Heat transfer mechanisms in highly porous fibrous insulation from vacuum to high pressures--experiment and modeling

Abstract

Highly porous fibrous ceramics are widely used as insulating materials in thermal systems due to their high temperature limits and light weight. An experimental apparatus, using the radial flow method, has been developed to evaluate heat transfer in highly porous fibrous ceramic materials. The experimental apparatus is constructed and instrumented to evaluate an insulation specimen at pressure ranges from 0.005 < P < 30 bar of pressure and for temperatures ranging from 300 - 1273 K. The design, construction, and instrumentation of the experimental apparatus are reported as well as validation of thermal conductivity measurements to manufacturer data. Analysis of experiments conducted near vacuum and at atmospheric pressure provide valuable insight into the contributions of solid conduction and radiation heat transfer. In addition to experiments conducted at the Colorado School of Mines, tomographic reconstructions of sample insulation material were analyzed to determine fiber size and orientation distributions as well as tortuosity and volume fraction of the solid and pores. A combined radiation and conduction computational model was developed to calculate thermal conductivity in ceramic fiber materials using the Rosseland diffusion approximation to evaluate thermal radiation transfer, and a tortuosity-weighted effective thermal conductivity to evaluate gas and solid phase conduction. Experiments conducted at elevated pressures provide insight into natural convection effects in the highly porous media. Nusselt correlations based on the Darcy-Rayleigh number were fit to the data and demonstrate how insulation density and fiber orientation impact natural convection. Together, the conduction, radiation, and natural convection model, informed by tomographic reconstructions, can predict directional thermal conductivity of fibrous blanket material within 6% of measured values, providing a useful tool for manufacturers to identify critical parameters of ceramic fiber insulation manufacture and installation to maximize insulating properties.