Optimizing the internal cooling circuits has become vital for the design of modern engines. Although three dimensional (3D) Computational Fluid Dynamics (CFD) proves to be a powerful tool, well suited to this kind of application, a very long time is still spent in the mesh generation step. This is certainly a major obstacle to its systematic use in the engine development process. This paper proposes an original approach enabling the 3D grid generation of complex internal circuits where traditional methods prove to be unusable within reasonable time. The unstructured meshes of tetrahedra obtained in this way are usable for fluid-dynamics calculation with N3S, a finite element code. As an illustration of the method, a complete example is presented from the mesh generation step up to the CFD calculation.