I'm standing in a Costco store, and they have a large walled off area for chilled produce. The entrance to this section is a square opening about 10 feet on a side. When you walk in, you notice a rapid change from room temperature to cold.

I've noticed that this temperature barrier is extremely thin. You can stick your arm in and feel your hand is cold while your elbow is warm. How does this even work? There is a moderate current of air moving across the entrance as well so I'm sure that's the key. But with no differing materials on either side like a glass door to insulate it, what stops the heat from spreading over a wider area? Is moving air alone an insulator? Even if convection is prevented, what about conduction?

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    $\begingroup$ Is there a wind curtain? (A box/rail above the door which blows a wall of air down. They're present at many store entrances) $\endgroup$ – user12029 Nov 21 '14 at 22:36
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    $\begingroup$ I won't speak for @Tesserex, but the walk-ins at all Costco's I've ever been to have wind curtains. The key is laminar flow of air which effectively seals in the cold air (as discussed here: poweredaire.com/AC-101.htm). $\endgroup$ – bobfandango Nov 21 '14 at 22:53
  • $\begingroup$ Yes I mentioned the current or air going across the entrance, sorry if it seemed like I meant through, I meant the air went over the plane of the entry so yes a curtain. $\endgroup$ – Tesserex Nov 21 '14 at 23:06
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    $\begingroup$ @NeuroFuzzy Might be worth an answer, given OP's response. Also, your website link on your user page is wrong (since you shifted to Jeckyl). There's some great stuff on your website for everyone to see, so fix it! $\endgroup$ – WetSavannaAnimal Nov 21 '14 at 23:38
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    $\begingroup$ Related: physics.stackexchange.com/q/69510/2451 $\endgroup$ – Qmechanic Nov 22 '14 at 0:32

The key here is the air curtain. You can be certain that if it didn't save Costco money, they wouldn't bother with it! It takes a bit of power to push air that much. Two very helpful diagrams are in the youtube video Powered Aire - Cold Storage Air Curtain:

Air curtain off

Air curtain on

In the first case, the air can mix and change temperature through convection, and it does so in a pretty drastic way. Convection is a much much better way to transfer heat.

Conduction occurs, definitely. You can see the lighter blue warmer air in the second picture, which is due to conduction and also probably mostly due to turbulent mixing in the airjet stream.

Another good video demonstration is with smoke and air bubbles:

Video of "bubbles vs. air curtain"


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