Input != Output in box fan land.
May 12, 2013 5:28 PM Subscribe
I have a box fan running in my window. It pushes out a lot of air when I hold my hand in the front but when I hold my hand on the back of the fan it barely feels like any air is being pulled in. Where is the extra air coming from?
Is it wrong to assume that that the air being pulled in would be equal to the air going out?
Is it wrong to assume that that the air being pulled in would be equal to the air going out?
Sticking your hand in front of the path of the fan is no way to measure the flow. There is no extra air.
posted by pompomtom at 5:31 PM on May 12, 2013
posted by pompomtom at 5:31 PM on May 12, 2013
Your fan is sucking air from a large volume of ambient air. That large volume of ambient air does not need to move much for a part of it to be sucked through your fan. If the air behind your your fan were constricted before hitting your fan, your hand would feel it more because you would have the same amount of air flowing through a smaller volume.
posted by dfriedman at 5:34 PM on May 12, 2013 [3 favorites]
posted by dfriedman at 5:34 PM on May 12, 2013 [3 favorites]
If you put your fan in front of the curtains, you'll see how powerful it is in taking in air.
It also starts making a very sad sound when you do this, so don't do it for very long.
posted by xingcat at 5:35 PM on May 12, 2013 [1 favorite]
It also starts making a very sad sound when you do this, so don't do it for very long.
posted by xingcat at 5:35 PM on May 12, 2013 [1 favorite]
The air sucked into the fan is coming from all around the back of the fan. The air pushed out is 'focused' by the fan blades. It's the same amount of air (e.g. in ft^3/second), but as the expelled air is pushed through a relatively smaller space, it moves faster through that space.
posted by carter at 5:55 PM on May 12, 2013 [5 favorites]
posted by carter at 5:55 PM on May 12, 2013 [5 favorites]
Your hand can't really feel airflow volume - it just feels air speed, kinda. If you think of a 1-ft diameter fan, the output near the fan is like a cylinder with that same 1' diameter, with pi*(1/2)^2=pi/4 cross-sectional area. The input is the entire hemisphere behind the blades, with (1/2)*4*pi*(1/2)^2 or pi/2 area.
Obviously, the edge of the fan blade is moving faster, and pushing air faster, so it's not like the air being sucked in or pushed out is a uniform speed. However, just looking at the inflow and outflow areas, the average speed in is going to be very roughly half that of the output speed.
Now, drag on your hand - the means by which you "sense" airflow - goes up with the square of speed. So if the average input speed is half that of the output speed, the drag in is going to be a quarter the drag you feel on the output speed.
This is completely back-of-envelope--my fluids professor is probably screaming right now--but it at least gets within an order of magnitude of explaining the phenomenon.
posted by notsnot at 6:44 PM on May 12, 2013 [1 favorite]
Obviously, the edge of the fan blade is moving faster, and pushing air faster, so it's not like the air being sucked in or pushed out is a uniform speed. However, just looking at the inflow and outflow areas, the average speed in is going to be very roughly half that of the output speed.
Now, drag on your hand - the means by which you "sense" airflow - goes up with the square of speed. So if the average input speed is half that of the output speed, the drag in is going to be a quarter the drag you feel on the output speed.
This is completely back-of-envelope--my fluids professor is probably screaming right now--but it at least gets within an order of magnitude of explaining the phenomenon.
posted by notsnot at 6:44 PM on May 12, 2013 [1 favorite]
This is a little like the Sprinkler Problem: Nobel Laureate Physicist Richard Feynman, in a memoir, wrote about an experiment he attempted on this topic. Picture a lawn sprinkler made of an S-shaped pipe where water is fed to the middle of the pipe and is sprayed out nozzles at either end of the S, causing the whole pipe to spin around the center axis from the force of the spray. The question was whether it would spin the other way if the water was being sucked into the nozzles instead. He didn't have a pump, so he built a sprinkler (the essential parts at least) and put the whole thing inside a glass carboy, half filled with water). Then he pressurized the whole thing, forcing water into the nozzles. It didn't spin, and then the carboy exploded, leaving the whole thing inconclusive.
But the problem (which dates in publication to 1883, well before Feynman's time) has a solution; the sprinkler barely spins, due to a few things including the generally unfocused nature of the draw compared to the focused nature of the output. Air is being drawn into your fan from a lot of directions-- top and sides of the box fan as well as the air directly behind it. That air is being organized and focused, to an extent, by the fan's action, into the blast of air from the front. The forces going in are adding up to the force going out, but the forces have very different directionality. And as mentioned above, your hand is less than ideal for measuring this. A streamer will detect forces at the edges of the fan, indirect angled drafts, on the drawing side, the opposite of which won't be present on the blowing side.
posted by Sunburnt at 8:42 PM on May 12, 2013 [4 favorites]
But the problem (which dates in publication to 1883, well before Feynman's time) has a solution; the sprinkler barely spins, due to a few things including the generally unfocused nature of the draw compared to the focused nature of the output. Air is being drawn into your fan from a lot of directions-- top and sides of the box fan as well as the air directly behind it. That air is being organized and focused, to an extent, by the fan's action, into the blast of air from the front. The forces going in are adding up to the force going out, but the forces have very different directionality. And as mentioned above, your hand is less than ideal for measuring this. A streamer will detect forces at the edges of the fan, indirect angled drafts, on the drawing side, the opposite of which won't be present on the blowing side.
posted by Sunburnt at 8:42 PM on May 12, 2013 [4 favorites]
Try licking your finger before holding it behind the fan. You'll detect lots of air flow. (Your finger will get cold.)
posted by Chocolate Pickle at 9:41 PM on May 12, 2013
posted by Chocolate Pickle at 9:41 PM on May 12, 2013
This thread is closed to new comments.
posted by empath at 5:31 PM on May 12, 2013