Paper presented at the 5th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 1-4 July, 2007.
Double skin façades have become an important and
increasing architectural element in office buildings as they can
provide numerous advantages such as energy saving, sound,
wind and pollutant protection with open windows, solar
preheating of ventilation air, night cooling and aesthetics.
In the present study, preliminary results of the fluid
dynamics behavior of a glass double skin façade equipped with
integrated movable shading devices are presented; the aim is to
optimize both winter and summer energetic performances. The
model is developed for a façade oriented towards the south
direction and taking into account the climatic data of Italy.
The double-skin façade constitutes an optical-energetic
system with several different layers; therefore, a spectral
modelling is necessary, considering that materials have optical
properties heavily dependent on wavelength.
A spectrophotometric campaign on glasses and opaque
materials has been conducted to evaluate transmittivity,
reflectivity and absorptivity of each layer, investigating also
their variations with light incidence angle. Optical
characteristics constitute the input data for a computational
fluid dynamics code, whose task is to model the façade cavity
airflow that results from many simultaneous thermal, optical
and fluid flow processes, which interact and are highly
dynamic. The solar radiation path with its multiple reflections
at the different interfaces have been taken into account,
employing a ray tracing method, integrated in the CFD code.
The simulation shows that the winter configuration of the
proposed façade enhances the solar heat gain, in spite of the
presence of shadings, placed however in horizontal position.
Heat is stored in the air that flows in the gap and its motion
could be driven by mechanical ventilation or by natural
convection; buoyancy driven flows resulted difficult to model
because of small driving forces that lead to numerical
instabilities. Besides, a strong thermodynamic coupling
emerged between the air flow through the naturally ventilated
double skin façade and the air temperature difference between
the cavity and outside.
Solar gains in buildings are desirable in winter-time, but
problematic in summer, as they may cause overheating and
discomfort; for this reason the external layer remains open in
the hot season, giving the air the possibility of escaping from
the gap and blocking, at the same time, the solar irradiation by
the shading devices, configured with a high tilt angle.
Results from the CFD package show that the air flow in
summer conditions, although limited by the absence of the
stack effect, contribute significantly to reduce the heat absorbed
by the inner pane, so reducing building cooling loads.