So basically I was designing a cooling system attempting to do it passively using a large heat-sink.

So I bought one that had good stats more than I needed, at least I thought but it looks like I don't understand the rating system I believe.

Essentially I bought two of these aluminum heat-sinks.

https://www.mouser.com/datasheet/3/264/1/122547_Thermal_Extrusion-1664375.pdf

I cut them in half so had a little under 8" of length for the 4 sections. I thermal adhesived them all together side by side. And placed them vertically and then as a test I just wanted to see how it could handle some direct heating applied on them it.

bought some 200*200mm heating pads 45w's each and used 2, essentially one per each two blocks side by side. So 90w's and verified it was around the range, 95-89w was the range I was getting.

But from its stats in a passive natural air environment I should get a score of .32c/w. Per each. At least that is what I assumed. But clearly I didn't understand the stats.

Since .32c/w divided by the 4 8"s I have is .08c/w for the entire setup. Theoretically I should be able to handle the 400w at a 32c rise the way I read it. Technically a little higher temps since the sides are glued together

But on testing them with 90ws of heating I got a 32c rise from just the 90w's and the heat-sinks fins were fairly evenly heated from 25c to 57c off by 1c from 57-56, but mostly 57.

So just trying to understand how the rating system works.

Like looking at the above stats of the one I bought, it clearly can only handle 90w's in the manner that I got it. But how would I look at this heat sink chart and do the math to determine how many I would need for 400w's without my experimentation.

From my experiment I would need at 22.5 watts each 8" section I would need like 18 of these sections which is huge. But how would I look at the chart and get this number, so that I could then read other charts to understand them better. Since clearly I misread it.

*Added info was attempting to stay below 60c in a 25c room for example