Do We Understand the Drying of Porous Materials?

Convective drying at pore scale is studied by a two-component two-phase lattice Boltzmann model at pore scale showing the important influence of capillary pumping from large to small pores and pinning of contact lines in dual porosity materials. Capillary pumping and pinning explains the first drying period with almost constant drying rate. The drying rate during the first drying period is found to depend on air velocity or Reynolds (Re) number, and a logarithmic relation between average drying rate and Re is found. This analysis allows to better understand first and second drying period and their dependence on air velocity and pore structure. In an upscaling example, the evaporative cooling effect of a two-layer porous pavement with optimal wetting protocol is analyzed for a square in Zurich. The top layer of the pavement enhances drying during first drying period due to capillary pumping, while the second layer prevents loss of sprayed water to the subsoil. The evaporative cooling from pavements enhances the thermal comfort, but has to be combined with other measures like shadowing from trees. The proposed multiscale approach upscaling from pore to continuum scale is believed to enhance the understanding of drying of porous materials and its application in urban and building physics.