A novel methodology for calculating thermal conductivity of natural hollow fibers with validation in nonwoven fabric structures

Natural fibers, especially hollow ones, are increasingly used in nonwoven insulation structures for their superior thermal insulation performance, renewability, and biodegradability, as hollow fibers offer lower thermal conductivity than solid fibers. However, accurately measuring the thermal conductivity of single hollow fibers, particularly given their extremely small diameter (10–50 μm) and thin wall thickness (around 1 μm), remains challenging, limiting the understanding of their role in composite materials. To address this, a novel approach combines experimentally measured bulk thermal conductivity with theoretical models for effective and radiative thermal conductivity in a numerical iterative process. Additionally, a theoretical framework was established to analyze composite thermal conductivity and was validated through experimental bench tests. After successfully predicting the thermal conductivity of a single hollow fiber, results indicated minimal anisotropy in the examined fibers that can be attributed to their thin wall thickness. Furthermore, the study demonstrated that fiber arrangement had little impact on thermal conductivity in highly porous structures, while decreasing fiber diameter significantly reduced radiative thermal conductivity due to increased scattering. These findings provide a comprehensive framework for evaluating the thermal behavior of hollow fibers and optimizing natural-fiber-based insulation materials, contributing to the development of more efficient and sustainable thermal insulation solutions.