Measuring the thermophysical properties of porous fibrous materials with a new unsteady-state method

In this article, the authors first introduced a theoretical model dealing with unsteady-state heat conduction in porous fabric to assess the effects due to local convection during the testing. A few important issues are analyzed including the criterion for local thermal equilibrium in the fibrous materials and the confidence time region (t _min–t _max) during measuring process. The influence due to different heat source capacities can be ignored if the measuring time is greater than the minimum time duration t _min , yet the heat loss via outside surface becomes negligible if the testing duration is below the maximum allowable value t _max. Accordingly an apparatus that can simultaneously measure two thermophysical properties (the thermal conductivity k and thermal diffusivity a) of fibrous materials is developed in this study, which then leads to the determination of the volumetric capacity via ρC = k/a. In order to minimize the influence of potential local micro heat convection and the contact resistance during heat transfer, some background, and stacking materials are adopted in the apparatus. The error range of the apparatus is estimated empirically based on the data from measuring some Perspex samples. Finally four kinds of polyester nonwoven fabrics with different porosities are tested using the device and the data analyzed and compared with theoretical predictions.