Thermal plumes and electric potentials generation in a porous medium locally heated from below

International audience; The mechanism of convection and electric phenomena around an isolated heat source in a fluid saturated porous media is of interest in geothermal processes and volcanology. Laboratory and numerical experiments of transient convective flows and induced electric potentials in a porous layer with a local bottom heat source are reported. Laboratory characterization of fluid flow in porous media has a severe limitation by non-transparent nature of the media comparing to the case for a single fluid. In this study as for the temperature characterization, we utilized gridded thermocouples for multiple temperature measurement. As for the flow characterization, we set multiple electrodes to measure flow-induced potential. Axisymetric laminar plumes were experimentally generated by a small electric heater in a tank filled with water-saturated glass beads. We determined the spread of the plume with horizontally-gridded thermocouples, while vertical sets were used to characterize the time evolution of the plume ascent. Vertically-aligned electrodes along the central line exhibit large positive increase associated with plume ascent. The magnitude is consistent with the temperature increase and applied power. To understand the development of the plume, 2D-3D numerical simulations have been performed for the same situations of laboratory experiments. The flow pattern is investigated for Rayleigh numbers up to 8000. Plumes ascent in two different regimes. For Ra1200, the velocity increases owing to the development of the thermal boundary layer, remains nearly constant during the rise, before decreasing at the top of the tank. Finally, the electric potentials induced by the development of the plume are analyzed. It is shown that the signal initially decreases when the plume is detaching itself from the bottom, but largely increases during the ascension of the water, which is systematically observed in experimental results. We consider flow-induced potential as a good experimental probe for fluid flow characterization in porous media. This study is the first step to further experimental and numerical works on convective cells generation and induced electric potentials in a stratified porous medium.