Angela Limare (Institut de Physique du Globe de Paris). Mixing in heterogeneous internally-heated convection.

Past laboratory experiments of thermo chemical convection have dealt with systems involving fluids with different intrinsic densities and viscosities in a Rayleigh-Bénard setup. Although these experiments have greatly improved our understanding of the Earth's mantle dynamics, they neglect a fundamental component of planetary convection: internal heat sources, mainly due to the decay of long-lived radioactive elements.

We have developed a microwave-based method in order to study convection and mixing in systems involving two layers of fluid with different densities, viscosities, and internal heat production rates, allowing to reach the high Rayleigh-Roberts and Prandtl numbers that are relevant for planetary convection.

I will illustrate the potential of our technique with a few examples of convection experiments of an initially stably stratified system where the lower layer has higher internal heat production and density than the upper layer. Due to mixing, the amount of enriched material gradually decreases to zero over a finite time called the lifetime. Based on more than 30 experiments, I will present the derived scaling law that relates the lifetime of an enriched reservoir to the layer thickness ratio, to the density, viscosity and internal heat production rate contrasts between the two layers, and to the temperature scale imposed by the global heating rate.