oxygen and obtain the highest possible gas temperature before the jet nozzle so that the highest gas speed can be obtained from the available pressure drop .
Our A/Bs probably work at 90% or thereabout burn efficiency , so there'll be the opportunity for some external flames as any unburnt fuel encounters external oxygen, so to answer your question, yes , external flames might indirectly "add??" to thrust .
Probably best to do the trial and error approach and feed in whatever amount of fuel produces best thrust irrespective of what the external exhaust plume looks like , though a nice "tight" bunsen burner type flame looks "best"
I'll try to optimise my "dry" thrust and then add on the 40% theoretical increase in thrust from the A/B , then try and obtain that "wet" thrust figure by adding ever increasing amounts of fuel , if I can't make that 40% increase then there'll be constructional issues that I'll have to look at, and even if I do get that 40% but at unrealistic fuel consumption , I'll have to reconsider a few things .
Yep , fun times ahead , keeping my fingers crossed :-)
Cheers John
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Post by finiteparts on Feb 3, 2022 22:26:06 GMT -5
Are these pressure waves simular to the pressure waves that are utilised in a two stroke engine expansion chamber exhaust (tuned exhaust) because in the two stroke exhaust you are utilising the pressure wave from the exhaust port opening and resultant release of the blow down pressure and with the use of specific cone angles and length of the exhaust pipe to actually reflect the pressure wave back to the exhaust port at just the right time before the port closes to push the small amount of escaped unburnt mixture back into the cylinder thus supercharging it. Are our cones that we are using on our D.I.Y. engine reheat pipes not causing any pressure waves to reverberate back and forth simular to the two stroke expansion chamber exhausts? Richard S.
Richard,
Yes, exhaust pulses are also the same standard acoustic waves, since the flows in your exhaust systems are not supersonic. Shock waves mark the sudden readjustment of a supersonic flow to a subsonic flow or vice versa. There are compression and expansion waves, that occur in subsonic flows, which are used in two stroke exhaust to "charge" the cylinder before the exhaust stroke completes. As for angles of the pipes, any sudden changes in area, pressure or temperature will cause a sudden change in the acoustic impedance and thus usually, the low angles used in two stroke exhausts are used to try to slow the flow down without causing a sudden change in impedance which would cause partial reflections of the wave. Usually the wave is desired to be reflected at the end of the pipe to get the timing right (tuning) so that the exhaust pulse and reflected wave are in resonance.
As John said, any fuel that is burned after the exhaust flow has passed the nozzle throat does absolutely nothing for thrust, other than add mass flow to the exhaust stream. I also agree with John on the limited value of trying to guess the equivalence ratio from the appearance of the flame. When the fuel is burned to a full stoichiometric value, the flame temperature is over 3800F, which appears luminous by itself. It gets hard to differentiate between this luminous gas and a still reacting flow. Also, the efficiency of afterburners are notoriously low, so you could also just be seeing locally rich flow on the outside of the stream finally finding oxygen. I agree that measuring the thrust is the proper way to assess the performance of the AB system.
Thomas,
Great job on your AB, it looks like it is really doing great. You might be getting near a limit on the fuel flow and thus getting screech when you start to go to a richer bulk flow, but you could just as easily just be getting to a condition where the afterburner ducts natural acoustic mode is inline with the fluctuating heat release. It is really hard to determine the reason the driving mechanism for screech, which is why it is still a very rich field of research and study.
I also thought that I would suggest that with all your instrumentation on your engine, you can direct determine if you have the properly sized nozzle area by looking to see if you have a shift in rotor speed, or a shift in compressor pressure ratio when you light off you AB. Just a thought.
- Chris
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