@Josef_Prusa, I think it might be useful to have the ability to set different fan speeds for the inner walls and outer walls of a print. I thought about this when watching a Melt Zone podcast from @Thomas_Sanladerer and Stefan (CNCKitchen). They were talking about how more fan can make prints look nicer but be weaker. Meanwhile, less fan can make the parts look worse, but be stronger. So, do you think it would be possible to set two speeds so that you can have a crisp outside and a well bonded inside? The infill would surely have the lesser fan speed as well. Basically, only the outer walls and perhaps the support material and bottom layer of a raft would have the high fan speed while everything else would be the slower fan speed.
This video is what inspired me: https://www.youtube.com/watch?v=IOz7vNxv43I
How small an area is your fan cooling? Probably a much larger area than the distance between your inner and outer wall. What really matters for strength is now much the plastic cools before the next layer is laid down on top, so the inner perimeter is going to get the same cooling that the outer one is getting while the outer one prints, before the next inner perimeter prints.
A better strategy, and one I’ve been thinking about how to implement for a long time, would be to cool the print using temperature-controlled air that will cool the plastic below its glass transition, but actively warm it if it drops below that temperature to help adhesion. This would give you the benefits of a heated chamber without requiring a full enclosure. You would probably need a stronger stream of air though, because the warm air would not cool the print as efficiently as cool air.
I understand what you are saying, but I believe that the cooling for the inner would be roughly about 2/3 of outer vs inner.
Air shielding.
If there are 3 walls or more, I figure any walls sandwiched between other walls would be mainly cooled indirectly and would maintain heat better to meld better with the next layer. The effect is less if there are only 2 walls unless there is infill which would provide a small amount of air shielding. Basically, this is the same idea as a draft shield helping to keep plastic warm. Since some infill or a wall would be acting as a draft shield, it should maintain heat better even if the fan is turned up for outer walls. At least that is my theory on the matter.