Investigation on airflow and heat transfer of a glazing facade with external louvers

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1 Investigation on airflow and heat transfer of a glazing facade with external louvers A. Tablada de la Torre, D. Saelens Katholieke Universiteit Leuven, Laboratory of Building Physics, Heverlee, Belgium J. Carmeliet Chair of Building Physics, Swiss Federal Institute of Technology ETHZ, Zürich, Switzerland M. Baelmans Katholieke Universiteit Leuven, Department of Mechanical Engineering, Heverlee, Belgium ABSTRACT: The application of large glazed surfaces in office and residential buildings demands the integration of shading devices in the design of the facade in order to reduce overheating and high cooling loads during warm periods. However, the physical phenomena occurring at the facade, between the external layer of louvers, the glazing pane(s) and the interior space are very complex and depend on numerous factors. The objective of this study is to investigate the influence of solar radiation on the airflow and heat transfer on a glazing facade with exterior louvers. The study is performed in three phases: (1) design and modeling of the experimental set-up, (2) laboratory experiments and (3) CFD validation. This paper focuses on the first phase. The experiment set-up consists on a vertical test opaque facade with an external layer of aluminum louvers which is placed in front of a solar simulator. 1 INTRODUCTION The application of large glazed surfaces in office and residential buildings demands the integration of solar protection or shading devices in the design of the facade in order to reduce overheating and high cooling loads during warm periods. Exterior louvers is one of the options to effectively reduce solar gains through glass windows. Detailed knowledge of the thermal and solar transmission properties of exterior louver systems is required. However, the physical phenomena occurring at the facade, between the external layer of louvers, the glazing pane(s) and the interior space are very complex and depend on numerous factors, like the direction of the incident solar radiation, the sky and wind conditions, the slat tilt angle and the glazing and slat material properties. The thermal impact of the facade system with exterior louvers is generally assessed by the Total Solar Energy Transmittance (TSET) or g-value, which describes the total fraction of incident solar energy gain through the facade system. The transmitted energy fraction consists of two parts: the solar transmittance (short-wave) τ and the secondary heat gain q i, which is the ratio of the long-wave and convective inward heat flow to the total incident irradiance on the whole facade system. g = τ + q i (1) The identification of the TSET value for facade systems is not straightforward, since the heat transfer through the non-planar, air permeable shading structure is complex. The optical characteristics of the different layers of the facade system define the fraction of the solar irradiance that is transmitted to the interior space or absorbed by the facade components. The solar transmittance is the result of direct, diffuse and reflected short-wave irradiation. On the other hand, the absorbed energy by the louvers and glazing layers turns to heat that is transferred both to exterior and interior space through long-wave radiation, convection, and conduction. In addition, forced convection due to wind action and natural convection due to temperature differences, the nature of the airflow around the louvers and the glazing surface will also influence the convective heat transfer phenomenon. Several analytical models have been developed in order to predict TSET in facades with exterior blinds (Parmelee & Aubele, 1952) (Pfrommer et al. 1996) (van Dijk & Goulding, 1996) (ISO DIS ) (Khun, 2006). However, simplifications have been made concerning physical and geometrical parameters in order to reduce computational load. In addition, most of the models are 1-dimensional although it has been stated that height plays an important role on the convective heat transfer coefficients (h c ) in facades with internal blinds (Collings, 2004) and on TSET values in double-skin facades with a shading screen (Manz, 2004). However, more recent studies have obtained h c values independently of height in double-skin facades (Safer, 2006). In single facade systems with slat-type shading devices the solar transmittance is responsible for most of the TSET. Secondary heat flow was consid-

2 ered negligible in a study by Eiker et al. for the facades with exterior blinds (Eiker et al., 2008). However, this is valid when the shading device works with the maximum efficiency. The tilt angle of the slat and the position of the sun may vary the value of the secondary portion of the TSET. In addition, the optical properties of the slats, e.g. dark color of slats, can increase the value of the secondary heat flux due to the higher absorbed radiation on the material (Simmler & Binder, 2008). Experiments on glazing facades with exterior blinds have been performed both at laboratory test facilities (Kuhn et al. 2000) (Eiker et al., 2008) and on actual facades on outdoor test facilities (Simmler & Binder, 2008) in order to obtain TSET values and to validate analytical and numerical models. The results are mainly based on the measured cooling power of a room at constant temperature. However, apart from the different slat-shape used on these experiments (blinds vs. elliptical louver in this study) (Fig. 1a), a detailed temperature profile on slats surfaces and the airflow pattern and velocity field between the slats and the glass surface have not been provided for CFD validation. The main objective of this study is, therefore, to investigate the influence of solar radiation on the airflow pattern and temperature distribution on a glazing facade with external louvers. Special attention will be paid to the airflow conditions around the heated slat surfaces and the resulting convective heat transfer at different heights of the facade system. The study is performed in three phases: (1) design and modeling of the test set-up, (2) laboratory experiments and (3) CFD validation. This paper focuses on the first phase which also includes CFD simulations and an investigation about the use of the non-gray discrete ordinates (DO) radiation model as implemented in Fluent (Fluent 6.2, 2005) for this type of facade system. The final goal of the research project will be to develop a global model for glazing facade systems with exterior louver-type shading devices. In this paper, first the test set-up design is explained. Then, the different phases of the CFD modeling are presented including the implementation of the radiation model and the simulation of the final test set-up. The paper ends with a discussion section and conclusions. 