A tomography-based effective thermal conductivity model for ceramic fiber insulation

Abstract

Ceramic fiber insulation is widely used in thermal systems due to its high temperature limits and light weight. In high temperature applications, radiation is the main mechanism of heat transfer in these materials due to high porosity (greater than 95 %), and orientation of the fibers in the construction of the insulation. A combined radiation and conduction computational model has been 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. 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 void fraction. The model is validated by manufacturer data. Together, the conduction and radiation model accurately predicted effective thermal conductivity of ceramic fiber materials in the temperature range of 300–1673 K. The tortuosity-weighted conduction approach allows the model to capture the directional dependence of thermal conductivity in the highly an-isotropic fibrous structure. Critical parameters of fibrous ceramics to maximize insulating properties are also identified.