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Obi Offiah
22nd Oct 2010, 18:05
Last year I read through a document from the early 50's about the characteristics of starting an afterburner at altitude. What I
can't understand is why it generally takes longer for AB light-off the higher up the aircraft is. What occurs in the AB duct that prolongs ignition?

The way I see it is that if fuel is added to the AB duct and it fails to ignite, it is simply blown out of the engine, so regardless of
how much fuel is added and for what duration, ignition should not take place no matter how much time has elapsed, however this is not the case in reality.

I initially thought that vapourised fuel would recirculate in a region down stream but in fairly close proximity to the flameholder. The recirculation would allow stable burning of the fuel to take place, however from what I recall the document mentioned delays of 8 seconds or more for AB light-off, so I doubt the vapourised fuel could be held in this state for such an extended period of time, without being blown out of the duct. So again I wonder what is happening in the duct that will cause ignition to be so laboured?

Intruder
22nd Oct 2010, 23:00
The air is thinner and colder, less optimal for ignition of the fuel. Ignition will occur only if the fuel/air ratio is in the correct range and the fuel is properly atomized. The lower air density affects the fuel/air ratio, and the low temp affects the atomization.

Machinbird
22nd Oct 2010, 23:41
I don't recall any significant difference in afterburner light times at altitude vs. sea level on the J-79s. Some had a modification so that the burner torch had to be lit each time (for night invisibility reasons). If the AB igniter got coked, you still could light off pretty promptly just off turbine torching. The J-79 had an excellent 4 stage burner that modulated nicely, not at all like the J-57s which were either on or off.
But it has been a long time since I have flown them:sad:.

boguing
22nd Oct 2010, 23:58
I like your recirculate 'zone' idea. How fast were the igniters?

Obi Offiah
23rd Oct 2010, 09:45
Intruder:

I appreciate your explaination, but what I don't understand is what occurs in the time period from 'no light' to ignition. If the high altitude region is less than optimum, in the extended time period that the AB is attempting to light, what actually happens/changes during this time for ignition to suddenly take place?

Machinbird:

When you mentioned turbine torching do you mean autoignition, or did the J-79 have secondary/backup method of AB ignition, the hot streak or hotshot technique?

boguing:

Unfortunately I can't remember the exact figures, but I think it was up to 8 seconds at 30 to 40 thousand feet.

matkat
23rd Oct 2010, 09:49
Well the spey as fittted to the the F4 did not have ignitors for the R/H the fuel was ignited by catalysts on the spray ring.

M2dude
23rd Oct 2010, 11:00
I think you will find that it is all about ESTABLISHING the ideal fuel/air mix and stabilising the resultant flame quickly. In the case of the afterburning 'long white pointed thing' that until 7 years ago carried passengers in shirt sleeved comfort at Mach 2, the burners were lit at FL 290 for transonic acceleration and lit almost instantly. AND the most inneficient means of ignition, using a swirled arc ignitor was used also, although the fuel flow control was really neat. If anyone is interested I can post some diagrams here to show how it was done with the 'rocket'.

Dude :O

Obi Offiah
23rd Oct 2010, 11:11
M2dude, yes, please post the diagrams if you can.:ok:

With the case I mentioned earlier, ignition occured around 8 seconds after AB was selected and stable burning took around 17 seconds due to oscilliations caused by the closed loop nozzle reacting to the changing pressures in the AB duct.

Intruder
23rd Oct 2010, 13:52
I appreciate your explaination, but what I don't understand is what occurs in the time period from 'no light' to ignition. If the high altitude region is less than optimum, in the extended time period that the AB is attempting to light, what actually happens/changes during this time for ignition to suddenly take place?
Fuel is flowing into the exhaust stream, but either does not ignite or does not all ignite, so the A/B flame pattern is not ideal and does not produce the extra thrust it is supposed to produce.

At some point the fuel will begin igniting, and eventually the proper flame pattern will be established, and the designed thrust will be produced.

boguing
23rd Oct 2010, 17:50
Didn't make myself clear. I assumed that the ignitor was an electrical spark generator, and wondered how many sparks per second it could achieve. If, f'rexample, it was once per second, I could see that it could be a matter of luck that the spark met a nicely blended air/fuel ratio?

Machinbird
24th Oct 2010, 03:38
When you mentioned turbine torching do you mean autoignition, or did the J-79 have secondary/backup method of AB ignition, the hot streak or hotshot technique?



Autoignition.

Having seen other aircraft's burners light inflight, I remember smooth opening of the nozzles and symmetrical balance of flame between the two engines.
The J-79 afterburner spark plug ignited a torch igniter which had its own fuel supply and as the afterburner selection increased, burner combustion initiated in the core of the flow and spread outwards until max thrust.