2 TEST SET-UP The objective of the experiments is to simulate solar radiation of a facade with exterior louvers. Two experimental set-ups are being constructed: (1) indoor opaque facade with external louvers in front of a solar simulator (sol-sim) and (2) two facades, one with and one without exterior louvers exposed to exterior environmental conditions in the VLIET test building. While the outdoor experiments will allow obtaining TSET values on an actual glazing facade with exterior louvers, the indoor experiments will provide data for CFD validation and for a more detailed analysis of the secondary heat gain (q i ). Only the indoor test set-up and its modeling is explained in this paper. The indoor experiment seeks the creation of a natural convection mechanism along the facade by means of the difference of temperature between the ambient air and the surface temperature of the facade elements (louvers and back-side wall). The facade elements are heated by the incidence of short-wave radiation from the sol-sim. 2.1 Solar simulator and louvers A sol-sim is installed at the laboratory of the Mechanical Department of Katholieke Universiteit Leuven, Belgium. The sol-sim attempts to imitate outdoor solar lighting conditions by emitting a shortwave irradiation beam. The simulator consists of 36 (6x6) Thorn CSI 1kW floodlamps, mounted on an adjustable steel structure as can be seen in Figures 1b, c. This structure allows the height and angle of the irradiance beam to be modified. At the top and bottom of the frame where the lamps are mounted, two ventilators are installed to extract the heat generated by the lamps. The spectrum of the individual lamps has been found to be a good approximation of the solar spectrum. The louvers on which the study is based are the type 'BS 100' which is produced by Reynaers Aluminium (Fig. 1a). They consists of a series of ellipse-shaped aluminum slats that are located externally on a vertical system and can be fixed or controllable. a b c Figure 1. a: Elliptical louvers. b and c: Solar simulator. 2.2 Test set-up requirements In order to achieve natural convection along the facade the design of the test set-up should fulfill the following requirements: The setup creates an airflow pattern around the slats that approximates real situations

3 The airflow pattern reaches a «steady state» in a realistic time Forced convection is reduced as much as possible The setup allows measurement of physical values: velocity of airflow, air and surface temperature. Some characteristics of the sol-sim and of the space where the test set-up is being installed can however produce undesirable effects: Heating of the lamp caps of the sol-sim may produce long-wave radiative and convective heat transfer Ventilators of the sol-sim may generate forced convection and enthalpy transport of ambient air Heating of the floor may lead to long-wave radiative and convective heat transfer The following design decisions are made in order to reduce these effects: Placing a glass pane in front of the sol-sim allows that part of the long-wave radiation emitted by the heated lamps is absorbed by the glass. A diminished long-wave radiative and convective heat flux will occur on both sides of the heated glass. Placing lateral walls and the glass pane reduces forced convection inside the test set-up from laboratory and sol-sim ventilation. Giving a white reflective finishing to the floor and lateral walls reduces surface heating and long-wave radiative and convective heat flux. The actual dimensions and the position of the setup design are chosen in order to optimize the uniformity/symmetry of incident radiation on the facade system. By varying the height of the sol-sim and the distance between the solar simulator and a provisional black-painted wall, an optimal configuration was obtained. Measurements were performed with a pyranometer, an infrared camera and thermocouples in order to test the uniformity/symmetry of incident radiation on the facade. The best results were obtained for the configuration as given in Figure 2. The test set-up is designed to allow a series of measurements of air velocity and temperature. For air velocity measurements, a 2-dimensional hot-wire sensor with a Constant Temperature Anemometer (CTA) is mounted on an automatic traverse system (fig. 3) which guides the probes to the pre-defined points. Measurements will be performed near the back-side wall and in between slats. 2.3 The test set-up Figures 2 illustrates the test set-up design. The set-up consists of wooden (insulated) lateral walls and floor with white reflective finishing, a back-side wall with opaque black finishing, a glass pane positioned ahead to sol-sim and louvers on a aluminium support structure (Reynaers BS 100). Figure 3. Automatic traverse system connected to CTA and computer. 3 CFD MODELLING 3.1 Turbulence and energy models 4.80 m 5.41 m Figure 2. Schematic 3D views of the test set-up and its position in front of the solar simulator. In this work, the Reynolds-averaged Navier-stokes (RANS) equations are used. The RANS approach has proven its efficiency for a wide range of engineering applications (Blocken, 2004). The realizable k ε turbulence model is chosen because this is the most widely used and validated model. The low Reynolds modeling is performed, so the near wall behavior is modeled by using a sufficiently dense mesh near walls instead of wall functions. This ensures that natural convection, which is produced in the near-wall region, is modeled. In addition, it has been proven that the use of wall functions to determine heat transfer coefficients can be responsible for considerable errors (Loomans, 1998) (Blocken et al., 2008).

