Evaluating the impact of airflow conditions and fabric thickness on nonwoven thermal resistance via local thermal non-equilibrium

Natural-based nonwovens featuring hollow fibers are investigated for eco-friendly cold-protective clothing due to their biodegradability and low thermal conductivity. However, predicting their thermal resistance under high-velocity airflow requires resolving complex fiber-air heat interactions. The present study evaluates the standard local thermal equilibrium assumption against a local thermal non-equilibrium framework. A two-dimensional numerical porous-media model was developed, coupling external turbulent flow with internal laminar flow. The energy formulation was modified via two user-defined functions that integrate radiative thermal conductivity and interfacial convection coefficients for multicomponent hollow fibers. The developed non-equilibrium model demonstrated excellent agreement with experimental measurements. Subsequent comparisons reveal that the validity of the standard thermal equilibrium assumption is regime-dependent and dictated by fluid residence time. Under horizontal airflow (1 m/s), extended residence times permit thermal equilibrium, rendering the equilibrium assumption a suitable approximation. Conversely, vertical flow shortens fluid residence time, inducing a thermal lag between solid fibers and interstitial air. Without capturing this phenomenon, the equilibrium model overestimates convective heat loss, diverging by 23.8% from non-equilibrium predictions for the thickest sample investigated at a 4 m/s vertical flow, and underestimates the insulation benefit of thicker fabrics. By resolving the finite rate of heat exchange, the non-equilibrium framework partitions the individual heat-transfer mechanisms. Utilizing this methodology prevents the under-prediction of thermal resistance in high-wind environments, providing a predictive tool for engineering cold-protective clothing.