Another point about the effects of local boiling can be found in the Nagayoka
curves. The thin sheath of steam insulates the metal from the water, so it gets
hotter, so the steam sheath gets a bit thicker, reducing the conductivity, so
it gets hotter......it can actually run away thermally and melt the copper
anode block. The Nagayoka curve plots thermal conductivity against temperature,
and actually goes to a negative slope, explaining this phenomena.
This is more likely to be a problem with vapour phase cooling, but can happen
in liquid cooled systems.
Interestingly, the problem didn't appear to be observed in the fireboxes of
steam locomotive boilers, but this may have been because the watts/sq inch (or
centimetre if you prefer!)were less, even though the temperatures were very
high.
73
Peter G3RZP
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