Why Is Fan Suction/Intake So Weak Compared to Exhaust?

[kyphysics]

I’ve Googled this and the general responses I’ve seen are that a fan’s intake is designed such that it “sucks in” air from all around it (left, right, up, down … with a very wide area that it draws from), but it exhausts/shoots out air in a “jet stream.”

But why is that? I’ve looked at a fan’s blades and frame and it still doesn’t make sense. Like my house fan’s exhaust/exit part is the same size as the intake. It’s not like the exit is “focused” through a narrow tube to concentrate the path or strength of the air. But, clearly, every fan does have stronger exhaust. If you stand it front of it, you can feel strong wind, but if you stand behind the fan, you literally notice nothing. You need to place a piece of lightweight tissue paper near the back of fan to even notice any air suction.

If it’s not the exhaust frame (like a narrow opening/tube) that creates a much stronger force, what is it that does this?

 

[Baluncore]

The fan lowers the pressure at the inlet. Air from all directions begins to flow toward the fan, turning gradually as it accelerates, onto the axis of the fan.

Once accelerated by the fan, the air has momentum that carries it away from the fan outlet, without any need to immediately spread out again.

 

[jbriggs444]

It feels like there must be a deeper reason for this.

In principle, the laws of physics are symmetric and time reversible. If the input to a fan is omnidirectional, there is no reason in naive principle why the output of a fan could not also be omnidirectional.

But in fact the output is directional. So it would seem that entropy has to figure in somehow.

A fair bit of skull sweat has gone into analyzing the conundrum. https://en.wikipedia.org/wiki/Feynman_sprinkler

 

[256bits]

So it would seem that entropy has to figure in somehow.

Should not the entropy from the coal power plant supplying the electricity to the motor to turn the fan more than make up the reduced entropy of the directional entropy of the blown air.

In fact, vegetation from millions of years ago converted the low entropy sun rays that hit the earth into energy for growth. We now use the stored energy from the dead vegetation as fuel for the coal fired power plant and shed high entropy heat into space.

 

[jbriggs444]

Should not the entropy from the coal power plant supplying the electricity to the motor to turn the fan more than make up the reduced entropy of the directional entropy of the blown air.

Yes, of course.

With a little bit of time to ponder, I have a different way of explaining this.

"Why was there not a directional flow of air directly into the intake of the fan?"

Because we did not set one up, of course. Out of all the possible configurations of air feeding into a fan, we automatically assumed the one with the simplest macroscopic description. A body of air with uniform temperature and pressure, stationary everywhere. Because that is how a body of air ends up if one does not disturb it. It is overwhelmingly likely to be found in a state with maximal entropy.

We have this fan. It adds macroscopic organization that had not been present previously.

"Why is there a directional flow of air from the output of the fan?"

Because the air does not immediately relax back into its maximally disordered state. It takes time for the metaphorical deck to become completely shuffled, one riffle at a time.

 

[DaveC426913]

Yes, as an open system, electricity enters the system and is converted to kinetic energy as air flow.

If you time-reverse it, you essentially have a wind turbine that extracts energy from fast moving air (slowing it) and converts it to electricity as output.

That's one aspect. The other is the entrainment and focus of air flow.

 

[berkeman]

Interesting YouTube video tracing out the airflow around a fan:

 

[marcusl]

Baluncore's comment is a large part of the answer. It is possible to have powerful air currents on the suction side by simply fitting a duct to the inlet. In fact, the usable regions of some of the wind tunnels at NASA Ames are located on the suction side. This includes the 80x120 foot tunnel, which is the world's largest wind tunnel--it's on the suction side of the giant fans that power the 40x80 foot tunnel.

 

[Ken Fabian]

Thinking about this, (without referencing any experts sources) it seems that vacuum is an absolute limit on the suction side but there are no hard limits to the compression. I don't know to what extent a fan can disperse air non-directionally at low velocity rather than make a fast stream of air

 

[DaveC426913]

Thinking about this, (without referencing any experts sources) it seems that vacuum is an absolute limit on the suction side but there are no hard limits to the compression. I don't know to what extent a fan can disperse air non-directionally at low velocity rather than make a fast stream of air

I think the theoretical upper limit would be Mach 1.

 

[jbriggs444]

I think the theoretical upper limit would be Mach 1.

