Airflow questions

Kmpkt

Innovation through Miniaturization
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KMPKT
Feb 1, 2016
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I am trying a design case idea where I have solid side panels and a front to back airflow pattern (think Compact Splash) with back to back GPU/Motherboard (think Dan A4). How close can I have a side panel to a top down CPU heatsink or non-blower style GPU cooler without cutting off cool air supply? There should be direct fresh air cross flow over both the CPU cooler as well as the GPU's heat sink and cooler. Exhaust in this case would be through the back only.
 

EdZ

Virtual Realist
Gold Supporter
May 11, 2015
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How close can I have a side panel to a top down CPU heatsink or non-blower style GPU cooler without cutting off cool air supply?
That entirely depends on the heatsink you are using. If you are using a top-down flow heatsink (e.g. stock Intel, Noctua NH L9i, etc) then you will need some clearance for the fan to pull in the passing air, and possibly internal ducting to prevent warm air from the sides of the heatsink recirculating back into the top of the heatsink. If you are using a fanless horizontal-forced-airflow heatsink (e.g. like the ones normally found in servers) and taking advantage of the airflow from one or more large case fans (I assume that's the goal of your front-to-back flow layout?) then you don't need any clearance at all.
 
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Necere

Shrink Way Wielder
NCASE
Feb 22, 2015
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I did a bit of research on this recently, and most if what I found was rule of thumb type stuff from the HVAC world. There, the recommendations are for as much as a full fan diameter's distance from a wall. For PC case design (especially SFF), that's pretty much a non-starter, and probably wholly unnecessary anyway. For a more usable rule of thumb, I've settled on approximately 0.25 * fan diameter, as this gives an equal area between the fan diameter area, and the area of the outer surface of an imaginary cylinder projecting from the fan to a wall. The idea being that the air has the same amount of space to flow through.

Needless to say, the actual performance is going to follow a curve, from no airflow at zero distance, to full airflow (equivalent to open air) at some distance (maybe the 1 * fan diameter mentioned). I haven't done those tests, but from what I've seen and read of people's experiments with the M1 and elsewhere, airflow definitely improves considerably from ~0.1 * fan diameter to ~0.2 * fan diameter (i.e, 10-12mm to 20-24mm clearance for a 120mm fan), so 0.25 * fan diameter would seem to strike a good balance between use of space and performance.
 
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Kmpkt

Innovation through Miniaturization
Original poster
KMPKT
Feb 1, 2016
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So as a follow up to this, if I have holes in the side panel this would obviously reduce the amount of space needed between the side panel and the topmost aspect of the fan. Would side panel holes negate the benefits of a front intake blowing fresh air across the CPU/GPU to exhaust at the back?