Re: spark gap question...

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>

Supersonic flow is certainly possible in a spark gap, without even working
hard. A 1/8" orifice driven from ordinary shop air at 60-80 psi will form
visible Mach diamonds; especially if the compressed air is humid, or you
look at the shadow of the air stream in strong sunlight (so you can see the
density discontinuities).  1/8" is 8.5E-5 square feet, and at 1000 ft/sec
(sonic speed, roughly), you're looking at around 5 CFM. The effective
orifice size is going to be different, and the fine detail of the orifice
has a huge effect (it changes what the shock waves look like.



However, before you start calculating madly, you should be aware that
trying to actual calculate the flows and speeds is quite complex,
particularly when you throw in the effects of the plasma, which radically
changes the density, temperature, and compressibility of the gas.
Supersonic nozzle flow is tough enough to calculate by itself.  This is an
area where empiricism (i.e. try it and see how it works) might be a better
approach.

And for the second part of the question, yes, the supersonic flow will blow
the spark out. This is used in air blast circuit breakers, as well as a
variety of other schemes, where the hot gas formed by the spark is used to
blow the spark out (as in an expulsion type fuse).  It's also been used in
high speed wind tunnels, where an ionized gas line is created across the
air flow( using a row of spark gaps), which then drifts downwind at the air
speed, allowing visualization of the air flow.

A gentleman named Erwin Marx actually did quite a lot with triggered
supersonic gaps.
http://home.earthlink-dot-net/~jimlux/hv/blastgap.htm

Tesla list wrote:
> 
> Original poster: "Lau, Gary by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<Gary.Lau-at-compaq-dot-com>
> 
> Let's say that the air passing through the gap is traveling at the speed
> of sound - 1129 ft/sec.  In a typical coil, the time from the start of a
> bang to first primary notch, where we'd like an "ideal" gap to quench,
> is about 40 usec.  Mach 1 air will travel 0.54" in this time, which is
> just about how far you'd want any ionized air to be from your electrodes
> to prevent re-ignition.  But is mach 1 airflow actually attainable in
> our gaps?  Also, is it a given that the plasma channel moves with the
> airflow, or could it simply exist wherever the current path is?  Ions
> from an *extinguished* arc would be swept away, but I don't know about
> an active plasma channel.
> 
> Gary Lau
> MA, USA
> 
> >Original poster: "Sundog by way of Terry Fritz <twftesla-at-qwest-dot-net>"
> <sundog-at-timeship-dot-net>
> >
> >Hi All!
> >
> >Isn't the plasma effectively remvoed by the high airflow?  I see no
> reason
> >the gap wouldn't quench well, if the spark by the trigger electrode is
> tho
> >only thing that lets it fire? With enough air, you can quench anything
> :)
> >
> >Shad
> >besides, it's less fuss than a rotary!