Abstract

Convective heat and moisture transfer coefficients are required to simulate the performance of building envelope systems, for example, in the simulation of the drying of wood or brick cladding wetted by driving rain. Such coefficients are dependent on the velocity and type of the air flow, the air and material temperature, the moisture content of the material and the relative humidity of the air. Convective heat transfer coefficient correlations are readily available for many geometries and air flow conditions, but primarily for mechanical engineering applications. It is not so for convective mass transfer coefficients. Building physicists must often put up with values from literature that are not entirely adequate or perform measurements for the conditions under study. The overall goal of this work was to study the feasibility and accuracy of using computational fluid dynamics (CFD) to calculate convective heat and vapour transfer coefficients. The objectives were: (1) to validate the CFD simulation results for boundary layer velocity and temperature profiles for laminar and turbulent forced convection, and for turbulent natural convection; (2) to simulate vapor transfer between air and a porous material; and (3) to compare the calculated convective heat and vapor transfer coefficients with literature experimental data. Several CFD simulations were performed to calculate the boundary layer velocity and temperature profiles in different configurations. The calculated convective heat transfer coefficients were compared with analytical, semi-empirical and/or experimental results from literature. A grid sensitivity analysis was performed to determine the grid independent solutions for certain cases. The overall conclusion was that CFD accurately predicted the boundary layer velocity and temperature profiles and the convective heat transfer coefficients for the cases studied. In order to simulate vapour transfer between air and porous materials, a model was developed using CFD coupled with an external vapour transport model. CFD was used to model heat and water vapour transport in the air, including both convective and radiative heat transfer, and heat transport within the material. Vapour transport in the material was calculated externally and coupled with the CFD solution at specific time steps. A transient case of air flow over a drying wood sample was simulated using the developed model. A sensitivity analysis was performed on relevant model parameters, such as the material properties of the wood and flow conditions of the air layer