Flame tube design issues?

[fidel]

I've done some minor fixes and temps are now fine. I'm pretty happy with how it runs and I've started to finish the afterburner. Is there any formula of the optimal length of the afterburner tube? I need to rebuild the fuel system to more easily control fuel pressure and flow. But I believe I can sort that myself. But what about length?

[Johansson]

May 11, 2016 14:38:18 GMT -5 fidel said:

I've done some minor fixes and temps are now fine. I'm pretty happy with how it runs and I've started to finish the afterburner. Is there any formula of the optimal length of the afterburner tube? I need to rebuild the fuel system to more easily control fuel pressure and flow. But I believe I can sort that myself. But what about length?

Check this NACA document out, hope you find it useful. Copy the adress below and paste it in your web browser, the "!" in the web adress seems to upset the link function somehow.

onedrive.live.com/redir?resid=355D2A43AD038D5F!205&authkey=!AHIMzssNnglquVY&ithint=file%2cpdf

Cheers!
/Anders

[fidel]

Another thing that I´m pondering about is load. Since I don´t have a tachometer I cant really tell what load I have on the engine rpm wise. I can of course use my ears and the boost meter, I haven´t been beyond 0.8 bar boost with it since I´m a bit worried I´ll blow it up. Is boost pressure a decent way to tell load on the engine? The turbo itself is a Garrett TB0312.

Here´s the compressor map: 

[racket]

Hi Fredrik

P2 can be a guide to rpm as long as you know where on a comp map you are flowing .

If for instance your T2 at 0.8 bar ( 1.8 :1 PR) is ~75 -80 deg C on a 15 deg C day , then you can assume the comp is flowing within its higher efficiency islands and you could reasonably assume that a 2.5 PR - 1.5 Bar P2 upper limit would be safe .

But if your T2 at 0.8 Bar P2 is ~100 C degrees then efficiency would be down ~60% and theres a good chance that you'd be flowing over in the choke region of the map where your rpm would be a lot higher than if T2 was at 80 degrees and efficiency in the 70's % , with efficiency in the 60% rage then even a 2:1 PR could be pushing the rpm limit .

But generally speaking a 1.5 Bar P2 is safe to use even if flows are "choking" .

Cheers
John

[fidel]

Wow, interesting. So I´ve never been above like 50-60% load?:-)

[fidel]

Any idea of bying a T2 temp meter? This way I could safely monitor load. As long as T2 is below 80C as you say 1,5bar should be ok. Or did I misunderstand?

[racket]

Hi Fredrik

I use something like this www.ebay.com/itm/Digital-Thermometer-2-K-Type-Temperature-Thermocouple-Sensor-Probe-TOOL-/121282223125?hash=item1c3cfbdc15:g:jjYAAOSwMmBViq~w

The sensing end I fitted into the engine housing at the diffuser outlet .

Yep , you're only at quite modest power levels , if you can get that ~80 deg C at your 0.8 bar P2 then it'll be safe to go to 1.5 bar, at 1.5 bar your T2 will hopefully be ~140 deg C on a 15 deg C day if flowing in the 70% effic island

Cheers
John

[fidel]

So got the new fuelsystem for the afterburner ready. Looks and works nice. Allthough I think the fuel need to be better atomized, since it´s a bit hard to ignite even at moderate speeds. What kind of nozzle is good to use. Is there a good way to manufacture a nozzle on your own?

[smithy1]

May 16, 2016 2:54:04 GMT -5 fidel said:

So got the new fuelsystem for the afterburner ready. Looks and works nice. Allthough I think the fuel need to be better atomized, since it´s a bit hard to ignite even at moderate speeds. What kind of nozzle is good to use. Is there a good way to manufacture a nozzle on your own?

Afterburners are finnicky things at best...too much fuel and you'll be lucky to light it....too little and the result is the same, it's a balancing game...A lot also depends on the air speed though the pipe and how much of a disturbance you can introduce with your flame holder. Fuel delivered in droplets which are too big will be hard to light and keep lit as well.

Do you have any picks of your A/B system handy??

I've finally been able to get the A/B working fairly well on my GT6041 Go-Kart...took a bit of work to get right and I had heaps of help from John and Andy M. Was all worth it in the end though...makes a beautiful noise now..! Well...at least I hope it does..!  

Cheers,
Smithy.

[fidel]

Here are some pictures, new video on youtube aswell but not with the burner lit. I haven´t been experimenting a lot, I just found it very hard to light. I think my nozzle is a bit too big and doesn´t atomize well enough. 

www.youtube.com/watch?v=nmqLHXoJ1cs

[fidel]

I could add that the system is built with a 3 bar regulator so fuel pressure is always constant. Flow is controlled with the valve, pump is just on/off by button.

[smithy1]

Hi Fidel,
I'm afraid an afterburner is a lot more than just a straight pipe with fuel thrown into it and attempting to light it up.

Firstly your pipe is not centralized to the turbine, this has it's own inherent issues. The length of the pipe is not overly critical, it can be too short though, we need to give the fuel time to burn.
Ideally the pipe should have a divergent, straight and convergent sections, or at least a "step" type flame holder section with convergent nozzle. You'll need a spray ring rather than just a spray nozzle, there is also no flame holder in your pipe. You may be able to light it and have flames etc....but to make any increase in thrust will not happen with that pipe I'm afraid.

The fuel pressure required at the A/B spray ring will not be too much as the resident pressure in the pipe is usually less than ~10psi, however, to atomize the fuel correctly/sufficiently you'll need a multi orifice spray ring and a decent amount of pressure behind it, the smaller the holes the better the atomization... but this will require higher fuel pressure to achieve.

The paper Anders links to is very informative, please take some time to study it. It was made in the 1960's but is still relevant today.

Cheers,
Smithy.

[fidel]

Winter is back and time to finish the afterburner. I´ve been using the engine to heat the garage, works well :-)

What nozzles is recommended for atomization of the fuel in the afterburner? I´ve been looking at regular high preassure oil burner nozzles but they´re pretty expensive.

What are your recommendations on that matter?

[BFTO]

Have you consider a fuel ring instead of a nozzle? cheap solution.

jetandturbineowners.proboards.com/post/2389

[fidel]

No I haven't until now. That seems like an easier solution aswell.I have plenty of copper tubing left so I'll give it a try.


Thanks