||Literature review: Heat transfer through two-phase insulation systems consisting of powders in a continuous gas phase.
Yarbrough, D. W. ;
||Oak Ridge National Lab., TN.;Department of Energy, Washington, DC.
||EPA-600/R-92-203 ;ORNL/M-2426; AC05-84OR21400;
Thermal Insulation ;
Heat Flux ;
Heat Transfer ;
Particle Size ;
Thermal Conductivity ;
Two-Phase Flow ;
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This review of the literature on heat flow through powders was motivated by the use of fine powder systems to produce high thermal resistivities (thermal resistance per unit thickness). The term ''superinsulations'' has been used to describe this type of material, which has thermal resistivities in excess of 20 ft(sup 2)-h-F/Btu (3.52 K-m(sup 2/)W) per inch (2.54 cm) of insulation thickness. The present report is concerned with superinsulations obtained using evacuated powders. The literature review has shown that the calculation of heat flow through gas-powder systems is highly developed. One major weakness in the calculational procedures is the absence of structural features for the powders, which are invariably characterized as regular arrays of spheres or cubes rather than random irregularly shaped particles. The effect of particle size distribution on the shape and size of void spaces is not modeled, although it affects the thermal conductivity of the gas. Calculations of thermal performance based on simplified descriptions of the porosity distribution can be used to show the dependence of thermal resistance on interstitial gas pressure. The literature reviewed in this report provides a basis for predicting the interstitial gas pressure at which thermal conductivity begins to increase. The objective is to design filler material for powder insulation systems with ultrafine void spaces that will permit pressure increases without dramatic thermal conductivity increases.