It is difficult to get a prop-driven airplane to exceed the speed of sound, yes. But a centrifugal fan should be able to produce a supersonic outflow.

 

[A.T.]

But a centrifugal fan should be able to produce a supersonic outflow.

There are continuous supersonic wind tunnels after all.

 

[A.T.]

I’ve looked at a fan’s blades ...

If it’s not the exhaust frame (like a narrow opening/tube) that creates a much stronger force, what is it that does this?

It is the design of the fan blades. They are not designed to accelerate the air uniformly in all directions, but rather with a strong bias towards the axial direction.

You can design the blades to have the opposite effect, and just spread the air radially (like a centrifugal pump, but without the casing). Here the flow speed at the central inlet will be higher than of the outgoing radial flow at the periphery.

 

[sophiecentaur]

But why is that? I’ve looked at a fan’s blades and frame and it still doesn’t make sense. Like my house fan’s

I have a simple explanation / description. At the input there is a maximum change in pressure of one atmosphere, which, even ideally, fundamentally limits the mass flow through an aperture.

Air from the output can be at any pressure your pump / fan can make it, so the pressure difference can be many atmospheres. This pressure can cause all the air mass to flow through a small and directional nozzle.

Hold a candle in front of a large loudspeaker cone and the flame is blown away from the cone. Pushed in from all directions and pushed out forwards.

So Blow is stronger than suck (in sloppy terms)

 

[russ_watters]

I have a simple explanation / description. At the input there is a maximum change in pressure of one atmosphere, which, even ideally, fundamentally limits the mass flow through an aperture....

So Blow is stronger than suck (in sloppy terms)

While that's can be true, for a desk fan the associated pressure change is very close to zero (vs atmospheric), so it's not a limiting factor and it's not an explanation of the phenomena.

 

[Ken Fabian]

I think it is as simple as the explanation found unsatisfactory in the first para of the initial post -

the general responses I’ve seen are that a fan’s intake is designed such that it “sucks in” air from all around it (left, right, up, down … with a very wide area that it draws from), but it exhausts/shoots out air in a “jet stream.”

Put the fan in a duct and the flow rates would be the same on either side of the fan. For the un-ducted fan the closer to the fan we get the less difference between the inlet and outlet side

 

[Baluncore]

Because kinetic energy, KE = ½·m·v², less energy is needed to draw in a fixed mass of air from a hemispherical shell at velocity v, than from half a hemispherical shell at velocity 2v. That can explain why air is drawn radially into a fan from all available directions.

If air was to enter the fan as a jet, then what would selectively accelerate only that jet, in a direction towards the fan inlet? How far back would the jet form?

 

[russ_watters]

Here's the way I would say it, following from our thread on suction: The definition of suction is the removal of air (in this case) from the suction side of the fan, creating an area of low pressure. This low pressure area/imbalance is addressed by the atmosphere pushing air towards it from all directions. The discharge side of the fan, however, is not merely a high pressure area, it's throwing the air away from it in a specific direction.

Maybe another way: The suction side of the fan creates a negative static pressure whereas the discharge side is creating a positive velocity pressure.

 

[sophiecentaur]

the associated pressure change is very close to zero

Yebbut Newton 1 will always apply.

If air was to enter the fan as a jet,

That would be like herding cats (or sheep) unless you contained the air in some sort of conical inlet funnel. Is that allowed in the OP's rules?

 

[sophiecentaur]

The suction side of the fan creates a negative static pressure whereas the discharge side is creating a positive velocity pressure.

The actual details will make a difference but, for the simplest model, the inlet aperture will have a maximum pressure difference of 1At and air will flow in from all directions. The actual mechanism is very complicated and PF spends days and days discussing things like 'how a plane flies' etc.. The output port will have an unlimited positive pressure and the direction of flow can be whatever you choose. If you use a fan or just use an unspecified supply of compressed air you can direct the air as you choose. A room fan (or aeroplane etc) does its best to direct the air efficiently and that is more a matter of selecting and directing the added momentum. Using a port of the right shape (modern jet engine) does this and the propulsion is more due to the ported fan than just the combustion gases; you could do similar with an electric motor.

 

[Majorana]

I’ve Googled this and the general responses I’ve seen are that a fan’s intake is designed such that it “sucks in” air from all around it (left, right, up, down … with a very wide area that it draws from), but it exhausts/shoots out air in a “jet stream.”

