MODELLING MECHANICALLY VENTILATED DOUBLE SKIN FACADES WITH INTEGRATED SHADING DEVICE

Proceedings of BS2015: 14th Conference of International Building Performance Simulation Association, Hyderabad, India, Dec. 7-9, 2015.

MODELLING MECHANICALLY VENTILATED DOUBLE SKIN FACADES WITH INTEGRATED SHADING DEVICE

Alessandro Dama1 , Diego Angeli2 ,

1 Department of Energy, Politecnico di Milano, Milano, Italy

2 Department of Sciences and Methods for Engineering, Università degli Studi di Modena e Reggio Emilia, Reggio Emilia, Italy

ABSTRACT
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.

INTRODUCTION
The assessment of the energy performance of buildings with Double Skin Facades 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. [Tanimoto 1997, Saelens 2002, Zollner at al.2002, Manz 2004, Park 2004, Jiru at al. 2006,] The CFD
modelling activity presented in this work aims at supporting the assessment of a simplified model, specifically developed for the integration of DSF component in the building dynamic simulation.
The thermal 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.
A former experimental validation of a simplified model was carried out using the dataset of a measurement campaign, which was performed in twin test facades on mechanically ventilated DSF adopting both Venetian and roller blinds. [Angelotti et al. 2007]. The aim of this work is to provide a more detailed assessment of the assumptions and of the hypotheses employed by the simplified model, in order to support its improvement and reliability. For the purpose of a better understanding of the thermofluid dynamic phenomena a CFD model was also developed.
The boundary conditions, both for the simplified and the CFD model, are the temperatures of the inlet air and of the surfaces inside the DSF channel. The mass flow rate is given and the validation is based on the prediction of the outlet temperature. In the following Section we describe the experimental case study and the models features and assumptions.

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