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Because at the sorts of air pressures that most PC cooling fans can provide, coupled with the fin density of the heat exchangers being used, a single radiator will typically achieve between 90-98% thermal transfer efficiency for the amount of air-flow that a single fan is capable of pushing through the singular radiator.
If you double up the radiators, one after the other, the second radiator is receiving air that is within 10% of the temperature of the coolant already. Worse, the second radiator will in turn double the air-flow resistance, effectively cutting the air-flow through the radiators by around 30%. So the 10% of possible fluid-air heat transfer capacity that the first radiator didn't achieve that you're counting on the second radiator to scavenge, gets totally overwhelmed by the fact that there's now 30% less air going through to pick up the heat.
i.e. you'd see ~20% higher water temps as a result.
Now if your fan is SO powerful that the first radiator's thermal transfer efficiency is low enough that adding the air resistance of the second radiator is minor in comparison to the amount of heat transfer that the second radiator is capable of scavenging, then that's where it'd work.
In my experience, this isn't going to happen for PC sized 12cm radiators until you start using >200CFM singular fans, or alternately double up some 100cfm fans in a push-pull arrangement, and even then, it's going to be borderline better than the single radiator with the single fan.
If you're going to use two radiators, they need to both be pulling their own clean air source for best effect.
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Hmmm, I suppose I should've explained it better.
Air has a finite thermal capacity per volume. It happens to be around 1250J/m³.K (round figure).
i.e. it takes around 1250 Joules to raise 1m³ of air by 1°C.
So if the fan is moving 1.5m³/min of air through the radiator, then the air-flow has an inherent thermal capacity of around 31.25W/C, or a C/W of 0.032. i.e. the radiator CANNOT perform any better than the inherent thermal capacity of the air. Even if the radiator was 100% efficient, the C/W will never be lower than 0.032C/W.
Now if the radiator is 90% efficient at this air-flow speed, this means that the radiator will have a C/W of 0.032/0.9 =~ 0.036. i.e. the equilibrium point will be such that the water warm up by 0.036°C per Watt of heat input (from CPU + GPU + pump + whatever else is being water-cooled).
If we now add on a second radiator, and this increases the air-flow resistance such that the air-flow through the two radiators is now 1.1m³/min, then the inherent thermal capacity of the air is now 22.9W/C, or a C/W of around 0.044.
i.e. even if we achieved a 100% thermal transfer efficiency of heat from the fluid into the air using the two radiators, at best we will achieve a C/W of 0.044, or about 20% worse than the 0.036 of the single radiator.
Now as air-flow goes up, radiator's thermal transfer efficiency goes down. There is eventually a cross-over point where adding a second radiator in air-flow series to the first radiator does help, but for PC water-cooling radiators, that doesn't occur until you start stacking extremely noisy fans, or using obscenely noisy single fans (Delta FFB/TFB, etc).
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