So I get the blades from a common desktop fan. I set them up so they’re positioned back-to-back inside a box which is open at either end. There is no way for air to be drawn into the system except from the open ends. The edges of the blades go right up to the edges of those openings.
What happens? Is there enough space between the fan blades for air to get sucked in? Do I feel a wind or do the blades rotate inside a box with very low air pressure?
What happens if I use a more powerful engine and more efficient blades - could I crush the box?
Assume the engine/motor is not heating the box.
Thanks! And apologies if the answer is straightforward and boring - I can but ask.
So two ducted fans fighting each other? You’ll generate a vacuum inside the enclosure. If they’re ordinary desktop fans, then it won’t be a very strong vacuum; there’s not a ton of engineering that goes into those blades (other than how to make them as cheaply as possible), and the motors aren’t very powerful.
If you carry this out far enough, you end up with the compressor section of a gas turbine engine, with multiple stages of rotors and stators. You can’t have two actual combustion-based gas turbine engines fighting each other for intake air though, so you’ll just have to bring mechanical shaft power in from somewhere else to drive the stand-alone compressor section on each end of the box. Gas-turbine compressor sections are really good at pulling in air; if you look out an airplane window at the beginning of the takeoff roll, sometimes you can see condensation forming at the engine inlet, indicative of a significant pressure drop (and associated adiabiatic cooling, which causes the condensation). So if you did this, you could expect to generate a pretty substantial pressure drop inside your duct. However, you probably couldn’t generate as great a pressure drop as these compressor sections do when installed on actual gas turbine engines; turbine compressors don’t like high-pressure-differential + low-flow conditions. Compressor stall tends to happen under these circumstances; you’ll see this phenomenon on automotive turbocharges operating at high boost + low engine RPM conditions, and also on jet aircraft operating at high thrust + low airspeed (max pressure drop in front of engine), as during or shortly after takeoff.
Are you postulating spinning fans inside a cylinder with a fan blowing outwards at each end?
If so, the answer is straightforward: you’ll create a low-pressure zone inside the cylinder, but it will be far from a vacuum. As the fans spin, they’ll blow some air outward, lowering the internal pressure. But they’ll very quickly reach equilibrium: they’ll keep blowing a small quantity of air out, but it will be equal to the volume of air that gets sucked in past the blades by the low pressure zone inside the cylinder.
How low the pressure gets depends on the fan blades themselves and how fast they spin. But air will “leak” back in no matter what, even if the fan blades seal perfectly agains the cylinder wall.
In theory, the cylinder could be crushed by the pressure differential, but it would have to be of very flimsy construction for that to happen. Even if the fan blades could pull what’s called a “hard” vacuum, the maximum pressure differential is 14.7 PSI. What’s more, a cylinder happens to be a good shape to resist the pressure differential; a box with a square cross-section and the same material/wall thickness as a cylinder would get crushed much sooner than a cylinder.
But to answer the question I think you’re asking, desktop fans can’t pull much of a vacuum. Not only do they have large gaps between the blades, but many of them use blades without airfoils—they’re just angled or helical blades that “bat” the air they encounter the same way you do when you open your car window and stick your arm out with your hand angled with respect to the air going by. That creates lift, but not nearly as efficiently as an airfoil does.
Interestingly, a specialized vacuum pump called a turbomolecular pump does use fan blades, but a whole lot of them in series. Turbomolecular pumps typically include some angled “batting” blades because, when the internal pressure gets low enough, there aren’t enough gas molecules for an airfoil to have much effect.
The two fans will generate a partial vacuum in the box (like the others said) - once this begins to happen, the rotational speed of the fans will increase - you might think the fans would slow down because they are fighting each other, but they will actually speed up because a)they’re operating in the sparser partial-vacuum atmosphere they have created (which offers less resistance to motion) and b) they are doing less actual ‘work’ - they are not pushing out so much air, so they are not reacting against that outflow.
