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.