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

[jim mason]

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









--
------------------------------------------------------------------------------
Jim Mason
Website: http://www.whatiamupto.com
Current Projects:
   - Gasifier Experimenters Kit (the GEK): http://www.gekgasifier.com
   - Escape from Berkeley alt fuels vehicle race: www.escapefromberkeley.com
   - ALL Power Labs on Twitter: http://twitter.com/allpowerlabs
   - Shipyard Announce list:
http://lists.spaceship.com/listinfo.cgi/icp-spaceship.com



-- 
------------------------------------------------------------------------------
Jim Mason
Website: http://www.whatiamupto.com
Current Projects:
   - Gasifier Experimenters Kit (the GEK): http://www.gekgasifier.com
   - Escape from Berkeley alt fuels vehicle race: www.escapefromberkeley.com
   - ALL Power Labs on Twitter: http://twitter.com/allpowerlabs
   - Shipyard Announce list:
http://lists.spaceship.com/listinfo.cgi/icp-spaceship.com