A little fluid (Air) dynamics question...

[jdm]

If two identical fans were stacked on top of each other, would the resulting air flow be larger than just one fan alone?

 

[mikef208]

I am not an aerodynamics major, but I will give you my 2 cents. I would think that if the fans were spining at the exact same rate that it may yield higher cfm, but I would think that if they were not spinning exactly toegther it would cause turbulant airflow which might hinder CFM.

 

[zogg]

Try it. You will find that two are definately better then one...are they double the output? No... but more then if you only had one fan.

 

[Def]

They create alot of turbulence stacked in series, and I really doubt you'd gain much if any output.

The top fan might just be cavitating from all the turbulence around it, not really doing much except adding alot of noise.

From my experience, it seems that stacking fans produced alot of noise and cut down on airflow.

 

[mechBgon]

In my experiments cooling a CPU heatsink, stacking fans actually made the temperature of the CPU go up.

 

[Duvie]

Stacked fans can result in a small increase over the one alone, but putting them side by side is the only true way to double the cfm over the one...

I have tried the sort of forced air type method...I placed a 80 mm fan at side of case with a custom tube fastened from the 80 mm fan to the 40mm fan of my HS at the time. The spread was about 4-5 inchs and resulted in slightly cooler temps. thinking this was from the access to outside case air alone I unhooled 80 mm fan and the result was warmer. Therefore it did some improvement. However the noise factor plus the pain of when I need to remove side of case squashed that idea. I have since added better case intake and exhaust fans and the case air is damn near ambient room temp.

 

[00aStrOgUy00]

yeah it will...i put 2 fans on top of my cpu, and the temp dropped 10%

 

[goshdarnindie]

With regards to fluid dynamics, there will only be an increase in pressure, not necessarily air flow. Now, an increase in air pressure may cause an increase in CFM through the two fans, but it will be insignificant compared to two fans blowing side by side.

To answer another indirect question, turbulent flow is a good thing because it means there is more contact with the surface that you desired to be cooled. Now Def is right, the top fan would probably just start cavitating, but turbulent flow in its own right is a good thing.

 

[jdm]

00aStrOgUy00, by 10% do you mean 10% of the difference from 0 degrees or from ambient temp?

 

[CurtisBilly]

Basically a jet engine works by the concepts of stacking fans...one big difference...after every row of blades, there is a stationary row of airfoils (blade-like) which redirects the air (since after the first row of blades the air is swirling in the theta direction as well as along the axis of the engine)...these are called stators/vanes depending on what section of the engine you are talking about.
No you would not get twice the airflow...although you would certainly be altering the flow. Basically a fan works by evacuating the air immediately in the fans path by pushing it out the back. In order for the surrounding air to remain at equilibrium pressure, more air fills in the "evacuated" space. This is a crude way to imagine it, since the situation is "steady state-steady flow".
Fluid Dynamics is a VERY difficult subject, one that I am learning currently in my internship job with a jet engine manufacturer/mechanical engineering curriculum).

By the way the compressor section of an engine is the part where the blades (fans if you will receive work input-the case we have with a DC fan). This section of the engine compresses, and heats up the air!

Hope this helps.

 

[bgatot]

How about if the other fan is some distance away from the first (say one or two inches, or more), and the area between them is sealed? Or not sealed?