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Would a fan such as this, with one fan set to intake and the other to exhaust, have the effect of circulating air such that air from the inside is being expelled by one blade while air from the outside is pushed inside? Or, would the fans, being in such close proximity, effectively cancel one another (so that one blade feeds a lot of air to the other, which feeds it back to the first, etc.)

The only pieces of information I've found on this were on websites like this that I think would make any claim they can to sell a fan.

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Precisely how effective a fan like that would be would depend on things like how powerful the blades are and how large the room is. That being said, the general idea is solid.

The velocity field of the air in an open space (like a room) is a continuous vector field. This implies that your fan would circulate the air throughout the entire room - though the circulation may only be significant near the fan if the room is particularly large or the fan is particularly weak.

More importantly, a fan like that allows air to be continuously exchanged between the inside and the outside. Have you ever tried to put a one-way fan in your window while the door to the room was closed? The fan tries to blow air into the room, but because the air has no easy way to escape, it simply increases the air pressure which fights the additional flow of outside air into the room. Try it, and watch what happens when you open the door - suddenly the air can escape, and the blast of external air will be remarkably noticeable.

If you don't want to open your door every time the fan is on, a dual intake fan will be vastly more effective than the alternative.

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  • $\begingroup$ Why does being a continuous vector field imply that the fan circulates air through out the entire room? If the fan were very powerful, wouldn't the velocity field still be continuous if the powerful exhaust and powerful intake just rapidly circulated a very small amount of air between themselves? $\endgroup$
    – AmagicalFishy
    Commented Aug 3, 2017 at 19:07
  • $\begingroup$ Apologies, I was sloppy, and used the phrase "continuous vector field" in a casual, rather than technical sense, but the point is this: Assume that you have a region where the air is circulating and a region where it is not. There must be a boundary between them, but at the boundary the nonzero viscosity of the air will impart flow velocity to the nearby, previously stationary air. The result is that you cannot have such a boundary in the steady state. $\endgroup$
    – J. Murray
    Commented Aug 3, 2017 at 19:20
  • $\begingroup$ @AmagicalFishy I'm not claiming that the fluid flow far from the fan won't be negligible, though. $\endgroup$
    – J. Murray
    Commented Aug 3, 2017 at 19:21

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