Small Gap between Fan and Heatsink?

[SpiderWaffle]

I was wondering if anyone has ever heard of tests being done on the optimal spacing, if any, between a fan and heatsink? I can't seem to find any such study, tests or specific theory about it anywhere.

For Push Configuration:
It's likely assumed that a minimal gap size is ideal, as to create more static pressure which seems reasonable. However, there's a big benefit to have from turbulent airflow vs laminar, (~2-3 times more effective) and this is what I think high end CPU sinks are mostly going for with fancy bends near the edges on their fins, like on the silver arrow; it's a lot easier to manufacturer than putting a small bump along the front of each fin.

If it is optimal to have the smallest gap, what do people do with the anti-vibration rubber pads that invarably add a small gap, are their any studies how on much they cost performance? Also some fan+heatsink combos come with a type of shrouding to further eliminate pressure leaks. How effective are these, has their been any testing? If it's effective it seems like a very reasonable mod to do, has anyone seen such a mod?

Thanks,
Spider-Waffle

 

[Ricedaddy]

As I understand it, a gap between heatsink/radiator and fan is desirable due to the design of most fans as it gives more uniform airflow through all of the fins rather than mostly around the edges. The large rotor in the middle of the fan can cause uneven performance (even if it's not really relevant in our uses).

This is provided that you don't have an "open" gap and proper flow to the heatsink or radiator. A lot of watercooling enthusiasts either mount spacers to the rads or gut old fans and mount them between the fan and the radiator so the airflow remains focused but doesn't leak out between the journey to the radiator.

Truthfully, I'm not sure it makes enough of a difference to matter, as there are much larger variables at work that have a greater impact on temperatures, like proper heatsink/waterblock mounting, case airflow, and the right amount of thermal paste.

 

[doyll]

Pretty much what Ricedaddy said, probably very little difference.

Spacing does not increase pressure or airflow, but it will even out the flow in radiator and does often lower noise level.
A seal is needed between fan and radiator to keep air from escaping.