[Beowulf] Re: overclocking with liquids

[Jim Lux]

At 08:21 PM 9/21/2007, richard.walsh at comcast.net wrote:
>  Jim Lux wrote:
>
> > The disadvantage of oil?  It's a mess if you have to remove the stuff.
>Why doesn't anyone ever mention higher heat capacity, relatively inert gases?
>How does the heat capacity of pure CO2 or N2 compare with air?

N2 is basically the same as air.  Almost all (diatomic) gases have 
essentially the same specific heat. One might be able to move more 
heat by using a denser gas, but the pumping losses and aerodynamic 
drag(windage) will be greater (i.e. it goes as rho)...

What you might want to look into is low viscosity gases. He and 
H2?  High speed turbogenerators are cooled with hydrogen to reduce 
the windage losses, but H2 has all sorts of practical problems (the 
molecules are so small it leaks out through tiny, tiny holes and cracks)

>  There must
>be other candidates that would give you 2 maybe 3 times the heat 
>transfer ability
>without being a mess.  Kind of like global warming ... ;-) ... but inside your
>computer.
>


But how do you measure heat transfer ability...?

If you've got to move a certain amount of heat off a surface, you 
have to run a certain amount of something by.  Either you pick a 
2x-3x denser gas and run at the same volumetric rate, or you use the 
same gas and run it at a 2x-3x higher volumetric rate. The mechanical 
work to move that mass past the device scales roughly as the mass 
rate.  (There are a lot of subtle practicalities, including looking 
at things the whether it's laminar or turbulent flow, what the 
Reynolds number is, etc.   A whole cluster could be put to work just 
doing the fluid dynamics calculations...)

>My friends at 3M must have thought about this ... maybe, I'll ask.
>
>rbw
>

James Lux, P.E.
Spacecraft Radio Frequency Subsystems Group
Flight Communications Systems Section
Jet Propulsion Laboratory, Mail Stop 161-213
4800 Oak Grove Drive
Pasadena CA 91109
tel: (818)354-2075
fax: (818)393-6875