As a conclusion we can say that Gambit allows to build easily a huge panel of geometry configurations, and to mesh them in many different ways. However, it's necessary to read a little about Gambit before going ahead, as some tools are not intuitive to use.
The two kind of meshes we developped present advantages and drawbacks. The block meshing technic allows to refine the mesh where necessary, and to choose  the type of volumic mesh, however the high number of faces makes it very long to impose the boundary conditions. The second meshing technic we used is very quick, and allows to impose the boundary conditions easily, however the user can't choose the type of volumic mesh, nor refine it in some particular locations.
When building a mesh for this kind of mixing reactor, the user has to find a compromise between the two technics : if he wants to refine the mesh close to the blades, he will have to choose the first solution, if he doesn't need it the second solution will be fine.