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A Preliminary Study on Bottom Air Intake - Page 2

post #11 of 28
Quote:
Originally Posted by doyll View Post

Quote:
Originally Posted by billbartuska View Post

A simple calculation would be extend an imaginary box that's the size of the fan from the bottom of the case to the surface the case is setting on.
If the area of the four sides of the box is, Oh, let's say 1-1/2 times the area of the fan (ie, area of 120mm, 80mm, etc. diameter) then restriction should be minimal.

That works out to a height of about 1-1/2 inches for a 120 mm fan.


The airflow into a fan is the circumference, not a square the size of fan, times the clearance.

120mm fan's circumference is 37.7cm
120mm square's circumference is 48cm

Circumference of square is almost a third bigger than the fan.
But the shape of the air's path under the case is bounded by the edges of the case (a square, or at least rectangle!)). I just moved the "square" to right under the fan where it's area would be the smallest and hence the required area would be the largest.. I suppose the calculations could be done with the surface area of a tube (120 mm, etc) but this is was just an exercise in approximation.

What do you come up with?
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My System
(15 items)
 
  
CPUMotherboardGraphicsRAM
FX6300 Black M5A99X EVO R2.0 Nvidia GTS450 Team Vulcan PC3 12800 
Hard DriveOptical DriveCoolingOS
Samsung 840 PRO Asus DRW-1608P (x2) Custom Water Cooling Win7 (Ult), Win 8.1 & Win Server 2012 R2 
MonitorKeyboardPowerCase
2 X Samsung 915N Ducky Shine III, Blue Cherry/Blue LEDs PCP&C 1kw Lian Li PC-71 (W/Window) 
MouseAudio
Logiteck G400s none 
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post #12 of 28
Quote:
Originally Posted by doyll View Post

The airflow into a fan is the circumference, not a square the size of fan, times the clearance.

120mm fan's circumference is 37.7cm
120mm square's circumference is 48cm

Circumference of square is almost a third bigger than the fan.

I believe area is measured in square centimeters, not centimeters, so
the units wouldn't match as well.
post #13 of 28
Quote:
Originally Posted by billbartuska View Post

But the shape of the air's path under the case is bounded by the edges of the case (a square, or at least rectangle!)). I just moved the "square" to right under the fan where it's area would be the smallest and hence the required area would be the largest.. I suppose the calculations could be done with the surface area of a tube (120 mm, etc) but this is was just an exercise in approximation.

What do you come up with?
No, the smallest area of intake on most bottom fan intakes is the circumference / perimeter of the fan itself times the space between case and floor, not the perimeter of the case. The perimeter of the case is much bigger than the fan perimeter.

The best area of fan airflow is the area of fan minus the area of motor hub in middle. Which makes a 120mm fan about 100sq cm and a 140mm fan about 140sq cm in area.

Think of the fan as a vortex. Or the drain in a sink. When sink is full the entire drain area is flowing, but as the water drops it reaches the point only the edges of the drain are flowing. A fan on bottom of case setting on a desk or floor is similar flow. Even having the case spaced off of surface the same distance as fan radius minus the radius of fan motor hub, which is the same width of airflow all the way in, is still restrictive because of friction loss on floor and bottom of case as well as loss making the 90 degree turn into fan.

Using the circumference of fan times clearance gives us the intake area to the fan over a flat surface, like desktop or floor.

Edited by doyll - 3/27/15 at 10:19am
post #14 of 28
Quote:
Originally Posted by 8051 View Post

I believe area is measured in square centimeters, not centimeters, so
the units wouldn't match as well.
doh.gif
Circumference / perimeter is not an area. It has no thickness or width. It is only a length, a single dimension. tongue.gif
Two dimensional is square units of measure, three dimensional is cubic units of measure. thumb.gif
Edited by doyll - 3/27/15 at 10:30am
post #15 of 28
Quote:
Originally Posted by doyll View Post

doh.gif
Circumference / perimeter is not an area. It has no thickness or width. It is only a length, a single dimension. tongue.gif
Two dimensional is square units of measure, three dimensional is cubic units of measure. thumb.gif

Didn't I write that area is measured in square centimeters?

Trying to equate measurements in different units doesn't make
any sense.
post #16 of 28
Quote:
Originally Posted by 8051 View Post

Didn't I write that area is measured in square centimeters?

Trying to equate measurements in different units doesn't make
any sense.
I should have said perimeter for the 120mm square, but in my defense the measurements I posts are lengths, not areas.
You quoted my post about circumference in relation to area. poke.gif
Quote:
The airflow into a fan is the circumference, not a square the size of fan, times the clearance.

120mm fan's circumference is 37.7cm
120mm square's circumference is 48cm

Circumference of square is almost a third bigger than the fan.
Are you confusing circumference / perimeter with area?
Because there is no sq cm in a circumference or perimeter measurement. tongue.gif
Circumference / perimeter is only one measurement .. and it takes at least two measurements at right angles to calculate area. biggrin.gif
Edited by doyll - 3/27/15 at 11:28am
post #17 of 28
Science going on in here!

I've found the bottom air intake noisy without some spacing between the fan and case surface. Just putting a 7mm radiator shroud between them had a significant effect in reducing noise. I suspect this is because of leading edge turbulence and not restriction - the restriction (filter / fan grill) still exists. It also doesn't occur in exhaust mode, not that you would ever want the bottom fan in exhaust mode.
post #18 of 28
Thread Starter 
Quote:
Originally Posted by Himo5 View Post

Seems to me we need to take dust filtering into consideration here. I also note that probably the most prevalent form of bottom intake fans are bottom mounted PSUs in desktops, where there is usually an unshrouded gap between the vent and the fan. I should think maximum possible efficiency in these cases would require cutting out the vent grill and replacing it with a DEMCiflex filter stuck outside the bottom of the case for easier cleaning.

I agree. If there are no vents, just a fan filter, it would have better airflow. I'm not sure how much better though, and I'm sure the answer depends on the air pressure produced by the fans. This could be an interesting test.
Quote:
Originally Posted by xaeryan View Post

Science going on in here!

I've found the bottom air intake noisy without some spacing between the fan and case surface. Just putting a 7mm radiator shroud between them had a significant effect in reducing noise. I suspect this is because of leading edge turbulence and not restriction - the restriction (filter / fan grill) still exists. It also doesn't occur in exhaust mode, not that you would ever want the bottom fan in exhaust mode.

I have the same experiences. I have actually found a significant decrease in noise just from moving the fans away from the case surface by 1 or 2 millimeters, which is astounding to me. Sort of makes me wonder if most of the sound is from the fan vibrating against the metal.
post #19 of 28
Spacing fans 5-15mm away from grill or filter definitely reduces noise. Most case front fans with HDD cage behind them benefit from more space between fan and HDD cage too. Many start with about 10mm, but I've found 15-20mm is usually much quieter.
post #20 of 28
Thread Starter 
Quote:
Originally Posted by doyll View Post

Spacing fans 5-15mm away from grill or filter definitely reduces noise. Most case front fans with HDD cage behind them benefit from more space between fan and HDD cage too. Many start with about 10mm, but I've found 15-20mm is usually much quieter.

How the distance of fans from the walls affect noise may be a viable topic for future tests.

Do you (or anyone else) know of a good way to test for noise other than using a microphone in a very quiet room?
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