Simulated Afterburner Flame?

[Lugebob]

I am still trying to simulate burner pops or to make a flame show for the Jet Luge, but still cannot get anything to light off when sprayed into the exhaust stream once the engine is at idle.

First: I do not want to add an afterburner. It is not practical due to size and the amount of fuel needed and I dont need the extra thrust. I tried to add a single flame holder to the exhaust cone, but it reduced the thrust and raised the temp of the engine. I have also hooked up a 700,000 volt stun gun via spark plug wires to the smoke tubes and can get a big a** arc between them in the spray, but still can light the kero or desiel fuel and even tried methyl alcohol.

I can get flame from the smoke injector as the engine is spooling up, but once to idle rpm or higher I just get smoke.. My assumption is that flame cannot be sustained at the high velocity of the exhaust, it just blows out to smoke. I have read about High velocity flame as used in welding and thermal painting and they all add oxygen. Could the exhaust be low on oxygen and that is why no flame.? What if I inject oxygen with the fuel spray? What if I premix it with about 50% Hydrogen Peroxide? (hmm maybe post in rocket forum)

Anyone have any theories? THings to try.?

If I can not get this to work in the exhaust stream I will need to do it with and external flame thrower near the engine.. but would rather not.

Bob

here are a few picts to visualize

[Lugebob]

PS. I have tried the injector tubes at various distances from the exhaust.. just makes more or less smoke.


bob

[NickC]

if you add oxygen you will start getting thrust though I believe. that is the concept in a jet-rocket which was used in early supersonic planes, i believe. An afterburner is the only way to go. Trying to make a non-combusting flame in a high speed stream of air that is low on oxygen won't be simple. If you make an afterburner that runs really rich you won't get as much exta thrust and you will have a big flame coming out of the back.

[racketmotorman]

Hi Bob , Ben
This looks an interesting excercise :D
My suggestion would be a "dump style" afterburner (without jetnozzle, just a plain tube) "displaced" a few inches downstream of the GT 's jet nozzle with an annular gap between the jetnozzle and the "afterburner??".
What you are looking for are the features of a dump style afterburner without the pressure drop variations across the turbine normally associated with an afterburner.
The annular gap should mean there will always be ambiant pressure conditions downstream of the jetnozzle .
The step in the "dump style " burner pipe will provide the necessary low velocity region required for flame holding .
Fuel delivery would be by a circular fuel manifold around the exit lip of the GTs jetnozzle with multiple holes that would delivery fuel equally around the outer surface of the jetnozzle efflux , the high velocity gases will provide excellent atomisation from the sheering action .
The end of the dump style burner would have a hole ~ 25 % larger in dia than the jetnozzle to allow the undisturbed jet efflux and fuel to enter .
eg , if your jetnozzle is 60mm dia then the dump entry would be ~75mm in dia and about the same distance axially downstream from the GTs jet nozzle . What you are looking for is an opening just large enough for the jet efflux and fuel to get thru without touching the edges of the hole .
The diameter of "the dump" would need to be rather large to get the most effective recirculation zone , as you're not interested in efficient pressure conservation this shouldn't be a problem to you .
The diameter of the burner tube body will need to be fairly large to avoid "choking" problems with attendant "back pressure" problems if the temperature rise of the gases is too high for a "small diameter pipe", a diffusing style pipe might be in order , but I'd be looking at at least 9inch dia at "the dump" and possibly something 15-18 inch dia at the downstream outlet over a length of about the same .
Ignition point would be somewhere in the outer wall of the burner body a couple of inches downstream of "the dump" , and within the recirculation zone of "the dump"
Just some thoughts :D
Cheers
John

[Lugebob]

Sorry I was vacant for a bit.... Thank you for the responses.... they are real food for thought... Racketmotorman, you hurt my head... I read and re read and now I am forced to study all that your were talking about.. More research... even if this does not work... I am sure learning alot here. Once all the vistors leave here next week I might have a go at biudling something..

I have also determined I need a better place to test. Spraying possible unburnt fuel in the driveway is probably not a good idea....

