Insulating Potatoes

name         Marcus
status       student
location     FL

Question -   I am doing an experiment to see which product 
will keep food hotter or colder. Aluminum versus plastic. I used 
2 potatoes and microwaved them for 5 minutes at the same time and 
then wrapped one in aluminum foil and one in plastic wrap. I did 
it 3 times and each time the aluminum kept the potato hotter 
longer. I did the same with a refrigerated potatoes and the 
aluminum kept it colder longer. However I wrapped a piece of ice 
in both aluminum  and plastic and the ice dissolved quicker in 
the aluminum. I do not understand why. Please explain.
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There are several mechanisms for heat transfer that can occur 
simultaneously. Convection, Conduction, Radiation. In addition, the 
heat capacity of the wrapper also comes into play. In the microwaved 
potato case, the plastic wrap (I assume transparent wrap) reflects 
no heat back into the potato, it has more contact with the hot 
potato than does the aluminum foil, and the average wrapper / potato 
distance is smaller than in the case of the foil. So all these 
factors tend to favor the aluminum foil as an insulator. On the 
other hand, when you wrap an ice cube in the two wrappers you add 
the much larger conduction of liquid water, rather than vapor, to 
the equation. The water then 'wets' the aluminum foil and provides a 
much larger heat transfer to the foil. Remember that the transfer of 
heat within the aluminum is going to be much larger than the heat 
transfer within a wrapper made of an organic film. So the heat 
"spreads out" much more in the case of the water foil interface.

Vince Caldery
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Marcus, I imagine the potato stays roughly dry, or at least the gap 
between potato and wrap is not flooded by water.
That makes one insulating layer of air.
The ice will not have that air layer: melting a little, the water 
wets across the gap
from ice to wrap, conducting heat rapidly between ice and aluminum.
The aluminum then spreads the "outwards cold flow" a little, helping 
air convection currents carry it away.

You have about 4 modes of heat transport:
  - radiation
  - convection
  - conduction
  - evaporation/condensation

and about five layers or zones to think about:
  - the object (potato or ice)
  -   gap1: the inside air blanket (or water-fill)
  - the wrap, heat transport thru it
  -(the wrap, heat transport laterally, relevant for aluminum foil)
  -    gap2: the viscous boundary layer of air crawling slowly over 
 the wrap on the outside
  - the infinite, freely-stirred thermal ambient outside that.

With all those parts, predicting insulation effects can get a little tricky,
even though the situation looks simple.

For ice in aluminum foil, the melt-water filling the inside conducts 
heat across gap1,
and dew-condensation from the air onto the foil conducts heat across gap2.
For a hot potato in foil, the potato is trying to cool by 
evaporation (among other modes),
but the aluminum foil blocks the water-vapor transport
before it can cross the more effective insulation gap, which is gap2.

Your potatoes sound like a good analogy for the use of vapor-barriers
in the thermal insulation of a house.  Otherwise it can be difficult 
to remember
whether the vapor barrier sheet goes outside or inside the insulation layer.

Jim Swenson
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