Investigation of Gravity-Driven Drainage and Forced Convective Drying in a Macroporous Medium Using Neutron Radiography

Abstract

The co-occurrence of gravity-driven drainage and forced convective drying in a macroporous medium is investigated in this study. The drainage and drying processes of fully saturated porous asphalt (PA) specimens placed in a custom-made mini wind tunnel are documented with neutron radiography (NR). Six PA specimens of dimensions 180×10×30mm3 with a maximum aggregate size of 8 or 11 mm are used in the experiments. In the first few minutes of each experiment, there is significant moisture loss in all the specimens due to gravity-driven drainage. Most of the residual water retention is observed at the bottom region of the specimens due to the strong impact of gravity-driven drainage in the upper regions. The specimens are subjected to many hours of airflow at their top surface; however, forced convection from turbulent airflow near the upper part of the specimens is found to have a minor influence on moisture loss when there are no water clusters in the upper regions of the specimens. This points to the strong resistance to evaporation in PA as a result of the large vapor diffusion lengths. By combining neutron radiography and microcomputer tomography (X-ray μ -CT) images, saturated and unsaturated flows in the pores are distinguished. Fluid flow path during air entry and water redistribution is further analyzed by reconstructing the real three-dimensional pore geometry of the specimens from X-ray μ -CT scans.