Experimental and numerical analysis of heat transfer and airflow on an interactive building facade
Introduction
Architectural design was oriented to more sustainable new concepts during the last decade. Many designers could realize their ideas through the use of high technology features for both aesthetic and technical point of view. Systems using Double Skin Facade (DSF) are becoming popular because they provide facade transparence as an aesthetic feature together with improved acoustic quality and reduced energy use.
A typical DSF forms the vertical enclosure of a building and consists of two parallel glazed units separated by a cavity. Each of these parallel glazing is commonly called a skin or a layer. The cavity enclosed between the two skins may or may not be ventilated. The cavity width ranges from a few centimeters to more than a meter for different facade types. The facade may be equipped with a solar radiation shading device (e.g. a Venetian blinder) outside, between, and inside the two glazing.
Some studies in literature used simple methods to calculate DSF thermal performance. In [1] the authors analyzed the energy efficiency of different kinds of one-storey multiple skin facades with a dynamic building energy simulation program. The analysis showed that the energy performance depended strongly on how the cavity air was used and most typologies could not lower the heating and cooling demand. Only by using sophisticated control mechanisms could multiple skin facades work efficiently throughout a year. The factors that influence the greenhouse effect in a DSF were identified in [2] using a one-dimensional approach under various operative scenarios. By analyzing the global behavior of a building with DSF, they found that greenhouse effect was moderately favorable to the building energy balance. A lumped variable procedure was used in literature [3] to provide, on a real time basis, optimal settings for various DSF components. A non-dimensional analytical method was also developed in literature [4] to calculate the overall heat transfer coefficient and energy balance of a DSF.
The literature contains only few experimental studies about flow and heat transfer issues in DSFs. In [5] the authors performed an experimental analysis on a single full size ventilated box window without shading devices. The authors concluded that complex flow phenomena were present in the facade cavity and the convection fluid flow in the DSF depended very strongly on the aspect ratio (width to height ratio). An idealized window with adjacent venetian blinder in laboratory was used in literature [6], [7] to evaluate the influence of heated and spinning louvers on the convective heat transfer of the external glazing. The data were used to validate a two-dimensional computational fluid dynamics (CFD) calculation where a simplified radiation method was implemented in order to take account of the diffuse radiation. Experimental measurements in laboratory had difficulties in generating appropriate boundary conditions, such as wind and solar radiation. It seemed also arduous to conduct onsite measurements, due to uncertainties in controlling the thermo-fluid boundary conditions. In addition on containing significant measurement errors, the experimental tests were generally very expensive [8] and the quality of experimental data was difficult to assure.
CFD method has also been used for studying airflow and heat transfer in a DSF. A building energy simulation program with a CFD program were coupled in literature [9]. The authors simulated a DSF with buoyancy driven flow and compared the results with experimental data from the literature, obtaining a good agreement. In [10] the authors investigated a compact DSF equipped with Venetian blinds and forced ventilation. The authors performed a three-dimensional simulation of airflow inside the cavity using a porous media model to substitute a homogeneous object for the real venetian blind but the solar radiation was not taken into account in the CFD simulations.
If the solar radiation models embedded in normal CFD programs were used to calculate radiative heat transfer, the computing time could be very long. This is especially true if one would like to generate a database by numerical simulations. Some researcher tried to determine the radiation separately to reduce the computing time. The solar heat gain coefficient (SHGC) of a multilayer shaded glazing system was evaluated in literature [11] by using the solar–thermal separation and layer method. The solar–thermal separation assumed that short wave solar radiation can be separated from long wave one. In the layer method the fenestration was broken up into a series of plane parallel layers and the system optical properties were calculated from the bi-directional optical properties of the individual layers through a scanning radiometer. Therefore through an optical analysis it was possible to determine transmitted, reflected and absorbed components of solar irradiation. Finally the absorbed component could be used as input into a thermal analysis that was solved independently [12].
From the above review one can conclude that understanding the complex nature of airflow and heat transfer in a DSF is far from being complete. No suitable tools are available for designing DSF with confidence, taking account of the complicated nature of airflow and heat transfer. The CFD approach seems the most sophisticated and comprehensive in studying the complex flow and thermal peculiarities of a DSF. Numerical simulations may be used to establish a database of the thermo-fluid performance of DSFs. Such a database will be useful to develop a simple tool for designers. In accordance with this purpose, the CFD tool should allow an intensively usage without excessive computational costs, and must be validated to ensure that the approximations used in the model are acceptable.
The study presented in this paper introduces and validates a procedure that firstly estimates with a simple analytical method the contribution of the solar radiation in the energy balance of a DSF. Then the energy is used in a CFD model to solve the complex flow and thermal features of the DSF, in order to find the global thermal performances of the facade.
Section snippets
Methods
Not many CFD studies are available for evaluating the thermal performance of a DSF in buildings because of the complicated airflow and heat transfer features. The radiation heat transfer models found in commercial CFD software are very computationally demanding. To overcome this limitation, this investigation assumed that optical analysis may be conducted separately from the thermal analysis. The effect of solar radiation on a DSF was considered through a separate program, WIS [13], that
Validation of the decoupling approach
The experimental measurements were conducted on the DSF as depicted in Fig. 2. Table 2 presents four cases of different environmental conditions for which experimental data were collected in different days and time. In this table, the I is the external solar irradiation, Text the external ambient temperature, Tint the internal room temperature, and To the air temperature at the inlet of the DSF. In Table 2 are also reported the results from WIS calculations referred to the layers absorbed
Results and discussion
As discussed in Section 1, a number of CFD simulations must be carried out to find a useful database in order to develop a simplified tool for designing DFS. The use of two-dimensional models is suggested if one wants to reduce computational costs. In this section two and three-dimensional simulations are compared to evaluate the importance of the flow features and assess as the feasibility in using two-dimensional model for the thermal and fluid flows in the facade. The same operating
Conclusions
This paper demonstrated the feasibility of using a decoupling method to separate the radiative heat transfer effects on the thermal and flow features inside a ventilated DSF.
The decoupling method allows studying the 3D flow and temperature distributions in a DSF in different conditions with reduced computing costs. The complex 3D features of the flow in the DSF cavities can be predicted with great details and the comparison between numerical results and experimental data for a commercial DSF
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2021, Building and EnvironmentCitation Excerpt :Jiru & Haghighat [20] reported errors of up to 5 °C for a simplified model based on a zonal approach at peak solar conditions. For a detailed study of ventilation in the air cavity in which one is interested in understanding airflow in the cavity including the impact of the external boundary conditions; which are dynamic in nature, a detailed numerical model is necessary such as those based on computational fluid dynamics – CFD [20–22]. The potential of ventilated façade for energy saving was investigated using the CFD approach and significant energy savings were achieved [14,23,24].