Pore-scale Simulation of Entrapped Nonaqueous Phase Liquid Dissolution Using a Coupled Lattice Boltzmann-Finite Volume Modeling Approach

For nonaqueous phase liquids (NAPLs) that are commonly found in contaminated groundwater environments, interphase mass transfer between the NAPL phase and the aqueous phase is a process of crucial importance for both accurate assessment of risk and the design of cost-effective techniques for remediation. The NAPL-aqueous phase mass transfer process has been studied extensively using macroscale laboratory experiments. However, these experiments alone do not provide a complete characterization of factors that affect the rate at which this process proceeds and they do no provide a means to connect to microscale processes that influence the observed macroscale behavior. We apply a pore-scale modeling approach to simulate the dissolution of a residual NAPL in a three-dimensional random sphere-pack medium. We generate residual NAPL distributions using a morphological approach and quantitatively characterize the entrapped nonwetting phase by calculating volume, orientation, interfacial area, and shape of isolated NAPL regions. We use a multiple-relaxation time lattice-Boltzmann approach to obtain a detailed aqueous phase flow field, then solved the advective-diffusive equation in the pore space using a high-resolution, adaptive-stencil finite-volume scheme and an operator splitting algorithm. The accuracy of the model is verified by comparison with three-dimensional benchmark problems. We show a good agreement between the mass transfer rates predicted in the computational approach and previously published experimental observations. Predicted results of the Sherwood number as a function of the Reynolds number (Re), Schmidt number (Sc), and NAPL saturation are compared to correlations developed by several investigators.