I just finished baking a lasagna, it’s resting now. Soon it will be devoured.
I’m curious, though; it was tented for a time with a sheet of aluminum foil. I took the foil off and crunched it up, as I’ve done a hundred time. It wasn’t hot, at all, despite the fact it had just been in an oven heated to 350 degrees for the better part of an hour.
How the hell is that possible?
Because a piece of foil has negligible mass and an enormous surface area relative to that mass. The tiny amount of heat energy it contains is radiated away in an instant. Just an extreme version of why a light frying pan cools quickly when taken off the heat and a heavy cast iron one stays hot for a long time.
I think I’ve heard that a single sheet of aluminum foil doesn’t have the capacity to hold much heat, and very rapidly cools off when exposed to room temperature air.
yeah as long as it gets heat from the warm/hot dish it is hot. by itself it cools quickly.
this is why auto radiators and various heat exchangers (like an air conditioner) have lots of thin metal fins because heat leaves it so rapidly.
No, we don’t expect the temperature of the radiator to drop so quickly.
When you properly understand equilibrium, you will see that the radiatior drops heat into the air faster if it stays up near boiling temperature… no wild changes in temperature are neccessary for high capability to lose heat… the heat can flow in from the coolant just as fast as it is lost.
Simply put, the foil effect, and the automobile radiator, both worked on “heat loss is roughly proportional to difference in temperature, and to surface area.”.
So… there can be a trick … the temperature drop is exponential decay… when you feel the foil is warm, it may stay warm for a while… But when the foil was at its hottest, it would lose heat very quickly , and so drop temperature quickly… Don’t use the behaviour when its warm to tell you about the behaviour when its hot.
To add to that, ovens heat primarily via radiation, and aluminum foil, being reflective, absorbs very little radiation.
I don’t think that’s relevant. The air inside the oven should be very close to the radiation temperature, because the walls of the oven absorb IR and transfer heat to the air via convection. And aluminum foil will quickly come to the same temperature through convection.
In most cases radiation heat transfer is insignificant compared to convective heat transfer below 1000F - rule of thumb for desiging boilers / fired heaters
Aluminum is also an excellent thermal conductor, so it’s able to give up its heat very rapidly.
But more to the point is the fact that there is very little of it.
The specific heat capacity for aluminum is 0.9 J/g K. Water’s is 4.2. So gram for gram, the water in your pan is going to hold 4.7 times more heat than the aluminum.
In addition to all of the other factors discussed so far.
no that wasn’t the point.
if heat is constantly supplied into the heat exchanger then its temperature may not change much at all.
the transfer of heat from thin metal surfaces is rapid.
Aluminum has a lower specific heat capacity than the lasagna (which contains a lot of water), so it gains and loses heat much more rapidly.
In addition, the thin layer of foil has a high surface area:mass ratio, which is also going to contribute to faster heat loss.