Good explanation. Layer below must be solid prior to printing the next layer, as the bottom layer must be in its final contracted (contraction occurs upon cooling) position, otherwise the next layer is put put down in the wrong place. The layer above must have enough thermal inertia to melt the layer below it - it is not that the layer below is still melted. https://hackaday.com/2016/01/29/keep-your-nozzle-hot-and-your-prints-cool/
Comparison of Shrouds and Fan Types
Scientific comparison - very well done! - [1]. 40 mm fan with funnel works almost as well as 50 mm blower
Fan CFM Measurement
Inflate a large plastic garbage bag with a small fan and measure time - [2]
