1,720,997 research outputs found
Wind and buoyancy driven natural ventilation in double skin façades
No full textThe computational fluid dynamics (CFD) modelling activity presented in this work aims at investigating the reliability of the assumptions employed in a double skin façades (DSFs) simplified model, developed for the integration of naturally ventilated DSFs in building simulation (BS) tools. The simplified fluid-dynamic model considers both the wind action, by means of pressure coefficients (Cp) on the openings, and the buoyancy inside the ventilated channel. Both the BS and the CFD models have been assessed using the database of an experimental campaign carried out in a full scale test facility, named ‘The Cube’, and made available by the Department of Civil Engineering of Aalborg University. The results show a good agreement between the CFD and BS models in terms of predicted temperature increase, with a maximum deviation of 15%. Both models exhibit a high sensitivity to the imposed differential wind pressure, which depends on the Cp source employed. The determination of proper Cp values for the specific case is, hence, a crucial aspect. Moreover, the CFD analysis offers a deeper insight on surface heat transfer and suggests the need to take into account the interplay between thermal and wind-driven convection, which ultimately gives rise to a mixed convection phenomenon
A new adjustable and extensible lamella device for permanent external solar shading systems
No full textA new mechanical device has been designed in order to improve the seasonal flexibility of permanent external shading systems based on lamellas. In fact the new device allows for varying both the tilt angle and the lamella extension.
The performances of lamella systems equipped with such a device was evaluated in combination with different glazing systems and compared to adjustable but not extensible systems. Beam and diffuse solar and light transmittances were calculated for different solar positions and the effective monthly averaged solar factor was derived for different sites.
The results show that the system with the new lamella device has the same capability to shade beam radiation while at the same time allowing for significantly higher penetration of diffuse radiation. This results in a higher potential for the exploitation of daylight and solar gains
Explicit versus implicit method for radiative heat transfer in gray and diffuse enclosures
No full textThe paper presents an explicit formulation of the radiative problem in gray enclosures. The method is based on a series expansion that derives the mutual radiation factors by means of summation of the contributions to the radiative thermal exchange given by the multi-reflection process. A mathematical proof that the infinite expansion converges to the well known solution, given by the implicit formulation and based on radiosities balance, is provided. Moreover a parametric analysis of the approximation induced by the explicit algorithm, as a result of the truncation order in the calculation of the series expansion, is carried out. The analysis shows how the precision of every approximation order is mainly related to the surface averaged infrared reflectance and is weakly dependent on the reflectance distribution. For high emissive and low reflecting surfaces, very few summations are required to meet high precision; the same level of precision is achievable also with medium and high reflecting surfaces, by increasing the truncation order of the series. More generally, the explicit formulation provides an alternative approach to problems involving multi-reflecting cavities in different engineering applications, from heat transfer to optics
Modelling Natural Ventilation in Double Skin Facade
Full text linkThe assessment of the energy performance of buildings with Double Skin Facades (DSF) requires proper dynamic simulation tools, based on models capable of predicting heat and mass transfer in the DSF 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 for the prediction of the mass flow rate in naturally ventilated DSF. The CFD modelling activity presented in this work aims at investigating the reliability of the assumptions and hypotheses employed in the simplified model, which was specifically developed for the dynamic simulation of heat transfer in buildings. Both the CFD and simplified models have been tested and evaluated on an experimental case study, using the database provided by a research program developed under IEA ECBCS Annex 43/SHC Task 34, reporting the results of a measurement campaign conducted on an a transparent naturally ventilated DSF tested in Denmark, in an experimental facility called "the Cube"
Modelling Mechanically Ventilated Double Skin Facades with Integrated Shading Device
Full text linkDouble 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
Modeling naturally ventilated double skin facades
No full textDouble Skin Façades (DSF) are nowadays very popular with architects and investors. The
DSF adaptive behavior may offer interesting opportunities for energy saving and comfort.
However the thermophysical behavior of DSF should be evaluated by means of building
dynamic simulation, using models taking into account the combined solar gains and heat
transfer through DSF.
In this paper a model for naturally ventilated DSF, based on the coupling between fluiddynamic
and thermal problems, is presented. The fluid-dynamic model considers buoyancy,
wind and friction forces and derives the mass flow rate as function of the temperatures. The
thermal model derives the air temperature profile along the channel as function of the mass
flow rate. In this way the convection inside the channel is modeled by a simplified integral
approach adopting average bulk temperature and superficial heat transfer coefficient
correlations. A first comparison between the model, the standard EN 13363-2 and
experimental data from a test facility is presented and discussed
Algoritmi di calcolo delle proprietà ottiche ed energetiche angolari di sistemi vetrati: influenza della polarizzazione
No full textParametric Analysis of a Double Skin Facade combined with Mechanical Ventilation
No full textIn the search towards energy efficient, comfortable and visual attractive facades, dynamically insulated multiple skin façade systems are currently under investigation. Among them, today, the most popular with architects and investors are the glassed double façade system, so called "double skin façade" (DSF). These façades might also offer interesting synergies as far as concern an integrated design of the building envelope and systems.
In this work, the energy performance of a common typology of mechanically ventilated DSF, combined with HVAC system, have been analysed by means of the dynamic simulation of the whole building. The simulations have been performed with TRNSYS and, for the DSF, a specific non standard component (type), developed by the Department of Energetic of the Politecnico di Milano, have been employed.
Annual energy demand for heating, cooling and lighting, as far as thermal comfort indexes, have been evaluated for an office module varying orientation, internal capacity, ventilation and shading control strategies.
The parametric study also consider two optimized single skin transparent facades in order to provide a comparative analysis among the façade systems under different operational conditions
Comparative Analysis of a Double Skin Facade through dynamic modeling
No full textDouble Skin Façades (DSF) are nowadays very popular with architects and investors. The
DSF adaptive behavior may offer interesting opportunities for energy saving and comfort.
However the thermophysical behavior of DSF should be evaluated by means of building
dynamic simulation, using models taking into account the combined solar gains and heat
transfer through DSF.
In this paper a model for naturally ventilated DSF, based on the coupling between fluiddynamic
and thermal problems, is presented. The fluid-dynamic model considers buoyancy,
wind and friction forces and derives the mass flow rate as function of the temperatures. The
thermal model derives the air temperature profile along the channel as function of the mass
flow rate. In this way the convection inside the channel is modeled by a simplified integral
approach adopting average bulk temperature and superficial heat transfer coefficient
correlations. A first comparison between the model, the standard EN 13363-2 and
experimental data from a test facility is presented and discussed
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