Thanks LawrenceC and AlephZero. Finally, the issue was merely a meshing problem. I am ussing gmsh, which allows to provide a 1d parameter of "typical length". Although this parameter is well conserved in 2d meshes, 3d meshes seem to result with very deformed elements (e.g. tetrahedron with one...
I am solving the heat equation in a non comercial C++ finite elements code with explicit euler stepping, and adaptive meshes (coarse in the boundaries and finer in the center). I am aware the CFL condition for the heat equation depends on dt/h**2 for the 1D, 2D, 3D case. When I solve the...