Fan is pushing in the front and sucking from the back

[johnthekid]

A standing fan is blowing air when we are right in front of it but we noticed there are dirts on the back of the fan. So, at the front, a standing fan pushes air and from the back the fan is sucking. Is this correct? If it does, why?

 

[Ibix]

Yes. Where else would it get the air to push at you except from behind it?

 

[russ_watters]

A standing fan is blowing air when we are right in front of it but we noticed there are dirts on the back of the fan. So, at the front, a standing fan pushes air and from the back the fan is sucking. Is this correct?

Yes.

If it does, why?

I'm not sure what sort of answer you're looking for with that question. The blades are curved to force air to move in a preferred direction.

 

[johnthekid]

Yes. Where else would it get the air to push at you except from behind it?

Yes.

I'm not sure what sort of answer you're looking for with that question. The blades are curved to force air to move in a preferred direction.

Sorry, I don't really get the mechanism of fans so I was curious at first. Thank you for these responses.

 

[johnthekid]

Fans also compress air but not at the same level as specialized air compressors, is that so?

 

[russ_watters]

Fans also compress air but not at the same level as specialized air compressors, is that so?

Yes, though for your typical desk or central air conditioning fan the air can be considered incompressible for calculation purposes. The key similarity then is that they both move air by creating a pressure difference from one side to the other (yes, it's a contradiction to say you can create a pressure difference without compression, but it is ignored because it doesn't affect the calculations).

 

[berkeman]

I don't really get the mechanism of fans so I was curious at first.

Cheaper fans use flat blades at an angle. Better fans use airfoils at an angle (like a propeller). Here is a good PF Insights article that may give you more of a feeling for how air moves around a fan/propeller and why the airflow over the back surface is slower/lower pressure (and may lead to more dust accumulation): https://www.physicsforums.com/insights/airplane-wing-work-primer-lift/
(Edited)

 

[A.T.]

A standing fan is blowing air when we are right in front of it but we noticed there are dirts on the back of the fan.

the airflow over the back surface is slower/lower pressure (and may lead to more dust accumulation)

The OP seems to describe the opposite observation: More dust on the side with faster flow / lower pressure. This is also consistent with my observations on PC case fans.

My intuitive idea was always that there is more turbulence / acceleration of air on the faster flow side, so the dust falls out in certain flow pockets. The dust distribution is not uniform, but has very specific patterns.

The internet also mentions different amounts of static electricity due to the different flow dynamics. But I'm not sure about that.

 

[johnthekid]

The OP seems to describe the opposite observation: More dust on the side with faster flow / lower pressure. This is also consistent with my observations on PC case fans.

My intuitive idea was always that there is more turbulence / acceleration of air on the faster flow side, so the dust falls out in certain flow pockets. The dust distribution is not uniform, bus has very specific patterns.

The internet also mentions different amounts of static electricity due to the different flow dynamics. But I'm not sure about that.

Right. I was slightly confused.

 

[Lnewqban]

A standing fan is blowing air when we are right in front of it but we noticed there are dirts on the back of the fan. So, at the front, a standing fan pushes air and from the back the fan is sucking. Is this correct? If it does, why?

The blades of any type of fan transfer kinetic energy into the mass of gas that is surrounding them.
That energy is observable as difference of velocity and static pressure between upstream and downstream of each blade.

Please, see:
https://en.wikipedia.org/wiki/Fan_(machine)

https://en.wikipedia.org/wiki/Axial_fan_design

 

[berkeman]

The OP seems to describe the opposite observation: More dust on the side with faster flow / lower pressure. This is also consistent with my observations on PC case fans.

Right. I was slightly confused.

Ah, got it. Well, I just checked our portable fan (which has flat blades) and indeed the dust accumulation is on the front of the fan, not the back. It must be due to impacting the air on the front side where the air is pushed away from the fan.

 

[Baluncore]

Referring to the front and back can be confusing because it depends on how you look at it. Maybe refer to the air inlet and outlet, the inlet side of the blade and the outlet side of the blade.
Dust accumulates faster on the outlet side of the blade, because, as it pushes the air through the outlet, that side of the blade is hitting particles suspended in the air.

 

[A.T.]

More dust on the side with faster flow / lower pressure. This is also consistent with my observations on PC case fans.

My intuitive idea was always that there is more turbulence / acceleration of air on the faster flow side, so the dust falls out in certain flow pockets. The dust distribution is not uniform, but has very specific patterns.

I opened my PC case and checked it again: Contrary to what I wrote above there is more dust on the side with slower flow / higher pressure, but the dust is more uniformly distributed here, so it doesn't stand out visually.

The dust on the side with faster flow / lower pressure is mostly near the blade tips (which are moving fastest), right behind the leading edge. Hence my idea about turbulence / acceleration causing deposition there.

 

[johnthekid]

Referring to the front and back can be confusing because it depends on how you look at it. Maybe refer to the air inlet and outlet, the inlet side of the blade and the outlet side of the blade.
Dust accumulates faster on the outlet side of the blade, because, as it pushes the air through the outlet, that side of the blade is hitting particles suspended in the air.

