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The air being heated naturally flows upwards, so blowing downwards in such a design is a very bad idea. Ideally (and for it to be truly a relevant design), the whole thing is silent at the very least while idle, using very little if any fan and relying on convection to do most of the cooling.

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So how many cubic feet per minute will convection cause in this case? 1? 10? Fans will move a hundred cfm or more without much noise.

Both of my recent hardware failures have been caused by dust accumulating in heatsinks, so having something which contributes to that is also a "very bad idea". Especially since the air passes through a relatively confined space which you can't regularly clear out.

What constitutes much noise is subjective, I suppose, but any fan moving hundreds of cfm will be very loud. A 140mm fan moving 20 to 30 cfm is still audible, if you're in a quiet room.

A common approach is to have an intake filter that catches dust. Another is to elevate the case a few inches above the ground.

I have intake filters on my PC, and it's also somewhere around 2 feet off the ground. Still cops a lot of dust.

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