View Full Version : Settle a argument

tony draper
7th Sep 2011, 13:15
Engineering question?,if I have say a two inch diameter twelve inch long plastic pipe open at both ends, over one end I afix a small fan(summat like a CPU Fan) rotating in such a manner when powered that air is sucked into said pipe and blows out tother end at a caculatable rate,now if one fixes another identical fan at tother end rotating in a manner that sucks the air out of the pipe,what effect if any would this second fan have on the volume of air being sucked in and blown out.

7th Sep 2011, 13:21
It would double the airflow, approximately, maybe, perhaps, possibly.


7th Sep 2011, 13:21
Could this be "an argument" rather than "a augument"?

Be careful Drapes, us grammar and spelling police are everywhere :E

7th Sep 2011, 13:21
In my view increase the volume. The friction along the tube will restrict the flow so the addtion of another fan will help overcome this friction and increase the volume.

No formulas to give it but that is my view.


tony draper
7th Sep 2011, 13:33
This being a open system makes it more difficult,logically the first fan can only shift a given amount of air into the pipe for the second fan to work on,were it a closed system ie if it were mounted in say a closed torus,would the air speed going round the torus increase.
One admits one does not know the answer oneself
Anyway it makes a change from talking about tits.:rolleyes:

7th Sep 2011, 13:35
Try reversing one of the fans.

west lakes
7th Sep 2011, 13:45
the first fan can only shift a given amount of air into the pipe for the second fan to work on,

Yes but as the first fan is open additional air can still be drawn in past it by the second fan.
If the first fan were switched off the second would still draw air in.

7th Sep 2011, 14:02
The result would be a nett decrease in airflow. This is because one fan would be more efficient than the other and cause a stagnation of airflow within the pipe.

7th Sep 2011, 14:24
Just a thought on this interesting problem

Unless carefully designed and power (high pressure/low pressure) matched it is likely that the air flow between the fans will become turbulent resulting in inefficiencies that will certainly mean the the throughput will not be doubled and might (as has been suggested) be less than one fan alone.

It would be interesting to look at the complex maths on this one but I suspect that the overall throughput with two fans (dependent on the length of the pipe) will be of the order of 1.2 the thoughput of one fan alone. Not a great deal of improvement really given the additional cost and electrical load of two fans.


7th Sep 2011, 14:41
With two equal fans running at the same speed at opposite
ends of a tube of constant diameter there would be no overall
increase in the speed of the air passing through the tube.
At least, I don't think so.
Charles Law, Boyles Law, Bernouilles Theorem???
Something like that MUST apply, surely?

flying lid
7th Sep 2011, 14:41
Hf = 4FLV(squared) / 2GD

Hf = Head loss due to friction
F = Flow
L = Pipe length
V = Velocity of gas
G = Force of Gravity
D = Diameter of pipe

Get out your "British Thornton" slide rule & work it out !!!!.


DX Wombat
7th Sep 2011, 15:01
us grammar and spelling police are everywhere :rolleyes:
That should be "We" not "us". :*

7th Sep 2011, 15:35
Why can't you just blow on your tea like everyone else?

7th Sep 2011, 15:45
I have staff to do that for me.

7th Sep 2011, 16:07
The grammar and spelling Police missed the thread title then?

Settle A argument? Or Settle AN argument?

DX Wombat
7th Sep 2011, 16:09
No Norm, just left it for someone else to have a turn. ;)

7th Sep 2011, 16:10
I'll get back in my kennel then. Thank you.

7th Sep 2011, 16:18
The angle of attack and general shape of the fan blades is designed to accelerate stationary air to velocity x. If the air is already moving at velocity x or somewhat below it due to pipe-induced friction losses the second fan's efficiency will be significantly decreased. As others have said, the fan will still accelerate the air, just not by a lot.

Um... lifting...
7th Sep 2011, 16:20
Where's reynoldsno1 when you need him?

I submit there will be slightly higher flow rate.

The flow in the pipe will already be turbulent, and the pipe walls will create losses. As I recollect, laminar flow pretty much only exists in an infinite system, which Mr. D pointed out this weren't.

The energy imparted by fan #1 will be diminished over the length of the pipe, so #2 will jolly it along some further. This is why pipelines have boost pumps along their length, regardless of the slope of the pipe.

OK, smart guys, given a cylindrical pipe of diameter d, what is the optimal depth of liquid in the pipe to maximize flow rate to say... drain a tank?
The answer is not d. Even if'n you can't do the math... why d'ye suppose that is? Asking Mr. google will be disallowed.

7th Sep 2011, 16:21
Is the pipe on a conveyor belt ?

Ancient Observer
7th Sep 2011, 17:54
There was a bloke working for BAe who did a phd on this sort of stuff. he might help.
However, he doesn't belong to pprune. Helpful, huh!

7th Sep 2011, 18:15
Issue a summons forthwith!

7th Sep 2011, 18:16
I agree with Um... lifting... about the result. I think you'd get an increase in the volume rate all right, but not a doubling. Exactly how far off depends on a lot of factors. There's the air friction problem, and then there's the question of pressure.

