Numerical studies with CFD approach of the heat and air flow transfers combined with solar radiation in double skin facades

Transcription

1 September 2004 Page 1 of 6 Numerical studies with CFD approach of the heat and air flow transfers combined with solar radiation in double skin facades Nassim Safer, Monika Woloszyn, Gilles Rusaouën and Jean Jacques Roux Centre de thermique de Lyon, INSA de Lyon 20, av. A. Einstein, Villeurbanne, France CNRS-UMR 5008 UCBL / INSA de LYON ABSTRACT: A comprehensive modelling of a compact double-skin facade equipped with venetian blind is proposed here. The impact of solar radiation, slat tilt angle, lateral distance (glazing-blind) and air flow rate on temperature and velocity fields inside the channel of the double-skin facade is analysed using CFD approach. Conference Topic: 5 Material and building techniques Keywords: double-skin facades, numerical simulation, CFD approach, discrete ordinate 1. INTRODUCTION Nowadays different kinds of transparent facades are erected in new architectural projects especially in Central Europe and also in France. These facades called double skin facades are composed generally of an internal and an external glazing, the channel formed by these glazing is ventilated in most of the cases. The majority of double skin facades are equipped with venetian blind inside the channel in order to control the daylight and overheating due to the solar radiation. The aim of these facades is, on one hand to increase the internal comfort and on the other hand to decrease the energy consumption. To reach this objective, the double skin façades behaviour must be studied accurately. The double skin facades behaviour depends on many parameters such as azimuth, geometry of facades, ventilation strategy, solar protections, etc. The location and the orientation of the double skin facades define the collimated beam radiation (direction of the solar radiation and incidence angle). Moreover the slat tilt angle and the lateral distance (external glazing-blind) influence the internal double skin behaviour. Finally, the air flow rate have an impact on the heat transfer coefficients. A correct modelling of the air flow and radiation phenomena combined with solar radiation is needed to give us a better understanding of the double skin facades behaviour specially in summer conditions. This work is dedicated to the study of the influence of these parameters on the velocity and temperature fields. It is a first step in establishing a global model of double skin facade. 2. CASE DESCRIPTION Several reviews describing existing types of double-skin facades can be found in the literature, see for exemple Faist [1], the Belgian Building Research Institute [2], Arons [3] and Safer [4]. Proposed classifications are generally based on geometrical parameters and channel ventilation strategy. Geometrical parameters include height and width of the facade, width of the channel, type and position of the solar protection, etc. Channel ventilation strategy include the type of ventilation: natural, mechanical or hybrid and the description of the air path: outside-channel-outside, outsidechannel-inside, etc. Of course the physical behaviour of the double-skin facades depends also upon a number of other parameters such as inside and outside glazing properties, solar protection type, etc. We are interested here in the compact one floor double-skin facade: 3 meters high and 1.5 meters length. The width of the channel formed between the internal and the external skin is 20 cm (see fig. 1). The studied double-skin facade is equipped with a usual venetian blind (25 mm width). The ventilation is external (the air circulating in the channel comes from and rejected to the outside) and the inlet and the outlet are placed respectively in the bottom and the top of the external glazing. However, the air flow inside the channel of the double-skin facade is essentially 2D and the 2D simulations is sufficient to asses the detailed double-skin behaviour.

