Modelling mechanically ventilated double skin facades with integrated shading device

Double skin façades (DSF), are typically composed by two transparent envelope elements separated by a ventilated airspace. Such a technology can be applied both in new and existing buildings and may combine architectural value with energy efficiency. In the most common mechanically ventilated configurations the DSF air inlet came from the indoor environment and the outlet air returns to the HVAC system. Integrated shading devices are positioned between the skins. The assessment of the energy performance of buildings with DSFs requires proper dynamic simulation tools, based on models capable of predicting DSF heat transfer under variable boundary conditions, at the price of a reasonable computational effort. Many DSF simplified models have been developed and implemented in building simulation tools, but the validation of these tools is still an open issue, especially in presence of shading devices. The CFD modelling activity presented in this work aims at supporting the assessment of a DSF simplified model, specifically developed for the dynamic simulation of heat transfer in buildings. Such a model is based on an integral approach to the vertical channel, which is assumed to be separated into two channels when the shading device is used. Averaged surface heat transfer coefficients, depending on the geometry and flow regime, are adopted in order to represent convection inside the channels, according to the available correlations. The dataset of a measurement campaign, which was performed in a twin test facility on a mechanically ventilated DSF adopting both Venetian and roller blinds, was used to validate both the CFD model developed for this study, and the implementation of a former simplified model suitable for building simulation. The CFD approach allows for an assessment of the assumptions and hypotheses employed by the simplified model. Moreover, the CFD analyses provide a deeper insight on important aspects such as, the presence and impact of recirculation, the development of velocity and temperature profiles