A numerical investigation of impact of architectural and climatic parameters of windcatcher systems on induced ventilation

A numerical investigation of impact of architectural and climatic parameters of windcatcher systems on induced ventilation
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  1 Proceedings of the ASME 2012 International Mechanical Engineering Congress & Exposition IMECE2012 November 9-15, 2012, Houston, Texas, USA IMECE2012-87139 DRAFT: A NUMERICAL INVESTIGATION OF IMPACT OF ARCHITECTURAL AND CLIMATIC PARAMETERS OF WINDCATCHER SYSTEMS ON INDUCED VENTILATION Mohammad Mehdi Maneshi Department of Mechanical & Aerospace Engineering University at Buffalo Buffalo, NY-14260 Email:   Amir Rezaei-Bazkiaei Department of Civil , Structural & Environmental Engineering University at Buffalo Buffalo, NY-14260 Email:  A. Scott Weber Department of Civil , Structural & Environmental Engineering University at Buffalo Buffalo, NY-14260 Email: Gary F. Dargush Department of Mechanical & Aerospace Engineering University at Buffalo Buffalo, NY-14260 Email:  ABSTRACT The large energy demand from the HVAC industry for residential buildings, along with the ever growing need for the utilization of renewable sources of energy, has brought considerable attention to the area of induced ventilation. Windcatchers are green architectural structures historically used for the passive ventilation of the indoor spaces with the minimal non-renewable energy consumption. In this paper, a computational fluid dynamics (CFD) model is used to assess a windcatcher’ s performance with different characteristics of the windcatcher design. For a single room with heat emitting objects, the effects of the windcatcher louver design and height were thoroughly evaluated in conjunction with a variety of dominant wind velocity and incident angles. A comparison was drawn between the ventilation efficiency for the cases of circular and rectangular windcatcher designs. Thermal discomfort (PD) values due to draft were calculated for different temperature and wind velocities. The developed model was employed to obtain the optimized configuration for the windcatcher-room system. The effect of ambient weather conditions on the results was investigated by performing the simulations for a range of air temperature and velocities. Results obtained in this paper provide windcatcher designers with valuable insights on the important design parameters such as windcatcher height, louver design and impact of ambient conditions. INTRODUCTION Increases in the cost of conventional sources of energy and natural resource exploitation has pushed building designers and engineers to rethink the value of utilizing natural ventilation techniques. Naturally ventilated buildings tend to rely heavily on the ambient weather conditions and the flow patterns around and inside the building. Therefore, it is important for designers to: a) have a clear idea of how the air flow circulation and thermal performance of a naturally ventilated building are affected by the external parameters and b) understand the tools on the structural and architectural level to enhance or control the ventilation. Windcatcher systems are probably the oldest natural ventilation systems that have been used historically to cool the single and complex residential application in hot and arid regions [1], [2]. Due to the expensive experimental procedures for evaluation of natural ventilation mechanisms, computational fluid dynamics (CFD) modeling has gained extreme popularity in the past decade or two to further enhance their design. The enhanced computing power coupled with robust CFD modeling has become a reliable tool for building designers and architects to come up with more creative natural ventilation strategies for different applications [2  –  7]. This paper is focused on analyzing and evaluating the performance  2 of four different windcatcher louver designs for a small conditioned room via CFD modeling. Hughes et al.,  [2] investigated the effects of 8 different louver designs for rectangular windcatchers where the louver angle of 35 degrees demonstrated optimal performance. It was verified that the windcatcher is the most effective when placed right at the windward edge of the roof where the positive pressure is maximum and, the least effective at the leeward edge of the roof. Therefore, the ideal placement for multidirectional wind towers is at the center of the roof. It also was pointed that in spite of studies demonstrating better performance of rectangular cross-sections compared to other designs, most of commercial wind tower designs are circular, most probably due to the ease of manufacturing circular designs. In his Reynolds Averaged Navier  –  Stokes (RANS) analysis of effects of wind direction (incident angle) and velocity on cross-ventilation, Nikas found good agreement between the volume flow rate aerating and induced velocity profiles from the CFD model versus experimental results [8]. The study was centered around investigating the details of natural ventilation processes by closely looking at the shape and size of the vortices and the effects of building inner topology on the ventilation efficiency. A maximum inlet wind velocity of 5 m/s was used in the wind