2 September 2004 Page 2 of 6 Table 3: The weathers data for Nice city (43 42 latitude and 7 16 longitude). Component Value Direct solar radiation 910 W/m² Incidence angle 66 Diffuse solar radiation 145 W/m² External temperature 302 K (29 C) External convective heat coeff. 10 W/m².K Figure 1: The double-skin geometry. The external glazing is a simple glazing (12 mm), the internal glazing is a double glazing ( mm) and the blind is made of aluminium. The radiative characteristics of these components are very important, especially for the detailed calculations of solar gains: - The absorption coefficient (the extinction coefficient) of the glazing is given according to three wavelength bands (see table 1). This splitting is required for a better representation of the solar radiation and of the infrared radiation. The emissivity of the blind is 0.15, Table I: The absorption coefficients of the glazing according to the wavelength bands. Band Absorption coefficient (m -1 ) µm µm 200 > 4.5 µm The refractive coefficient is defined as the ration between the velocity of light in the vacuum and the velocity of light in the material. Table 2 gives this property for each component of the double-skin facade. Table 2: The refractive coefficients of the components of the double-skin facades. Component Refractive coefficient air 1 glazing 1.5 slat 1.44 For a better understanding of the overheating inside the channel of the double-skin especially in summer conditions, we simulate the pressure-velocity and temperature fields inside the channel for a south oriented facade at solar noon and the weathers data corresponding to Nice city (see table 3) situated in the south of France. The interior air characteristics are supposed to be constant: they are given in table 4. Table 4: The interior data. Component Value Internal temperature 298 K (25 C) Internal Convective heat coeff. 2.5 W/m².K Finally, the velocity at the air inlet is set to 0.1 m/s and the inlet temperature correspond to the external temperature since the ventilation is external. In practical cases, the value of the velocity inlet is about 0.10 to 0.15 m/s [2]. 4. MODELLING Nowadays, the double-skin facades are mainly used in the new architectural projects and several literature reviews describing the double-skin behaviour equipped with venetian blind has been done. These studies showed that there is a lot of very different approaches for the simulation of the heat and air flow transfers in the double-skin facades. These approaches are based on different modelling degree (different level of simplification). Arons [3] has developed a simplified numerical model based on an electrical analogy associated to a nodal model. His model is intended to predict the energy performance of multiples types of double-skin facades. Same approach was proposed by Di Maio [5], These simulations are carried out with the simulation code Matlab-Simulink. His model is derived from the heat balances and airflow models. Some more detailed models uses the CFD approach for accurate representation. For instance the finite element method to solve the non linear equations of fluid flow and radiation heat transfer was used by Ye [6]. The advantage of the finite element method is not only easy numerical resolution. It can also be straightforwardly adapted for different geometrical configurations. Ye studied the effect of blade-to-plate spacing and the slat tilt angle on the air flow and the convective heat transfer coefficients. This investigation neglect the effect of radiation exchange between the glazing and the blind and also the turbulent effect since the author consider that the air flow is laminar.

3 September 2004 Page 3 of 6 We are interested here in the impact of the solar radiation, the slat tilt angle, the lateral distance (blindglazing) and the flow rate on the heat and the air flow transfers using the CFD approach. This approach means that the equations governing the fluid flow (the continuity equation, the momentum conservation and the energy conservation) are numerically solved in each small volume (mesh) of the computational domain. The pressure-velocity and temperature fields inside the channel of the double-skin facades are computed using FLUENT [7]. The finite-volume method is the base of this commercial solver and the solution is provided by the SIMPLE solving algorithm. This algorithm is based on a predictor-corrector and was developed by Patankar [8]. 3.1 Air flow modelling Several conditions need to be met by the numerical model in order to represent correctly the airflow within the channel. As the channel of the double-skin is mechanically ventilated, the air flow inside the channel is turbulent. This turbulent effect is augmented by the presence of the blind inside the channel. This turbulent flow generates turbulent quantities and additional equations need to be introduced for the solving of the system of equations. These additional equations are the turbulent kinetic energy and dissipation turbulent flow, they are defined by a turbulent model ( realizable k-ε model for our case). The realizable k-ε model constitute a revised standard k-ε model [9] who is widely used in computational fluid dynamics and validated for turbulent classical flows. The revised model consists on a new equation for dissipation rate and a new realizable eddy viscosity formulation [10]. 3.2 Radiation modelling For the better understanding of the impact of the radiation on the double-skin behaviour, the global solar radiation composed by the direct solar radiation (collimated beam) and the diffuse one (see fig. 2) as well as the long wave radiation of each components of the double-skin facade (internal and external glazing, slats) must be modelled accurately. The radiation modelling consist on the solving of the Radiative Transfer Equation (RTE) taking into account phenomena mentioned above. Figure 2: The solar radiation components. The Discrete Ordinate Method (DOM) is used to solve the RTE [11]. The continuous hemispherical space is divided into a finite number of discrete solid angles, each solid angle being associated with a particular direction fixed in the global Cartesian system. In this study the following hypothesises are assumed: - The external glazing and the internal glazing are modelled as a semi-transparent material, - The air inside the double-skin and the air of the double glazing are modelled as a transparent material, - The blind is modelled as an opaque wall. 5. RESULTS 5.1 Impact of the direct solar radiation To illustrate the impact of the direct solar radiation on the double-skin behaviour, we compute the velocity-pressure and temperature fields inside the channel of the double-skin according to three distinct configurations. The first one is computed according to the standard weather data (see table 3). The second one consists in the modification of the incidence angle (the others conditions are unchanged), the computations are done for IA=24. The last one is computed without the solar radiation. The figure 3 shows the temperature profile inside the channel according to these three configurations. Figure 3: Temperature profile inside the channel according to the three configurations at 2 m height. The angle between the slats and the incident beam radiation has an impact on the temperature fields inside the channel of the double-skin since the highest values of temperature occur in the first configuration when the incidence angle is 66. In fact, the absorption of the slats depend on the incidence angle: when the direct solar radiation on the slats is close to normal, the slats absorb more and their temperature is high. The double-skin behaviour depends also on the velocity fields inside the channel. The figure 4 shows the iso-velocities inside the channel according to these three configurations and allows a better understanding of the air flow transfer. Important oscillations due to the solar radiation can be seen in figure 4 ((a) and (b)). these oscillations disappear in

