Backwards blown Ramjet

[Harabec]

I just finished my ramjet on Sunday and I had it goung for about 5-10 minutes then the flameholderblew out of the combustion chamber backwards! Luckily no one was near the blast. We were in an open feild when this had happend. When I did finally find my flameholder,it was pretty much disintegrated. What happend?

[larry cottrill]

I just finished my ramjet on Sunday and I had it goung for about 5-10 minutes then the flameholderblew out of the combustion chamber backwards! Luckily no one was near the blast. We were in an open feild when this had happend. When I did finally find my flameholder,it was pretty much disintegrated. What happend?

Harabec -

This is important, and I'm not trying to be funny: What do you mean by 'backwards'? I would expect it to blow out the front end, since there is a thrust force acting on it, so if it blew out the rear I would call that 'backwards'. I would also call it very strange! But, maybe you thought it should follow the flame. So, you need to tell us: did it blow out of the front or rear of the engine? Did the engine immediately flame out, or did it keep running until fuel cutoff?

At any rate, it sounds like either or both of the following: Your bracing of the flameholder against the thrust force it bears was inadequate; OR, the location of the flameholder and its bracing was actually inside the flame zone and the materials failed due to erosion by oxidation. This would mean that you were not getting flameholder action there, but rather forward of that point, in the diffuser section. That is not impossible, especially if your input airspeed was lower than expected in the design.

What material was your flameholder? I have shown experimentally in pulsejets that stainless will not survive full immersion in lean combustion flame, but will undergo rapid oxidation. Were you getting any white sparks out the tail end before the flameholder broke loose? That would indicate oxidation and erosion within or behind the flame front.

See: http://www.jetzilla.com/jetZilla.html#Article_1

L Cottrill

[Harabec]

I was using Typo 347. The type that was recommended. The flameholder WAS blown clear out of the rear of the engine. I am not trying to be funny about this. I thought that my friend had made a bad weld or that it over heated. What I am still trying to figure out is why it was blow out of the back. I know that it would have gone out of the diffuser and not the nozzle because thrust is acting apon it. The flameholder that I was using was an experimental design. It involved the use of the cylindrical flameholder with a 45 degree angle at the rear. I have built 1 other Ram Jet using the cylindrical flameholder with great success. Could my experimental baffle have been the cause? When it happend there was a falmeout and when the flameholder blew out,the fuel was still on UNTIl the holder was out of the engine. I have an emergency cut-off valve that I got from a milltary surplus airfeild from an F4 Phantom. If the valve was not on there might have been some enen more disartarous effects. If you want my experimental Flameholder design I can try to email it to you(If my scanner secides to work..). My E mail is Shadow12078@hotmail.comYes there was a shower of sparks coming form the tail of the engine. I have always tinkerd around with engines. Jet( an old f4 fpantom compressor) and piston. So what yor saing is that I mess up the Flame holder placment and I used the wrong type of metal? What type of metals should I use? I did not use stainless steel. I do not know why the sparks came out of the tail. I think It was not due to oxinztion on the flame holder. Mabe the weld gave way and thats what caused my shower of sparks.

[Harabec]

I was using another expeimental design with a large nozzle and a medium size diffuser(Nozzle diameter 9cm Diffuser diameter 6cm) I was testing it on a test stand that I made from and old Cement slab on some pallents for height. I was using standerd super gasoline. I can email it to you. My email is on my first reply.

[Harabec]

Yes there was a shower of sparks coming form the tail of the engine. I have always tinkerd around with engines. Jet( an old f4 fpantom compressor) and piston. So what yor saing is that I mess up the Flame holder placment and I used the wrong type of metal? What type of metals should I use? I did not use stainless steel. I do not know why the sparks came out of the tail. I think It was not due to oxinztion on the flame holder. Mabe the weld gave way and thats what caused my shower of sparks.

[larry cottrill]

Yes there was a shower of sparks comiing form the tail of the engine. i have always tinkerd around with engines. Jet( an old f4 fpantom compressor) and piston. So what yor saing is that I mess up the Flame holder placment and I used the wrong type of metal? What type of metals should I use?

The shower of sparks, rearward ejection of the part and eroded material all has to mean that your flameholder was totally immersed in flame - which means, (a) it never had a chance to act as a real flameholder at all, but simply as a drag element in the flow stream; and (b) you were getting combustion significantly ahead of your flameholder's location!

