MODELING AND EXPERIMENTAL DETERMINATION OF CONVECTIVE VAPOUR TRANSFER COEFFICIENTS

Convective surface coefficients are commonly used in whole building simulation to model the indoor and outdoor heat and mass transport boundary conditions. There are some standards and guidelines available for convective heat transfer coefficients, which generally include correlations for simple geometries. The convective vapour transfer coefficients are not so well documented and are often calculated with the heat and mass transfer analogy, and then subsequently applied for cases in which the analogy is not valid. As a step towards the development of guidelines on the use of convective vapour transfer coefficients in hygrothermal models, an experimental setup and numerical modeling results are presented. An experimental setup was designed for the purpose of validating two numerical models: 1) a coupled CFD-diffusion model and 2) a vapour diffusion model. In the CFD-diffusion model, heat and mass transport in the air domain is solved using CFD, while, in the material domain, vapour transport is modelled. This model allows the prediction of the convective vapour transfer coefficient. It was found that the vapour transfer coefficient changes in time evolving to constant values depending on air speed, and to a lesser degree on the relative humidity of the air. In the vapour diffusion model, a convective vapour transfer coefficient was imposed and the change in moisture content was determined. The experiment consisted of a wind tunnel placed in an environmental chamber with climate control capability. The air flow in the wind tunnel was driven by a variable-control fan that allows for a range of speeds in the laminar regime. Convective vapour transfer coefficients were determined using the experimental setup for a number of humidity and air flow conditions over gypsum samples. The results were compared with the numerical results for the purpose of validation and sensitivity analysis.