4 September 2004 Page 4 of 6 the last configuration when the double-skin behaviour is computed without the direct solar radiation. 5.2 Impact of the slat tilt angle To shows the impact of the slat tilt angle on the double-skin, the pressure-velocity and temperature fields inside the channel were computed according to several slat tilt angles (0,30, 45 and 60 ) for the standard weather data (Incidence angle = 66 ). Figure 6: Temperature profile inside the channel according to several slat tilt angles at 2 m height. Figure 4: Iso-velocities inside the channel for the three configurations respectively from left to right. The figure 5 shows the velocity profile along the channel placed at m and m from the external glazing for the first and the last configuration (respectively with and without direct solar radiation) and allows the quantification of these oscillations. With the direct solar radiation, the velocity profile is not constant on the height. For 2 meters height, the velocity value is about 0.15 m/s on the right part of the channel while the velocity value tends to zero on the left part of the channel. Without the direct solar radiation, the velocity is much mode uniform on the height, the velocity value reaches 0.22 m/s beside the external glazing (on the left part of the channel) and tends to zero on the right part of the channel. The figure 6 illustrate that the slat tilt angle has only a little influence on the temperature fields within the channel and the difference is less than 5K. It is due essentially to the buoyancy effects inside the channel and, therefore to a good mixing of the air and high heat transfer between the components. Also, we are interested to the velocity and the temperature profiles along the channel. The figure 7 shows that the oscillations disappear when the slats tend to be closed. Figure 7: Velocity and temperature profiles along the channel placed at m form the external façade (respectively (a) and (b)) for 30 and 60. Figure 5: Velocity profiles along the channel placed at m and m form the external façade ((a) within the direct solar radiation (first configuration) (b) without the direct solar radiation (the third one)). 5.3 Impact of the lateral distance (external glazingblind) In practice the blind is in general placed beside the internal glazing. To show the impact of the position of the blind inside the channel on the doubleskin behaviour, additional simulations were carried out for different positions of the solar protection.

