Fan ducts/spacers: 6 fan diameters ideal length?

[magellan]

I remember reading an UK engineering white paper on axial fan technology (for industrial applications) that stated something to the effect that 6 fan diameters is the ideal duct distance to have a fan from any restriction (incl. things like elbows). I wish I could find it again, but I lost it in a HDD crash. I'm not sure if this applies to heatsinks though and I'll bet you would lose air velocity if you were to try it.

So what is the ideal length of an axial fan ducted to a heatsink? Does it differ w/fans of different diameters?

 

[RnRollie]

it all depends on the properties of the fan & the HS , how much "power", flow, pressure, fan design, etc

I think the 6x refers to avoiding noise in an airducting system.
http://www.engineeringtoolbox.com/air-ducts-sizing-d_1736.html

For the fans commenly used in PCs/Coolers/Radiators its about the hub diameter.
But the easiest is : slightly more as the thickness of the fan.. which means for a 2.5cm "thick" fan about 3cm.

The question is WHY? What do want to achieve?

 

[magellan]

it all depends on the properties of the fan & the HS , how much "power", flow, pressure, fan design, etc

I think the 6x refers to avoiding noise in an airducting system.
http://www.engineeringtoolbox.com/air-ducts-sizing-d_1736.html

For the fans commenly used in PCs/Coolers/Radiators its about the hub diameter.
But the easiest is : slightly more as the thickness of the fan.. which means for a 2.5cm "thick" fan about 3cm.

The question is WHY? What do want to achieve?

The whitepaper I read referred to duct length, not width/diameter.

I wondered if longer/thicker ducts/spacers between heatsinks and axial fans might
not be more efficient, maybe even quieter.

 

[Conumdrum]

Maybe you haven't heard of Martinsliquidlab.org. Look in the archive. Shroud info for a PC. Not a lot of gain unless you got high CFM fans, which we have moved away from over the years.

 

[magellan]

Maybe you haven't heard of Martinsliquidlab.org. Look in the archive. Shroud info for a PC. Not a lot of gain unless you got high CFM fans, which we have moved away from over the years.

Martinsliquidlab had one interesting test. Like you said it came to the conclusion that ducts around the thickness of the fan body (38mm for his tests) were best. I'm familiar w/the ducts he was using though, and I don't know if they're a very good design as they taper from one end to the other.

He had some other interesting tests about push, pull and push-pull configs on a radiator w/fans of the same size, but differing thicknesses, CFM ratings and static pressure ratings.