Experimental and simulation study on validating a numerical model for CO2 density-driven dissolution in water

Carbon dioxide density-driven dissolution in a water-filled laboratory flume of the dimensions 60~cm length, 40~cm height, 1~cm thickness was visualized using a pH-sensitive color indicator. We focus on atmospheric pressure conditions, like in caves where CO2 concentrations are typically higher. Varying concentrations of carbon dioxide were applied as boundary conditions at the top of the experimental setup, leading to the onset of convective fingering at differing times. The data were used to validate a numerical model implemented in the numerical simulator Dumux. The model solves the Navier-Stokes equations for density-induced water flow with concentration-dependent fluid density and a transport equation including advective and diffusive processes for the carbon dioxide dissolved in water. The model was run in 2D, 3D, and pseudo-3D on two different grids. Without any calibration or fitting of parameters, the results of the comparison between experiment and simulation show satisfactory agreement with respect to the onset time of convective fingering as well as the number and the dynamics of the fingers. Grid refinement matters in particular in the uppermost part where fingers develop. The 2D simulations consistently overestimated the fingering dynamics. This successful validation of the model is the prequisite for employing it in situations with background flow and for a future study of karstification mechanisms related to CO2-induced fingering in caves