Radiative Heat Transfer in Porous Media

Abstract

Radiative heat transfer in porous media can be a significant mode of heat transfer in many applications, ranging from low-temperature insulation to combustion. A porous medium may be considered a collection of elements or particles. The shape and size of these particles may be obvious, or some approximations may have to be made to break the structure into a collection of particles. Different sizes of particles and the application of the Raleigh, Mie, and geometric scattering theories are examined. The chapter discusses those systems where a large number of porous-media applications lie in the large-particle range. The question of dependent versus independent scattering (or absorption) is also taken up in detail. Direct simulation methods are discussed as a technique for solving the class of problems that cannot be solved by continuum methods and to establish the range of validity of independent scattering and develop correlations to model-dependent scattering. The ray-tracing Monte Carlo method is used to examine the thermal radiative transfer through packed beds of large particles. Opaque, semitransparent, and emitting particles are considered. A technique for continuum modeling of dependent radiative heat transfer in beds of large spherical particles is presented. The scaling factor is found to depend mainly on the porosity and is almost independent of the emissivity. A novel method for modeling the effect of solid conductivity on radiative heat transfer is also presented. This method combines the Monte Carlo method of solving the radiation problem with a finite-difference solution for the temperature distribution in a spherical particle, to model the effect of solid conductivity on radiative heat transfer.