tunnel and CFD analysis which resulted in a maximum induced flow rate of 0.345 m 3  /s to the building for a zero-degree incident wind angle. The induced flow rates decreased with an increase in the incident wind angle. The numerical study confirmed that not only the building opening dimensions and incident angle but also the magnitude of wind velocity play an important role in the air exchange rates in the building. It also was concluded that, although the inner geometry of the building does not affect the average air-change rate, it has considerable effect on refreshing the air in inner regions of the building envelope. In our study, the selection of inner columns ’ dimensions and attention to the form and size of formed vortices around the windcatcher system were inspired  by Nikas’s study.  The effects of the wind incident angle and magnitude on the quality of air exchange rate obtained form a windcatcher’s performance also have been confirmed in other similar studies [9], [10]. Li modeled a 50×50 cm square windcatcher with incident wind velocities in the range of 0.6-5 m/s and four different incident angles. It was concluded that the induced flow velocity to the building increases with increase in the inlet velocity. Slight decrease in the induced flow due to increase in incident angle was reported for wind speeds below 3 m/s. The maximum induced flow rate was reported for the wind speed of 6 m/s in zero-degree incident wind angle to be 0.19 m 3  /s [9]. Elmualim employed an experimental wind tunnel and CFD modeling to compare the performance of four-section circular windcatcher systems to rectangular ones [11]. Pressure coefficient distributions, internal air speed and volumetric air flow rate in the test room were measured for the sake of comparison with the CFD model. Similar to the previous studies, increases in induced flow rates was observed with an increase in wind velocity whereas a decline in induced flow rates occurred with increasing incident angle from zero to 45 degrees. A maximum normalized induced flow rate of 2.5 m -3  /m 2 .s was reported through the rectangular design for the inlet velocity of 6 m/s in zero-degree wind attack angle versus approximately 0.5 m 3  /m 2 .s for the circular design. The flow catching efficiency of the rectangular design over circular was more pronounced for higher inlet wind velocities. A ventilation rate of 5 air changes per hour with the air velocity of 3 m/s was achieved with the circular windcatcher. Experimental wind tunnel and smoke visualization methods were employed by Elmualim [12] to investigate the effects of air flow control mechanisms and heat source inside the room on the windcatcher performance. A 50×50 cm rectangular cross-section windcatcher of 150 cm height was analyzed. It was suggested based on the findings that the level by which the windcather system lowers the indoor temperatures in the presence of the heat sources is mainly dependent on the outdoor air temperature values. Maximum internal temperature observed in the presence of heat sources was 30-32  C when the external temperature was in the range of 20-22  C. The introduction of the heat source inside the room increased the induced flow rates specially for the cases with lower outside air wind velocities. With an 8  C temperature difference between internal and external temperatures, the windcatcher was capable of reducing the indoor temperature by approximately 6  C. It was concluded that the windcather installation was appropriate specially for overheated summer days. In another study, the efficiency of installed windcatchers was compared to the openable windows in identical conditions which resulted in considerably higher indoor induced flow rates with windcatchers on the roof versus windows on the wall [13]. In their analysis of a one-sided wind tower, Montazeri and Azizian studied the effects of placing objects, shorter and taller than the tower opening size, upstream [14]. Based on the wind tunnel analysis, they found that placing a short upstream object before the windcatcher reduces the circulation area thus increases the effective inlet area that consequently contributes to higher performance levels. A taller upstream object on the other hand, leaves the tower in the wake region of upwind where the tower works like a suction device. In another study, an experimental and CFD analysis were performed on a two-sided windcatcher system in a channel flow experiment. The stack effects of a one-sided systems was compared to a two-sided system which resulted in less fluctuant performance levels achievable with two-sided system in different wind speeds and incident angles [15]. Montazeri [16] carried out another study on the effects of openings and incident wind angles on windcatcher’s performance with an array of two to twelve-sided windcatchers in a wind tunnel. It was concluded that the induced air flow rate decreases with increase in the number of openings. However, it was suggested based on the results that the sensitivity of the windcatcher’s  performance to the incident wind angle decreases with increasing number of openings. Montazeri also stated that  3 the efficiency of the windcatcher’s performance strictly depends on creating maximum pressure difference between