Just checked my fan, there are dust on the blades back and front and also on the air outlet (front) and inlet (back). Basically almost every parts of the fan. Its just for some reasons there are slightly a bit more dust on my fan's air inlet, including the motor area, than on the air outlet. So, dust are both sucked and blown away when the fan is operating. My initial thought is that dust are only sucked from behind when the fan is working.

 

[Herman Trivilino]

I opened my PC case and checked it again: Contrary to what I wrote above there is more dust on the side with slower flow / higher pressure, but the dust is more uniformly distributed here, so it doesn't stand out visually.

The dust on the side with faster flow / lower pressure is mostly near the blade tips (which are moving fastest), right behind the leading edge. Hence my idea about turbulence / acceleration causing deposition there.

This dust pattern is also observed on the flat blades of ceiling fans. Well known to those of us fighting the never-ending battle of keeping them clean.

 

[Herman Trivilino]

So, at the front, a standing fan pushes air and from the back the fan is sucking. Is this correct?

Not to me. The fan pushes air at the front but it doesn't suck air from the back. It creates a lower pressure on the back side and thus the atmosphere pushes air into that region of lower pressure.

 

[johnthekid]

Not to me. The fan pushes air at the front but it doesn't suck air from the back. It creates a lower pressure on the back side and thus the atmosphere pushes air into that region of lower pressure.

It seems to be kind of looks like more of a semantic issue.

 

[weirdoguy]

It creates a lower pressure on the back side and thus the atmosphere pushes air into that region of lower pressure.

Isn't that 'sucking' by definition?

 

[russ_watters]

It seems to be kind of looks like more of a semantic issue.

Isn't that 'sucking' by definition?

Yeah, some people don't like the implication of negative pressure in the word; Absolute pressure can't be negative. But to engineers all fluid pressure is relative so you can have negative pressure (or positive vacuum!), thus a vacuum "sucks". Semantic preference aside, it's something frequently seen in the field so it needs to be understood both ways.

 

[Herman Trivilino]

It seems to be kind of looks like more of a semantic issue.

Isn't that 'sucking' by definition?

Not to me. For example you don't suck fluid up through a drinking straw. You lower the pressure at the top of the straw so that the atmosphere can push the fluid up the straw.

Vacuum cleaners are another example. They don't suck the dirt into the machine. They lower the pressure so that the atmosphere can push the dirt into the machine.

But to engineers all fluid pressure is relative so you can have negative pressure (or positive vacuum!), thus a vacuum "sucks".

I agree that the notion of negative gauge pressure needs to be understood. But without the surrounding atmosphere there would be no negative gauge pressure and thus things like drinking straws and vacuums wouldn't function.

Either way you look at it you need the surrounding atmosphere to push the fluid. If one wants to refer to that as "sucking" that would of course be a matter of semantics, but it can leave the learner with a fundamental misunderstanding of the physics of how these devices work.

 

[Lnewqban]

I opened my PC case and checked it again: Contrary to what I wrote above there is more dust on the side with slower flow / higher pressure, but the dust is more uniformly distributed here, so it doesn't stand out visually.

The dust on the side with faster flow / lower pressure is mostly near the blade tips (which are moving fastest), right behind the leading edge. Hence my idea about turbulence / acceleration causing deposition there.

At the tip of each blade, there is also a reversed flow or vortex moving air and dust from downstream to upstream due to pressure differential.

 

[russ_watters]

Not to me. For example you don't suck fluid up through a drinking straw. You lower the pressure at the top of the straw so that the atmosphere can push the fluid up the straw.
....

Either way you look at it you need the surrounding atmosphere to push the fluid. If one wants to refer to that as "sucking" that would of course be a matter of semantics, but it can leave the learner with a fundamental misunderstanding of the physics of how these devices work.

I mean, to me that's just rejecting the word "suction" without recognizing what it actually means. I don't see how understanding the definition can cause a misunderstanding. But maybe that's part of the semantics.

 

[A.T.]

At the tip of each blade, there is also a reversed flow or vortex moving air and dust from downstream to upstream due to pressure differential.

Yes, that's what I meant by turbulence.

 

[Herman Trivilino]

I mean, to me that's just rejecting the word "suction" without recognizing what it actually means. I don't see how understanding the definition can cause a misunderstanding. But maybe that's part of the semantics.

Emphasizing that the atmosphere pushes the fluid up your drinking straw is part of understanding how the process works. Describing it as sucking the fluid glosses over that fact and students don't get an understanding of what it means to "suck".

Anyway, describing it as a push by the atmosphere instead of a suck by the drinker is the way it's done in college-level introductory textbooks.

I do take your point that that's what it means to suck, but I don't think an uninitiated student would understand that meaning.

Here's another example. To treat an ear infection it is sometimes necessary to place a tube in the ear to promote drainage of fluid trapped behind the ear drum. What does that tube do? Does it allow atmospheric pressure to enter the ear and push the fluid out? Or does it allow your Eustachian tube to suck it out?

I suppose you'd say it's a semantic difference, but without the atmosphere pushing on the air in the inserted tube the Eustachian tube would not be able to "suck" the fluid out, making the inserted tube useless.

