[AR] Re: Igniter Popping Sound
- From: "Graham Sortino" <dmarc-noreply@xxxxxxxxxxxxx> (Redacted sender "gnsortino@xxxxxxxxx" for DMARC)
- To: "arocket@xxxxxxxxxxxxx" <arocket@xxxxxxxxxxxxx>
- Date: Sun, 21 Sep 2014 17:58:03 -0700
Thanks very much. I read back through the post and Bruno had mentioned that the
popping sound is usually uncombusted gasses so perhaps the theory about the
pressure lines being too large makes sense. It's very strange that I don't see
this on every test.
Regarding erosion, if it is happening it's very slight. The wall on the fuel
orifice side looks a bit smoother and the edges are slightly more beveled then
on the ox side but I'm not sure if it's actual erosion or me just imagining it.
Is a "blank" beswick fitting something like this?
http://catalog.beswick.com/#WORKFLOW;PRODUCT,MH,FILTER_FITTINGS. Apologies, I
wasn't quite clear what blank meant.
Regarding the mixture ratios, test 5 looked pretty good but the pressure was
only about 40% of design. The color was nice but that was because it was near
dusk. On my most recent test last weekend where I heard the popping (I haven't
had a chance to post it yet) I'm estimating that I'm running at a 1.5 and 2.0
O/F ratio but I still get the popping periodically. My worry was that I was
overcompensating by putting too much GOX in, which caused the popping, however,
I see in your original email postings you said that you went up to about 1.8 -
1.9 without any issue. So it doesn't sound like putting too much GOX in would
cause a problem.
Best,
Graham
On Sunday, September 21, 2014 4:56 PM, Robert Watzlavick
<rocket@xxxxxxxxxxxxxx> wrote:
Graham,
Check the aRocket archives for some posts on 5/19/2012-5/20/2012
titled "Combustion popping with igniter". I had some similar
symptoms that may or may not be the same thing you're seeing. I
never determined the exact source of the popping but only that it
went away with a few changes, mainly mixture ratio and slowing
down the GOX jet.
I was having bad problems with wall erosion in the igniter that I
think might have been causing the popping as bits of metal melted
away from the wall and went downstream. The fix was to move the
GOX orifice upstream just a bit by inserting a blank Beswick
fitting in between the orifice and igniter chamber. The GOX
orifice is about 0.030 inch or so but the hole in the wall of the
igniter chamber is 0.100 so the space in between acts as a big
manifold and slows down the GOX flow. The fuel orifice still
sprays on the wall directly. Once I made that change, I was able
to run the igniter several times in a row with no erosion.
If you think it's resonance in the pressure tube, you might
consider going to 1/16 inch SS capillary tubing. In general, for a
pressure line exposed to hot gas, you want as little volume as
possible in both the line and the transducer. For some of my
tests, I tried a new (cheaper) 1/4 NPT pressure transducer that
had a large empty space inside the fitting. During one test, I
noticed the tubing get red hot near the chamber and afterwards,
the transducer body was full of black sticky gunk. I changed back
to the #10-32-style Kulite/Endevco transducers and haven't had any
problems since.
As far as jet position vs. the spark plug, it seemed to work best
for me when the two jets were 90 degrees apart, and perpendicular
to the chamber axis. I tried a version where I angled the jets
downstream a bit (trying to reduce erosion) and it didn't even
start at all - it just made a weird sound:
http://watzlavick.com/robert/rocket/igniters/tests/20120519-run3-cam1-igniter5.mp4
I never figured out why that one didn't work. For the ones that
did work however, I was able to put the spark plug upstream
without any problems.
BTW, I think Test 5 on 2014-05-10 looks the best to me and seems
to have the best combustion. The ones after that may be too
lean. Did you have any erosion on the 5/18 tests?
-Bob
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