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Old 10-22-2019, 10:15 PM
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Why Don't Fans Suck As Strong They Blow?


Maybe the answer is obvious and eludes me, but: Why is it that a fan's suction side seems nowhere near as strong as the stream leaving the fan? And yet, wouldn't the flow rate in = flow rate out? Seems logical to me. Why is that not so?
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Old 10-22-2019, 10:23 PM
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An old thread: https://boards.straightdope.com/sdmb...d.php?t=435479
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Old 10-23-2019, 07:30 AM
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The other thread that Reply linked to explains it nicely: intake air is sucked in from all directions, and outflow air is all pushed out in one direction. You see this phenomenon any any pump inlet and outlet: fans, vacuum cleaners, and even jet engines. For jet engines, it helps explain why foreign objects lying on the runway or carrier deck are a major concern: with intake air getting sucked in from every direction, junk lying around can literally get sucked up off of the ground and into the intake as the aircraft rolls by. Here for example is an F-16 parked on the ramp and undergoing some engine tests. The ramp is wet and the air is humid. The engine is creating so much suction in front of the intake that a miniature tornado forms, with pressure so low that visible condensation of the air occurs. You can see how that tornado bends down toward the ground, and it's actually sucking up water from the puddles there. Now imagine there is loose change or a screwdriver on the ground; there's a fair chance it's going to get drawn up into the intake and cause some expensive damage.

Here is an aircraft carrier crew performing a FOD walk, inspecting the flight deck for loose debris that could potentially cause a very bad day for a pilot.
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Old 10-23-2019, 07:59 AM
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Quote:
Originally Posted by Machine Elf View Post
The other thread that Reply linked to explains it nicely: intake air is sucked in from all directions, and outflow air is all pushed out in one direction. You see this phenomenon any any pump inlet and outlet: fans, vacuum cleaners, and even jet engines. For jet engines, it helps explain why foreign objects lying on the runway or carrier deck are a major concern: with intake air getting sucked in from every direction, junk lying around can literally get sucked up off of the ground and into the intake as the aircraft rolls by. Here for example is an F-16 parked on the ramp and undergoing some engine tests. The ramp is wet and the air is humid. The engine is creating so much suction in front of the intake that a miniature tornado forms, with pressure so low that visible condensation of the air occurs. You can see how that tornado bends down toward the ground, and it's actually sucking up water from the puddles there. Now imagine there is loose change or a screwdriver on the ground; there's a fair chance it's going to get drawn up into the intake and cause some expensive damage.
Also see the inlet of a big turbofan when spooling up:

https://youtu.be/cA6_U9uGHA0
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Old 10-23-2019, 08:29 AM
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Since the question has been answered, I’ll point out that fans of the Montreal Canadians do indeed suck as well as they blow. ;-)
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Old 10-23-2019, 09:15 AM
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This has always intrigued me. The answer that the air on the intake is coming from all possible directions, whereas it's concentrated and directed on the outlet is fairly obvious, IMO. The other thread also discusses why that's the case, which starts getting kind of complicated. What I'm curious about is if there's any possible way to reverse the phenomenon such that the intake is directed and the outlet is diffused? Sure you can use baffles and vanes to diffuse the outlet, basically chopping it up as it exits. If you use an electrostatic(?) "blower" such as in one of those fanless ionic air purifiers, would the outlet air be diffused the same way as the intake if the wires were just out in the open rather than in a case to direct the airflow? It sounds to me like it may still "line up" the airstream on the outlet.

In either of these cases, the inlet is still totally diffused. I just find the notion that moving a fluid from low pressure to high pressure creates this different response on either side. Fluid dynamics eh? I assume liquids behave the same, though we don't usually observe it since water, for instance, is either totally contained on both sides in pipes, or it's pumped into air which changes the equation. Still, you don't see "streams of suction" in water either.
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Old 10-23-2019, 09:52 AM
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Quote:
Originally Posted by jjakucyk View Post
What I'm curious about is if there's any possible way to reverse the phenomenon such that the intake is directed and the outlet is diffused?
You mean like a turbo expander ? Where the gas goes from high pressure to low pressure ?

Quote:
I assume liquids behave the same,....
Liquids are incompressible and donít behave the same.