The torch igniter, staged lightoff of the burner, plus good flameholder design are likely the reasons I have good memories of the J-79 afterburner performance. :)

M2dude
24th Oct 2010, 11:00
As promised, here are a few diagrams of the Concorde reheat (afterburner, for our American friends) system. The ORIGINAL design was done by SNECMA, but due to them getting into all sorts of trouble with the fuel injection system and flame stabilisation, Rolls Royce baled them out, and it became a Rolls Royce/SNECMA design. (The core engine was a 100% Rolls design, with no French input whatsoever. However some engine sub-assembles were manufactured by SNECMA).
The basic way the afterburner worked was by spraying the fuel FORWARDS intially at high pressure, against the jet stram about one inch, until it hit the anvil. . As the fuel strikes the anvil it is blown back by the jet stram and atomises, passing over the of the spray ring and the over the flame holder. The ignition operated by passing 15KV across a dual cylindrical tube, the resulting arc was 'swirlied' into the fuel stream by blowing engine 5th stage HP compressor air into the tube (there were 7 stages in all).
The key to successful ignition was a healthy spark, a good supply of air to the ignitor and accurate scheduling of fuel flow. (This was scheduled against dry engine flow as a funtion of total temperature). The other important factor (as with any afterburner) was correct and rapid operation of the exhaust nozzle. Fortunately, Concorde used it's primary nozzle for control of engine N1 anyway, so adapting this to operate as an afterburning nozzle also was a relative walk in the park, and it operated superbly.
During the light up phase of 3.5 seconds, the fuel ratio is a fixed 0.45 (ie. reheat fuel is 45% of dry fuel). After the light up phase the full scheduling commenced. As far as the FLIGHT RATING figures go (not take-off) the ratios were 0.6 at a TAT of 54 deg's C, falling linearly to 0.3 at 107 deg's C and above. (Remember that Concorde used afterburning really sparingly, just for take-off and then transonic acceleration; cut off at Mach 1.7 altogether. :)

Dude :O
http://i1237.photobucket.com/albums/ff476/riconc11/Concorde/ReheatInjection22A-1.jpg

lomapaseo
24th Oct 2010, 13:04
As usual lots of really informative stuff on how things work :ok:

Of course a couple of layers below the introductory stuff might be the answer to the original posters question, which has to do with the variations in timing of the on-off event (mostly the on).

In my simplistic view it looks like

1. Turn on the ignitors

2. Turn on the fuel

3. Allow the fuel to stabilize its spray pattern in the engine exhaust air

4. Sense the light-off (pressure/temperature/N1) adjust the nozzle for optimum pressure/N1

The design keys are

powerful ignitors

Spray Nozzles

Flame holders

The system keys are the feedback loop that monitors pressure/temp/N1 against the nozzle position (either open/closed or variable)

The answer to the original posters questions versus altitude might indeed be a function of the time that it takes to achieve the desired spray patterns (feel free to add/correct my simplistic views)

mihigo37
6th May 2011, 19:25
Can someone please send me some figures of turbine engine augmentor. I am working on augmentor ignition research, and want to compare varous military turbine engines (F135, F100, F101, F118, F119). I know this is not easy to find but in case someone can, I would appreciate.

Thanks

gums
6th May 2011, 20:12
Salute!

'bird never flew the motors with the "hard light" such as the J-57 and J-75, and I did. They used a "hot streak ignition" that injected a bunch of gas up by the turbine outlet stage which burned very rich and then ignited the massive amount of JP-4 being sprayed by the afterburner ring(s). The exhaust nozzle opened before final ignition, so we had a brief lull, then a BOOOMMMM!!! Unless you went to an airshow or flightline before 1984 ( when the F-105 was retired), you prolly never saw/heard that.

The J-79 and later motors did not do this, plus they had variable stages of augmentation and neat nozzle controls.

Best bet is to go the f-16.net and look up "TEG" on the user list, then send a private message. His callsign stands for "That Engine Guy", and he has had more experience with more engines and prolly knows more than anyone here on Pprune. No offense to all you airline folks.

Gums sends...

bearfoil
6th May 2011, 20:28
A guy keeps his F100 nearby in Ione. He does a rare air show, and the Viggen next to him is stationed at McClellan. The "Hun" most definitely makes a boom.
If not ready for it, one expects to see bits of aluminum starting to fall. Impressive.

gums
6th May 2011, 21:07
Salute!

Look at this reference about an afterburner, then surf the whole manual.

We called the motor the J-58, but the manual refers to another designation.

SR-71 Online - SR-71 Flight Manual: Section 1, Page 1-22 (http://www.sr-71.org/blackbird/manual/1/1-22.php)

Gums sends...

P.S. Lottsa interesting charts and stuff about mach numbers and altitude and in other sections of the flight manual.......

antic81
6th May 2011, 21:21
Heres a nice little video of a J-79 Engine...
Well I thought it was anyway! :}

YouTube - J79 - Turbine Engines: A Closer Look