Contributions of micromechanics in the characterisation of heat transfer: a multi-scale model to take into account the variability of the thermal conductivity of a biosourced wall.

Abstract

The building sector is facing the environmental challenges that we are facing nowadays, both in terms of low carbon and summer comfort. Bio and geo-based materials can meet these challenges. Nevertheless, a lack of hindsight on their behaviour leads to a regulatory vacuum that hinders their use. Indeed, the characterization and testing of these composites are relatively time-consuming and even costly. In this context, we propose a numerical modelling by finite elements of the heat transfer through a multilayer bio and geo-based wall. This simulation is carried out in particular on a heat wave episode. This allows us to understand the impact of the variability of the thermal conductivity of the wall by taking into account the dependence of physical quantities on temperature. The originality of the study lies in the multi-scale aspect. On the one hand, analytical approaches derived from micromechanics make it possible to take into account the effects linked to the microstructure of the composites (nature/form of the aggregate, nature of the binder) and to the increase in the thermal conductivity of the air with ambient temperature. On the other hand, these results feed a macroscopic modelling of the bio-sourced wall. Given the high porosity of the materials studied, of the order of 60 to 80% for vegetal concretes for example, this multi-scale study highlights the need to take into account the effects of temperature on the thermal performance of bio-sourced walls.