By that I mean literally sucking up stuff. The vent fan above my stove only seems to actually pull anything in while it’s on low; setting it to high makes it louder but stops pulling any smoke or steam up through the hood. I’m just curious how the hell that works; shouldn’t a faster spinning fan suck MORE? Is there some property of aerodynamics that was forgotten when they installed this shit?
Fan blades are basically spinning wings or airfoils.
Depending on their design or how expensive they are, they may rely on pushing air rather than aerodynamic effects at low speeds, and they’re always optimized for a specific rpm.
As it speeds up, the aerodynamic flow takes over, with the rotors creating a pressure differential that pulls air through.
As it gets faster and faster, eventually, that pressure differential reaches the next rotor and the entire thing stops being as effective because now the the second rotor is stalled out. (Only they’re all stalled out because any given rotor is both leading and trailing.)
Said another way, each rotor is passing through the wake of the previous rotor and not pushing more air because the air is already moving with the wing.
I like this answer. The only thing I would add is that when the fan blades are all stalled, it might seem then that drag and energy consumption should reduce, since there’s not much air moving. But in a cruel twist (fan pun intended) of aerodynamics, the useless spinning of stalled fan blades still causes parasitic drag. So not only does the fan not move air, it’s also consuming more energy than spinning a solid disk of the same moment-of-inertia.
When the engine fails for certain single-propeller aircraft, there’s sometimes a mechanism to lock the propeller to make it stop rotating, since it would otherwise “windmill” in the air and waste the precious kinetic energy that’s keeping the plane aloft. Or so I’m told.
that’s a great addon, thank you.
Side note; that’s why they get noisier, too.
deleted by creator
Constant Speed is probably what I was thinking of. And speaking of multi engine failure, you’ve just reminded me of the demise of TransAsia Airways Flight 235 where the right engine feathered itself erroneously, but then the crew misdiagnosed the situation and shut down the left engine. Mentour Pilot made a video on that particular accident.
This was a really good explanation. Ty
Maybe the output side is clogged up and the attempt to push more air through than it can handle causes turbulence that results in slower air flow, or affects what air it is sucking up so that it pulls more from the sides rather than below.
Have you tried changing the filters?
I’m guessing you probably don’t have the air intake part of the equation. A lot of newer, more powerful hoods require an air exchanger that blows in outside air to equalize the pressure from the hood blowing air out.
The faster the hood blows, the more it struggles to pull air as the pressure in the room drops. An exchanger fixes that.
At worst though, it should be pulling in as little air as when on low, not less.
I think it initially does, but then the difference in pressure basically works against it, as the pressure inside drops
deleted by creator
Cavitation occurs in a liquid. When you’re pushing air, you may bump into other types of problems like stalling. If the fan is connected to a pipe, you can also get repeating surges of airflow.
Does it exhaust through the wall or just back into the room? Is is set up right?
Every microwave hood that’s come with the apt or home I’ve had has always been installed wrong and has been blocking airflow.
It’s in an apartment so that might be it.
deleted by creator