I am trying to mesh a single heat sink fin surrounded by a body of air (as shown in the first attached picture) and I can't manage to get a fully quadrilateral mesh. I want the fin to have a coarser mesh than the surrounding air, which seems to cause a lot of trouble. The fluid section always ends up with triangular elements (picture 2), and face meshing does not give the option to force quadrilateral elements (picture 3).

Is there a way to have two different quad meshes on these bodies? I have combined them both into a single part so does that make a difference? I am hoping to have a mapped mesh on both outer sides so I can implement a periodic boundary condition also. I am a bit inexperienced when it comes to meshing so sorry if this is a basic question, any suggestions would be greatly appreciated. Cheers!