Each fan moves air by creating a pressure difference between the front and the back. The first fan is pressurising the air in the pipe, and the second fan has to deal with that too. Simplified example with fake figures:
- first fan can produce a 10 kPa pressure differential in the pipe, so the pressure is 10 kPa just after it; at least part of this is "back pressure";
- there's a "head loss" due to friction in the pipe, say 3 kPa, so pressure just in front of the second fan is 7 kPa;
- the second fan is blowing out in to the open air, not in to a partly-closed pipe, so it produces less of a pressure differential of its own - say 8kPa
- The second fan also has to deal with the pressure behind it: it could end up spinning faster than is good for it, not "biting the air" properly;
- so, between the inlet of the first fan, and the outlet of the second fan, you have less of a pressure difference than you'd get by doubling the ability of the first fan.
After that, it's the pressure difference that determines the velocity (V) of the air flow (using Bernoulli's equation for compressible fluids) and then the flow rate (Q = VA, where A = area of the pipe). I think.

7th Sep 2011, 18:26
I have a little experience with this sort of thing. Here's what happens: The flow increases slightly but never as much as the designer hoped. You need a lot of horsepower on the second fan to get a significant increase.

After an excellent landing etc...

7th Sep 2011, 18:35
I have no idea of the clever fluid dynamics stuff, but to double the airspeed of a flying machine, you need a lot more than double the power. I would therefore suspect that the second fan in the FSL's example would increase the air flow by a very small amount.

If you want more airflow in your tea-cooling machine, use a bigger fan at the input end.

flying lid
7th Sep 2011, 18:57
So you have 2 small electric fans and a bit of tube-----

(RTF=ready to fly, EDF = electric ducted fan)

MIG-29 Twin 70mm 360 Degree Thrust Vectoring RTF EDF JET! Flight Review in HD! - YouTube


7th Sep 2011, 19:00
Many years ago I tried the dual cascading fan in the bedroom window approach (both fans being the same model). The fan in the back simply increased in speed as it became unloaded relative to the incidence angle on its blades.

Of course it did drop the pressure behind the first fan so the airflow went up along with noise and the vibration. I tried all sorts of tricks twisting the blades until i finaly broke one off.

Now-adays I suspect the enjune designers put stators in between the fans and change the incidence angle on the rearmost fan blades to up the mass flow ... of course their blades break off every now and then as well.

Noah Zark.
7th Sep 2011, 19:38
I always wonder how them Cessna 337's work!

( Sorry about the aeronautical content!)

7th Sep 2011, 19:47
Anyway it makes a change from talking about tits.:rolleyes:

I'm totally lost, can we talk about tits now please? ;)

7th Sep 2011, 20:00
Lid's equation is the basic formula guys but as we all know it is more complex than that... I mean we haven't spoken about the efficiency of the blades or the turbulence and then there are questions of compression and compressabilty (this might be a subsonic or supersonic setup)...

It sounds like a lot of Reynolds to me!


7th Sep 2011, 20:50
Angle of attack :
I assume it depends whether blades turn in the same direction or not.

Limit Layer :
some micro holes in the tube should change the result.

Hf = 4FLV(squared) / 2GD

Does it work the same if tube is positionned vertically vs horizontally ? :p

tony draper
7th Sep 2011, 20:53
I dunno, I only start the arguments round here.:rolleyes:

8th Sep 2011, 21:31
One expects that the close-coupled nature of the two fans will lead to significant amounts of inter-modulation of the physics rules for interaction between the two devices. The net result will be unpredictably unstable variable behaviour in airflow through the whole system, ranging from perhaps 0.8 to 1.8 times the flow possible with a single fan.

Basically this smacks of a tuned resonant system without any included mechanism for fixation & management of the tuning principles. If thoughtful sensor-based dynamic control were added, some interesting results might possibly be achievable, including: continuously variable flow rates between 0 and 2 x, power usage efficiency improvement whereby the 2-fan system moves more air per unit of energy than a single fan - while adding some fail-soft redundancy, and (if extra expense is incurred), a pushme-pullyou system where airflow volume might be variable from +v to -v in an automatically or remotely controllable manner.

unstable load
9th Sep 2011, 20:31
In other words, Admiral Draper.... Blow on yer tea, it seems to be more effective.....

9th Sep 2011, 23:06
Probably more effective to fix a second tube and fan alongside the first one.

Put wheels on the box and you might be able to drive it round the floor with the increased thrust - at least until the mains plug gets pulled out of the socket.


10th Sep 2011, 00:49
Possibly some glaringly obvious science being ignored in my theory but:

The input fan will have a maximum volume of air it can gather, dependent on it's design, speed etc. and will be able to impart a maximum speed to that volume of air, as the air progresses down the tube it will slow and pressure will increase. Now place an identical fan at the extraction end and there will be an optimum length of tube where the speed and pressure will remain constant throughout. If P1 is pressure delivered by the input fan and P2 is pressure at the far end of the tube then P2 will be less than P1, now introduce a fan at the opposite end and for a given length of tube P1 and P2 will be the same?