I currently do some experiments with ducted propellers in which I try to figure out which effect a nozzle has on the thrust produced. My theory goes like this: If I reduce the exit diameter of the duct the pressure is going to decrease and the air velocity and thus the thrust is going to increase. Consequently, you would want a small exit diameter.
However, in my experiments I measured the thrust of a 12x12 inch propeller at around 5500 rpm and got 20 N without a nozzle (just a constant diameter duct) and only 4 N with a nozzle reducing the exit diameter to 50 percent of the prop diameter.
These results are contrary to my theory! Does anybody have an explanation for this ? And what should I change to actually increase the thrust compared to the prop without a nozzle ?