ORIGINAL: freakingfast
ORIGINAL: Sport_Pilot
Ever covered the hose (inlet or outlet) on a vacuum cleaner? It speeds up, less load....the impeller blades are stalled.
The fan speed increases because it is not allowed to move air. So a small amount of air recirculates between the blades and around the edge of the fan. The fan increased the pressure on the outlet side and decreases the pressure on the inlet side, the fan
blades are now operating in a partial vacuum, even if blocked on the outlet side, so there is less load. It has nothing to do with a stalled condition.
Lots of info, search: stalled axial centrifugal impeller
stalled axial centrifugal impeller
I stand a bit corrected, but a stall with constricted air flow is not the same as full flow stall. The drag is low at shut off conditions because it is operating in a partial vacuum and thus less drag. Also the stall is only partial and the blades are doing work to keep the outlet pressure up. See the chart on the axial fan link and stalled condition is considered at the right side of the chart.
http://www.comairrotron.com/axial_fan_stall.shtml