I’m spurred to wonder this by a “ten years ago” deal my wife just shared on Facebook. She was an undergrad then (this is before I knew her), and she and some friends were bringing the ice cream bars back to share with someone or something like that.
Anyway, one of the pics shows one of her college friends smiling sheepishly for the camera and is captioned “[friend’s name] took one for the team and spent time and energy holding the ice cream bars in front of the air conditioning vent.” So I wondered: the air coming out of that vent is cold, but is it below 32 degrees cold, even right out of the vent? I suppose it might be, but I’m not sure. If it’s not, would it be better to move air that’s, say, 40 degrees, rapidly over the bars, or keep them in air that’s 70 degrees but more still?
And wouldn’t the best idea of all have been to wrap them in a blanket or something?
The air coming out of your car’s AC is not colder than 32 degrees.
I’m not an expert on automobile AC systems, but it’s my understanding that they generally aim for an outlet temperature of somewhere around 40 degrees. I’ve also heard that as long as the outlet temperature is 40 degrees below the ambient temperature that the AC is considered to be operating ok, but I think some folks disagree with that. In other words, if it’s 100 degrees out, the AC isn’t going to be able to maintain a 40 degree outlet temperature, but as long as it maintains 60 degrees or cooler, it’s not considered to be a problem with the AC.
Maybe one of our resident car experts can give some better info on that.
Anyway, the answer to whether it is better to have ice cream in 70 degree stagnant air or 40 degree moving air depends on how fast the air is moving. If the 40 degree air were still, then obviously the ice cream would melt less quickly in it than the 70 degree air. The faster the 40 degree air moves, the more heat it can draw out of the ice cream, so at some point it breaks even, where the ice cream would melt equally at 40 degree moving air as 70 degree stagnant air. Past that break-even point, the 40 degree air will always draw more heat out than the 70 degree stagnant air.
I’m bad enough at thermodynamics that I am not even going to attempt to calculate exactly where that break-even point is.
My gut feeling is that you don’t need a whole lot of moving air to draw a lot of heat out of the ice cream, so most likely you are better off not putting the ice cream in front of the vent.
The relative humidity of the air is important as well, my first instinct is that the water in the ice cream would quickly evaporate, only remaining in it’s melted state briefly. What remains is still frozen. I can even imagine the solids remaining behind forming a crust over the frozen parts, slowing down conduction.
Even if the ambient air is extremely dry, the low temperature of the ice cream bar means that the micro-climate (at the immediate surface of the bar) will be at very high relative humidity. You won’t see much moisture loss until the bar is melted and its temperature somewhere close to ambient.
Simple solution. Buy 2 identical ice cream cones, put 1 by the vent and 1 away from the wind. Watch what happens. You can eve buy more cones and try it at different fan speeds.
In my youth I sold Ice cream on a beach. I had a soft ice cream machine and I was trained to pile all the ice cream up on top of the cone (except for pretty girls of course).
I would sometimes see a family about 500 yards or so away, send a small child to collect some cornets. If there was no wind, there was a good chance that they would arrive reasonably safely. If it was at all breezy, as it often was, the child would arrive with much of the ice cream running down their arms.
We have air flow past the ice cream … and if that air is dry we’ll continually be picking up water vapor from the ice, more so if it’s already melting. That was the key here, the ice cream doesn’t appear to be melt even with 40ºF air flow over it. Something else has to be going on.
it will speed up the melting. the simple fact that you’re blowing air across it (regardless of how cold it feels to you) will increase the rate of heat flow.
Except as stated upthread, there must be an equilibrium point. Surely 40 degree air moving past the ice cream bar at 1 FPS is melting the ice cream more slowly than stagnant 90 degree air?
the stagnant air immediately surrounding the ice cream is not 90 degrees. air is a terrific insulator. Alton Brown even did a segment about this, but this is the closest I could find:
Okay, but in reality the air is not going to be perfectly still. So it’s more like slower moving 90 degree, or 70 degree, air vs. faster moving 40 degree air. And I still think wrapping them in a blanket would be the best move of all.
The ambient air will have to be very dry. According to the standard psychrometric chart, 90F air at ~12% relative humidity will generate condensation if cooled to 32F. So if your ambient relative humidity is more than 12% at 90F, then you’re going to add moisture to your ice cream sandwich, rather than drying it out. There are places with humidity that low, but they’re relatively rare.
If the AC’s evaporator is only cooling the air to 40F, then it’s not drying it out enough to prevent condensation when it’s further cooled to 32F. Hope you like soggy ice cream.
I am not a physicist, but I think that the airflow would speed up the melting. From the perspective of the ice cream, it’s warm/hot air. It would be akin to how a hair dryer heats up your skin, even though it has convection.
It won’t be 90 degrees, but it won’t be stagnant either. As the icecream cools the air immediately adjacent, that air will fall because it will become more dense, and it will be replaced by warmer air moving in from above and to the side.
You may be going the wrong direction here. Let’s assume the air coming out is 40ºF and at 100% RH. The air will start immediately mixing with the 70ºF air, the two will have a temperature above 40ºF, which means the RH of this mixed air will be below 100%. Evaporation will occur, and even if the air is then 100% RH, it is immediately push downstream being replaced by more air below 100% RH.
I understand you point, however it’s invalid with the A/C unit. Even if our original air is 90ºF at 100% RH, the output will be 40ºF and 100% RH. The extra water condenses in the A/C unit and drains out the piping provide for this, not on the surfaces where the cooler air is going.
That doesn’t mean the water is rapidly evaporating after it melts, and I have been specifying that this all depends on the RH. If the air is indeed leaving the A/C unit at 100% RH, then we may well see the melting first and not notice the evaporation. However I’m taking the OP at face value where we have a car full of young ladies exclaiming “It’s like totally not melting”. I’m only suggesting under certain circumstances this can be explained as I have above, The water melts but then quickly evaporates giving the impression that it’s staying frozen, when it’s actually “sublimating” instead.
One thing for absolute sure, ice cream in a 40ºF air flow will not stay frozen.
