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Post by Johansson on Dec 31, 2013 9:21:25 GMT -5
Thanks Stoffe, and happy new year! I cut a couple of hours of sleep last night and worked on the afterburner instead, first I finished the fuel manifold. After that I cut the A/B apart to fit a flame holder and grind the internals a bit for better flow. Here I have smoothed the internal passages a bit. I made a three legged flame holder from SS2368 heat resistant stainless steel tubing, I cut the tube in half and used three halves. Now the afterburner is very close to finished, all that is left to do is to fit a spark plug bung to it. Cheers! /Anders
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Post by Richard OConnell on Dec 31, 2013 10:19:28 GMT -5
The "high profile" layout on the fuel lines really gives the AB a unique look! Great job as always, Anders
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Post by finiteparts on Dec 31, 2013 11:17:18 GMT -5
Hi Anders, Beautiful work...one thing though...you have six AB injectors and only three flame gutters. I would be concerned that the injected fuel wouldn't get entrained into the flame holder wake. And since the freestream velocity of the flow in the AB will be much higher than the turbulent flame speed, it will not burn in the confines of the AB and just spray out the exhaust nozzle to burn outside. After burners are notorious for having very low combustion efficiencies due to low cross stream mixing and thus emit large amounts of vaporized yet unburned fuel. In AB design, you usually aim to have the injectors feed the flame holder wake, which usually results in them being inline with the gutters (unless you have residual swirl). Contrary to popular belief, the goal is to have as much of the reaction as possible occur before the exit plane and not to have long flames shooting out. So, I was wondering if it might be a good idea to switch to a six gutter design?
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Post by finiteparts on Dec 31, 2013 14:03:06 GMT -5
I see that you have welded the AB together just ahead of the gutters...since this is an area that sees extreme thermally driven stresses, I would want to have a flanged connection so that if the flameholders did crack or break, they could be repaired or replaced.
Just a thought...
Good luck!
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Post by Johansson on Jan 2, 2014 14:38:09 GMT -5
Thanks Richard, it looks a bit special indeed. Finiteparts: Good input there, I only had one size of ss2368 tubing available and 6 "gutters" would probably choke the flow too much. But, with an unknown amount of exhaust swirl is it even possible to predict where the gutters should be placed relative to the injectors? Welding the A/B is because we had a distortion in the old kick afterburner that made the flange leak and toasted my friends leg during one run, so it feels safer to weld the entire afterburner. Cheers! /Anders
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gidge348
Senior Member
Joined: September 2010
Posts: 426
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Post by gidge348 on Jan 4, 2014 21:19:38 GMT -5
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Post by finiteparts on Jan 5, 2014 12:57:35 GMT -5
Hi Anders,
You are correct...predicting the swirl might be tough and it would vary as a function of engine speed...that is why many ABs use v-gutters in the shape of a ring. The radial position of the flow would be more easily chosen since there should be very small deviation from the upstream fuel injection location...I think for your design, if you got close to being inline with the injection, you would be in a likely position to entrain fuel into the flame holder wakes.
I bet if you calculated the blockage area, six gutters would be well under the exit area of the turbine which is really where you are more likely to choke the flow. Remember, area of a circle goes up as the square or the radius, so when you expanded the AB flow area with the divergent section, you gained area quickly.
I can see how a thermally distorting flange could cause problems...I guess if you have to repair the AB, it would be as easy as cutting it at the weld and then just re-weld...
I agree with some of the other members that a cooling liner should be used. The thousands of tiny holes are primarily for changing the acoustic signature of the AB boundary as a means to reduce high frequency pressure pulsations (called "screech" and due to combustion-pressure coupling (Rayleigh Criterion for combustion)). Since the projected area of full scale AB is so large, the high frequency pressure pulsations, even if they are small in magnitude, create large cyclic forces on the AB outer walls. The use of the "screech liner" damps out these pressure pulsations and reduces the cyclic forces that would cause fatigue failure of the AB parts.
With our small ABs, these cyclic forces would be small enough to manage without the need to modify the acoustics of the AB...so the liner could just be used as a cooling feature without holes or a small number of strategically placed holes. A small amount of exhaust flow to cool the liner would go a long way. The liner acceptable temperature can be much higher since the pressure load on it is much less than the outer tube which has to act as a pressure vessel. Since the load on the cooling liner can be higher, the use of stainless steel sheet would be acceptable since a long fatigue life isn't required.
Just an aside, the main purpose of using the high nickel alloys is the high strength, oxidation resistance and fatigue capability...not thermal capability. As an example, 304 SS has a melting temp around 2550-2650F, while Inco 718 melts at 2300-2437F. Other high temp alloys are similar. So, if the part doesn't have high loads (since 304 SS strength drops quickly around 1200F), doesn't stay at high temps for long times (driving the oxidation and fatigue)then the use of stainless could be viable. So, it would be important to make sure that the cooling liner isn't mounted in such a way as to impose high thermal stresses. Holding it fixed at one end and letting it float on the other end with some angled mount should keep the loading low. If you check some of the early turbojet engines with ABs, you will see that many had solid tubes acting as the liner with the "cooling" air exiting at the nozzle section.