It’s questions like this that lead to “interesting” results. Richard Feynman once looked at a rotating lawn sprinkler, and asked, “what would happen if you put one in a tank of water and applied a vacuum to the hose?” - basically inverting the setup. That train of thought eventually lead him to a Nobel Prize.
I agree that there would be a partial vacuum between the fans and the system would reach an equilibrium where the air pushed outward by the blades would equal the air which slipped inward past the blades. I think it’s worth pointing out that, once this equilibrium were reached, you would feel no outward airflow in front of each fan. If you put your face in front of one of the fans, you’d see the blades spinning but feel no wind on your face. Whether you could crush the box depends on the efficacy of the blades. Simply increasing the power of the motors wouldn’t be enough if the blades are inefficient. Think about the demonstration where you boil some water inside a metal can and then seal it up while it cools. That creates a partial vacuum, much less than one atmosphere of pressure differential, and it’s more than enough to crush the metal can. With efficient enough blades and powerful enough motors, just a few pounds per square inch might do it.
Is this true? i know there would be no net air movement. Would there be air movement outward on the outer edges of the blades, where the fans move the air the fastest, and inward at the center, replacing the moved air?
Additional question - is the outcome of this back to back fan arrangement any different that having each fan backing onto a sealed box? Imagine that instead of the two fans back to back, there is a solid wall in between them. Wouldn’t the results be the same?
I’m not sure - on the one hand, you’ve got two fans - double the effort - trying to evacuate the box - on the other hand, they’re each fighting one another - I am not sure if those two things balance out, or add up.
when a fan blade can’t move the air, because of back pressure, it stalls.
This is “stall” as in what an airplane does if the wing angle is wrong, Not stall like an engine does if it can’t turn. The wing still moves: the fan still turns: the wind doesn’t generate lift: the fan blade doesn’t generate push.
I just asked a mechanical engineer, who works in industrial HVAC. His response… In a perfect world, 2 fans back to back would result in no air movement. We also could not differentiate between 2 fans back to back, and 2 fans back to back with a barrier between them.
In reality, imperfections, (not perfect seals around the blades, differences in the fans, irregularities in blade speed, etc) mean that there will be air flow of some amount, both outward and back in to replace it.
In spherical-cow land, a wall in the middle shouldn’t have any effect, because the setup is symmetric, and so the net axial air movement should be exactly zero on the entire central plane. In the real world, there’d probably be some turbulent effects there, so the wall would make a difference, but it’d be really difficult to say a priori what the difference would be.
The blades of cheap desktop fans are generally constant-thickness plastic with a relatively sharp edge on them. Sharp-edged airfoils are highly susceptible to stall when operated at any significant angle of attack - so I suspect such fans are operating in stall just about all the time.
An airfoil in stall does not produce zero lift. The lift of an airfoil is mostly due to the pressure drop on the upper surface of the foil, but a significant portion comes from the pressure increase on the lower surface of the lift. In stall, you do lose most of the lift that was being produced by the pressure drop on the upper surface, but you don’t lose any of the lift from the lower surface.
Bottom line, the fans will still generate a pressure drop in the enclosure, even if they aren’t moving any air.
That can’t be right; hamburgers are made of cows and they’re round (Wendys excepted) and chicken nuggets are square-ish. It seems to me you may have cows in your chicken coop…
Dammit, now I want to try the reverse lawn sprinkler experiment.
Oh, and while Feynman definitely won the Nobel Prize, I’ve never heard that the lawn sprinkler thought experiment in any way led to that work. I always understood it to be just another example of him pointing out misconceptions, in people who know how to algebraicly manipulate equations, but don’t understand what the equations actually mean.
I no longer have my copy of his book, but IIRC, you are correct that this was not what lead to the discovery for which he got the Nobel prize. I seem to recall that it had to do with wobbling plates.
Wasn’t the hooking up the sprinkler in reverse a debate among the freshmen in his dorm (?) and they decided to actually try it in one of the labs, but the professor got angry at them?