4 To model natural convection a specific process of simulation is applied to ensure convergence (Fluent 6.2, 2005). First, a steady calculation is started with a reduced gravitational acceleration of e-5 ms - 2, lowering the Rayleigh number by an order of 5. When this phase converges, the results are then used as an initial condition for the unsteady calculation in which the actual value of the gravitational acceleration is used. The unsteady calculation is started with a first order discretization and followed by a second order discretization. 3.2 Implementation of the radiation model Fluent (Fluent 6.2, 2005) states that the discrete ordinates (DO) model is the only model that allows specular reflection (next to diffuse) and the calculation of radiation transport in semi-transparent media, such as glass. DO is also the only model that allows computing non-gray radiation using a wavelength band approximation for the radiation spectrum. Not only the optical and thermal properties of materials, but also incident external radiation can be specified within every band. DO model has been validated in literature. Choudhary & Malkawi used this model to simulate a room with and without a glass facade and good results were obtained when compared to actual measurements (Choudhary & Malkawi, 2001). The DO model solves the radiative transfer equation for a finite number of discrete solid angles. The fineness of the angular discretization is controlled by the user. This radiative transfer equation is transformed into a transport equation for radiation intensity: I r, s s + α + σ I r, s ( ( ) ) ( ) ( ) = s 4 s 4π 2 CbT σ α n + I d (2) ' ' ' ( r, s ) Φ( s. s ) Ω π 4π 0 where: I = radiation intensity, depending on position r and direction, α = absorption coefficient, n = refractive index, T = local temperature, Φ = phase function, s ' = scattering direction vector, Ω ' = solid angle. The equation can be written for each wavelength, but are solved for wavelength intervals. The boundary condition parameters can be defined specifically for each band. The treatment within a band is the same as that for the gray DO model. The essential parameters needed that define a boundary condition are: theta and phi division (define the number of control angles used to discretize each octant of the space), theta and phi pixels, diffuse fraction and external incident radiation beam (beam direction, width). Due to the presence of a semi-transparent medium in this work, a pixilation of 3 is recommended. The diffuse fraction (d f ) can be specified for each surface and it specifies the amount of radiation that is reflected and transmitted diffusely. The other part is then reflected and transmitted specularly. The default value is unity, which means that every incoming radiation is treated as diffuse. For semi-transparent materials at the border of the outside of the computational domain, the possibility of specifying an incoming incident radiation beam exists. This beam is described by its intensity in Wm -2, the beam width and the beam direction. The beam direction enables the user to define the spatial coordinates of the direction vector of the incoming incident radiation Preliminary analysis: modeling the sol-sim A set of simulations was performed in order to identify the parameters needed both for a correct use of the DO model and for the correct imitation of the sol-sim into the model. The sol-sim is modeled as a semi-transparent wall and three different strategies were explored: (1) to model the sol-sim with an external incident radiation beam and ambient surface temperature as a boundary condition (called radiation thermal condition), (2) with an external incident radiation beam and actual surface temperature as a boundary condition (called temperature thermal condition) and (3) with an external radiation temperature instead of an external radiation beam. Other parameters like diffuse fraction and external incident radiation beam were also tested. It was found that the use of an external incident radiation beam and the 'temperature thermal' wall condition for the sol-sim surface leads to the best results. We further observed that when using 'adiabatic' or 'convective thermal' conditions, the problem is not able to attain convergence and results illustrate