Now I agree that fuel cooling the liner might not be the best choice due to coking of the fuel, but if you wrapped water cooling lines on the outside of the AB liner I think the big gain in heat capacity could be beneficial and you could even inject the steam formed in the cooling tubes upstream of the nozzle to get the mass flow benefit. I also think that using steam to cool the inside of the AB might not be the best choice. If you just perform a simple adiabatic mixing calculation, you see that the change in enthalpy by using water involves a change of phase, thus absorbing a larger amount of heat than using the cooling liner. Now this involves some assumptions, but fundamentally, the use of a "coolant" that involves a latent heat of vaporization may limit you to such a small flow that you can not provide full cooling coverage for the same amount of heat loss that would otherwise be used if you went with the cooling liner approach. Secondly, if the water did not vaporize to steam prior to impacting the hot, THIN walls, the severe thermal stress formed at that point would likely cause cracks to form in very short order.
With all this being said, I have been designing industrial gas turbine combustors for years and never stop being amazed at how hard it is to predict...I have seen combustors that wouldn't light off...some that held flames on steps where parts mate together and burned up in seconds (nice to watch on video if it isn't your program! hehee!)...among others. All this with one of the best design teams in the world and almost unlimited budgets to run reacting CFD models, thermal analysis, etc. So by no means am I saying that your AB will not function properly, I was just relaying what others have had work in the past. I think the best way to do combustion designing is just what you are doing...build it-test it...fix what burns off...test again! So good luck...I look forward to seeing it completed.
gidge348...if you cut the dome section off of that combustion chamber, it would work as a cooling liner if it fit your AB tube size...but I think I would try to roll my own stainless first and keep that combustion chamber as a nice piece of desk art.
~ Chris
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Post by finiteparts on Jan 5, 2014 13:05:20 GMT -5
Here is a shot of the Pratt&Whitney J-57 engine afterburner arrangement in the F-100 Super Sabre...note that it lacks the tiny holes used for damping screech. ~ Chris
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Post by Johansson on Jan 5, 2014 16:17:20 GMT -5
Hi Chris, I am impressed by your knowledge and experience with combustors, it would be awesome to be able to work with your hobby even though it might take some fun out of the hobby part. The afterburner liner idea, water vapour injection and all that is interesting but not very doable on our jet kick, we have had loads of problems with every single engine part during our ice racing years so we are keeping the "good to have" ideas to a bare minimum while focusing on the "need to haves". With that said please keep the ideas coming, it is very interesting to hear what´s in the heads of you guys here at JATO. Cheers! /Anders
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gidge348
Senior Member
Joined: September 2010
Posts: 426
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Post by gidge348 on Jan 5, 2014 20:42:45 GMT -5
Hi Chris Thanks for the really interesting details AB & combustion liners as you see I like asking silly questions and getting very intelligent answers. I was mainly thinking that something like this could be used as part of the trial and error that goes into developing a DIY engine and for $20 or so it is not a big price to pay. Probably worth more as scrap? Cheers Ian...
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Post by Johansson on Jan 12, 2014 14:51:43 GMT -5
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Post by stoffe64 on Jan 12, 2014 17:30:45 GMT -5
Hello anders, you welded the connection combustor casing to compressor outlet, what about distortion?,or have you calculated that? Cheers stephan
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Post by racket on Jan 12, 2014 17:35:38 GMT -5
Hi Anders
Cutting off the comp housing outlet and welding on the elbow saves a lotta work constructing the usual bolt on adapter, also makes for a much more compact and neater finish , ......... nice :-)
Cheers John
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Post by Johansson on Jan 13, 2014 13:27:59 GMT -5
Hello anders, you welded the connection combustor casing to compressor outlet, what about distortion?,or have you calculated that? Cheers stephan The comp housing is so thick that there wasn´t any danger of distortion, I´ve seen much smaller housings being welded without problem. Hi Anders Cutting off the comp housing outlet and welding on the elbow saves a lotta work constructing the usual bolt on adapter, also makes for a much more compact and neater finish , ......... nice :-) Cheers John Hi John, Thanks! I´ve learned this from the turbo car builders, it is normal practice to cut the comp housing outlet and weld V-band flanges to it. Before I weld the second housing I need to preheat it since the massive housing demands massive amps when it is cold. With a small WP-9 air cooled tig torch it doesen´t take long before it is too hot to hold... Cheers!
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Post by stoffe64 on Jan 13, 2014 13:50:04 GMT -5
Hello anders, no i meant that when the engines are running they get warmer and start to expand'if they are too tight the engine seize in the compressor'i saw such happen on youtube with a diy engine,it was rigid mounted so it couldnt move so it seized up
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