Ostensibly, you're wasting air flow, if it's outside of the fins, since the only contact the air MIGHT have is with the tip of the fin, and the heat transferred would be negligible. The whole point of the air flow is to get lots of contact with the largest heated surface as possible.

TTFN (ta ta for now)
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It takes very little gap to divert the bulk of the airflow around the fins.

I agree, that's why modern car radiators have ducts, so the air can't dodge around the cooling fins.

Cheers

Greg Locock


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wishfully blowing air over a heat source is probably doomed to failure, worrying about how close can the cooling fins be is probably moot.

if you can't use something like a CPU fan, then ducting is probably required.

you'll probably have to test whatever you do, maybe you can CFD it ?

another day in paradise, or is paradise one day closer ?

Some resources that may be helpful… RE Avionics/electronics cooling...

This is NOT a simple subject... especially when air density and incoming air temperature is also factored-in: air-cooling effectiveness can fall-off like a streamlined rock thru finned heat sinks...

SAE/TP 2008-01-2919 ENABLING MUCH HIGHER POWER DENSITIES IN AEROSPACE POWER ELECTRONICS WITH HIGH TEMPERATURE EVAPORATIVE SPRAY COOLING
MIL-B-23071/* BLOWERS, MINIATURE, FOR COOLING ELECTRONIC EQUIPMENT
ARINC 728-* AVIONICS REFRIGERATION COOLING SYSTEM (ARCS)
SAE AIR64 ELECTRICAL AND ELECTRONIC EQUIPMENT COOLING IN COMMERCIAL TRANSPORTS
SAE AIR1277 COOLING OF MILITARY AVIONIC EQUIPMENT
SAE AIR1811 LIQUID COOLING SYSTEMS
SAE AIR1957 HEAT SINKS FOR AIRBORNE VEHICLES
SAE/TP 2008-01-2919 ENABLING MUCH HIGHER POWER DENSITIES IN AEROSPACE POWER ELECTRONICS WITH HIGH TEMPERATURE EVAPORATIVE SPRAY COOLING

And there is always vibration and thermal expansion/contraction effects to worry about...
PREVENTING THERMAL CYCLING AND VIBRATION FAILURES IN ELECTRONIC EQUIPMENT ISBN 0471357294
VIBRATION ANALYSIS FOR ELECTRONIC EQUIPMENT, 3ED, ISBN 1601192916
SAE AIR1557 [CX] HIGH AND EXTENDED VIBRATION ENVIRONMENT

Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

Use a closed cell foam between the fins and the casing if you are worried about contact problems.

Cheers

Greg Locock


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