Numerical simulations with the discrete element method (DEM) and corresponding experimental investigations were carried out to understand and to quantify the heat transfer in indirect heated rotating drums. Monodisperse glass spheres (diameter 2 mm) were used and the bulk movement was kept within the rolling motion mode (rotational speed between 1 and 9 rpm). The focus is on the heat transfer between the covered wall and the particles in contact with this wall, as well as between the particles on the free bed surface and the adjacent fluid. Radiative heat transfer has been neglected due to the low maximum temperature within the system (474 K). Effective heat transfer coefficients for the heat fluxes mentioned were derived from the DEM simulations, considering the actual particle velocities on the free bed surface and on the wall. The particle movement and the heat transfer resulting from the simulations show good agreement with the experiments in general and thus allow the calculation of the effective heat transfer coefficients for the range of parameters considered in the current study. (C) 2015 Published by Elsevier B.V.