It can't be an impeller because the vanes are almost perfectly radial, and not shaped to draw (or expel) air, and it is very heavy looking. The inertia would be tremendous!
My first thought was an air-raid siren too, although I am going to back the suggestion of it being an idler wheel from a tracked vehicle. This would explain why the vanes are much thicker at the outer edge than in the centre, and why the whole thing is a bl**dy heavy-looking piece of cast.