Review on thermal transport in high porosity cellular metal foams with open cells

Thermal radiation in ultralight metal foams with open cells

Nomenclature a, b = partial wave coef cients C = radiative cross section F = orientation distribution function fv =  ber volume fraction G k = asymmetry scattering factor; Eq. (23) Ib = black-body radiation Ib k = Planck function i k =  ber-scattering intensity distribution K k = extinction coef cient kr = radiative conductivity k0 = propagation constant L = specimen thickness M = number of  ber polar orientations mg = molecularmass of gas-phasematerial ms = molecularmass of solid-phasematerial N = number of  ber sizes n = real part of complex refractive index P = size of the equivalent pore spaces inside aerogel pk = scattering phase function q = heat  ux r = radius of  ber T = temperature xi = fractional volume of  bers of radius ri a = size parameter, 2p r / k C k = modi ed extinction coef cient; Eq. (22) c = ratio of speci c heats at constant pressure and constant volume d s = solid fraction of matrix medium g = scattering angle

Conduction and Radiation Heat Transfer in High-Porosity Fiber Thermal Insulation

A brief analysis is presented of how heat transfer takes place in porous materials of various types. The emphasis is on materials able to withstand extremes of temperature, gas pressure, irradiation, etc., i.e. metals and ceramics, rather than polymers. A primary aim is commonly to maximize either the thermal resistance (i.e. provide insulation) or the rate of thermal equilibration between the material and a fluid passing through it (i.e. to facilitate heat exchange). The main structural characteristics concern porosity (void content), anisotropy, pore connectivity and scale. The effect of scale is complex, since the permeability decreases as the structure is refined, but the interfacial area for fluid–solid heat exchange is, thereby, raised. The durability of the pore structure may also be an issue, with a possible disadvantage of finer scale structures being poor microstructural stability under service conditions. Finally, good mechanical properties may be required, since the development of thermal gradients, high fluid fluxes, etc. can generate substantial levels of stress. There are, thus, some complex interplays between service conditions, pore architecture/scale, fluid permeation characteristics, convective heat flow, thermal conduction and radiative heat transfer. Such interplays are illustrated with reference to three examples: (i) a thermal barrier coating in a gas turbine engine; (ii) a Space Shuttle tile; and (iii) a Stirling engine heat exchanger. Highly porous, permeable materials are often made by bonding fibres together into a network structure and much of the analysis presented here is oriented towards such materials.

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Porous materials for thermal management under extreme conditions

Nanofiber technology (fiber diameter less than 1 micrometer) is under development for future Army lightweight protective clothing systems Nanofiber applications for ballistic and chemical/biologic

/pdf/application-of-nanofiber-technology-to-nonwoven-thermal-nfq1a7sxw7.pdf

Application of Nanofiber Technology to Nonwoven Thermal Insulation

The validity of an improved theoretical model for radiative heat transfer through high-porosity e ber insulation is examined by comparison of experimental measurements with theoretical predictions of heat transfer. Radiative thermal conductivity of an optically thick medium is modeled by a diffusion approximation in which the spectral extinction properties are calculated by utilizing a rigorous treatment of the e ber medium scattering phase function and the composition of the e ber material. A semiempirical model is used to calculate the e ber-matrix conduction. The accuracy of the model is tested by comparison with experimental heat transfer data measured in vacuum for temperatures from 400 to 1500 K for three types of bonded silica e ber thermal insulation materials having different e ber size distributions. The validity of the modeling approach is demonstrated by the excellent agreement between the theoretical predictions and the experimental results.

G. R. Cunnington

Papers

Heat Transfer in Fibrous Insulations: Comparison of Theory and Experiment