Natural convection flow in internally heated vertical cylinders is investigated experimentally and numerically. Experimental facilities restricted the Rayleigh number range from 3 x 10(10) to 1 x 10(14), covering the laminar boundary layer regime and the first transitions to turbulent flow. Laminar and turbulent natural convection in cavities differ significantly with regard to the influence of boundary conditions. For the numerically calculating turbulent flow, several low-Reynolds number k-epsilon models are tested. They all fail to correctly predict experimentally observed laminar-turbulent transitions.