Heat Transfer in Microsphere Cryogenic Insulation
Multilayer insulations, while thermally very effective under controlled conditions, often suffer considerable deterioration and poor predictability in thermal performance when installed on cryogenic systems. These undesirable features are caused by a number of factors, namely the difficulty in installing the multilayer in certain geometries, the strong dependence of conduction on compressive load, and the highly anisotropic thermal conductivity . Notable improvement in these features may be realized through the use of an insulation consisting of packed, hollow glass spheres, typically of sizes ranging from 15 to 150 µm in diameter and coated on the exterior with a film of low emittance such as aluminum. While the effective thermal conductivity is five to ten times that of multilayer, the isotropic nature, ease of installation, and repeatable performance make this system an attractive alternate to multilayer in many applications. The good thermal performance of the microsphere* insulation comes from the constriction resistance to conduction, radiation attenuation by the metal coating, and the light weight and reduced heat capacity of the hollow spheres. Typical performance characteristics of this system are presented in Table I in comparison with those of multilayers.
KeywordsHeat Transfer Hollow Sphere Effective Thermal Conductivity Optical Thickness Total Heat Transfer
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