 

[russ_watters]

Describing it as sucking the fluid glosses over that fact and students don't get an understanding of what it means to "suck".

Right, that's why I'd teach the proper definition.

Here's another example. To treat an ear infection it is sometimes necessary to place a tube in the ear to promote drainage of fluid trapped behind the ear drum. What does that tube do? Does it allow atmospheric pressure to enter the ear and push the fluid out? Or does it allow your Eustachian tube to suck it out?

I suppose you'd say it's a semantic difference, but without the atmosphere pushing on the air in the inserted tube the Eustachian tube would not be able to "suck" the fluid out, making the inserted tube useless.

You wrote that backwards, didn't you? I think fluid goes out through the inserted tube, not through the Eustacian tube.

Anyway, the main reason the word is used/useful is it describes the goal or which side of the device is the main point of the device. You stand in front of a fan (blows) and behind a vacuum pump (sucks). It's a single-word way to describe that.

 

[Herman Trivilino]

You wrote that backwards, didn't you? I think fluid goes out through the inserted tube, not through the Eustacian tube.

Not backwards but I did get it a bit wrong. The fluid drains out, not through the inserted tube or the eustation tube, but into the ear canal and out of the ear.

Ear tubes (tympanostomy tubes) allow fluid to drain from the middle ear creating a new pathway through the eardrum directly into the ear canal, bypassing the blocked Eustachian tube to relieve pressure and prevent infection. The fluid exits the ear externally, often seen as clear or cloudy drainage, indicating the tube is working effectively.

So rephrasing my original question, what is the function of inserted ear tube? I'm saying that it allows the atmosphere to push the fluid out from behind the drum into the ear canal.

Are you claiming that the ear canal "sucks" the fluid out from behind the ear drum? And that stating it that way is only a semantic difference? If so, what is the function of the tube? And if there were no atmospheric pressure the fluid would get "sucked" out anyway?

I'm saying that the fluid is not being sucked out, which is the entire reason for the doctors to insert the tube. Because the tube provides a path for the atmosphere to push the fluid out.

It's not a semantic difference. It's a correct description of the purpose of the tube.

 

[russ_watters]

Not backwards but I did get it a bit wrong. The fluid drains out, not through the inserted tube or the eustation tube, but into the ear canal and out of the ear.

So rephrasing my original question, what is the function of inserted ear tube? I'm saying that it allows the atmosphere to push the fluid out from behind the drum into the ear canal.

Are you claiming that the ear canal "sucks" the fluid out from behind the ear drum? And that stating it that way is only a semantic difference? If so, what is the function of the tube? And if there were no atmospheric pressure the fluid would get "sucked" out anyway?

I'm saying that the fluid is not being sucked out, which is the entire reason for the doctors to insert the tube. Because the tube provides a path for the atmosphere to push the fluid out.

it's not a semantic difference. It's a correct description of the purpose of the tube.

I mean, that's still pretty much a mess, so you're arguing against your point. The tube is a pressure relief device. Excess pressure in the inner ear is relieved by the tube, allowing fluid to drain out through the tube. The atmosphere isn't doing the pushing (the fluid is escaping TO the atmosphere), the excess pressure in the inner ear is doing the pushing. There's no "sucking" going on.

I'll say again: the reason "suck/suction" or "blow" is used is it's describing an action by a device. The atmosphere is the steady thing that is always/already there, so it's ignored. Then you add a device to make a change and describe what that device is doing. You wouldn't say the atmosphere is sucking fluid out of the ear - that makes no sense. Similarly, saying "the atmosphere pushes air/dirt into a vacuum cleaner(fan)" completely ignores what the vacuum cleaner(fan) is doing to make that happen. That's why it's a less useful/descriptive and more cumbersome way to describe the situation and that's why engineers don't say it that way.

 

[russ_watters]

Hopefully the OP is still finding this instructive....

Then you add a device to make a change and describe what that device is doing. Saying "the atmosphere pushes air/dirt into a vacuum cleaner(fan)" completely ignores what the vacuum cleaner(fan) is doing to make that happen. That's why it's a less useful/descriptive and more cumbersome way to describe the situation and that's why engineers don't say it that way.

Here's an example clip from a fan balancing report I was looking at yesterday:

Setting aside that I don't think there's a single-word you could use as a label in place of "suction" to say "the atmosphere pushing-in side of the fan", there's a real practical reason why we don't say that. It's because we neither know nor care about what atmospheric pressure is, and it would be cumbersome to measure, report and use in the calculations. That's why we ignore/zero-out atmospheric pressure when measuring or describing fan performance.

 

[Herman Trivilino]

Excess pressure in the inner ear is relieved by the tube, allowing fluid to drain out through the tube.

No. The fluid runs out of the ear through the ear canal. Read my post again.

 

[renormalize]

... The tube is a pressure relief device. Excess pressure in the inner ear is relieved by the tube, allowing fluid to drain out through the tube. ...

View attachment 368723

No. The fluid runs out of the ear through the ear canal. Read my post again.

@Herman Trivilino, do you disagree with the illustration? It clearly depicts the fluid to be flowing through the ear tube in order to reach the ear canal.