Re: Metal Selection for gap.

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

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).