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Name: Marcus
Status: Student
Grade: Other
Location: FL
Country: United States
Date: April 2006


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.



Replies:
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


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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