I do have another question though.. What If? What if I did want to put an afterburner on the JetCat....? It is already peaked out at 113,000 rpm producing 50+ lbs of thrust on a cool day.. EGT at this MAXes out around 700c

What would I have to do to the "turbine" (mechanical or ECU programming) to add an afterburner? Must I detune it? I do not wish to grenade this $5000.00 engine that runs 4 inches from my head...

thanks

bob

[racketmotorman]

Hi Bob
A well constructed and controlled afterburner shouldn't change your engines running parameters one little bit , BUT, trying to run a GT with/without afterburning using a fixed area jet nozzle is very dangerous if you're looking for maximum thrust .
The jetnozzle will need to be "large" to get those very high temp but very low density gases thru it when running "wet" , but if for any chance the afterburner should flameout , the gas producer will overspeed instantly because of the sudden increase in the pressure drop across the turbine which will now be running with less "back pressure" in a "dry" condition but with a "wet" sized jetnozzle.
Afterburners need very good controls and variable area nozzles if one wants maximum output with any degree of safety .
I'd recomend you stay well away from afterburners with your particular setup .
You either need to run the gasproducer at lower than peak output so as to have a bit of a safety buffer in case of an afterburner flameout , or take the risk of an engine blowup if you run full gasproducer rpms.
Not a choice I'd be taking ,
If you want to fit an afterburner, LOL ....."for demonstration purposes only" then you need to set your engine up with the afterburner attached , and set your engines main fuel flow rate at a level which producers your max rpm , thrust and exhaust temperatures will naturally be low because of the "oversized" jetnozzle , then when the afterburner is lit , your comp discharge pressure will drop along with rpm , depending on the comp design and a large number of other factors,and the thrust should increase to a sort of "intermediate" level , more than your present 50 lbs but not to a level theoretically possible if you were running max rpm .
Its a complex situation and frankly I don't think its worth all the trouble , at best its a case of "two wrongs trying to make a right ".
I hope this hasn't complicated matters too much :D
Cheers
John

[Lugebob]

Ha! Complicated yes, but a really cool learning exercise, that has not cost me.... due to your experience.. I think I am actually starting to get a good grasp on the theory now.

While talking to the factory tuner, trying to get him to push the engine a bit more and still keep it safe, he told me that due to the relatively low EGT that I have at MAX RPM I could bump her up another 500 rpm,,,,, but absolutely no more.... He said that the Turbine blade tips are already nearing supersonic and that even a momnetary over-rev would produce spectacular results that I would not want near my head. He told me that when I send it back for the 25 hour Maintenance Cycle he would play with a slightly longer and tighter exhaust nozzle if I wanted. He may have been simplfying for my benefit though..

Either way.. I still need a fire show. To bad I cannot program the the ECU to semi flood the engine in shots while it is spooling up... I had a great fire show on startup one day when the air outside was so thick you could cut it. I am just looking for 4 quick blast of fire about one ft in diameter that will roll backward.

Anywho... this new engine may be heading back to factory sooner than later... there is a harmonic vibration and noise that occurs as it is spooling up that cannot be detected once at idle or above.... One of the cool things about this engine is that it actually is under warranty....


thanks for the help
bob

PS incase you have not seen this. I leave you with a little video of what I do for fun... look out for the guy in blue...

http://www.jetluge.net/video.html/christylugevid.wmv

[racketmotorman]

Hi Bob
Low exhaust temps generally mean you could get a bit more thrust from a slightly smaller diameter jet nozzle , this will lower the pressure drop across the turbine meaning you'll need to burn more fuel to get the thermodynamics to compensate , but if the NGV set and compressor diffuser are setup for a particular flow you could send the comp into difficulties with changes .
Theres no way your turbine blades would have been getting near supersonic , at the temps of the turbine gases sonic is near 2,000ft/sec , and at those sort of tip speeds your blades would have been coming out the side, I doubt if your turbine tip speeds would be anywhere near even 1500 ft/sec , probably closer to 1,300 ft/sec max.
If you semi flooded the engine during spoolup , you'd send the comp into surge and possibly wreck your engine :-((
Cheers
John

[Jim Berquist]

Lug!

All you want is a Flame show? Have you ever held a Bic lighter in front of a can of WD-40? You can purchase WD-40 by the gal. Add a pseudo After Burner to your exhaust and inject WD-40. A spark plug should ignite. On it's own, it shows bright yellow/white flame. Being sucked out by your exhaust, it may change some. A round gas burner of the right size may work. May look like a MINI Bat Man Thing!

God I love being ignorant!


Jim