What is the airspeed of a swallow?

[Phinance]

Can some of you physics/engineer types check my math? I did some QuickMath(tm) to determine the air speed difference between a 60mm and an 80mm fan at equivalent air flow ratings. Everybody talks about how 80mm fans with a funnel don't give the same results as an equivalent (in cfm) 60mm fan, so I thought I'd work up just how fast the air needs to be moving to have equivalent velocity:

Diameter (mm) 80 60
Surface Area (mm^2) 5026.5 2827.4
Surface Area (ft^2) 0.0559 0.0314
CFM Rating 38.0 38.0
Linear Distance (ft) 680.4 1209.6
Velocity (ft/s) 11.3 20.2

Basically, I assumed (big assumption) that a fan has its entire rated diameter available in a circular opening to move air (you would want to find out the exact usable diameter of a particular fan and subtract out the center hub for better results). After that, it's a pretty straigh-forward volume problem (how long is that column of air...). Basically, to get an 80mm fan to move air at the same speed (pressure) as the black-label Delta 60mm, you need to push ~68 cfm of air! Oddly enough, that's about the rating of the big Delta 80mm...

 

[Biochem101]

funnels will only change the velocity of the air flow, not the volume, you have to up the rpms of the fan to create more flow/min or volume delivered. Increased Velocity = greater noise.

There also comes a point where you get dimished returns in the relationship of the RPMs to the flow of the fan (probably what your concern is)

Going from 60mm to 80mm , you will get diminished returns and increased noise from harmonics and tryin to pull air faster through the hole.

Did I just repeat myself, I think I did. Damn.


Lesson: get a bigger fan.

 

[Phinance]

The gist of my thought for the brains roaming free here is that pressure will build up in the mouth of the funnel (back pressure to the funnel, static pressure to the fan) causing the fan's efficiency to decrease and the overall volume delivered to decrease. However, I'd be curious as to whether this impacts the speed of the air leaving the mouth of the funnel. Hence why I'd guess that you would almost have to use the big delta to equal the cooling power of the little delta.

 

[Biochem101]

AS I think you know, the speed of the air leaving the mouth of the funnel is in relation to the amount of air it takes in, this ratio is dependent on the entrance and exit surface areas.

Yes, the diminished returns for the amount of air you wish to pull through the apparatus is dependent on the 1-shape of the funel, 2- the efficiency of the fan, and the flow or speed of the air you are puling, Surface Area air resistance tends to be exponetial in its effects.

That help ?

 

[WarCon]

First off the air coming out of most of these fan blades is pointed out and away at about a 45. In a funnel, that means the first thing the air encounters is a nearly perpendicular wall, this will obviously raise the air pressure (creating backpressure) and make the blades spin in a backwash which makes them spin faster, but move much less air.

The ideal would be laminar flow until it hits the heatsink, but that is impossible with the turbulence created in your funnel.

Your equations work for drawing air I believe, but the above factors further lessen the efficiency and lower the ideal outputs.

I had an interesting idea though. What if your fan stood several inches above a spiraled inward fin design. Above and below the fan you put a set of vanes whose purpose would be to create a mini cyclone of air. You could use a quiet 120mm set to draw right out of your case. I was just thinking more air would be drawn from low to high because of the low pressure it would create at its center. Only issues would be case design and other cooling fans. Properly set they could vastly aid the cyclone or inproperly, totally disrupt it. You could make the fins out of very thin copper and you could make the spiral almost reach to the fan holder and thereby increasing surface area. Besides it would look cool.........

................edit...................
Oops, forgot to ask which do you mean, an african or an european swallow? . ...........

 

[damocles]

An African or a European swallow?

 

[Aboroth]

Is it unladen?

 

[Demon-Xanth]

If the swallow is carrying a cocanut it'll be slower...


Typically fans don't do backpressure well. So thier CFM rate will be heavily affected by obstructions (like a funnel). A squirrel cage fan will be less affected by restrictions than a standard fan but has that little disadvantage of being large.

Without doing more math than a Geforce3 during a round of Q3A there is no real conclusive way to say 60mm fan A will do better than 80mm fan B with funnel C on HSF D. The particular HSF used WILL affect the results.

 

[ManBOO YA]

You can stack the 80mm fans to increase your air pressure so u decrease back flow