Cylindrical Duct with blockMesh
 

Hi everybody,
I am new of OpenFoam and I am trying to do a very simple geometry with blockMesh. I am trying to build an half of a cylindrical duct (180 degrees) and for this reason I divide my cylinder in 4 sector. I write my blockMeshDict and it works but automatically OpenFoam creates 3 default Faces corresponding to the divisions between the sectors and I don't understand why.
I put here my blockMeshDict (there are only three sector in this version but the problem is the same.
I feel a bit stupid asking so simple thing but any idea would be very appreciated.
convertToMeters 0.001;

vertices
(
(0 0 0)
(0 0 0)
(0 5.2 0)
(0 5.2 0)
(-0.65 0 0)
(-0.459 0 0.459)
(-0.459 5.2 0.459)
(-0.65 5.2 0)
(0 0 0.65)
(0 5.2 0.65)
(0.459 0 0.459)
(0.459 5.2 0.459)

);

blocks
(
hex (0 1 2 3 4 5 6 7) (50 45 1) simpleGrading (1 1 1)
hex (0 1 2 3 5 8 9 6) (50 45 1) simpleGrading (1 1 1)
hex (0 1 2 3 8 10 11 9) (50 45 1) simpleGrading (1 1 1)
);

edges
(
arc 4 5 (-0.60 0 0.248)
arc 6 7 (-0.60 5.2 0.248)
arc 5 8 (-0.248 0 0.60)
arc 9 6 (-0.248 5.2 0.60)
arc 8 10 (0.248 0 0.60)
arc 9 11 (0.248 5.2 0.60)
);

boundary
(
inlet
{
type patch;
faces
(
(4 0 1 5)
(5 0 1 8)
//(8 0 1 10)
);
}
outlet
{
type patch;
faces
(
(6 2 3 7)
(9 2 3 6)
(11 2 3 9)
);
}

wall
{
type wall;
faces
(
(0 4 7 3)
(1 2 9 8)
);
}
freestream
{
type patch;
faces
(
(8 5 6 9)
(5 4 7 6)
(10 8 9 11)
);
}
);

mergePatchPairs
(
);