Heat transfer and airflow characteristics in multilayer porous clothing under wind

Layered clothing is commonly used in human daily activities. However, characterizing the associated heat transfer mechanisms remains challenging due to the complex interplay between multiple air gaps and air-permeable fabric layers. This study investigates heat transfer from the skin to the ambient environment in a two-layer clothing system comprising two air-permeable fabric layers and two air gaps. The experiments were conducted using a sweating guarded hotplate to measure the heat flux, air gap temperatures, and fabric surface temperatures. A steady-state numerical model was developed for the skin-clothing-environment system. The fabric layers were modeled as porous and air-permeable media using the Darcy-Brinkman-Forchheimer formulation. The model showed good agreement with the experimental data. Subsequently, the validated model was used to analyze the effect of ambient air velocity, air gap thickness, and fabric layer arrangement on the heat transfer performance through the fabrics in a vertical configuration with perpendicular airflow. Results show that as the ambient air velocity increases from 0.1 to 6.0 m/s, the airflow in the outer air gap transitions from natural convection to mixed convection, and finally to forced convection, as quantified by the Gr/Re 2 ratio. A non-monotonic (U-shaped) relationship was identified between the total heat flux and the inner air gap thickness, which indicates that the air gap thickness strongly regulates the relative contributions of different heat transfer modes. In addition, the arrangement of fabric layers with different permeability significantly affects the heat transfer performance, while the influence of thermal conductivity is relatively limited. Specifically, positioning high-permeability fabrics as the outer layer, rather than the inner layer, enhances convective heat transfer and reduces the total thermal resistance. These findings clarify the governing heat transfer mechanisms in multilayer clothing systems and provide guidance for the fabric selection and clothing arrangement to improve thermal comfort.