A numerical analysis of the air ventilation management and assessment of the behavior of double skin facades

Highlights

• 2D CFD simulations for thermal behavior and flow in the double skin facade.

• Investigation of solar radiation transmitted through the double skin facade.

• Study of management of the ventilation channel with a specific index.

Abstract

In the case of ventilated double skin facades, good management of shading devices and rational use of the ventilation channel are the two most effective ways to reduce the penetrating undesirable load inside interior spaces, especially during warm periods. This paper examines the effect of solar radiation, incidence angle and slat angle on the thermal properties, as well as, the solar transmission into the interior environment for a double skin facade equipped with a venetian blind. In addition, the ventilation efficiency was examined as a function of material emissivity, inlet air flow rate and inlet air temperature. To complete this operation the CFD code, FLUENT 6.3, was used with a realizable k–ɛ turbulence model. In order to solve the radiative transfer equation and to quantify the solar and thermal radiation, the discrete ordinates (DO) radiation method was implemented using a two-band spectral model. It was concluded that, the solar transmission is influenced by the solar radiation, the incidence angle, the slat angle and the material emissivity. The results showed that, when the slat angle exceeds 60°, the heat flux transmitted reach minimum values. The results indicated also, that the dynamic insulation efficiency, ɛ_d is independent of the ventilation strategy when θ > 75°, and its assessment is important, when the incidence angle is 0° and the slat angle is 85°. Thus, the ventilation channel is necessary when the solar radiation incident on the exterior glazing is maximal and the slat prevents that the solar radiation penetrates into the interior environment.

Introduction

The improvement of the thermal comfort, the visual comfort and the buildings ventilation equipped with high glazing surfaces and located in the hot regions, generates significant energy savings. That encourages the domain specialists, to search the ways to deal with this problem, by using several technical and systems, such as: the double skin facade (DSF) and the shading devices. The DSF is a special type of envelope, where a second skin, usually a transparent glazing, is placed in the front of a regular building facade. The air space in between, called the channel, can be rather important (up to 0.8–1.0 m) [1]. In the channel, the solar shading device such as the venetian blinds is often used to improve the DSF performance. As the goal is always, conserved energy and ensure the thermal comfort in the indoor space, several constraints affect the DSF behavior, must be analyzed and managed well, such as: the shading device performances and the ventilation channel. However, the presence of a shading device renders the process of the heat transfer and the airflow, complex. The shading device is an important component of the DSF. The shading device provides protection from direct sun radiation and overheating in summer, thus reducing the cooling loads for the building. Several parameters have been examined in order to assess the shading device performances. The slat angle effect and the distance between the shading device and the glazing were investigated by Gratia et al. [2], Mauricio [3] and Jiru et al. [4]. Other important studies, such as, Pfrommer et al. [5], Rosenfeld et al. [6], Yahoda et al. [7] and Chaiyapinunt et al. [8], were interested in the definition of the solar-optical properties of a shading device, such as: the solar absorption and the solar transmission. These quantities help to assess the DSF thermal performances, especially when we use, the thermal–solar separation technique. Concerning the ventilated channel and its thermal behavior, there are three different operating modes for the ventilated channel: natural, forced or mixed. The natural ventilation is driven by two mechanisms: the difference in the wind pressure or the buoyancy effect produced by the temperature difference between the air channel and the exterior environment. The mechanical ventilation is only used when the natural ventilation driving forces, become insufficient to achieve the desired performances. According to the ventilation type and the system design, the airflow can be oriented upwards or downwards. The experimental results show that, the use of the DSF can reduce the energy consumption until 15%, while using a natural ventilation and until 30% in the winter while using a passive solar heating, Xu et al. [9]. For optimization and study the DSF behavior, numerical and experimental models have been used. Network models, Tanimoto et al. [10], the lumped and the analytical models, Park et al. [11], the dimensional analysis, Balocco et al. [12], the airflow network models coupled with the energy simulation, Gratia et al. [13] and the zonal models, Jiru et al. [14]. However, for mechanically ventilated facades, an experimental study, well detailed by using the computational fluid dynamics (CFD) was led by Manz et al. [15] and for naturally ventilated facades [16].

The study of Safer et al. [17] is concerned for a three-dimensional simulation with a CFD tool for a DSF equipped with venetian blinds. Ji et al. [18] investigated the coupled heat transfers by convection, conduction and radiation in a DSF. Recently Darco et al. [19], proposed a methodology for modeling the solar irradiation with a CFD tool. The authors deducts that the slat angle variation minimizes the solar transmittance into the interior environment by 85%. In terms of energy savings and comfort conditions, several studies have been conducted, such as the works of Corgnati et al. [20], Perino et al. [21], Perino et al. [22] and Serra et al. [23]. In the absence of the experimental tests, the aim of this work is to evaluate and improve the DSF behavior in term of a specific index, in this work, it is the dynamic insulation efficiency, thus the quantification of the flux penetrating inside the local with a CFD tool and taking into account several parameters that influence significantly the DFS behavior, such as: the incidence angle, the slat angle, the material emissivity and the mass flow rate.