Experimental and numerical pore network simulation studies on convective drying of capillary porous media under slow isothermal conditions are presented in this work. As a physical model of a real capillary porous medium, two dense packings of particles filled with monodisperse spherical glass beads (mean diameter 0.8 mm) and initially saturated either with distilled water or with a salt solution are prepared. Two controlled drying experiments with these packings are carried out using a custom-made setup installed in a lab-scale X-ray microtomograph. Based on in-situ tomograms (voxel size 16.4 μm) the time evolution of three-dimensional (3D) structures of liquid and salt deposit in the packings during drying are characterized. The respective results clearly demonstrate the formation of capillary liquid rings at the wedge-shaped pores located near the particle-particle contacts. The rings remain connected over a long distance to the packing surface during a significant period of drying. To highlight the crucial impact of liquid rings on drying, a 3D discrete pore network model that explicitly accounts for the ring effect is developed. Pore network simulations in the presence and absence of this effect are compared with measurements in terms of drying kinetics and saturation profiles. It is found that liquid rings act as additional hydraulic pathways for moisture transport from the interior of the pore/particle network to the surface and thus notably accelerate the drying process, and they lead to a spatially homogeneous distribution of the liquid down to low saturation. This study can be considered as a step forward in discrete modeling of drying of capillary porous media with 3D secondary capillary structures and should be of interest for various applications in the field of complex multiphase flow phenomena in porous media.

本文介绍了慢速等温条件下毛细管多孔介质对流干燥的实验和数值孔隙网络模拟研究。作为真实毛细管多孔介质的物理模型，制备了两个填充有单分散球形玻璃珠（平均直径为 0.8 毫米）并最初用蒸馏水或盐溶液饱和的颗粒的致密填料。使用安装在实验室规模 X 射线显微断层扫描仪中的定制设置，对这些填料进行了两次受控干燥实验。基于原位断层扫描图像（体素尺寸 16.4 μm），表征了干燥过程中填料中液体和盐沉积的三维 (3D) 结构的时间演变。相应的结果清楚地表明，在位于颗粒-颗粒接触附近的楔形孔处形成了毛细管液环。在相当长的干燥时间内，这些环在很长的距离内与填料表面保持连接。为了突出液环对干燥的关键影响，开发了一个 3D 离散孔隙网络模型，该模型明确说明了环效应。将存在和不存在这种效应的孔隙网络模拟与干燥动力学和饱和度分布方面的测量值进行比较。发现液环充当了水分从孔隙/颗粒网络内部传输到表面的额外水力通道，从而显着加速了干燥过程，并且它们导致液体在空间上均匀分布直至低饱和度。