But why is that? I’ve looked at a fan’s blades and frame and it still doesn’t make sense. Like my house fan’s exhaust/exit part is the same size as the intake. It’s not like the exit is “focused” through a narrow tube to concentrate the path or strength of the air. But, clearly, every fan does have stronger exhaust. If you stand it front of it, you can feel strong wind, but if you stand behind the fan, you literally notice nothing. You need to place a piece of lightweight tissue paper near the back of fan to even notice any air suction.

If it’s not the exhaust frame (like a narrow opening/tube) that creates a much stronger force, what is it that does this?

I definitely remember to have read, in several technical aviation magazines, that in a jet engine (both turbofans and turbojets) the total thrust comes predominantly from the intake, and only in lesser percentage from the exhaust. Noteworthy, I think...

 

[Baluncore]

I definitely remember to have read, in several technical aviation magazines, that in a jet engine (both turbofans and turbojets) the total thrust comes predominantly from the intake, and only in lesser percentage from the exhaust.

I think you should check your memory.
If you look at the balance of momentum, in a turbojet engine, the net thrust comes from the output (exhaust).
With a high bypass turbofan, greater thrust will come from the differential pressure across the first compressor stage, that is, a ducted propeller.

 

[Majorana]

I think you should check your memory.
If you look at the balance of momentum, in a turbojet engine, the net thrust comes from the output (exhaust).
With a high bypass turbofan, greater thrust will come from the differential pressure across the first compressor stage, that is, a ducted propeller.

I am definitely sure about my memory, and in the meanwhile I remembered better. One of the sources - not a magazine - were some books about the SR-71 Blackbird (equipped with the Pratt & Whitney J-58 engines) written by pilots and engineers that flew that aircraft. Another source is a classic reference book in aviation, "The Jet Engine", by Rolls-Royce. I just checked it, 5th edition, chapter 20 "Thrust distribution" (pages 207 to 213).

 

[sophiecentaur]

the total thrust comes predominantly from the intake,

That 'bald' statement has implications that don't really make sense. Taken in full context there could be some sense but it implies that the engine 'sucks itself along'.

 

[Majorana]

That 'bald' statement has implications that don't really make sense. Taken in full context there could be some sense but it implies that the engine 'sucks itself along'.

I am not any fluid mechanics physicist, much less I intended to baldly state anything. I only cited the statements found in the specialized press, written by engineers. You should argue with them, not with me.

 

[sophiecentaur]

You should argue with them, not with me.

I am only arguing with the small subset of uncited information that you are providing to sustain your view. Give us some substantial evidence and we will be able to discuss it.

 

[Majorana]

I am only arguing with the small subset of uncited information that you are providing to sustain your view. Give us some substantial evidence and we will be able to discuss it.

The books on the SR-71 Blackbird are in another house and I cannot get them right now, but they are all well known (look in any of the books by Ben Rich or Brian Shul). The book from Rolls-Royce, "The Jet Engine", I clearly cited the edition, chapter and page number, you can read it by yourself.

 

[Baluncore]

The book from Rolls-Royce, "The Jet Engine",

That analysis makes some questionable assumptions.
11. When applying the above method to calculate the individual thrust loads on the various components it is assumed that the engine is static. ...
12. ... Since the pressure and the velocity at the inlet to the compressor are zero, it is only necessary to consider the force at the outlet from the compressor.

 

[Majorana]

That analysis makes some questionable assumptions.
11. When applying the above method to calculate the individual thrust loads on the various components it is assumed that the engine is static. ...
12. ... Since the pressure and the velocity at the inlet to the compressor are zero, it is only necessary to consider the force at the outlet from the compressor.

I am unable to dispute an analysis from Rolls-Royce engineers. It's my educate guess that they should know quite something about jet engines. Have a happy weekend!

 

[A.T.]

the total thrust comes predominantly from the intake, and only in lesser percentage from the exhaust.

It's not clear what exactly you mean here. The net thrust is related to the difference in flow speed between inflow and exhaust

Another source is a classic reference book in aviation, "The Jet Engine", by Rolls-Royce. I just checked it, 5th edition, chapter 20 "Thrust distribution" (pages 207 to 213).

The book is talking about the local axial gas forces on the engine parts. The axial forces on the tubine and nozzle point backwards (see Fig 20-1), so these axial forces are not contributing to thurst "in lesser percentage", but are rather opposing thurst.