[AR] Re: Igniter Popping Sound
- From: Norman Yarvin <yarvin@xxxxxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Mon, 22 Sep 2014 00:03:03 -0400
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.)
On Sun, Sep 21, 2014 at 07:40:49PM -0700, Graham Sortino wrote:
>Thank you Norman -
>
>In the picture it is a bit hard to see but the spark plug is actually
>about 7-8mm downstream of the orifices. Your analysis of the orifice
>positions is correct. One of my thoughts was moving the plug a bit
>further downstream to give the propellants a bit more time to mix.
>
>I understand your point about non-deterministic flow patterns
>oscillating between too rich and too lean. Is there a better approach
>for producing a more deterministic mixture ratio at the spark plug?
>
>On Sunday, September 21, 2014 10:02 PM, Norman Yarvin <yarvin@xxxxxxxxxxxx>
>wrote:
>
>
>
>Looking at the photos, your spark plug is upstream of the two
>orifices, in 'dead space'. It also looks like the two orifices are
>pointing directly at each other, both at 90 degrees to the igniter
>axis, and 180 degrees apart from each other. (I figure this is what
>you mean by 'unlike impinging', but of course I can't tell from the
>photos whether this is precisely the case.) That's a really good way
>to produce nondeterministic flow patterns: which way the splatter will
>go depends on how, precisely, the two streams meet: what the mixture
>ratio is going to be at the spark plug is anyone's guess, and could
>easily oscillate between too rich and too lean.
>
>If I've misinterpreted, and the two flows are set to miss each other
>(and it sounds like this is Robert Watzlavick's design), there will be
>less nondeterminism, but I still wouldn't like to predict what the
>mixture ratio will be at the spark plug. Besides oscillation between
>too rich and too lean, that dead space also could get filled with
>nonburnable combustion products, which then got cleaned out by eddies
>of the flow, followed by a minor explosion and a return to the
>nonburnable state; you could have oscillation that way.
>
>A 'popping sound' is, of course, a small explosion. Which is sort of
>okay for an igniter, since the whole point of having an igniter is to
>make the 'hard starts' small and containable. But of course you want
>just one of them, followed by smooth combustion, not a "machine gun"
>like sequence of them.
>
>
>--
>Norman Yarvin http://yarchive.net/blog
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