Dissolution-driven convection in a reactive porous medium

Abstract

The heating from above of an initially homogeneous layer of solid crystals, saturated liquid and glass ballotini (an inert matrix filler) is considered both experimentally and theoretically. The heat flux causes crystals at the top of the layer to dissolve, forming liquid which, being more concentrated and dense than the interstitial liquid below, drives convection in the lower layer. Mixing of this high-concentration liquid into the lower layer leads to precipitation, thereby releasing latent heat which raises the temperature of the lower layer. Dissolution of solid crystals from the top leaves behind a closely packed layer of glass ballotini overlain by a layer of clear liquid, both of which deepen with time. The initially homogeneous porous medium thus develops into a three-layer stratified system of (from the top): clear liquid; clear liquid with close-packed ballotini; and the evolved initial assemblage of solid crystals, ballotini and saturated liquid. Data from laboratory experiments compare well with analytical and numerical results from a one-dimensional theoretical model. The model is based on the concept that the heat supplied from above is used entirely for the dissolution of solid crystals at the upper boundary of the lower layer. The resulting compositional convection redistributes the dissolved salt uniformly throughout the lower layer, where it partly recrystallizes to restore chemical equilibrium. The crystallization and associated release of latent heat leads to a gradual and uniform increase of both the solid fraction and temperature of the lower layer. Some geological consequences of the model are presented in the concluding section.