Moisture Transport of Free Water during Drying of Beijing Poplar Based on Darcy’s Law

Moisture transport during wood drying is critical for drying efficiency and product quality. The free water migration in Beijing poplar was investigated based on Darcy's law. Single-sided NMR was employed for non-destructive depth-resolved moisture content measurements at 60 °C and 80 °C. Based on a seepage model, the transport coefficient and migration intensity were characterized, and the effects of temperature, wood position, and anatomical direction were analyzed. The results showed that moisture content exhibited a stratified distribution with lower surface and higher interior values. Elevated temperature enhanced free water transport, with the transport coefficient at 80 °C approximately 30%–35% higher than at 60 °C. Sapwood showed higher transport capacity than heartwood, and radial migration intensity exceeded tangential intensity and decreased with depth. Darcy's law agreed well with experimental data above the fiber saturation point but showed reduced applicability below this threshold.