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?
The probably if you probably don’t have the intake part of the equation. A lot of more 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.
FuglyDuck@lemmy.world 20 hours ago
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.)
litchralee@sh.itjust.works 19 hours ago
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 previous kinetic energy that’s keeping the plane aloft. Or so I’m told.
AnAustralianPhotographer@lemmy.world 14 hours ago
That could happen with propellors that have Constant Speet Units. (Propellor pitch is able to be changed) The act is called ‘feathering’ . This can happen on multi engine aircraft and reduce the drag of the side with the failed engine. Cheaper propellors are fixed pitch and no means exist to change them.
FuglyDuck@lemmy.world 18 hours ago
that’s a great addon, thank you.
Side note; that’s why they get noisier, too.
afk_strats@lemmy.world 19 hours ago
This was a really good explanation. Ty