Radial diffusers are used for the loading and unloading of thermal energy storages and have a significant influence on the thermal stratification. To improve the quality of the stratification, the understanding of the complex flow phenomena is necessary. The first part of this contribution considers an isothermal flow in the radial diffuser with particular attention to the flow in the inlet path as well as the boundary layer treatment. A boundary layer resolving mesh is necessary to observe a symmetrical outflow.