Laboratoire Matière et Systèmes Complexes

Christophe Gissinger, Laboratoire de Physique statistique (LPS) École Normale Supérieure, Paris

Instabilities in Electromagnetic Pumps

Abstract : It is well known that a time varying magnetic field acting on an electrically conducting fluid causes motion within this fluid. Such electromagnetically induced flows are found in many applications, such as induction melting, magnetic levitation, or electromagnetic stirring. These types of induced flows also appears in nature, for instance in internal salty oceans of Jupiter’s moons. During this seminar, I will discuss both theoretical and experimental aspects of these electromagnetically driven flows. In a first part, I will present numerical modeling of an Annular ElectroMagnetic Pump (EMP) similar to the ones used in some secondary cooling system of fast breeder nuclear reactor. For some parameters, a large scale instability is observed in the fluid and leads to a dramatic drop of the flow rate developed by the pump. For sufficiently turbulent flows, this instability yields a surprising behavior : in a large region of the fluid domain, the fluid moves backward compared to the propagation of the driving electromagnetic wave. These results are understood in the framework of a simple model based on magnetic flux expulsion. Finally, a laboratory experiment is presented. A liquid metal is confined between two concentric cylinders and submitted to rotating magnetized discs, generating an electromagnetic forcing similar to the one used in electromagnetic pumps. Preliminary results indicate a driving of the fluid in good agreement with theoretical predictions. For some parameters, it is possible to observe unstable regimes, characterized by some intermittency of the flow.

In collaboration with P. Rodriguez-Imazio, S. Reddy, M. Pereira, F. Petrelis and S. Fauve.

Left : Schematic view of a typical electromagnetic pump. Right : Numerical simulation showing the velocity distribution of the axial flow in such a pump when submitted to magnetic flux expulsion.