[AR] Re: Igniter Popping Sound
- From: "Graham Sortino" <dmarc-noreply@xxxxxxxxxxxxx> (Redacted sender "gnsortino@xxxxxxxxx" for DMARC)
- To: "arocket@xxxxxxxxxxxxx" <arocket@xxxxxxxxxxxxx>
- Date: Mon, 22 Sep 2014 06:24:48 -0700
Thanks, I think I now understand your point about head-on impingement.
Actually, I hadn't noticed/realized that Bob's igniter wasn't impinging head on
and instead having the flow's miss each other.
What I could do is move the fuel orifice downstream a few mm's of the ox
orifice so that the gas jet pushes the fuel forward. Then I can put the
spark-plug downstream a bit further so it can ignite after they are mixed a bit
more. Originally, I planned on impinging them at an angle in the chamber but
after Bob's experiences I decided to avoid it.
One other suggestion I was given by the person who is letting me test my engine
on his farm is to inject the gas into the rear of the igniter (axial with the
igniter chamber) and then inject the fuel perpendicular to that so the gas
pushes the fuel towards the igniter
It sounds like I may need to make a few versions to figure out what works best.
Thank you again for the helpful advice.
On Monday, September 22, 2014 12:04 AM, Norman Yarvin <yarvin@xxxxxxxxxxxx>
wrote:
Oh, duh. I'd been looking at the third photo on your page; the second
photo gives a better idea of the spark plug location.
Still, moving the spark plug farther downstream does seem like a good
idea, whether you put it as you have (giving the propellants more time
to mix), or whether I have (making the mixture ratio seen by the spark
plug more deterministic); these are basically the same reason.
Having the propellants impinge on each other is common in main chamber
injectors, but in that case they're meeting at a relatively narrow
angle, so the result is fairly predictable: motion in much the same
direction they were injected at, but with mixing/splattering. With
propellants meeting each other head-on, there's no predictability: one
might imagine ideally they'd splatter out in a sheet perpendicular to
both jets, but even with perfect machining that perfect meeting can
never really happen, due to eddies in the chamber pushing the jets
around before they make contact. They might miss each other entirely
(though this would be more probable if they were two liquids; in your
case the oxygen is gaseous, so will spread out); the liquid jet might
punch through the gaseous jet; they might meet and glance off each
other. This unpredictability can lead to oscillations. Robert's
approach of not trying impinging, but just letting the ethanol jet
splatter against the opposite wall, seems better. (Of course then you
get the oxygen flow hitting the wall, too, and thus should probably
adopt his countermeasure to this. Or maybe put the oxygen orifice in
the end plate, so that it comes in axially to start with.)
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