I'm doing some research on heat pipes (check here for wiki). One of the things I'm planning to do is try some different fluids in the pipes. Water (commonly used) has a freezing point that's too high, so I'm looking into fluids that don't freeze up in winter and disrupt the equilibrium in the pipe.
I've randomly picked ethanol so far, but I'm looking for suggestions from someone more knowledgeable then me.
I'm mainly interested in vertical heat pipes that don't need a wick like here on wikipedia, but rely on gravity to get the drops of condensed fluid back to the warmer area.
Potential things that could matter:
Viscosity of the fluid
Bubble forming at boiling point (bubbles can form an isolating barrier perhaps?), is there a fluid that boils in vacuum with very small bubbles?
All suggestions are very welcome, please explain why you think a certain fluid is a good choice.
Thanks in advance,