The cooling of turbine blades in turbines is enhanced by providing the cooling ducts with ribs, so-called turbulators. It is investigated how these ribs influence the heat transfer of the cooling air on the blades. A model is given to study this problem such that it lends itself to a numerical approach. A detailed discussion is given of the problem involved. It is shown how the ideas are implemented in a numerical code. The results of the simulations are assessed showing a practical way to test the quality of these cooling ducts.