Experimental study on natural ventilation performance of one-sided wind catcher
Introduction
Buildings in hot and humid climates have been traditionally cooled by ventilation. Wind catcher or what is called Baud-Geer in Persian language has been employed in arid central regions of Iran and the neighboring countries to provide natural ventilation and passive cooling. In these regions due to the hot summer time, the buildings used to have special architectural features and components in order to protect the occupants from the harsh outdoor environment. The function of the Baud-Geer in these regions is to capture wind from external air stream and induce it into the building and courtyard in order to cool the occupant directly by increasing the convective and evaporative heat transfer from body surface. It cools the occupant indirectly by removing heat stored in the building structure. When wind is blowing over the tower and the building it severs, a wind pressure develops on various apertures. Air enters from the windward openings, with positive wind pressure coefficient, and leaves the leeward openings, with negative or lower values of the pressure coefficients [1].
Shapes, heights and internal structures of Baud-Geers were not only influential to the volumetric air delivery and cooling capacity of the building, but were an indication of dignity, wealth and social position of the house owners. They can be beautiful objects, feasible architectural feature additions to buildings and are inherently durable [2], [3]. Nowadays due to geographical coordinates, wind power and the direction of blowing wind, wind catcher designs differ in the height, the cross section of the air passages, the placement and number of the openings and the placement of the wind catcher with respect to the structure it cools. The wind catcher systems come in various configurations to suit various building type and requirements such as the incorporation of solar panel and light pipes to boost stack effect [4], [5]. Wind catcher is normally a tall structure with the height from 5 to 33 m mounted over the building roof. With taller tower capturing winds at higher speeds and with less dust [1]. The number of directions in which Baud-Geers face is different; therefore these towers are often classified to the number of their openings. One-sided, two-sided, four-sided (triangular cross section) and six-sided (hexagonal cross section) Baud-Geers are more common in Yazd. From many old buildings and houses equipped with Baud-Geers, Mahmoodi's house, Lariha's house and Dowlat-Abad garden Baud-Geers are the most famous ones which are the favorite tourist attraction areas in Yazd. Fig. 1 shows several old Baud-Geers with different number of openings.
In the modern design of wind catchers, the two ventilation principles of wind tower and passive stack are combined in one design around a stack that is divided into two halves or four quadrants with the division running the full height of the stack [6], [7]. As the wind direction changes so do the functions of each of the halves or the quadrants in the wind catcher. This causes the wind catcher to be operational which way the wind is blowing.
Several academic studies have been reported in the field of hydrodynamic analysis of Baud-Geers for natural ventilation. Bahadori [8] in his first study determined the pressure coefficients of a 1:100 scale model of a wind tower attached to a house by testing it in a wind tunnel, where the atmospheric wind was simulated by means of horizontal rods and a vertical screen. The effects of adjoining house and courtyard in the pressure coefficient distributions were also investigated. The tower corresponded to a full-scale height of 10 m with openings on all four sides. Karakatsanis et al. [9] improved earlier model studies carried out by Bahadori [8]. In that study a model of the same building, but of different model scale, was tested in the boundary layer wind tunnel laboratory. Pressure coefficients were determined and natural ventilation through the building was estimated. Elmualim and Awbi [10] carried out experimental investigation and CFD simulations to evaluate the performance of square and circular section wind catchers. Results showed that the efficiency of four-sided wind catcher is much higher than that for the circular one for the same wind speed. They explained this is due to the fact that the sharp edges of the square one create a large region of flow separation and higher pressure difference across the device. The hydrodynamic analysis of flow in wind catchers was done by Montazeri and Dehghan [11] using CFD technique. That work was based on the numerical solution of the two-dimensional Navier–Stokes. They concluded that the separated flow and wake region near to the lower edge of wind catcher opening cause the induced capacity of wind catcher to be decreased considerably. Elmualim [12] analyzed performance of a wind catcher natural ventilation system which was installed in a real room. The results showed the chosen ventilation design provided a substantially greater ventilation rate than an equivalent area of openable window.
Most of previous related studies [8], [9], [10], [11], [12] have looked at the ventilation performance of four-sided wind catchers. Short-circuiting (the air entering through supply openings and leaving through another without flowing inside the house) is a harmful phenomenon in these structures. In the present study, one-sided wind catcher with a single opening was investigated. In this type of wind catcher, there is no short-circuiting and entire air, which is passed through the channel, flows inside the room. Moreover, many attempts have been made to estimate ventilation air flow rate as a main parameter in the passive cooling systems [9], [10], [11], [12], [13]. In the present study, a scale model of Kharmani's School Baud-Geer was employed and using the accurate measurements, the induced air flow rate into the test room and the pressure coefficients around all surfaces of its channel were measured for different values of approaching air incident angles. Also using measured pressure coefficients, the theoretical values of ventilated air flow were estimated to evaluate ability of simplified models in natural ventilation studies.
Initially and prior to all testing, smoke visualization tests were carried out in order to recognize the flow pattern in and over the wind catcher model. Further, flow visualization tests helped to identify the supply and extract segments during all tests.
Wind catchers are traditionally used in places of high urban densities where surrounding buildings obstruct free stream air flow [14]. Due to placing of urban wind catchers in the boundary layer of atmospheric winds, in this study, the effect of this phenomenon was also examined. The experiments were also conducted when the wind catcher with its house model was placed in the wake of upstream objects, resembling neighboring buildings.
Section snippets
Experimental procedure
In this study, a 1:40 scale model of an ancient one-sided wind catcher was employed. The experimental investigations were conducted in an open working section subsonic wind tunnel located in the thermodynamics laboratory of the school of mechanical engineering of Yazd University. This wind tunnel is designed for the experimental testing of natural ventilation devices and has a test section with height, width and length of 46, 46 and 360 cm, respectively. According to the dimensions of models and
Natural ventilation efficiency
Natural ventilation efficiency of one-sided wind catcher can be defined as follow:where Q0 is the air flow rate through the channel of wind catcher for the wind angle of 0° and Q will be the flow rate in various conditions which both were determined experimentally. In the one-sided wind catcher, entire ventilation air is passed through the occupant zone, so neglecting the extract/supply segments; natural ventilation efficiency could describe its performance
Results and discussions
Hydrodynamic analysis of one-sided wind catcher can be very useful to evaluate performance of wind catchers with different configurations. Therefore in this study, various tests were carried out on its model.
In order to recognize the flow pattern in and over the wind catcher model, smoke was used. To flow visualization, both roof and one side of a one-sided wind catcher model were built of glass and the model was exposed under zero incident angle. Fig. 8 shows the flow field in front face and
Conclusions
In central or arid regions of Iran and the neighboring countries, wind catchers which are named Baud-Geers in Persian literature, were one of the main components of the old buildings. In the absence of modern air conditioning systems or mechanical driven air ventilation equipments, they were responsible to capture wind from any direction and guide it to the house or occupants zones. In this study, a 1:40 scale model of Kharmani's School Baud-Geer was employed and the induced air flow rate into
Acknowledgments
The authors would like to thank the thermodynamics laboratory of the school of Mechanical Engineering of Yazd University for providing the test rig and experimental facilities.
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