The average effective thermal conductivity Keff is measured for forty-eight multilayered needle-punched nonwoven samples. Samples are produced using glass and ceramic fibers layered in several different constructions and punched with needles with varying numbers of barbs. The thermal conductivities are determined at steady state, using a Holometrix guard hot plate at an average applied temperature of 455°C. Statistical results show an ability to predict effective thermal conductivity with greater than 88% accuracy. Important parameters of the model include fabric weight, thickness, porosity, and structure, along with the applied temperature. Results also show that the nine-barbed structure with the highest ceramic content has the greatest potential for thermal insulation at elevated temperatures.

A theoretical equation of the combined thermal conductive, convective, and radiative heat flow through heterogeneous multilayer fibrous materials is presented. Samples whose properties are analyzed by this equation were constructed from glass and ceramic webs and used in an earlier work to experimentally determine their thermal conductivities. In that experimental work, overall effective thermal conductivities were determined using a guarded hot plate instrument with temperatures ranging from 430 to 480°C. In the theoretical equation presented here, thermal convective heat flow is ignored because of fabric structural conditions, and the conduction component of the overall conductivity is determined by Fricke's equation. Furthermore, the results of Fricke's equation and the overall effective thermal conductivity are used to estimate the radiative thermal conductivity of the samples.Heat transfer through porous materials can be attributed to the simultaneous operation of three mechanisms: solid-to-solid conduction, gas conduction, and radiation [3, 10, 13, 17]. Viskanta [ 18] stated that due to interactions, these three mechanisms, strictly speaking, are not separable. In some special instances, however, treating total, heat transfer as the sum of the three independent contributions can be a reasonable approximation. As a general principle, separation of conduction and radiation is not valid. If the distance between the two surfaces is small and if they are separated by a transparent medium, only then is it a tolerably good approximation to ignore the interaction of conduction and radiation [18]. Radiation leaving a surface and passing through the material may pass through the voids in the porous materials, be transmitted through the particles, be absorbed by the particles and subsequently re-emitted, or be scattered by the particles [ 18].Heat transfer by simultaneous conduction and radiation in thermal radiation absorbing, emitting, and scattering materials was investigated theoretically by Viskanta [18]. He considered a one-dimensional system consisting of two diffuse, nonblack, isothermal, parallel plates separated by a finite distance. The space between the two plates was filled with an isotropically scattering material, and the problem was formulated exactly in terms of integro-differential and integral equations. The results both defined and illustrated several mechanisms of radiant energy transfer and showed how one mode of heat transfer influenced the other.The &dquo;state of the art&dquo; for heat transfer in materials of this type was reported by Wechsler and Glaser [20]. When radiation is coupled with the other modes of heat transfer, the energy equation, which is normally a differential equation, becomes a nonlinear integro-differential equation. This comes about because the radiative contribution to the total energy flux is due, in part, to the geometric configuration of the system and reflection [ 19]. There are no general solutions available for integrodifferential equations, but a few attempts ...

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.

The experimental and theoretical air permeabilities of twelve multilayered. needle punched nonwoven samples are compared in this paper. Samples are made from ceramic and glass with varying layer structures. The density and diameter of all fibers used to make individual webs are approximately the same, and so the fabric samples made from these webs are assumed to be homogeneous, multilayered structures. The porosity of all fabric structures is greater than 0.96. Thus, the theoretical air permeability is based on a modified Kozeny equation. Experimental measurements involve the Frazier air permeability tester.

The theoretical permeability of multilayered nonwoven fabrics was studied using a modified Kozeny equation. The Kozeny equation is based on the concept of a hydraulic radius, i.e., a characteristic length parameter. It is limited to structures with porosities less than 0.94. The structures used in this research are intended for use as high temperature insulation and all had porosities of greater than 0.96. The Kozeny equation was therefore modified to extend its usefulness to fabrics with higher porosity. Fabric construction parameters, along with fabric and fiber properties were used as inputs to this model and theoretical air permeability was determined. The effect of number of barbs and layering structure on the determination of theoretical air permeability was also considered and discussed. Statistical analysis was performed showing that fabric thickness, number of needle barbs, mean pore size and fabric density are significant factors in predicting theoretical air permeability.