File constant/polyMesh/boundary must be edited. First remove empty patch (bkg_mesh_walls), rename non-empty patches and edit their properties^12.5:

3
(
    rotor_inner_wall
    {
        type            mappedWall;
        sampleMode      nearestPatchFace;
        sampleRegion    rotor;
        samplePatch     outlet_wall;
        offset          ( 0 0 0 );
        nFaces          45595;
        startFace       4446459;
    }
    stator_inner_wall
    {
        type            mappedWall;
        sampleMode      nearestPatchFace;
        sampleRegion    rotor;
        samplePatch     outlet_wall;
        offset          ( 0 0 0 );
        nFaces          117235;
        startFace       4492054;
    }
    outer_wall
    {
        type            wall;
        nFaces          239746;
        startFace       4609289;
    }
)

Patch rotor_inner_wall is adjoining rotor region and is of type mappedWall because heat transfer from the fluid to the solid and vice versa is assumed. Patch stator_inner_wall is adjoining stator region and is of type mappedWall. Patch outer_wall is standard wall.