The Central Dogma

[Mrvile]

I think I've finally figured out why my Panaflo feels like it pushes less air than my AddA. This may not be correct, I iddn't do any research (I do all my research here on the boards) and is mostly logic mixed in with some stuff I remembered from Physical Science last year. Anyway...

I have two fans, a 66cfm 120x25mm AddA fan, and an 86.5cfm 120x35mm Panaflo M1 fan. However, the AddA always feels like it's blowing more air than the Panaflo, and I never figured out why. Did I get a dud? Is the PSU undervolting it for some reason? Hoping that it was none of the above, I gave it some optimistic thought and came up with this.

* * *

VOLUME:
I've noticed that my Panaflo spins at much lower rpms than the AddAs (the Panaflo is one of the first fans to stop spinning when I turn the comp off). However, since the Panaflo is deeper (38mm instead of 25mm), the blades are angled more to pick up and push more air per spin. In other words, the 38mm fan doesn't have to spin as many times per second to move the same amount of air as the 25mm fan.

Here it is mathematically:

x = amount of air pushed per spin of an 80cfm, 38mm fan
y = amount of air pushed per spin of an 80cfm, 25mm fan

x = 2y
1000x = 2000y

Something like that. Basically I'm saying that the x fan spins half the speed of the y fan, but since it pushes twice as much air as the y fan, it only needs to spin half as fast to get the same cfm.

* * *

PRESSURE:
But why does the AddA feel more powerful than the Panaflo? When you put your hand in front of the fan to do a "feel" test, all you feel is the air pressure and not really the volume. Since the AddA pushes smaller amounts of air per spin, it has to run at higher rpms in order to move the same amount of air. So the AddA pushes air at a higher speed, resulting in higher air pressure than the Panaflo, and when you feel the pressure against your hand it seems as though the AddA is stronger.

* * *

So I'm hoping that's what's really happening. Any physics majors want to correct me feel free, I'm not the brightest bulb in the pack, and I'm kinda biased into thinking positively. As for the pressure part I'm not sure if I explained it correctly but it was the simplest way to put it in words. Also I really didn't plan this out so if it's kinda jumbled well I was just typing what came to my head at the time. Any thoughts?

-V

 

[CycloWizard]

The pressure that you feel is a result of the velocity of the air. This velocity will depend on the rotational speed of the fan blades.

 

[kornphlake]

try this test, blow up a baloon, stick a drinking straw in the opening and let the air out, you'll feel a jet of air coming out the end of the straw. Now blow up the same baloon, put the opening inside a paper towel roll and let the air out, you won't feel as strong a jet of air coming out of the larger diameter tube. So which is actually better for cooling? Try putting a drop of water on a piece of glass and blow air through a straw as hard as you can and record the time it takes until the drop has evaporated, now put another drop of water on the piece of glass and blow through the paper towel roll and see how long it takes to completely evaporate the drop. let us know what you come up with, I've got no fundamental idea what's better, a large volume of air at a low velocity or a high volume of a low velocity, but I have a gut feeling that it's going to be approximately the same.

 

[Mrvile]

CycloWizard pretty much sums all that up in a few words.

Anyway kornphlake, I don't really have time to try that little experiment but I figure it also has a lot to do with what you'll be using the fan for. For instance, if you used it to cool the CPU, you'd want the air moving as fast as possible, to get the hot air off the heatsink asap. So a lower cfm, higher rpm fan would probably work a bit better. However for a case fan, you'd want a lot of new cold air coming in and a lot of hot air going out without sacrificing silence. A higher volume, lower rpm would probably work better here. I don't think the difference would be all that noticeable, but that's my theory.

Also, what if your CPU was only say 90cm wide, and you had a big 120mm fan blowing onto it? A lot of the air would be wasted. In this case you'd probably again want a smaller fan spinning at higher rpms to cool it, so that all the air would make it onto the heatsink, and the air would be exhausted quickly.

 

[CycloWizard]

The key is that the velocity near the part you're cooling will be proportional to the flowrate of air. For a fixed flow area (i.e. between the fan and the part), a higher volumetric flowrate increases the velocity (velocity ~ flowrate/area). Higher velocities increase convective heat transfer, which is how heat is moved from the part to the surroundings. The higher this rate of heat transfer, the better off you are.

One is quieter while moving more air due to a more efficient fan design. It is able to generate more thrust/higher flowrates at lower RPMs. The shape of the impellers (fan blades) are really the deciding factor here.

 

[Mrvile]

True, although the pressure is definately lower.

 

[shira]

Originally posted by: Mrvile

PRESSURE:
But why does the AddA feel more powerful than the Panaflo? When you put your hand in front of the fan to do a "feel" test, all you feel is the air pressure and not really the volume. Since the AddA pushes smaller amounts of air per spin, it has to run at higher rpms in order to move the same amount of air. So the AddA pushes air at a higher speed, resulting in higher air pressure than the Panaflo, and when you feel the pressure against your hand it seems as though the AddA is stronger.

I got my bachelors in physics, not that that makes me any more qualified to figure out what's happening with your fans than anyone else. But I think it's something like what you're surmising:

If the air coming off the fans were colored green, I think what you'd see is that your AddA fan has a relatively "sparse" vortex of air rotating at relatively high angular velocity and moving at relatively high speed away from the fan. Your hand detects the high-speed of the air coming from the AddA, but not density of the air vortex. Your Panaflo produces a relatively dense vortex of air, rotating at a relatively slower angular velocity than the AddA, and moving at a relatively lower speed away from the fan. Your hand can sense the lower velocity, but cannot factor in the higher total density of the vortex.

 

[mrscintilla]

Conservation of Mass. You guys should all remember this from your undergrad physics!!!
SAme volume of water going through tubes of different diameters. The thinner tube will pass water faster. But the same volume of water pass through per unit time. Therefore, if the same volume of air is pushed through, a wider-diametered fan will be like a fatter tube with slower speed of air passing through. The pressure you felt is correlated with the speed of the air.

 

[CycloWizard]

Originally posted by: Mrvile
True, although the pressure is definately lower.

Really, the fan design probably just fans out the air flow at the effluent so there is a maximized pressure drop through the fan. This will maximize velocity near the surface. So, the pressure will be relatively high at the surface but low at the exit. 😛