Imagine an empty coke can placed on the hob of a cooker. Place your hand on top of the can and turn on the cooker (low heat, one assumes). See how long it takes until the can gets too hot to touch.
Now, would it slow down that process if the can had cut-outs removed around the cirumference so that the top was connected to the bottom by small ‘bars’ of metal? Another way of describing this; as if there were four or five rectangular windows cut in the can around the middle. A subsidiary question was; would the ‘bars’ between the top and bottom half of the can get hotter than the rest of the side walls?
I guess the questioner was asking whether the reduced amount of metal connecting the top and bottom of the can would slow conduction. The subsidiary question seemed to be querying whether the heat would somehow ‘channel’ through the narrower parts of the conduction route.
I have my own theories on this, but I am keeping them to myself as my theories are always wrong… and I mean, always…
In an ideal steady-state model, ignoring radiation and convection, the heat conduction is proportional to the temperature difference and the conducting area divided by the length.
Ok, I’ll take a shot. Aluminum is a very good conductor of heat, and air is a rather bad one. However in this case the mass of Al is quite small, and heat convection is likely to win out over heat conduction. In other words, as the heat travels up the aluminum, much of it will be lost to the air because the surface area is quite large relative to the mass of the metal. That “lost” heat will heat the air and continue upward, both inside and outside of the can. Much of the heat on the outside is truly “lost”, but the hot air inside the can will continue to heat both the air and the metal above it. In effect, convection (the heated air) has become the predominant heat transfer driver.
When you cut out strips of the metal, you reduce the conduction heating effect, and more importantly, you allow more of that trapped convective heat to escape. There’s no question that the top of the can will heat slower in that case.
