Microscopic Modeling of Air Migration during Air Sparging

Air sparging is an emerging soil remediation method to decontaminate saturated granular soils and groundwater with volatile organic compounds (VOCs). The efficiency of an in situ sparging system is controlled by the extent of the contact between injected air and contaminated soil and pore fluid. Characterizing the mechanisms governing the movement of air through saturated porous media is therefore critical to the design of an effective cleanup treatment system. In this paper, an overview of the existing conceptual models and related microscopic research on air sparging is provided. Then, main issues associated with microscopic modeling of air sparging, including simulation of microstructure of porous medium, migration of fine particles along the air flow path, air flow pattern transition criteria, and critical size of air bubbles or clusters are discussed. Finally, three-dimensional (3D) networks of pore bodies connected by pore throats representing the main skeleton of the pore structure of porous media are recommended as the prototype of a pore-scale model to account for these irregularities in channel geometry. Methods used in constructing a 3D network are briefly summarized. According to the air-water two-phase flow theory, the transition criteria between two flow patterns observed during air sparging tests-bubbly flow and channelized flow-are discussed. The manuscript also discusses the critical size of a bubble or a bubble cluster during air sparging in a given soil. A method to estimate the critical size of a bubble or a bubble cluster in a 3D network model is suggested. Hence, this paper lays a basis for microscopic research on air spatial distribution, air saturation, air flow pattern transition, and air flow rate during the air sparging.