Morgan
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Heat transfer using methanol
Can someone offer an explanation why this engine runs fast, then slow, then fast again? There's a liter or so of methanol in the bottom of the tank
and the snorkel is the only opening for intake and exhaust. It's better viewed full screen and the video becomes brighter after ~30 seconds into the
run, it was shot at dusk to highlight the "invisible" fire/flames. It doesn't have any moving parts, just methanol in the bottom of the tank.
I know the temperature of the tank and alcohol are rising with time, but is there some latent heat of vaporization thing going on to cause the slowing
down phase? I don't really understand the thermodynamics but most probably the alcohol eventually reaches the boiling point and is sloshing around
providing heat from combustion and cooling from evaporation.
Somehow it falls out of balance and then gets back on track as if more than one thing is going on.
http://www.youtube.com/watch?v=OzHX5o9zRbk
Not the best article but the section on volatility food for thought.
"Referring to Figure 2-2, gasoline has a latent heat of about 140 Btu/lb; methanol, 474 Btu/lb; and ethanol, 361 Btu/lb."
http://journeytoforever.org/biofuel_library/ethanol_manual/m...
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12AX7
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Perhaps excessively high temperature causes too much vaporization, which makes the mixture too rich, so it slows down? Evidently, it's a feedback
system, and the delay around that loop is too slow to keep itself nicely in balance. You could probably use a pump to circulate the fuel over a hot
or cold surface, controlled by the exhaust temperature, to speed up and stabilize that. Or just do it open loop altogether by fuel injection (ha!).
Tim
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AJKOER
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If the metal in the tank starts to reacts with the CH3OH, and if the reaction is endothermic, the temperature will drop.
Or, there could been an exothermic reaction going on with the CH3OH that ends (perhaps a passification layer is formed).
If the tank is Cu or Fe, there may be some reaction with FeO (or CuO) decomposing the CH3OH at an elevated temperature, which could impact the system
temperature.
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Morgan
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That could be possible Tim. I've noticed some of the little boats I've been playing with will speed up several times after idling for a bit. Other
times they don't go through any phase, just instant on as with this aluminum bottle and two 8 inch nails for ballast. The snorkel a piece of plumbing
pipe. Imagine the delicate balance of fueling as the 100 ml of methanol is sloshing about yet the combustion steady. A fuel feed system would be
better of course. But it's nice when they run with such simplicity too.
http://www.youtube.com/watch?v=NLFL7GJcj-I
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Morgan
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The metal tank with fast, slow, fast behavior is stainless steel although the patina is gone from it glowing red hot in the past.
I thought about the little aluminum boats I play with possibly reacting with the hot methanol, to the extent of wondering if the addition of aluminum
powder or magnesium might somehow help. I've fizzed magnesium in methanol before, the reaction quite vigorous if the surface clean. Then too I thought
about some copper mesh I have, if it could partially crack the methanol to some extent.
It would be nice to have a more perky fuel, but all the added difficulties and hazards make the simple thought improbable/impractical.
I wonder if methanol absorbs heat at a steady linear rate thoughout and up to the boiling point?
Thanks for the comments.
Heat transfer tidbits. The part on boiling interesting.
http://en.wikipedia.org/wiki/Heat_transfer
[Edited on 21-5-2013 by Morgan]
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Morgan
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Here's the same tank again in a water bath exhibiting the same fast, slow, fast behavior. It would be nice to understand how that shifting is brought
about, the underlying reasons.
http://www.youtube.com/watch?v=KUYgTN1erJc
Another tidbit
http://en.wikipedia.org/wiki/Critical_heat_flux
[Edited on 21-5-2013 by Morgan]
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Morgan
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Pool Boiling Heat Transfer - Dragonfly Education
https://www.youtube.com/watch?v=N1yZwRcQSZw
http://en.wikipedia.org/wiki/Nucleate_boiling
[Edited on 10-5-2014 by Morgan]
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