[Gasification] thermoacoutics, rijke tubes, sondhauss tubes, pulse jets-- how do we make them not work?

[doug brethower]

Tune it with an expansion chamber patterned after two stroke exhaust. Many  
innovative designs to increase flow/power back in the day.  Newer designs  
focus on the sound quality aspects as well.
On Wed, 16 Feb 2011 01:11:42 -0600, jim mason <jim at allpowerlabs.org> wrote:

> any ideas out there for how to make a pulse jet robustly not work?
>
> we're having lots of problems with thermoacoustics now that we've put
> a tall stack on our gasifier swirl burner.  we're using a tall stack
> now to keep startup gasses away from the operator.   the swirl burner
> is 5" in diameter, 10" tall.  we've added a 4 foot or so stack of the
> same 5" diameter.   it looks like this
> http://www.gekgasifier.com/forums/showthread.php?t=501  (though this
> one without the joint at 10")
>
> when run under high load, it howls rather loudly.  if art, it would be
> fabulous.  if a genset, not so good.
>
> darrel tonight explained that we have made a rijke or sondhauss tube.
> or really, an unintentional pulse jet.  read about them here.
> http://en.wikipedia.org/wiki/Rijke_tube.   scroll down 3/4 to see
> sondhauss close bottom tube, exactly as we have.
>
> this apparently can happen with any pipe of one or two open ends, with
> a big temp difference between the ends.  a great enough temp diff
> between the ends alone will set up the standing wave.  you can make
> the effect more pronounced with heat exchange meshes inside that
> augment the pulse in one direction, but not the other.
>
> turns out the best place to site a mesh, or apply your heat source is
> about 1/4 length of the tube. (at least with the two end open type)
> hmmm, we put our always on silcone nitride igniter right about the 1/4
> length point.  didn't think of that one.  no idea if it is enough heat
> to be making a difference in the resonance though.  we should try
> turning it off and seeing if the resonance changes.
>
> an important thing that makes them work is the push and pull against
> atmosphere.  this suggests that one way to "detune" the tube would be
> to put holes in the sides around the top portion of the tube.  make
> the pulse hit atm at difference lengths, and thus scatter the wave,
> like one does in a car muffler.
>
> or, maybe a flare outward would work?  but that seems to only help
> impedence matching, and make the trombone louder.
>
> we put criss-cross baffles down the tube tonight.  that helped quite a
> bit, but didn't solve it fully.  also made more higher pitched tones.
>
> this pulsing is also likely why we have flame backing up into the
> mixing section/blower/etc in manners we never have before.  i we did
> most our testing on the blower and flame arrester, integrated
> flare/exhaust with just the short stack.  the big resonance problem
> did not start until we put on the 4' extension.
>
> i wonder if we could insulate our way out of this problem.  insulate
> the stack from bottom to top, even out the temp differentials so it
> can generate less sharp thermal pulses.
>
> any other ideas or experience for how to make your pulse jet not work
> under any circumstance?
>
> jim
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> --
> ------------------------------------------------------------------------------
> Jim Mason
> Website: http://www.whatiamupto.com
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>