Determination of Radiative Thermal Conductivity in Needlepunched Nonwovens

Abstract

Radiation heat transfer is found to be the dominant mode of heat transfer at temperatures higher than 400-500K [11]. Convection heat transfer being negligible in nonwovens, effective thermal conductivity is given by the sum of its conduction and radiation components. In this research two methods were identified to determine radiative thermal conductivity of needlepunched samples made from Nomex fibers. The first method involved the determination of radiative thermal conductivity using effective (total) thermal conductivity determined using a Guarded Hot Plate (GHP) instrument. In the second method radiative thermal conductivity was estimated using the extinction coefficient of samples. The extinction coefficient was determined by using direct transmission measurements made using a Fourier Transform InfraRed (FTIR) spectrometer. Results confirmed that radiation was the dominant mode of heat transfer at temperatures higher than 535 K. The conduction component of effective thermal conductivity did not change much in the range of densities tested. Empirical models for predicting the temperature difference across thickness of the fabric and the radiative thermal conductivity with R-square values of 0.94 and 0.88 respectively showed that fabric density, fabric thickness, fiber fineness, fiber length, mean pore size and applied temperature were found to have significant effect on the effective thermal conductivity and its radiation component. Though a high correlation between the results of Method 1 (Guarded Hot Plate) and Method 2 (FTIR) was not seen, the absorbance measurements made using the FTIR spectrometer were found to have significant effect on the radiative thermal conductivity.

Modeling the role of microstructural parameters in radiative heat transfer through disordered fibrous media

“…Traditional studies of radiative heat transfer in fibrous materials have been based on developing ''radiation thermal conductivity" values, which were often obtained via empirical methods or rigorous theories requiring input values that are often not readily available [4][5][6][7][8]. Radiation thermal conductivity is often calculated as the ratio of the measured heat flux to the temperature difference across the thickness of a medium.…”

Fabrication of Poly(methyl methacrylate) Nanofibers and Polyethylene Nonwoven with Sandwich Structures for Thermal Insulator Application

“…As shown in Fig. 3 Moreover, in fibrous insulation materials because of low fiber volume fraction, heat conduction through the solid phase (the fibers) is not significant and conduction through air is usually considered poor (0.17 BTU·in/hr·ft 2 ·°F) at room temperature. This can be explained by the fact that an increase in the density makes the fibrous structure of nonwoven mats more denser.…”

“…One of the key solutions to eliminate this problem is thermal insulation. 3 Some parameters such as an average diameter of fibers, weight, thickness, crystallinity, and pore sizes of fibrous insulators can be affected by overall heat transferred in each mode. Thermal insulation is used in various applications for decreasing heat transfer between a substance and its environment.…”

A simple simulation method for designing fibrous insulation materials

“…There are also analytical studies dealing with the effects of fiber orientation and fiber length on thermal conductivity (see for instance [8,9]). There are also numerous predominantly experimental studies reporting on the thermal insulation properties of different fibrous materials obtained, for instance, by a guarded hot plate apparatus [10,11]. In such studies, performance of the material in blocking conductive and radiative heat transfer is often lumped together in the form of an effective conduction-radiation thermal conductivity [10,11].…”

“…There are also numerous predominantly experimental studies reporting on the thermal insulation properties of different fibrous materials obtained, for instance, by a guarded hot plate apparatus [10,11]. In such studies, performance of the material in blocking conductive and radiative heat transfer is often lumped together in the form of an effective conduction-radiation thermal conductivity [10,11]. Improved testing procedures and more advanced macroscale numerical simulations have also been developed for studying the combined conduction-radiation heat transfer through fibrous media with a specific attention to the effects of operating temperature and pressure on the performance of high-temperature insulations by [12,13].…”