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If heat transfer by convection is greater why is food wrapped in Aluminum foil? Wouldn't that result in the food losing more heat to the foil and ultimately in the air in the form of convection?

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  • $\begingroup$ Reflective foil, when used correctly (i.e. by wrapping an item loosely) greatly reduces both convection and radiation. $\endgroup$ – CuriousOne Mar 1 '16 at 7:50
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While this overview is on an entirely different subject, I thought the answer was close enough where the information was relevant. The human body, on average, in 68 degree temperature, loses about 65% of it's heat to radiation.

Other factors apply, like when sweating vigorously, evaporation becomes the biggest factor, and in cold water, conduction becomes much bigger, but in general, convection, wind currents, etc, is 2nd to radiation or 3rd behind radiation and evaporation.

If you're talking hot food, 140 degrees (serving temp) or 165 degrees or more (cooked thru temp), the radiation rate, T^4 is greater but not that much greater. 52% higher than body temperature at 140 degrees, 92% higher at 165 degrees, but those numbers are in a vacuum. Radiation works both ways, there's a slightly greater variation than that in air temp, but the precise math on this I can't do, but it's safe to say hot food will radiate heat something around or a bit less than twice as fast as the human body will, that rate obviously slowing down as it cools. That's not super fast, which is why a cup of coffee doesn't cool down immediately, but leave it for 20 minutes and by then it's cold. Try an experiment, put aluminum foil over a hot cup of coffee before walking away next time. :-)

When wrapped in Aluminum foil, the foil does 2 things. The foil reflects some of the IR radiation that radiates from the food back at the food, that slows the radiation rate. Aluminum foil is also fairly rigid and bumpy, so it traps air around the food and trapped air is a good insulator. That significantly reduces convection cause trapped air is a very good insulator. Ideally you want the foil to be within roughly 1/4 inch of the food to eliminate circulation under the foil. You don't want the foil smooth and flat, touching the food over most of the area, nor do you want more than 1/2 inch of space between foil and food cause that will allow circulation of air currents between the food and the foil.

One simple solution is to crumple (but don't puncture) the foil before wrapping the food, creating small air pockets. It tends to crumple naturally, but this is a situation where the crumples will keep the warm in longer, where as a perfectly neat fold pressed against the food will be less effective. Some people say, shiny side of the foil towards the food, dull side outside because the shiny side is better at reflection.

Hope that wasn't too badly written, I put it together kinda quickly.

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Well, the aluminium foil does not contribute a lot in convection dissipation because it only hit the surface of your object. This surface is not more thermally conductive than its inside. You'd be right if the aluminium foil could get deeper into the object acting like a "heat radiator".

Overall, the foil is effective by reflecting part of the infrared spectrum and does not contribute to convection. Also, because its low emission coefficient, it prevent food to loose energy from radiation.

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Concerning the claim in the title : it's obviously depends on other factors. E.g., water or air environment have not the same efficiency for convection; the environment can be opaque or transparent. And radiation energy varies as $T^4$.

Concerning the food: I would say aluminium foil is primarily used to protect from air, water and light rather than to keep the heat. see wikipedia.

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Does an object lose more heat in the form of convection or radiation?

In general, that must depend entirely on the specific circumstances.

why is food wrapped in Aluminum foil?

I believe food is wrapped in aluminium foil primarily to prevent the food drying out.

It is used when cooking food since aluminium foil stands up to heat better than many plastics, is cheap, easy to shape and seal and relatively non-toxic (when used that way with non-acidic foods)

It also reduces convection losses by maintaining a (admittedly noncontinuous) layer of still air adjacent to the food.

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