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Liquid nitrogen quickie

Liquid nitrogen quickie - Physics Forum

Liquid nitrogen quickie - Physics Forum. Discuss and ask physics questions, kinematics and other physics problems.


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Old 02-06-2004, 05:21 PM
muhaha
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Default Liquid nitrogen quickie



I know it's dangerous but i was thinking about building my own liqiud
nitrogen cooler(based on THG video of the topic), and have one more
question to resolve:

What happens with the closed cilinder completely filled with lN when
in contact to ~70W dissipator.

Thanks for any suggestion on the topic.
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Old 02-07-2004, 12:34 AM
tadchem
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Default Liquid nitrogen quickie


"muhaha" <[Only registered users see links. ]> wrote in message
news:dea12ae6.0402060921.4d00186e@posting.google.c om...

That will depend on the configuration of the cylinder and the "dissipator"
(whatever that is - I assume it produces 70 watts of output, so it *is* a
'heater').

If the cylinder is completely closed and sealed, then the pressure will
increase until either the cylinder fails (probably catastrophically!) or
until thermal equilibrium is reached. At room temperature nitrogen would
have to be pressurized to over 1000 atmospheres of pressure (100,000 kPa, or
15,000 psi) to maintain the same density as liquid nitrogen.

Otherwise the nitrogen will leak out.

If the geometry of the heater permits, the nitrogen will form a gas layer
(research the "Leidenfrost effect") that will slow down the transfer of heat
from the heater to the liquid.

Liquid nitrogen ("LN2"), poured onto a flat surface at room temperature,
will NOT touch the surface, but as it *nears* the surface it boils fast
enough to produce a layer of nitrogen on which the droplets "float" and
skitter around like water droplets on a hot griddle (exactly the same
effect).

If you are careful enough to use only a small amount (less than 1 ml) you
can actually pour LN2 into the palm of your hand and feel no more than a
slight chill. Be VERY careful, though. I used to give demonstrations using
LN2 in which I would freeze flowers (roses, carnations) in under 3 seconds
cold enough to *shatter* them with a touch, and I froze bananas hard enough
to use them as hammers, driving 3-penny nails into 2x4's.


Tom Davidson
Richmond, VA


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