When I stop by McDonalds on a hot sunny day I get an McFlurry along with the horribly unhealthy meal. While eating the meal the Mcflurry tends to melt a bit. One of my cup holders is inline with an ac vent. My ac blows cold air but not freezing air so it is still warm enough to melt the icecream. Away from the vents the air is warmer.
The question is will the icecream melt faster under a constant flow of cool air or in warmer standing air?
I know you physics people want temperatures and cfms but I just don’t have those sorry.
Unless the wind is at spectacular speeds, it tends to cool things. This is why there’s a fan on your car radiator, and it’s put up front. The wind from the motion of the car and the fan combine to put new air molecules on the little vanes and absorb heat from the coolant.
The upshot being that standing cool air would be better than standing warm air, and I can’t help but think that moving cool air would do better than standing cool air. There’s got to be a little evaporation happening with ice cream, too, I’d think.
I’ll need to be convinced of this. I know in the winter, that wind makes you feel colder. That’s because you are a warm object and the wind is blowing the heat away. Wind will have no effect on the temperature of an object if the wind and the object are the same temperature.
With regards to the ice cream, assuming the AC is warmer than the ice cream, it will make the ice cream melt faster. Whether it will melt faster than if it is sitting in warm air cannot be determined unless we know how warm the air is and how cool the AC is and how fast the AC is blowing.
Yeah, the unknowns here are important. How warm the air is and how cold and how fast the A/C is blowing has to be known. And solkoe is right, moving air tends to equalize temperatures, not cool things down; blowing cold air on a warm object cools it down, but blowing warm air on a cold object warms it up.
That said, I have to admit that this is a damn good question. My take on it is, if the car is fully cooled off (cold as it’s gonna get), you’re probably better off pointing the A/C away from the shake; if you’ve just gotten it going, you’re better off with the A/C blasting the shake once the initial hot blast from the A/C during startup has dissipated . YMMV.
I think what Cardinal was trying to say, is that unless the FAN is going at spectacular speeds it will tend to cool things. As in going so spectacularly fast that there’s enough air friction from the blades to actually warm the air moving past them by some significant amount. But I could be wrong.
Ummm…you’re wrong. We tend to feel fans cool things because in most cases they cool us…but not always. If you were in an oven and a fan were blowing on you, you would get hotter faster than if there were no fan…in fact, that’s how convection ovens work, and that’s why they cook faster than a regular oven.
Ummm…How am I wrong? I didn’t say anything. I just paraphrased/clarified someone else. Unless you’re saying that I incorrectly summerized what Cardinal was saying. Is that what you meant?
What many here are forgetting (Cardinal excepted) is that moving air aids evaporation, which is a very good way of cooling something. Warm air will very likely still cool something if that thing is wet.
There are a lot of considerations here. I’m not familiar with a McFlurry. But if it’s in a cup, consider that even in otherwise still air, a layer of cold air will be created and will settle just above the ice cream (cooled by it). If you blow air over the cup from the air conditioner, you’ll keep shifting the near-ice-cream temperature layer of air off and replacing it with warmer (albeit still cool) air.
I don’t know how it will all balance out, but there are considerations galore here.
A fan on a car radiator cools the radiator because the radiator is hotter than the ambient air; drawing more air across it with a fan enables the transfer of heat from the radiator to the air to occur at a faster rate, because it’s taking away the air that has already been warmed up.
Ice cream is colder than the ambient temperature, which means there will be a tendency for heat to flow out of the air (even if that air seems cool to the skin) and into the ice cream. If you blow a fan on the pot containing the ice cream, you’re removing the air that has already given up some of its heat to the ice cream, and replacing it with more air that has not given up its heat; it will melt faster.
Now this still hasn’t got to the OP’s question which is whether moving cold air is better than comparatively still warmer air. There isn’t going to be a single, simple answer to this, as it will depend on how warm is the still air and how much cooler is the moving air. Also, droplets of condensation on the outside of the cup will provide better evaporative cooling in a stream of moving air than in static air.
So the answer is that it could be either; depending on how hot it is elsewhere in your car, and how cold, and how fast, is the air coming out of your AC vent.
But fortunately there’s a very much easier solution to your question; buy two McFlurries next time and expose one to each set of conditions. Science demands it.
It does come in a cup. As Mangetout points out, this means moisture will condense on it, and the moving air will cause that to evaporate, cooling the cup. But I don’t know whether that will counteract the effect of blowing warmer-than-freezing air across the ice cream–or, if I remember my physics class correctly, the heat that’s released when the moisture condenses in the first place.
That’s a good point; I had totally forgotten latent heat of condensation; I would imagine that this ought to be more or less equal to the heat removed by evaporation, in which case the whole issue of condensation would be pretty much a red herring.
We do well to note that whereas we describe the still air as warm and the flow from the AC as cool, from the McFlurry’s perspective both are hot (the still air being hotter).
Quite so, in fact, the idea of ‘cold’ is often a big distraction in these types of question; ‘cold’, of course, does not have actual physical existence in the same way that ‘heat’ does. ‘Cold’ is merely the absence (or dimimished presence) of heat, so the cold environment in a fridge (or in this case the cold air leaving a car AC vent) doesn’t ‘add cold’, it removes heat, but it can only do this to an object hotter than itself.
Wait, what if there was a fan blowing on the ice cream and the fan was on the treadmill, and the treadmill was going the same speed as the air blowing out of it?
(Please, for the love of god, don’t answer that question, I think we all know what will happen if someone tries)
If the treadmill accelerated to match the speed of melting, then the fan couldn’t possibly cool it, unless it had powered wheels (the ice cream, that is), in which case the spoon would be the probably point of failure, but only relative to the ground. I think that’s all correct, but someone please check me.