Air Displacement: Effects of Multiple Fans

[EvanJ]

Ok, I have a question I have been wondering about.

If you have a tube and insert one fan into it. The fan turns to create a certain air displacement. If you put 2, or 3 etc. fans into the tube spinning at the same speed, would it displace more air? or would each fan simply move the air through the tube at the same rate one would.

Thanks.

 

[0xDEADBEEF]

Fans create a pressure differential, more fans move more air, up to an upper limit.

 

[EvanJ]

ok. so would there be a way to mathematically figure out the thrust of additional fans?

basically what i am trying to accomplish, is how many small fans (propellers) connected in series in a tube (vaneaxial like), would create the same thrust as one bigger propeller. taken that each are moving at the same RPM.

can this be done without experimentation?

 

[russ_watters]

It will move more air. How much exactly depends on the particular specs of the fan and resistance in the tubing.

Do you have the fan curves for the fans?

 

[EvanJ]

I have not gotten that far in my design unfortunately. That is also one other thing I am going to need to figure out. I'm trying to figure out whether multi-blade compressor type propellors, or 2,3, or 4 blade rc airplane propellers, or even fully custom propellers would work best. Its going to have to withstand high rpm i would guess to be able to sustain the same thrust as a full size propeller.

 

[russ_watters]

Without knowing exactly what you're tyring to do, I think I can still say you're going about this the wrong way: there are thousands and thousands of different fans out there for any purpose you can think of. There is no reason I can think of to try to design the fan itself rather than taking your specifications and picking the fan that best matches them out of a catalog.

Is this an attempt at a low-cost ducted-fan for an RC plane?

 

[Uglybb]

There is a much bigger problem!

Turbulance from the first fan
(http://www.arising.com.au/aviation/windturbines/wind-turbine.html)

The most likely results are decreased effieciency and noise .. worst case rear fan destruction.

You need the fans side by side and organize a manafold either infront or behind the fans depending on whether you are after the sucking or propulsion

 

[EvanJ]

I have attached the reason to why I am asking all of these questions (ignore all the small labels). It is my concept for a powered hang glider harness. The design is still in its most primitive stages, but I do not want to continue with it if there are big issues to start with.

 

[Uglybb]

Wont work as shown without careful consideration of the turbulances in the tube most likely you will just produce a lot of vibration and noise in the rear fans without careful work.

If all you are trying to do is control glide your idea will be fine but human powered flight (http://en.wikipedia.org/wiki/Human-powered_aircraft) is all about lift efficency because the power any given person can produce is limited.

Does not matter what blades etc you use by the laws of physics you can not get more energy out than you put in so power to weight ratio for lift is the guiding factor.

 

[EvanJ]

Ok. In this case there will be an engine, so power i do not believe will be a problem. but i do see the problem of turbulence.

I found this website: old.greenheck.com/technical/files/engineering/04fall_eng.pdf which can help prove the general concept, but would adding 3 fans have triple the pressure of just one fan?

Would putting each propeller on its own free spinning axis work help with the turbulence? or am i missing the point...do you have any recommendations to decrease the turbulence? Which things do i need to consider?

 

[Uglybb]

You can use a flow straightner between fans but I suspect you will lose a lot of energy. They are nothing more than a grill of small length pipes or hex or octogons in your tube

these will give you the idea:
http://www.evga.com/forums/tm.aspx?m=108486&mpage=1
http://softsolder.com/2010/12/10/simple-air-flow-straightener-for-simple-fans/

My real question is why you are going for an axial fan.

Look at the fan types http://en.wikipedia.org/wiki/Mechanical_fan

A centrifugal fan produces more pressure for a given air volume isn't that what you want?

The actual fans would also be a hell of a lot easier to find as scrap .. they will be in any industrial blower or evapouritive cooler.

 

[EvanJ]

Wow! those air straighteners work amazing! i will definitely consider that in my design.

Also, I do realize that centrifugal fans produce more pressure, but they are bulky. Weight needs to be a minimum for a hang glider.

I do not believe you can connect centrifugal fans in parallel and make it effective (although i may be wrong). Then I would have to use 2 (one on each side replacing the tubes in my sketch) and i would believe that 4 and 4 axial props in series would produce more power than 2 centrifugal fans alone.

But that said, I do believe the weight would be the biggest problem.

