Three kinds of ramjets

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Three kinds of ramjets

Post by Mark » Tue May 03, 2005 1:29 pm

I started thinking about flame holders and how they might benefit a pulsejet. Anything that aids mixing has a potential to help, whether it is the in/out reversing flow, the often abrupt transitions in diameter, the heat from the walls, etc.
Flame holders not only "hold the flame", but they store heat and to some degree behave as a permanent spark plug in that the heat they provide most probably further speeds up the reaction, an intimate mixing of heat throughout the reaction vessel. When you grind gunpowder, you want it powdered very fine and intimately mixed or else it doesn't burn very fast. Perhaps the flame holder can be seen as another ingredient that you want intimately mixed, intimate heating.
I remember a straight tube pulsejet that I made, it was so "slippery" of course, (it's easy to make a straight tube too!); anyway, everything shot out the tail end before being completely burned, ... bang, pop, but never enough compression or burn rate reaction to catch the wave and run. By corking the tail end, enough compression/confinement/backflow could be gained so that it ran, the bang start method. Yet I wonder now if some sort of small obstruction that works well in both directions, remember the backflow phase must not be weakened deliteriously, well just something to act as a reactor to burn the mix before it escaped prematurely in a "new" philosophy of duct design is was I was dwelling on.
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larry cottrill
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re: Three kinds of ramjets

Post by larry cottrill » Tue May 03, 2005 1:46 pm

Mark -

I would often get my tiny pulsejet ideas to pulse [with ram air] by introducing a "draggy flameholder", usually a non-streamlined "bluff body" [a term offered by Graham Williams]. One of the best, though, was a wad of stainless steel wool scrubber, in the front of the chamber right behind the air inlet. You could still see light peeking through it, but it was dense enough to slow and turbulate the air so you could maintain combustion right up at the front end of even a tiny chamber [half a CO2 cylinder, typically].

So, I definitely believe in the value of a flameholder in a tiny pulsejet. This is, in fact, the theory that we should use for a flameholder in a very low-speed ramjet engine. It should not be a miniature diffuser [as in the textbook subsonic ramjet drawings], but rather a bunch of holes that act as tiny nozzles, so there is actually a pressure drop and velocity increase from the diffuser into the combustion chamber, exactly as in an ordinary gas burner.

In a classic ramjet, you have the problem of the possibility of the incoming air literally blowing the flame out the back. But in a VERY slow ramjet, the problem is just the opposite - the flame will want to move forward into the diffuser section. So, your flameholder should be a fairly high-drag barrier with fairly high velocity jets that isolate the combustion zone from the slightly higher pressure diffuser zone. This is precisely why Maggie Muggs is designed the way she is - to achieve that separation via a slight pressure drop.

L Cottrill
Maggie Muggs - the crude flameholder creates a slight pressure drop to isolate combustion from the diffuser zone. Drawing Copyright 2003 Larry Cottrill
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