This kind of started when I was doing research about the Thermalright TRUE CPU heatsink. As you can see in the picture, it comes with fan bracers on both sides to hook up two fans. In general, common sense dictates that two fans should be set up like this:
> * > (“>” indicates the fan is blowing towards the left, “*” indicates the heatsink.)
However, in laboratory tests, this does not increase the amount of cooling substantially (for the sake of this thread, let’s assume that as “cfm” [cubic feet per minute, or the amount of air pushed by the fan] increases, the amount of cooling also increases.) Therefore, putting fans in the order above is not much more effective than a single fan, and therefore the sucking action of the left fan contributes very little additional CFM.
In a separate experiment, I know that having fans in this setup:
*> <
decreases the CFM towards the heatsink. Therefore, even a slight decrease on the intake side of the fan decreases the CFM of the main fan.
Therefore, my question is: What are the physics of two fans oriented like this:
*>>
with two fans on the same side blowing in the same direction? Would I get twice the CFM, or some lesser/greater amount? What other positioning would result in the highest CFM?
Most fans (although this can vary a bit according to fan design) have a focused area of high pressure on the output, and a diffuse area of low pressure on the input.
In your second example, the fan on the right is creating a low pressure area across the entire input of the fan of the left, decreasing its efficiency.
Creating a focussed area of high pressure on the input (as in your third example) will help, but perhaps not as much as you hope.
In theory you should get the maximum cooling by facing both fans away from the heatsink in opposite directions. Neither will be at full efficiency, since they’ll create low pressure on the input of each other, but the total amount of air moved over the heatsink will be maximised.
I think most case companies have the fans as >* >, where the second fan is blowing out the side of the case, making sure that there is total airflow through the case, instead of the same heated air being pushed onto the heatsink over and over. You get the compressed air through the vanes of the heatsink, but new air is being used for the most part.
