Do you have heated seats? Have the seat pre-warmed, set the pizza there, and throw a coat or other insulating cover over it.
Yes, because wind chill measures how fast a human body chills compared to the equivalent heat loss in still cold air. At higher temperatures, body heat loss due to wind is negligible.
The principle still applies to pizza boxes, except we’re talking about chilling a box well over 115°F with air at 115°F. how fast does it lose heat compared to still air? I.e. what’s the still air equivalent (wind chill) of a stiff breeze against a hot box vs. not moving? Does it result in “pizza box wind chill” equivalent to the air temperature in the car, say 75°F?
Initial first-principle guess suggests that the result would be the box much more quickly cools to 115°F and then stays there… Assuming the blast is a uniform 115 and not mixing with ambient air to actually be a cooler breeze than that. The basic principle of wind chill says that the two factors are air speed and temperature difference.
I found a student project online, (PowerPoint download) with the following goals. I’m not really sure if students at BYU actually understand science (obligatory joke as I attended an actual, reality-based university, the University of Utah, which happens to be a rival school) but they have interesting results anyway.
Their goals:
They heated pizzas to 71 C, placed the pizzas on a wood table then measured how long it took to reach 39 C, and found that the box with foil took 10 minutes longer, and it looks like 37 minutes.
Their conclusions was “The foil did not reduce heat transfer due to radiation it actually served as an inhibitor of convective heat transfer”
The conditions in the OP doesn’t seem to be possible, specifically, that there would be still air above the seat, given that the interior is being heated up with forced air on the floor. While the fan would not be directed at the seat, I don’t see how to prevent air movement across the pizza box.
The reason wind chill is a thing is that in stationary air a heat source can create a bubble of warm air around it that is warmer than the ambient temperature, and so keeps the heat in. So just doing a quick though experiment without any math involved. I it seems to me that the question would be whether when you put the pizza on the cold still area, does the pizza on the seat warm the cool still air around it to a warmer temperature than the heat on the floor. My common sense feeling is that it doesn’t, so the pizza is better off on the floor. Also note that the air on the seat is still probably moving around a fair amount since its picking up some of the air currents floor ventilation.
Put the pizza on the seat in the still air to keep it warmer, it will cool faster on the floor under the blowing air. You’ve all known why since you were children:
When your plate of food is sitting on the table too hot to eat how do you cool it down quicker? You blow on it. Even though you’re blowing warmer than ambient air across it, it cools down faster than it would sitting in still air with an intact boundary layer.
That’s why I would stress a practical resolution of attempting to measure the temperature of the box itself. If the outside layer of the box never heats beyond 115F, then sticking it into a 115F air stream can’t hurt (provided the flow of air leaking through into the box is negligible, that is).
And also provided that steam leaking out of the box is negligible, too.
Cool air sinks and warm air rises. Don’t put your pizza on the floor.
Actually, the still air around a person/object gets heated and rises. But of course, it does not immediately jump a foot away from the object- it flows upward, and the flow rate presumably is dependent on the temperature difference plus other factors like the shape of the hot object. (How “aerodynamic” it is for an upward wind?) and it is replaced by cold air entering the bubble from below, and getting heated in turn.
Real wind chill simply speed this process of replacing hot air with cold more quickly, thus accelerating the rate at which heat is transferred by ensuring the air envelope is generally cooler. (Plus changing from a vertical heat flow to a horizontal one)
I think we want to stick to basic principles and first order effects. Otherwise, too many variables. This can only truly be solved by experimentation. Buy two pizzas. Domino’s probably has a two-for-one special.
Yes, the same puzzle as we are discussing now, but with air at 98.6°F. Also, moist air (as in, our breath) probably has a higher heat capacity than dry air when exposed to something hot. So your car should have a dehumidifier built in if there are melt puddles on the floor mats. Call Toyota and see if that’s an option…
I think that the thermal conductivity of the surface you put the box on is favor of the position on the seat,
Moving warm air should cool faster than still cooler air. Especially since the difference in temperature is small (relative to the hot pizza) Again in favor of the seat.
I don’t see how it could even be close, but to be able to say something definite you should get an IR thermometer and 2 pizzas
Calculating stuff like this is much too complicated for me, I spent enough time on thermodynamics to know you should just test it empirically. (100$ for the IR gun an 20$ for the pizza should be doable)
- the humidity of the air in the car vs. air from the heater.
- the moisture content of the pizza.
- the material of the floor and seat
- the construction of the box
all have potentially more impact on the results than the position relative to the vent.
