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RE: Metal Selection for gap.
Original poster: "Luke" <Bluu-at-cox-dot-net>
Well that gave a lot of food for thought.
Time to shut up and do some thinking etc......
Thanx.
But from what I have learned recently isn't the anode actually the
hotter electrode of the gap? Yes, I know they keep trading places since
we are dealing with a/c. But everyone refers to the cooling of the
cathode when it is the anode that receives the electrons and heats up
more as a result of that. The cathode drop is higher than the anode
drop but the anodes temperature is higher due to the impact of the
electrons on the surface. It is probably of no consequence but just
wanted to point that out.
Luke Galyan
Bluu-at-cox-dot-net
http://members.cox-dot-net/bluu
-----Original Message-----
From: Tesla list [mailto:tesla-at-pupman-dot-com]
Sent: Monday, March 08, 2004 6:44 AM
To: tesla-at-pupman-dot-com
Subject: Re: Metal Selection for gap.
Original poster: Kurt Schraner <k.schraner-at-datacomm.ch>
Luke,
specific heat is not the only physical property, determining the
temperature of the electrodes. For the STATIONARY condition it's even
not
relevant, but HEAT CONDUCTIVITY of the material is. Of course,
stationary
heat transfer, is not exactly, what we have in spark gaps, and so you
are
right to consider SPECIFIC HEAT as well. And DENSITY would needed to be
taken into account further, because
density * electrode volume * specific heat[J/kg]
--- electrode mass [kg]---
determines the heat[J] storage capacity of the electrode.
But that's only the minor part of the problem: Electrode temperature is
determined by the far more complex INSTATIONARY heat transfer...
1.) from spark to electrode metal
2.) to accumulation of heat in the metal over spark's time
(Well: periodically repeated by the BPS...)
3.) to transfer the heat by conduction to the whole electrode surfaces
4.) to transfer the heat by convective heat transfer to the forced
air stream, flowing around the electrode (laminar or turbulent)
...and from my own experience with heat transfer, you would only need to
consider the first and the last one, for the speed of heat transfer to -
and away from the electrode, because heat transfer resistance from a gas
to
a solid metal surface (and vice versa) is order of magnitude less than
inside the metal.
Obviously pt.4.) is depending on air flow around the electrode, how
much,
and in what geometrical arrangement... Heat transfer to flowing fluids
can
be threated following Nusselt's theory.
Pt. 2.) and 3.) depend on electrode material properties, and they are
relevant for the instationary part of the problem, which would become a
quasistationary one, taken a number of cycles of the BPS(-> so depend on
BPS as well)
But that's not all, regarding quenching, considering what Bert Hickman
reported recently, in thread "quenching question" - I cite Bert (from
today's mail):
"1. No portion of the electrodes can remain incandescent. This is often
a
problem, since an incandescent cathode spot is normally created whenever
we
form an arc. If the cathode spot cannot be cooled quickly, it remains a
profuse source of thermal electrons. The free electrons are then
accelerated by E-field in the gap, where they can retrigger spark
breakdown, and arc reignition. Using more massive electrodes, external
cooling, and selected refractory metals or excellent thermal conductors
(copper, molybdenum, tungsten) helps. It's also important to prevent
buildup of oxidation byproducts since these can also become sources of
thermal electrons."
We see: here is another material property, taking serious influence on
quenching of a TC sparkgap (may be it's not the last ;-)... ). So far we
have the following metal properies inside the system determining gap
quenching:
specific heat
density
thermal conductivity
"cathode spot cooling ability"
It would perhaps be a valuable simulation project, in order to get an
idea
about the thermal parameters of a forced air cooled multigap à la Terry
or
TCBOR or ....?, - not to mention the quenching behavior. Some properties
of
5 commonly used gap metals at 20C:
Density specific heat thermal cond. Tmelt
kg/liter J/kg K W/m K C
Copper 8.9 390 393 1083
Tungsten 19.3 142 197 3380
Brass MS72 8.56 390 92 920
SS V2A 7.88 500 21 1400
Zinc 7.14 376 109 419
I'm shure, other members of the list are able to provide deeper insight
on
the relevant problems, but wanted just to point out, using specific heat
as
a selection criterion for gap material, not to be adequate.
Best regards
Kurt
Tesla list schrieb:
>Original poster: "Luke" <Bluu-at-cox-dot-net>
>After a short time the electrodes will fully heat up regardless of the
>specific heat. Never thought of it that way.
>So would a lower specific heat be able to move the heat into the air
>more rapidly? Since the surface of the electrode exposed to air would
>release heat and be slightly cooler than the metal under the surface.
>If the material had a lower specific heat, it would allow the heat
>energy from beneath the surface to raise the temperature of the surface
>quicker. And since higher heat differential (surface to air) would
mean
>faster heat dissipation.
>Seems the lower specific heat would win out once the temperature
>stabilized.
>Luke Galyan
>Bluu-at-cox-dot-net
>http://members.cox-dot-net/bluu
>-----Original Message-----
>From: Tesla list [mailto:tesla-at-pupman-dot-com]
>Sent: Sunday, March 07, 2004 7:39 AM
>To: tesla-at-pupman-dot-com
>Subject: Re: Metal Selection for gap.
>Original poster: "Jim Lux" <jimlux-at-earthlink-dot-net>
>In the "transient" case, where you're starting up, true, a high
specific
>heat will keep things cooler longer.
>But, eventually, you'll reach equilibrium, at which point the specific
>heat
>won't make any difference. Heat in=heat out and temperature
essentially
>determines heat out (for constant airflow).