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Post by Johansson on Dec 18, 2013 9:40:47 GMT -5
Hi John, We will use plain washers to shim the two cradle halves apart so the A/B can expand freely, but I could´t guess that we would need as much as 7mm clearance. No wonder the life span of an afterburner is so short... Cheers! /Anders
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Post by racket on Dec 18, 2013 15:58:32 GMT -5
Hi Anders
Yeh , those expansion distances get rather large at elevated temperatures , ............when I made the turboprop pulsejet engine I had a "crade" at the tail end and there were witness marks indicating ~20mm longitudinal expansion even though it was only the combustion chamber section that got really hot , I'd be giving your kick cradle at least 8mm diametric clearance just to be on the safe side as theres going to be a fair bit of axial expansion and you don't want things binding up and tearing the cradle apart or buckling the softened afterburner shell and splitting a weld ............heh heh , the smell of Olov's leg flesh burning might be a bit much for the spectators to stomach ;-)
Cheers John
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gidge348
Senior Member
Joined: September 2010
Posts: 426
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Post by gidge348 on Dec 18, 2013 22:21:01 GMT -5
Not sure if this is a dumb question or not, but why not have an inner diffuser and outer casing to the AB like large military jets?
You could deflect some of the coolish turbine exhaust gasses between the outer casing and inner diffuser to form a boundary layer to hold the super hot AB gasses away from the diffuser so not wasting energy heating up the outer casing?
Or even in this application where you have 2 turbines feeding 1 AB... duct one to the inside and one to the outside?
Ian...
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Post by racket on Dec 19, 2013 0:33:14 GMT -5
Hi Ian
The screech liner in full sized engines has millions of small holes in it and is by necessity made from an Inconel material , its not easy to replicate for a DIY'er .
To get maximum thrust from the afterburner all of the "air" exiting the turbine wheel needs to be "burnt" , and considering the relatively short length of DIY afterburners and the extremely limited time frame for combustion, its necessary to have combustion taking place with all of the "air" as early as possible within the afterburner , diverting air to cool the wall will reduce maximum thrust
My thoughts on a solution to the extreme metal temps for a DIY afterburner would be a water spray onto the inside wall of the AB near the flameholder , the water would spread out across the surface of the metal and flash off as steam which should produce a "steam blanket" between hot combustion products and the wall .
Considering most DIY afterburners only ever operate for less than a minute at a time it'd only require a couple of litre of water and a small pump producing 20 or so psi of pressure to get it into the afterburner , the steam would also contribute to the mass flow and increase thrust .
Cheers John
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gidge348
Senior Member
Joined: September 2010
Posts: 426
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Post by gidge348 on Dec 19, 2013 3:37:24 GMT -5
Thanks John, makes sense.
Like I said these are probably dumb questions but I was just wondering though by using water as the medium that would the flashing off, wouldn't that draw a lot of BTU energy from the AB and so shrink the volume and so the velocity of the exhaust?
Would it be viable to wrap the fuel line for the AB around the outside of the after burner starting at the back and moving to the front.... sort of like a liquid rocket.
Then the energy would be recycled by pre heating the AB fuel to probably several hundred degrees and make atomisation and burning better? While at the same time insulating the outer casing?
Ian...
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Post by racket on Dec 19, 2013 14:39:10 GMT -5
Hi Ian
This is how I reason it out ..........if the AB stainless tube weighs about the same as the amount of water being injected over the minute of operation , the amount of heat required to raise the tube to >1,000 deg C would be more than the heat required to turn the water into steam at 100 deg C , the steam blanket produced would insulate the metal from any further conduction heating though there would still be radiant heating .
LOL, I guess it'd be a trial and error process to determine the heat balance.
Wrapping the AB with the fuel line would be a problem , the fuel flow rate is very low compared with a rocket, whilst the surface to be cooled is much greater than a rocket combustion chamber ...........if the fuel vapourises in the delivery line there can be all sorts of combustion problems as per the Schreckling FD3 micro turbine engines with a similar setup in their combustion chambers
Cheers John
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gidge348
Senior Member
Joined: September 2010
Posts: 426
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Post by gidge348 on Dec 19, 2013 18:48:01 GMT -5
Thanks John, all this stuff is a learning experience for me and I appreciate your patience in explaining things. Cheers Ian...
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Post by racket on Dec 19, 2013 23:05:48 GMT -5
Hi Ian
No worries , only too happy to explain :-)
Cheers John
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Post by Johansson on Dec 20, 2013 4:09:36 GMT -5
Water steam insulation would be an excellent option, not this year but perhaps later when we have more time time on our hands. Good heat shields are bulky and adds lots of weight to the rear of the kick, so with steam cooling a thin aluminum shield covered in a fibre glass blanket should work.
A true A/B with internal ducts and bleed air would be even better but as John points out we need every gust of air coming from the slightly undersized compressor.
Cheers! /Anders
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Post by Johansson on Dec 21, 2013 14:57:11 GMT -5
Last night I started making the fuel injectors for the afterburner, six 1.5mm injectors will provide all the fuel it will ever need. I should probably have settled with 1mm holes but I managed to snap all 1mm drills. Free hand drilling 1mm holes after a couple of beers is not the best idea... Cheers! /Anders
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Post by racket on Dec 21, 2013 15:43:40 GMT -5
Hi Anders
This beast is starting to look serious ............she's heavy duty construction :-)
Cheers John
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gidge348
Senior Member
Joined: September 2010
Posts: 426
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Post by gidge348 on Dec 21, 2013 19:44:47 GMT -5
Looking great Anders, how is the list of jobs still to do ..... getting smaller I hope? Yep beer tends to kill drill bits Ian...
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Post by Johansson on Dec 26, 2013 15:32:25 GMT -5
Hi John and Ian, Yeah, the afterburner gets sort of a steam punk look with the injectors. The list is getting shorter but boy it is still a lenghty one, all the small bits and pieces at the end tend to take forever to finish. We still have hope in the time plan so keep yer fingers crossed. I spent an hour today making the first of two fuel injector plenums, with one plenum on each side it will be possible to remove the injectors from the afterburner. Cheers! /Anders
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Post by Johansson on Dec 29, 2013 14:19:57 GMT -5
Today we fitted the syringe injectors, spark plugs and propane nozzles to the combustors. Cheers! /Anders
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Post by stoffe64 on Dec 30, 2013 2:59:06 GMT -5
impressively simple as Always it looks like when you guys do something, looking good Anders! i have been trying to figure out myself how to cut the parts to make a workable flametube, still not sure how to;( by the way, what will happen to the old jet kick Engine??....sell it or keep it as a spare?
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