5 September 2004 Page 5 of 6 The figure 8 illustrates the temperature profile at 2 meters height for two slat tilts angle (0 and 60 ) for two blind position: - The blind is placed at 0.01 m from the external glazing, - The blind is placed at 0.19 m from the external glazing (at 0.01 m from the internal glazing). 5.4 Impact of the mass flow rate Even if the channel of the double-skin facade is equipped with mechanical ventilation, the buoyancy effect is able to modify the air flow and the thermal transfers. However a threshold exists when the effect of the forced convection will be predominant. To investigate the establishment of this threshold, the pressure-velocity and temperature fields inside the channel of the double-skin facade were computed for higher air velocity at the inlet (air velocity was doubled V=0.2 m/s). The standard double-skin facade with the blind placed in the middle of the channel was simulated here. Figure 8: Temperature profile inside the channel according to different blind position at 2 m height for 00 and 60. The figure 8 shows that the slat tilt angle has only a little influence on the temperature fields within the channel since the difference is less than 3K. This figure shows also that the position of the blind has an influence on the air temperature inside the channel Therefore, the air temperature is high when the blind is placed beside the internal glazing. To avoid the overheating inside the channel for the summer conditions, it would be better to place the blind beside the external glazing. Figure 10: Velocity and temperature profiles along the channel of the double-skin facades (respectively (a) and (b)) placed at m form the external façade. Comparison between figures 5 and 10 shows that the frequency of the oscillations seen in figure 5 tends to decrease when the flow rate increases. Its shows that the effect of the forced convection begin to be more important than the buoyancy effect. CONCLUSION Figure 9: Velocity profile inside the channel according to different blind position at 2 m height for 00 and 60. The figure 9 illustrates the velocity according the y axis (V y ) at 2 meters height. This figure allows the detection of several re-circulation zones beside the internal glazing when the blind is placed at 0.1 m form the external glazing and beside the external glazing when the blind is placed at 0.1 m form the internal glazing. The re-circulation zones are mainly due to natural convection: the stack effect close to the blind resulting from important temperature gradient. The double-skin facades behaviour depend on several linked parameters. The direct solar radiation is an important one since it modifies significantly the velocity and the temperature fields inside the channel. In this case, the buoyancy effect are important and predominant, even when a mechanical ventilation is used. This predominance is changed towards the forced convection when the mass flow rate increases. The slat tilt angle effect on temperature and velocity fields inside the channel is limited. Nevertheless, it is still a very important parameter for global approach of buildings since its regulates the solar radiation transmitted to the inside. The blind position was also investigated. In the summer conditions, the blind placed beside the external glazing is the most appropriate since the air temperature inside the channel decreases. However,

6 September 2004 Page 6 of 6 this results must be complemented by some computation especially in winter conditions. The next step of this work will be the conception of a simplified model to be implemented within a global building energy simulation code. The final objective is to asses the contribution of double-skin façade to energy savings and to limiting the overheating problems in summer. The good predictions of the pressure-velocity and temperature fields inside channel and the establishment of the main parameters influencing the double-skin behaviour will be our support to design this simplified model. It is anticipated that the simplified model will be based on a few vertical zones. Some comparison with the experimental data are also planned. [9] W.P. Jones and B.E. Launder, The prediction of laminarization with a two-equation model of turbulence - Int. J. Heat Mass Transfer, vol. 15 (1972), [10] T.H. Shih, W.W. Liou, A. Shabbir, Z. Yang and J. Zhu,. A new k-ε eddy viscosity model for high Reynolds number turbulent flows - Computers Fluids vol. 24 (1995), [11] R. Vaillon, Méthodes numériques en rayonnement thermique : la Méthode des Ordonnées Discrètes, Juin ACKNOWLEDGEMENT This study is financed by The French Environment and Energy Agency (ADEME) and SOMFY International. REFERENCES [1] A.P Faist, La façade double-peau - Laboratoire d énergie solaire et de physique du bâtiment (LESO/PB) EPFL Lausanne, Rapport de projet, juin1999. [2] Belgian Building Research Institute (BBRI), Source book for a better understanding of conceptual and operational aspects of active façades - Department of Building Physics, Indoor Climate and Building Services, Belgium, [3] M.M.D. Arons, Properties and applications of double-skin facades - University of Minnesota, Massachusetts Institute of Technology, Master of science in building technology, June [4] N. Safer, Etude bibliographique : la façade de type double-peau - Centre de Thermique de Lyon, INSA de Lyon, Rapport intermédiaire n 1 ADEME/SOMFY, Juillet [5] F. Di Maio and A.H.C. Van Paassen, Simulation of temperature and air flow in a second skin façade - Proc. of the 7th inter. conference on air distribution in rooms ROOMVENT (2000), [6] P. Ye, P.J. Harrison and P.H. Oosthuizen, Convective heat transfer from a window with venetian blind: detailed modelling - ASHRAE Transactions, vol. 105 (1999), [7] FLUENT Inc., Fluent user s guide, version Lebanon, USA (2001). [8] S.V. Patankar, Numerical Heat Transfer and Fluid Flow - Hemisphere Publishing Corporation, Washington, USA (1980).