a cooling anomaly due to an energy imbalance Preliminary analysis: modeling the louvers Heat transfer between the individual slats and the exterior environment is one of the crucial mechanisms which influence the airflow and temperature distribution on the whole facade system. Special attention is given to the modeling of the louvers. The slats are modeled both with and without the interior air volume in the slats (fig. 4). The effect of the d f of the slats is analyzed in section 4.2. Figure 4 shows that the surface temperatures of the slats are clearly influenced by including the interior air volume in the slats. For the case with interior air volume, the maximum and average surface temperature of the slats are higher compared to the case without air volume. This may be explained by thermal stacks effects in the case with interior air volume. We further observe that the left corner of the slat with interior air volume shows the highest temperature (329 K) due to the direct radiation on the corner. We conclude that in order to simulate accurately air flow and heat transfer in and around the slats, the interior volume of the slats needs to be included in the simulation domain. For the included case the thermal condition of the surface should be

5 'coupled'. This is applicable when the wall (slat surface) has a fluid or solid region on each side, which is called a two-sided wall. In this case, the two regions are the air on the exterior and interior of the slat surface. No additional thermal boundary conditions are required, because the solver calculates heat transfer directly from the solution in the adjacent cells (Fluent 6.2, 2005). Temp. BC T=370 K Irradiance bean: 800 W/m 2 First band Pressure outlet T= K Pressure outlet T= K Coupled BC ε=0.82 Glass Coupled BC ε=0.82 Aluminium Adiabatic ε=0.82 Plywood a k Excluded Interior a b Operating T= K Adiabatic ε=0.82 Plywood Surface Temperature (k) x-position Included interior Excluded interior b Figure 4. Slat/air temperature (k) for included/excluded interior. a: contours of temperature. b: surface temperature. 4 CFD SIMULATION OF FINAL TEST SET-UP 4.1 Mesh and boundary conditions Figure 5 illustrates the boundary conditions and the grid. A 2D hybrid (structured / unstructured) grid was designed with quadrilateral cells close to the walls and slats (high resolution) (fig. 5b) and triangular cells in the remaining space. The grid has, in total, 47,920 cells. A sensitivity grid analysis were also performed and low dimensionless wall distance (y + ) were obtained on the cells adjacent to the slats surface (1 < y + < 2.5) and back-side wall (0.2 < y + < 4.4). This strategy considerably reduce the total amount of cells without affecting accuracy (Blocken et al. 2007). Figure 5. Test setup. a: boundary conditions and grid (solar simulator inclined surface). b: detail of grid around/inside slats. 4.2 Parametrical comparison The comparison between different cases are based on the variation of the following parameters: Diffuse fraction (d f ) of the semi-transparent surface that simulates the sol-sim (further referred to as sol-sim surface) equal to 0.2 or 1 d f of the glass surface equal to 0.5 or 1 d f of the slat surface equal to 0.7 or 1 Absorption coefficient of the glass (1m -1 ): 0.099, 0.99 or with spectral bands (Table 1) It was found that the d f of the sol-sim surface has little effect on the surface temperature of louvers and back wall, neither on the airflow pattern. However, it influences the glass and floor temperature. With d f = 0.2 (high specular incident radiation on the visible band) the amount of heat absorbed by the floor reduces, while the amount absorbed by the glass increases, especially, on the upper part of the pane. This is expected since most of the irradiance follows the beam direction normal to the sol-sim surface contrary to the case of d f = 1 (completely diffuse). Regarding the values of d f for the glass pane and the slats surfaces, we found no differences concerning airflow pattern around the slats and the back-side wall. However, when the d f of the slats is equal to 0.7 the surface temperature of the slats reduced with about 3 K and the surface temperature of the backside wall increased with about 2 K due to the specular reflection from the slats.

