I’ve been misinformed then if that’s the case. I’ll have to go back and look for the source.
When it was cold enough to store my frozen food in the garage, I decide to defrost 4 years of ice buildup in my chest freezer. I found that pointing a household fan down into the open freezer (after turning it off, duh!) the ice melted much faster. Air may be a lousy conductor of heat, but also - my theory - it creates far less vigorous convection currents than water also, so the hot air is replacing the cold air surrounding the ice slower - a forced air flow will add heat much faster that standing air.
@Cheesesteak, thanks for running the experiment. How tall were the glasses you used - tall enough that the air at ice level was still ?
Air circulation would have a major impact on the relative speeds of melting. Take an extreme example of a fan directed into the glass: in that case, the ice/water mix will probably take longer to melt, because the air must warm up the mass of water as well as the mass of ice. The lower conductivity of air will be outweighed by the great mass to be warmed.
(ETA: and now I see md-2000 said much the same thing I did.)
Agree
Dry air at 32F Thermal Conductivity = 1.390e-2 BTU/hr-ft-F and wet saturated air at same temperature = 1.387e-2
Well, sure - but that’s not why…
If you direct forced-air at a lump of ice, the air will contact the entire 3-d surface of it. If you direct forced-air at the surface of the water in a glass full of a water-ice mixture, the air only contacts the (much smaller) area of the top of the glass.
The OP seems to describe a cup without any forced convection. Since the cup is open at the top, cool air will sit stratified in it (as opposed to for example the buoyant convection in a hot coffee cup). Forced air isn’t part of this case, is it?
@Cheesesteak, thanks for running the experiment. How tall were the glasses you used - tall enough that the air at ice level was still ?
Air circulation would have a major impact on the relative speeds of melting. Take an extreme example of a fan directed into the glass: in that case, the ice/water mix will probably take longer to melt, because the air must warm up the mass of water as well as the mass of ice. The lower conductivity of air will be outweighed by the great mass to be warmed.
Bolding mine.
The ice was entirely within the cup at the beginning of the experiment, but with plenty of space around for air to circulate. My ice cubes are very square so they were stacked like blocks at the start.
Of course, not much air would circulate since cold air is more dense than hot air, it will pretty much just sit where it is.
I would agree that it would melt faster but you will have less practical material available for heat absorption. So if you just want to keep ice longer drain the water but if you want more cooling potential save the water.
Good point!
For many analyses, you can get a correct result if you think of coolness as the stuff itself, the agency flowing around that you can manipulate, rather than just thinking of it as the absence of heat and only analyzing heat as the stuff itself. Letting water out of a cooler, when that water is colder than the environment, is throwing the coolness away.
In fact, in many contexts, you can get the right answer by thinking of coolness as “stuff”, when you can’t get it by thinking of the heat as “stuff”. For instance, when you drain the water out of your cooler, you’re removing “coolness”, and also removing thermal energy.
Hasn’t anyone here ever sped up the defrost of a turkey by keeping it in cold water?
There are lots of meats in sealed packages that I’ve defrosted quickly in a bowl of cold water that would have taken much longer left on counter and gotten some areas cooked doing it in the microwave.