After going through the CFD results of a drone project I am involved in, I noticed that the data shows significantly increased (+10%) thrust generation at the second stage of a counter-rotating, 2-stage, ducted fan. Both stages use identical airfoils and spin at the same rpm. They also have low solidity, meaning they look more like propellers than airliner fans, as the blades take up a small proportion of the disc area.
Generally, propellers are less efficient when the incoming airflow has a high velocity, so I would expect, in any N-stage fan using otherwise identical stages, for the N stage to produce less thrust than the N-1 stage, but apparently I am missing something.
In a previous answer here Peter Kämpf mentioned that the first stage of the Kuznetsov NK-12 turboprop pre-swirls the flow to improve thrust creation conditions at the second stage. How does this process work? Does it require a specific design of the stages to work in tandem?