An analytical and numerical investigation into conductive-radiative energy transfers in evacuated honeycombs. Application to the optimisation and design of ultra-high temperature thermal insulation

Abstract

Ultra-high temperature thermal and latent energy storage technologies offer a potential solution to the decarbonisation of the energy sector. However, uneconomical thermal energy losses are a barrier to their development and where significant, their minimisation requires knowledge about the different modes of thermal transport and their interaction in a given structure. In this paper, evacuated rectangular honeycomb structures are investigated as candidates for efficient ultra-high temperature thermal insulation.First, a theory is laid out along with its numerical implementation for the modelling of coupled non-linear conduction and radiation in three-dimensional cuboid multi-media structures with rarefied gaseous and opaque or semi-transparent solid media. The model is then applied to the study, optimisation and design of efficient ultra-high temperature thermal insulation based on rectangular honeycomb structures.A dimensionless number Nrc is defined and an analytical thermal analysis of rectangular honeycombs is developed which correlates Nrc to the optimal geometry and equivalent thermal conductivity of a honeycomb insulator. A numerical optimisation procedure is then presented and the correlations are validated, and thus constitute simple practical design tools for honeycomb insulators. Finally, it is shown that with an appropriate choice of geometry and materials, thin-walled honeycombs have the capacity to outperform existing high-temperature thermal insulation technologies with thermal conductivities lower than 0.01 W/m/K at 1600 K. In particular, a wall thickness of 50μm on a titanium alloy honeycomb would suffice to outperform ceramic fibre insulation over its entire range of operating temperatures.