z wrote:

we did a project on window type fans undergrad. turned out that
diameter was the big determinant of flow, higher speed on the same fan
didn't increase the flow much; but it did increase the turbulence,
which might be just as useful if you're not sitting in the direct
blast of the fan. of course, that increased noise also, which makes it
sound like there's more flow.
also, of course, ducting/shrouding the fan gave big increase in flow
vs turbulence, but everybody probably knows that already.

With an open fan, I imagine one limitation is turbulence on the sucking
side of the blade if the air can't change direction fast enough to fill
in the vacuum smoothly. If a fan had an intake duct to give the air a
couple of feet (maybe more) to accelerate toward the fan, maybe the
blades could grab more air at high speed. Increasing diameter would be
more efficient.