 

[sophiecentaur]

You could take a lesson from the existing designs, I think. The theory is very complicated and very non-intuitive and you wouldn't want to repeat everyone else's past mistakes would you?
I have seen a few ducted fan systems (looks like it from the ground anyway) on microlights around here but never a centrifugal fan on any aircraft. This may be because thebasic requirement for airflow in ventilation is not the same as for propulsion.

 

[EvanJ]

I agree. centrifugal would not be a step in the right direction. But most ducted fans only have one blade. I'm trying to decide whether i could put in 4 blades (like in my sketch posted earlier) or if that would have to many problems, or i would not get power equal to one big propeller, and i should scrap the idea all together. I am more or less looking for something different and "cool" instead of the typical design.

 

[sophiecentaur]

Well. "Cool" and efficiency don't very often go together.
I'm sure that multiple fans in tandem must have been tried and rejected long ago. I think this must be something to do with the speed of air across the blades and the necessity of keeping the tip speed reasonable to avoid turbulence.

Turbines, however, have several layers of blades but a turbofan engine only has one set of blades used for actual propulsion afaik.

 

[EvanJ]

Yah i see what you mean. What is not being used is probably that way for a reason because i know I am probably not the first one to think of this idea. I may just make a small prototype and see what happens. It just frustrates me that if it has been tried and deemed unsuccessful, i wish i could find some report to saying the testing and conclusion.

 

[sophiecentaur]

I think you'd have to go back to the early 20th Century for that. Possibly not available via Google.

 

[Uglybb]

I can't believe you think there isn't much information out there on ducted fans they are used by most of the jet fighter looking RC models

google "rc plane ducted fan" youtube will also show numerous designs and looks at operations.

And I was going to say someone was sure to have tried what you were talking about ... its petrol driven not human powered but same principle

http://www.youtube.com/watch?v=_AXuy2-YoQY&playnext=1&list=PL7B069B9F7A30CAFC

 

[sophiecentaur]

I wonder if the designs for rc models will scale usefully to 'full size'?

 

[256bits]

I am quite sure the extra fans will not impart extra momentum to the air.
They have to be of different design and /or rotate at different speeds.

Try you own experiment:
Take 2 fans exactly the same.
Power the first.
Put second fan in the airflow of the powered first fan.
Ideally, the second fan will now rotate at the same speed ( assume no energy losses) as the first.
Ereeka - you have just constructed a primitive "automatic transmission".

Now, mechanically have the second fan rotate at the same speed as the first with a connecting shaft for example. Do you now think that there will be an extra air flow displacemnt by adding axial fans in series in a duct?

The second fan has to be of different fin design and/or rotational speed to impart any extra momentum to the air flow. You could be up to a challenging task.

Note that if you add many many fans in series arranged with an offset of tip to tip, you will end up with an auger or archimedes screw.

Have you ever heard the low frequency thump thump beat of large ventilation systems? Ramp that up with a higher rpm system and that type of feedback may just blow your system apart as mentioned before in previous posts.

 

[sophiecentaur]

Certainly the pitch would need to be different.

 

[Studiot]

One point that has only been skirted round so far.

How do you propose to drive the fans, they will not turn on their own?

 

[jakers82]

sorry to interrupt this thread without any help to anyone , but i had a question? can a fan/seperater, spinning at 25000 rpm powered by a 1.25 hp motor moving 70cuft of air per minute through water, clean a 2500 sq ft home in an hour?

 

[russ_watters]

Clean?

 

[jakers82]

yes. clean. if I am able to pull the air in my home through water as a filtering system at 70cf per minute, how
lomg would it take to clean a sealed 4000 sgft home?

 

[russ_watters]

Well, water doesn't make for a good filter and 70cfm is not a lot of air (you should calculate how long it takes to run all of the air in your house through it), so I'd have to say never.

Also, that fan's performance is terrible. 70cfm should only take a tiny fraction of a horsepowe unless for some odd reason you use a rediculously high pressure.

 

[sophiecentaur]

yes. clean. if I am able to pull the air in my home through water as a filtering system at 70cf per minute, how
lomg would it take to clean a sealed 4000 sgft home?

As the pump performance is measured by volume, you would need to know the heights of the rooms in order to get a volume from the area measurement you give. I know this is important because the majority of rooms in my house are more than 3m high and the house we used to live in had somerooms that were were not much more than 2m high. Our heating bills are shameless!