the inlet opening and exhaust. It also was confirmed that it is the air movement around the building that determines the optimal placement of the tower and opening sizes for the best performance. Moreover, Montazeri concluded that the rectangular windcatcher exhibites a better efficiency than the circular one for the range of wind speeds and incident angles studied. Kalantar took into account the effects of the wind tower height, wall material properties, wind velocity, humidity, ambient and inside air temperatures to assess the cooling efficiency of an evaporative cooling tower [17]. It was demonstrated that the modeled cooling system is capable of reducing the indoor temperature by as high as 10-15   C and the main area of influence exists in the first 2 meters of the tower’s height. Liu [18] considered a number of layers and lengths of louver designs in a CFD model to study the effectiveness of the windcatcher design in a typical office building for efficient contaminant removal. It was confirmed by the results that the maximum airflow is achieved as a result of enhanced resistance and the short circuit at top louvers. The windcatcher exhibited the highest ventilation rate when the louver length was equal to the reference length. Liu concluded that using a leeward side opening as a complement to the windcatcher design enhances the concentration removal and flow pattern uniformity. STUDY APPROACH AND ANALYSIS Except for a small number of studies focused on the indoor air quality and thermal comfort achieved by the adoption of windcatcher systems in residential buildings, the majority of the research has focused on building a knowledge inventory of flow patterns and accuracy of the CFD model predictions compared to limited experimental results. From this previous work, variation in ambient air temperature, humidity levels, building floor dimensions and height, interaction between different stories, inducing effects of neighboring buildings, and the amount of solar radiation absorbed by the building can potentially alter the obtained results in the literature to a considerable extent. The central focus of this study has been put on evaluating the performance of two different louver designs for both rectangular and circular windcathers in a modeled room via CFD analysis by controlling three main parameters; wind speed, air temperature and architectural features of the windcatcher. FIGURE 1. Windcatcher-room system schematic with heat emitting objects Physical Domain Studied In this research, the ventilation efficiency of a windcatcher has been studied for a simple room model. The room modeled is a    single story building with the only opening being the windcatcher system in the middle of the roof. To consider the effects of indoor human occupancy and equipment, it was assumed that there are two cylindrical (individuals) and cubic (computers) heat sources in two sides of the room (Figure 1). These objects are assumed to generate a constant heat flux of 450 W/m 2 . The only architectural elements considered in the room are two columns of 20 by 20 cm, on the side walls with 10 cm protrusion from the wall surface which are similar to the values used by Nikas [8]. A schematic of the room-windcatcher system is depicted in Figure 1. Windcatcher Design The most well-studied physical aspects of windcatcher design are the size and number of  4 openings [14  –  16] and louver design [18]. Louver design has been focused mainly on the impact assessment of number of openings on It is noteworthy that the combination of these physical parameters and ambient weather conditions can severely affect the windcatcher performance predictions, a matter that has not been thoroughly addressed in the literature at this time. FIGURE 2. Geometrical parameters of windcatcher design The focus of this study has been put on evaluation of a combination of physical and meteorological parameters. We chose to analyze three physical parameters; cross-section (rectangular versus circular), windcatcher height (1.9 to 1.18 m) and louver design (5 versus 10-level) and two main ambient weather characteristics; wind speed and ambient air temperature through the course of this study for a single room. A list of different design scenarios (cases) in terms of geometrical configuration of the windcatcher is presented in   Table 1. ‘  Theta ’   is the louver inclination angle equal to 45 degrees for all designs, “  L3 ” is the width/diameter of the windcatcher   cross section equal to 50 cm, and ‘  t  ’   is the opening gap for each louver in the windcatcher which is equal to 4.3 cm. For a description of each of the dimensions refer for Figure 2. Each parameter’s  significance will be further studied in the results section. TABLE 1. Dimensions of different windcatcher designs Case Louver level Cross section L1 (cm) L2 (cm) I 5   Rect. 118 168 II 10 Rect. 140 190 III 5   Circ. 68 118 IV 10   Circ. 90 140 V 10   Circ. 140 190 Louver designs with 5 and 10 layers were selected for comparison for both rectangular and circular windcatchers (cases I and II versus III and IV). To analyze the performance of each of these louver designs, a range of incident wind speeds and attack angle were analyzed in the initial tier of design to obtain the