well, I found the problem but
 

well, I found the problem but this patch doesn't do what I want. What kind of patch I can use in this case?
the fan is inside a cylinder volume with an inlet, outlet and a lateral patch. Maybe the problem is the lateral patch, where a zero gradient velocity and pressure is a non-phisical condition. So I tried to change it into a wall( a fixedVelocityOutlet with uniform (0 0 0)) and the solution goes worse with a reverse flow from outlet to inlet. Then I changed the inlet in totalPressure and the outlet pressureOutlet that have a phisical meaning for a fan, but the flow trough the cylinder is very small, like it was choked. Now I don't know what I can do.. I hope there are someone that can help me.. thanks