6 We further found that he airflow pattern and surface glass temperature are highly sensitive to the choice of absorption coefficient. We therefore concluded that actual coefficient values of the simulated glass, which are dependent on the spectral wavelength, should be used according to the DO non-gray band model as shown in table 1. We summarize the final settings of the CFD model the non-gray DO model is used, with the absorption coefficient of the glass defined per wavelength band an irradiance beam of 800 Wm -2 is applied on band 0 corresponding to the visible part of the spectrum. Irradiance of 100 and 50 Wm -2 for bands 1 and 2 respectively according to the lamps irradiation wavelength distribution is used the 'temperature thermal' wall condition is used for the sol-sim with surface temperature = 370 K an 'adiabatic' wall condition is used for the floor and back-side wall surfaces 'coupled' wall conditions are used for the glass and slats surfaces the air volume inside of the slats is included in the computational domain diffuse fraction of the sol-sim = 1 except for the band 0 = 0.25 diffuse fraction of the glass = 1 except for the band 0 = 0.5 diffuse fraction of the slats = 1 except for the band 0 = 0.75 Figures 6 and 7 illustrate contours of temperature and velocity vectors for the whole test set-up and details of top, middle and bottom slats. Figure 8 illustrates the values of surface temperature along the back-side wall and on each slat. The airflow pattern and temperature distributions are found to be realistic. We observe that air flows upwards along the heated surfaces (sol-sim, glass pane and back-side wall) and flows downwards in the central region towards and through the gap at the bottom of the glass pane. Regarding the airflow pattern (fig. 7), we observe that the air flows through the slats and then upwards into the cavity between louvers and back-side wall. The air velocity close to the back-side wall is 0.2 ms - 1 at the bottom and 0.49 ms -1 at the top of the louvers. When looking at the flow patterns between the louvers, we observe that the flow pattern gradually changes with height. Table 1. Glass absorption coefficients per spectral band for DO radiation model. Simplified version from Ozisik, 1985 and Maatouk, Bands Wavelengths (µm) Absorption coefficients (1m -1 ) 4.3 Final CFD model Figure 6. Contours of temperature (K) and velocity vectors on the whole domain k Figure 7. Contours of temperature (K) and velocity vectors around top, middle and bottom slats (total 16 slats).

7 At lower slat positions, the air flows in between the louvers more uniformly, while at higher positions a vortex between the slats is generated. The vortices develop close to the bottom surface of the slats and gradually gain size and strength with height, moving slightly towards the back-side wall. The main flow pattern can be explained by thermal stack governed by the heating of the upper sides of the slats and the back-side wall (Fig. 8). Several aspects concerning the CFD DO radiation model need further study. The glass pane unexpectedly performs as a barrier to the short-wave radiation (band 0, visible) even if the absorption coefficient was set m -1. Further the properties of the semi-transparent surface of the sol-sim have to be defined. Finally, the surface temperature values and the air velocity near the slats and the back-side wall needs to be validated with the measurements. Height (m) Surface temperature (K) Slat surface Back-side wall Figure 8. Values of surface temperature (K) along the back-side wall and on each slat. 6 CONCLUSIONS In this paper, the modeling and design of an indoor test set-up in order to investigate the influence of solar radiation on the airflow and heat transfer on a glazing facade with exterior louvers. The CFD simulations focused mostly on the correct implementation of the radiation model. The discrete ordinates (DO) model was selected, which allows the modeling of semi-transparent media and of the solar simulator (sol-sim). Furthermore, it enables the use of a non-gray band model in which incident radiation and radiation material properties are defined in specific spectral bands. Several parameters were tested in order to accurately include the sol-sim as a boundary of the computational domain. The solsim surface is modeled as a semi-transparent medium, at which an incident irradiance beam with known diffuse and direct radiation intensities is imposed. Other boundary conditions were also investigated to identify the degree of sensitiveness of certain parameters. Further refinement of the implementation of the integrated radiation/convection model on CFD will be possible and necessary after comparing the CFD results with the experimental data. The results obtained will be used to validate the CFD simulations and the global model for glazing facade systems with exterior louvers. 5 DISCUSSION It was found that modeling the sol-sim with 'temperature thermal' wall condition with a correct surface temperature value was essential to obtain realistic results. The high surface temperature of the solsim influences the airflow pattern both out and inside the test set-up. In addition, including the interior air volume of the elliptical slats in the simulation domain was also necessary in order to obtain realistic results. The incident radiation and long-wave radiative heat exchange result in a non-uniform temperature profile on the slat surface which may influence the airflow pattern and convective heat transfer close to the glass facade. Moreover, a clear blocking effect of the shading device is observed as expected. REFERENCES Blocken, B Wind-driven rain on buildings. Ph.D. thesis, Leuven: K.U.Leuven. Blocken, B., Carmeliet, J., Stathopoulos, T CFD evaluation of wind speed conditions in passages between parallel buildings effect of wall-function roughness modifications for the atmospheric boundary layer flow. Journal of Wind Engineering and Industrial Aerodynamics 95: Blocken B, Defraeye T, Neale, Derome D, Carmeliet J High-resolution CFD simulations of convective heat transfer coefficients at exterior building surfaces. 8th Symposium of Building Physics in the Nordic Countries, June 2008, Copenhagen. Choudhary, R,. & Malkawi, A A methodology for microlevel building thermal analysis: combining CFD and experimental set-ups. University of Michigan. Collins, M., Convective heat transfer coefficients from an internal window surface and adjacent sunlit Venetian blind. Energy and Buildings, 36:

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