If you built this to a standard plan, the only thing I can think of is that your air going in is a little slower than the minimum speed the engine is designed to work at. The combustion is happening somewhere ahead of the rear of the diffuser, so the flameholder never has a chance to work, and just heats up from being totally surrounded by flame. Then, oxidation occurs due to the excess air available in good lean running. I'm guessing that if you had stronger airflow going in, you'd have been OK with whatever material you had.

Probably the ultimate material for total flame immersion is Inconel - if you can form it and weld it [and afford it - but a small piece for a flameholder shouldn't kill you]. I am speaking theoretically; I have samples of it, but no experience with it. I know it can take FAR harsher conditions than any stainless. Titanium is another possibility, but again is expensive and you have to know how to weld it, which can be very tricky - the weld temp is high, and its burning temp in air is somewhat lower. It has to be totally shielded from air during welding.

From what you describe, I think the real answer is getting your air supply up to speed, though. You need enough pressure head at the rear end of the diffuser to keep your flame back in the chamber, with your flameholder keeping it from getting away completely. of course, you already know that; it just sounds like you weren't quite achieving it. Try getting this engine going again with exactly the same setup as before, and see if you can look in there with binoculars from the front and side and see where that flame front is [of course, I have no idea if that is really possible to observe].

But, my specialty is valveless pulsejets. You need to hear from those that hang out in this forum, to see if they even agree with my appraisal of what you describe. I'm not the one you should be listening to.

L Cottrill

[Harabec]

I dont agree. I could see down the front of the engine. The flameholder WAS not in the flame.

[Harabec]

I was using 2 leafblowers. They were set in a parell way. One behind the other. The flameholder was just thrown out of the back. The shell and stand survived. In my theory, the flameholder was ejected due to to much air/rapid combustion and to lean of a flame that melted the welds. My friend is a professinal welder. he is the one who helped my build this Ramjet and my other Ramjet. Also my expeimantal plans for ramjets/flamholders are avalible from me. Use my E mail. If you would like them, I can send them. If I was to try to rebuild this engine, I would have to grind the welds. Anyway you were to look at it, this engine is a goner. The nozzle is now warped beond useabilty. I am right ow trying the Heat Hammer process in order to get the correct shape. The diffuser and comustion chamber survied. We did grind the nozzle off. We did fabricate a new nozzle but are keepnig the old one to repair it then re use it. The new nozzle is not on yet. It is on the bench wiating for new parts. The new nozzle will not be put on untill the orginal nozzle cannot be repaired..

[Harabec]

Yeah but wouldnt the flameholder wouldve bolwn forward?If if blew backwards then the ignitor was to far forward. But that isnt the case. When ever i design my flameholders the ignitor is 90% of the time close to the back. I have discoverd that by doing this in corf=dinnation with the case desing I have reached optinum performance with these experiments.

[Viv]

Interesting, did the flame holder show any carbide precipitation, did it look pitted like orange peel? what filler rod was used to weld the 321 stainless?

It sounds like the weld metal disolved to me.

Viv

[Harabec]

I dont understand. What is carbid precipitation? We weleded it with a TIG welder. There were a few holes in the rear of the flameholder, but those were the hole that I had drilled in an attempt to get a better cooling rate on the flameholder.

[Viv]

I dont understand. What is carbid precipitation? We weleded it with a TIG welder. There were a few holes in the rear of the flameholder, but those were the hole that I had drilled in an attempt to get a better cooling rate on the flameholder.

Stainless steel is an alloy and a lot of its make up does not mix that well or is stable when you have mixed it all up:-)

Basically the carbon floats to the serfice and is lost when the metal gets to a certain temperatur, even worse things happen in other grades, imagine the metal heated up below its melting point but high enough for some things to start melting and becoming mobile.

Its like bread dough with gravel in:-) the bread dough is the soft steel and the gravel is the molydenum, cromium, tungsten and so on, this is why its a bitch to machine, its soft and grabs the tool sticking to drills and clogging them, the hard stuff wears the edge off the tools making them blunt!

321 grade is worse than all the rest, dot punch it with a centre punch prior to drilling? bad mistake! the metal work hardens were it was punched so the drill bit wont go through it:-( when cutting you need an agressive cutting rate to get under the work hardened area formed when cutting.

321 has to used over 500c as other grades will fail in time

Viv