A new mobile bed heat exchanger is presented in this work which is composed of a flowing granular material in a rotating drum and a cylindrical pipe with potential interest in different energy applications as cooling, heating or heat recovery processes. An optimal design of the device requires a characterisation of the phenomena involved at the interface between the granular flow and the pipe. The process is modelled by the discrete element method and a global classification of the flow patterns around the pipe is presented with respect to the three main control parameters of the problem: the Froude number, the diameter ratio and the relative filling height of the drum. The second part is devoted to the characterisation of the structure of the flow at the interface (velocity field, density field) in particular in a so-called Biflow regime where granular motion occurs above as well as below the pipe which is favorable to transfer by convection. A typical behavior at the interface with the pipe consists of a zone I with high velocities of particles at the top of the pipe, a second zone with quasistatic particles or low velocity particles at the front and at the bottom of the pipe and a last zone III of depletion of particles at the back of the pipe. The Froude number has a limited effect on the features of this structure on the first layer in the range of Froude numbers considered whereas the relative height is a more determinant parameter to control the relative magnitude of velocities in zone I and zone II as well as the extent of the depletion zone. This first hydrodynamical characterisation can shed light on the dynamical regimes with improved transfer between the particles and