The direction doesn't matter nearly as much as blowing the fan through a large-ish opening. You want the air from the fan to accelerate other air on the way ou

The direction doesn't matter nearly as much as blowing the fan through a large-ish opening. You want the air from the fan to accelerate other air on the way out/in. This is the same principle as turbofan engines, in which a higher bypass ratio (bigger fan) attached to the same fuel burning jet core is more efficient. Why? Because the relationship between velocity and volume is not linear. Takes more energy to move the same amount of air the faster you accelerate it. So ideally you want to position the fan in a large opening far enough back that the fast moving air can get a bunch of other air moving as well. Trade the high velocity fan air for a much higher volume.