I am having a hard time parsing your thread. It has some misunderstandings stated as facts.
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Old 10-23-2019, 09:53 AM
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Originally Posted by jjakucyk View Post
What I'm curious about is if there's any possible way to reverse the phenomenon such that the intake is directed and the outlet is diffused?.
That's what vacuum cleaners do. Some of them direct the exhaust also, but others like some shop vacs disperse the exhaust out of multiple vents. Shop dust collectors may exhaust into a large exposed filter bag. The problem is that nothing is physically impelling the input air so not very far from the nozzle the air will get pulled in from all directions, but inside the vacuum cleaner hose the air is very directed by constraint and moving at high speed.

Last edited by TriPolar; 10-23-2019 at 09:53 AM.
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Old 10-23-2019, 10:08 AM
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I was about to state that, although a gas can be compressed hundreds of times ambient...whatever the barometer says...the lowest pressure is of course high vaccuum.

0X<X<500X

But that's not the question, is it.
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Old 10-23-2019, 10:12 AM
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Originally Posted by am77494 View Post
Liquids are incompressible and don’t behave the same.
Liquids do behave the same as has been described in this situation: low pressure at a liquid intake results in liquid being drawn in from all directions, and outflow momentum results in liquid being expelled all in a single direction. This is why pop-pop toy boats work.

Quote:
Originally Posted by TriPolar
others like some shop vacs disperse the exhaust out of multiple vents.
Ridgid sells an exhaust attachment for their shop vacs that diffuses the exhaust. This is handy when you don't want a high-speed exhaust plume blowing things around in your shop or garage. The pic at that link shows a caged chunk of porous foam at the free end of the diffuser. If you remove the foam, you find that the cage is holding a small cone out at the distal end, with the apex of the cone pointed toward the shop vac. The exhaust air ends up being directed radially instead of axially, so the flow cross section quickly grows and results in nicely reduced velocities just a few inches away from the diffuser.

AFAIK there's nothing comparable for un-diffusing the inlet flow.

Last edited by Machine Elf; 10-23-2019 at 10:13 AM.
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Old 10-23-2019, 10:16 AM
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Originally Posted by Enola Straight View Post

But that's not the question, is it.
Itís hard to understand the question, I agree.

Also in condensing steam turbines, the inlet to the turbine is at high pressure but there are coolers at the outlet, to cool the steam so that it turns into liquid water; thereby creating a near vacuum at the outlet.
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Old 10-23-2019, 03:47 PM
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I think the key is the order in which things happen to the air molecules. At a fan inlet, the molecule has not yet been in the fan, so the fan has not given the molecule momentum in a particular direction. The molecule figures out its own direction, so to speak, by the group action of all the molecules crowding to get to the fan inlet and replace the molecules the fan has inhaled already.

It's a different picture entirely for the exit of the fan. The molecules there were just inside the fan. Often, all the molecules have been given momentum in the same direction, away from the fan parallel to its axis. Now we are dealing with molecules the fan already steered.

If you design the fan outlet differently by making it a semipermeable hemispherical dome, so that molecules were evenly distributed over and steered in all the possible directions that generally get further from the fan, then you would feel a similar flow field around fan inlet and outlet. The asymmetry you originally observed and asked about wouldn't be there in that case.

Last edited by Napier; 10-23-2019 at 03:48 PM.
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Old 10-23-2019, 06:31 PM
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Quote:
Originally Posted by Enola Straight View Post
I was about to state that, although a gas can be compressed hundreds of times ambient...whatever the barometer says...the lowest pressure is of course high vaccuum.

0X<X<500X

But that's not the question, is it.
But it does affect the effect.

Flow will also be less on the suction side because the maximum difference in pressure is less than 15 psig. The max d/p on the outlet side can be much greater.
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Old 10-23-2019, 06:42 PM
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Another thing that really sucks is a turbo-fan engine. It still blows more than it sucks but the engine front of the engine cowl is designed to maximize acceleration of air into the engine and the sucking is felt quite a distance away.
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Old 10-23-2019, 06:50 PM
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Quote:
Originally Posted by Jinx View Post
Maybe the answer is obvious and eludes me, but: Why is it that a fan's suction side seems nowhere near as strong as the stream leaving the fan? And yet, wouldn't the flow rate in = flow rate out? Seems logical to me. Why is that not so?
Um...just turn the fan around. Your blowing is now sucking
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