[AR] Re: Igniter Popping Sound
- From: Robert Watzlavick <rocket@xxxxxxxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Sun, 21 Sep 2014 15:55:55 -0500
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
On 09/21/2014 02:08 PM, Graham Sortino (Redacted sender
gnsortino@xxxxxxxxx for DMARC) wrote:
I’ve been having some intermittent issues with my latest igniter where
I get a loud popping noise that sounds something like a car backfiring
or a gun going off (see test 3
http://wiki.fubarlabs.org/fubarwiki/GOX-Ethanol-Igniter-v2.ashx#CABE-AF-BA:_Test_D_Slightly_Less_Poor_Mixture_Ratio_3
or http://youtu.be/y098qOkC7Vk). I had this problem earlier on and I
thought I had fixed it but it seems to be giving me problems again.
I suspect this is due to combustion instability but I’m struggling
figuring out what the root cause of it is. The interesting thing is
that sometimes one test will not have the popping and then without
changing any parameters the next test will have the popping noise.
Needless to say I’m struggling to come up with an explanation
I have a few theories
1.Originally I assumed this was caused by having a too low O/F
(GOX/Ethanol) ratio so I kept upping this to about 1.3 - 1.5. Now I’m
wondering if increasing the ratio too high causes the popping sound.
2.The impingement arrangement is unlike with 1 Fuel and Oxidizer
orifice impinging perpendicular to the axis of the combustion chamber.
The Oxidizer orifice is directly upstream of the igniter so the gas
velocity is probably pretty high (see:
http://wiki.fubarlabs.org/fubarwiki/GOX-Ethanol-Igniter-v2.ashx#Ports_10).
Perhaps this is not helping mixing and moving it upstream to slow down
the velocity may help.
3.The spark plug igniter is at almost at same point along the chamber
as the Fuel and Oxidizer. This may mean that the propellants have not
yet had enough time to mix. So perhaps moving downstream a bit would help?
4.The chamber pressure orifice is fed from 1/8” NPT stainless steel
tube, which has about a 0.2” diameter. On my previous engine I had
used a small diameter copper pipe to feed out to the pressure
transducer but I changed to stainless steel tubing because it was more
convenient for connecting/disconnecting from the engine. Perhaps the
larger diameter is having an effect on the combustion chamber.
I’m planning on rebuilding my igniter as I’m no longer confident that
the current version is working correctly but before I do I wanted to
see if I could get any feedback on what could cause the popping noise.
Thanks as always.
Graham
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