Thermal Insulation in Flow Systems: Combined Radiation and Convection Through a Porous Segment

The present work reports analytical findings on a new concept for thermal insulation in flow systems. The basic operating principles are illustrated by a duct flow system with a nonradiating gas passing through a porous segment, where convection and radiation take place. The radiative properties of the porous segment are computed from electromagnetic theory assuming that the porous medium is of homogeneous composition. The coupled energy and radiative transfer equations are solved numerically by iterative method in conjunction with over-relaxation. For the limiting case of no scattering, the present results based on the two-flux radiation model agree well with the exact calculations reported recently. It is found that in order to have higher recaptured radiation and temperature drop, a porous medium with large optical thickness and low scattering albedo is preferred. Two illustrative examples indicate that the pressure drop which resulted from installation of the porous segment is of little concern, especially in view of the resulting huge energy savings.