(I also am missing the speed/volume from the blower)
I’ll muddy the water even further… I know this violates my own “KISS” request, however my assumption was everyone would understand the floor @ the heater discharge was warmed to 115 deg at the time of pick up. but at the time of the OP it never occurred to me that conduction may play the dominant role here (I was convinced convection was the primary culprit) now I’m not so sure. The OP was not put forth in such a way as to preclude conduction, so let me reframe the question in order to remove that (and all other) variables…
A hypothetical:
ALL THINGS BEING EQUAL except the airflow and temperature over the pizza box, which condition produces the hottest pie upon delivery?
A) In an atmosphere of 115 deg F with an airflow of 80 CFM
OR
B) In an atmosphere of (stationary!) 50 deg F with zero airflow
It’s understood that this presents an obvious bias towards convection, but one thing at a time! Hopefully the whole convection vs conduction issue will eventually get sorted out.
Per the NWS:
WindChill = 35.74 + (0.6215 × T) − (35.75 × Wind sfc 0.16)
- (0.4275 × T × Wind sfc 0.16)
*** the 0.16 is a superscript ***
We can’t possibly know unless we know how effective the box and the still air inside the box is at insulating. Because the airflow isn’t acting on the pizza, it’s acting on the box. Depending on whether the outside of the box is more or less than 115F (and by how much) may alter whether the airflow has a cooling effect, or contributes to insulation by raising the temperature of the box, thus lowering the temperature differential between pizza and box w/ air in between, and so reducing the rate of heat transfer.
This is the NWS’s verbatim statement from their website, and it is complete hogwash. Ask any biker Take a ride in 55 deg weather in summertime casual outerwear. Keep it up long enough, and you’ll be damn near hypothermic.
The wind chill factor and the more recent “heat index” are calculated by the NWS and are designed around the vague notion of the average human’s perception of temperature. The NWS’s arbitrary stop point of 50 deg is a prime example of our desire to forgo common sense in favor of something science-y sounding.
Basically it’s great entertainment for viewers of The Weather Channel, but it doesn’t mean that the laws of thermodynamics stop at 50 deg.
ALL THINGS BEING EQUAL except… doesn’t cover this?
Is the box air and moisture tight or a real pizza box?
The real pizza box scenario argued for gtreater cooling impact of the stream of less cold air, as the faster moving air stream above creates relatively lower pressure above the box, sucking air into the box from the cooler below, across the pizza, also with some evaporative cooling, and out the box. Also any moisture saturation of the cardboard will be subject to more evaporative cooling with blowing.
In fact if the top of the box does absorb some warm steam from the pizza into the cardboard then I think a stream of 115 degree air can actually cool the box below 115 due to phase change …
It is a run of the mill cardboard pizza box.
I’ll admit that I only had thermodynamics in engineering physics, so the calculations are beyond me as well. I was also surprised when I went out in the real world and discovered how artificially constructed our engineering problems had been.
The best unanswered question here is how hot the top of the box will become. If it never gets above 115°F in the 15 minutes it takes to go home then it being in the warmer place would be in the warmer location.
And how emissive it is, being constructed of cardboard with air within the corrugated core of the lid.
Not really, as the boxes could be equally well or poorly insulating, and how insulating the boxes are is going to be a major factor.
That said, two things. First, I know that it’s not really about pizza, but I should point out that the pizza doesn’t come out of the oven at 350, if it did, it would be a dried up husk. It’s usually more like 180 or so. So, we are dealing with a significantly less temperature difference than originally assumed by the problem.
Now the second point is that there really isn’t a simple equation for this. Any equation would have to take into account and model a whole variety of factors, with input variables of not just temperature and airflow, but also the conductivity and heat capacity of many different surfaces as well as things like humidity and even atmospheric chemistry. Such things are usually much easier empirically tested than mathematically calculated.
That said, we can simplify things a little bit. For instance, if the heat coming out of the air vent is 180F or higher, then it will keep the pizza warm indefinitely. If the air coming out is 50F, then it will cool rapidly, with more airflow making it cool faster.
So, there is a point where the vent temperature will be high enough to keep the pizza warmer with it blowing on it than if it were in still air. Where that point is is going to be nearly impossible to calculate, and will change based on a number of factors. I would say that a 115F heater is not going to cut it, it’s going to cause more heat loss than it contributes, but that’s a WAG.
I remember a similar question, maybe it was on here, about ice cream, and if you wanted to get an ice cream cone home on a hot day, should you hold it in still air or in front of the AC vent? And I’m pretty sure the consensus there was that it would melt faster in front of the AC vent.
I think that the top of a pizza box can get hotter than that. The rule of thumb is that most people cannot keep their hand on anything over 140 without instinctually pulling away, and I’m pretty sure that I’ve felt the top of pizza boxes at that temperature.