near-optimal physical configuration for the windcatcher. Incident wind speeds of 5, 10, 15, and 20 m/s were used to cover a vast range of common windcatcher design conditions. The net induced air flow rate entering the building was considered as the main criteria for the comparison of different windcatchers and their efficiency in catching the outer flow. Incident wind angles of 0 and 45 degrees were selected for comparison. First, the model was run for taller rectangular windcatcher designs (cases I and II) with the combination of different wind speeds, incident angle and louver designs. The initial results for rectangular system were then compared to the runs for the circular windcatchers of lower height (cases III and IV) to compare the differences with the change in the windcatcher height. Followed by the comparison between the ventilation efficiency of the first four cases, with successively decreasing height, a series of simulations were performed for the case V, the circular design of the same height as case I, to provide a comparison between the performance of different cross-sections with the same height. The results from this stage were analyzed for the inlet mass flow rate capturing capacity to select the choice design to analyze the indoor air quality performance by considering the effects of radiation model and occupant internal energy production rates. CFD MODEL To construct a well-defined CFD model and for ease of grid generation, the whole fluid field was divided into three domains. The first domain consisted of the room, as defined in Figure 1, in which further thermal model analysis were conducted. The second and third included a small domain surrounding the windcatcher, and the ambient air surrounding the entire system. These domains were defined on the basis of the room hydrodynamic length scale, H, which is 8 meters. The ambient air inlet plane is 3H from the leading edge of the room and the outlet is 5H downstream from the room’s trailing edge to ensure uniform flow further downstream. The far-fields, i.e. the air surrounding the top of the roof and the room side walls, were considered to be 4H apart to be fairly undisturbed from the viscous effects induced by the room in the flow field. Because the problem is symmetric, only one half of the solution domain was modeled and solved. To numerically solve for the flow field, a very smooth tetragonal grid was generated with the goal of  5 placing the nodes more densely near solid surfaces, especially the windcatcher and near high ventilation zones such as windcatcher louvers and interfaces to the room and etc. To model near wall behaviours of the fluid and to meet the criterion,    , a boundary layer grid was generated to ensure a minimum of 5 nodes inside the viscous boundary layer. The final mesh generation around the windcatcher is depicted in Figure 3. FIGURE 3 .  Grid formation around the windcatcher   The induced flow pattern and temperature distribution inside the room and outside are governed by the general transport laws and conservation of mass, momentum and energy. The model applied here, includes the numerical capability for solving the continuity, Navier-Stokes, and energy for turbulent flows. The general form of all possible transport properties obey the Eqn. (1) in which   is the considered variable and    are the velocity components in    direction and   is the fluid density.            (1) Where    is the source term and    is a constant term related to a variable  .   The   Model The    model belongs to the class of two-equation models, in which model transport equations are solved for two turbulence quantities - i.e.,   and    in the    model. The    model is the most widely used complete turbulence model and it is incorporated in most commercial CFD codes. As is the case with all turbulence models, both the concepts and the details evolved over time; but Jones and Launder [19] are appropriately credited with developing the “standard”      model, with Launder and Sharma [20] providing improved values of the model constants. From these two quantities can be formed a length scale (      ), a timescale (     ), a quantity of dimension     (      , etc. The transport equations for k and epsilon are:                                   (2)                                             (3) Where:                            ,              In which   is the coefficient of thermal expansion, and    is the turbulent Prandtl number for energy. For the standard and realizable models, the default value of    is 0.85, and the constants are empirical values calibrated for most compatibility of turbulence regimes.                  The Boussinesq Buoyancy Approximation The basis of this approximation is that there are flows in which the temperature varies little, and therefore the density varies little, yet in which the buoyancy drives the motion. Thus, the variation in density is neglected everywhere except in the buoyancy term. If     denotes the density at the reference position where temperature is    , for small temperature gradient inside the fluid we can write             The buoyancy term           is the same order of magnitude as the inertia, and the acceleration or the
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