ABSTRACT

Highly porous cellular materials and foams have many interesting combinations of physical, mechanical, electrical, and thermal properties such as high stiffness, thermal and electrical conductivity, and “ow permeability along with low speciœc weight. The development of plastic, metallic, and ceramic highly porous materials [1-3] has enabled and improved the performance of aerospace, chemical, energy, industrial, thermal, and other technologies. Of primary interest in this discussion are cellular materials (foams) of high porosity(ϕ > 0.8) that are œnding increased uses in thermal management applications such as thermal insulations [4], heat exchangers [5-7], solar energy collection and storage [8,9], hydrogen storage [10], wicks for heat pipes [11], energy conservation/waste heat recovery [12], burners for combustion enhancement [13,14], cooling systems [15], thermal control of electronics [16,17], thermal protection systems (TPS) [18], and many others. Foams have

16.1 Introduction .......................................................................................................................... 535 16.2 Overview of Transport in Porous Materials ......................................................................... 536 16.3 Local Thermal Equilibrium Model ...................................................................................... 537

16.3.1 Fluid Mechanical Characteristics ............................................................................. 538 16.3.1.1 Permeability and Inertial Coefœcient ......................................................... 538 16.3.1.2 Thermal Dispersion ................................................................................... 539

16.3.2 Effective Thermal Conductivity ............................................................................... 539 16.3.3 Radiative Transfer .....................................................................................................540 16.3.4 Total (Equivalent) Conductivity ............................................................................... 543 16.3.5 Applications ..............................................................................................................544