Transient convection experiments in internally-heated systems

Radioactive decay of unstable isotopes is one of the main heat sources in the early stages of planetary formation as well as in the mantle of terrestrial planets. Laboratory studies characterized by Rayleigh and Prandtl numbers in the range relevant for planetary bodies had remained beyond the ability of the experimental approach until the development of a new technique based on microwave heating. Using this technique, we performed a series of experiments focused on the thermal evolution of an internally heated viscous fluid cooled from above. We established a steady-state scaling law relying the internal temperature variation to the Rayleigh number and we showed that this scaling law remains valid during the transitory regime provided both internal heating and secular evolution of the temperature are taken into account. The result is a parameterized model describing the average internal temperature of the fluid as a function of time in terms of experimental conditions and fluid properties. • We generated a uniform and stable volume heat source in a large volume tank, based on absorption of microwaves guided through an innovative design of microwave circuits. • Automatic laser scanning of the tank coupled with image acquisition and processing enables us the measurement of the 3D temperature field in the convective fluid from which we extracted the volume average temperature and surface heat flux evolution in time. • We validated a transient scaling law for the time evolution of the volume average temperature in an internally-heated convective system.