Abstract

Air infiltration has an important impact on the energy consumption of buildings. Influenced by the heat exchange in it, the temperature profile in the building envelope deviates from that of conduction. Therefore, energy analysis of a building needs to consider the coupled process of conduction and infiltration. A numerical model is presented to study the heat exchange performance of the building envelope, based on air flow and heat transfer through porous media. The governing equations are derived using the volume average method, and one-medium treatment is adopted for the description of heat transfer in the porous insulation. Computational fluid dynamics approach is used to solve the equations. As Darcy's term is the dominant factor in the momentum equation, an easily-implemented pressure correction method is presented. The model has been applied to an exterior wall, under four infiltration path conditions. Factors influencing the heat exchange performance have been discussed. The presented model has been verified by comparing its results with the experimental data. Investigation is also conducted for the dynamic insulation, which is a potential implementation of the heat exchange process in the building envelope. Numerical simulation is first performed to study the heat transfer in dynamic insulation, under both transient and steady-state boundary conditions. The results show that the steady-state analysis is a good approximation for the estimation of heat loss through the dynamic insulation. A steady-state analytical model is hence derived for the thermal performance of a dynamic insulated wall. An analytical model is also presented for the heat exchange performance of air infiltration in a conventional wall, by dividing the wall into ventilated and non-ventilated area, and treating the ventilated area as the dynamic insulated wall. The results in the study show that the heat exchange performance in the building envelope is first determined by the air flow rate, with the secondary impact from the air flow path. The influence of permeability of the material is also important as it is related to the air flow rate. However, influence of porosity of the material, and indoor-outdoor temperature gradient, is not significant