Thank you for your reply. 

I applied the curvature functions on the duct surface setting the min/max element size and the reference angle.

I also tried with the Meshing tool (workbench), third figure from the top, but not always the mapped mesh is generated on the duct whereas a blue circle appears near the function, without showing any warning messages. Moreover, it takes almost the whole day to generate the mesh. 

Thanks in advance,