Determining Radiative Heat Transfer Through Heterogeneous Multilayer Nonwoven Materials

Abstract

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 ...

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