4d ago

Can someone explain to me how a ventilation fan can suck LESS when on high compared to low?

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?

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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.
  
  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. (I believe some propellor are able to have their pitch changed by a maintainer on the ground so the aircraft can be optimized for climbing, cruising or a combination of both)
  'Reverse Thrust' is also possible in so e Other aircraft. the blade reverses to the point where it pushes the wrong way so the engines assist breaking* after landing. * Typo, but I'm leaving it in =)
- This was a really good explanation. Ty

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

Not exactly a scientist, and I agree with the other long answer about blades disrupting each other, so I'm replying to see if I can help you visualize/understand some of the forces at play.
My source is a Tom Clancy book about a sub war.
Typically the subs are stealthy, however if I remember correctly, I'd they decided to gun the engines (think something like 105% on the reactor), they could spin the blades really fast.
But compressing the water as the blade spins makes some of it high pressure and the other side of the blade low pressure.
This would reduce the pressure low enough that the water would become a gas and in doing so, make a lot of noise and was called cavitation.
When the sub commander did this, it did mean the ship could go very fast, but was noisy as hell and anyone near would know they were around, but if someone shot a torpedo at you and you had to get out of range.....
Before readying the other reply I thought something like this might have happened and disrupted the smooth laminar airflow.
I know aircraft have over speed and under speed warnings for propellors too, but I thought that was more to do with not breaking them than 'grabbing the air ' efficiently. Supersonic propellor tips aren't a good idea.
- 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.

https://en.m.wikipedia.org/wiki/Stall_(fluid_dynamics)

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