Methodology for the Evaluation of Natural Ventilation in ... - Cham

design. The depth of natural ventilation effectiveness in the single sided case is usually said to belimited to approximately 2.5 times the height of the space (Awbi 2003).In open floor plans, cross-ventilation can be considered when there are openings on two sides ofa space or building. In wind and combined wind-buoyancy driven cross ventilation flow, airenters through openings on one side of the building or space, traverses the space, and exitsthrough openings on another side of the building or space, or through the atrium or upper floors.In cross ventilation, the airflow will be driven by wind if there is no significant heightdifferential between inlet and outlet window openings and no vertical connection between floors.The airflow penetrates more deeply into the space than for single-sided ventilation, so the floorplans can be deeper. For cross-ventilation to work effectively, an open floor plan with fewinternal partitions is desired. The maximum depth of the space for effective ventilation by thismethod is said to be approximately five times the height of the space (Awbi 2003).Often atria are included in the design of naturally ventilated office buildings because they notonly increase the amount of natural daylight within the space, but also offer the potential tocontrol and passively enhance buoyancy-driven ventilation. An increase in atrium height canenhance the buoyancy-based ventilation. This enhancement can be obtained by providing anincreased height differential, and can be magnified by the use of glazing at the top of the atriumto increase the temperature of the air near the exhaust opening through solar gain. The use ofglazing makes the stack flow more effective due to the increase in buoyancy-driven flow.Additionally, in the winter the atrium can act as a buffer to the external environment. However,care must be taken when designing stack vents and atria, since the position of the opening canreduce or even reverse the impact of the stack flow.2.3.2 Window CharacteristicsIn naturally ventilated buildings purpose-provided openings allow fresh outside air to enter andexhaust air to exit a building in prescribed locations. Not only how the air enters and exits thebuilding, but also how much air enters, is impacted by the window type, its location, and itsorientation with respect to the dominant wind direction. Summer, spring and fall windconditions are usually of most concern to designers using natural ventilation techniques, asoccupant comfort is more dependent on passive ventilation to cool the building during monthswhen cooling is required.Each window type has its own effective opening area, or the percentage of the overall windowarea through which air can flow, and amount of leakage. Both of these factors can impact theselection of a window for a given climate and application. The three main types of windows aresliding, hinged, and rotating. These classifications also relate to the type of opening; simpleopening, vertical-vane opening, and horizontal-vane opening (Allard 2002). Windows that use atrack are either on a horizontal track, such as sliding windows, or a vertical track, as is the casewith hung windows. Track mounted windows are often referred to as simple openings. Sidehingedcasement windows or vertical-pivot windows pivot or hinge on the vertical axis.Horizontal-vane openings are similar to their vertical counterparts, except that the pivot or hingeis on the horizontal axis. Rotating windows can have either vertical or horizontal pivots, but thepivot is located at the center of the window, so that the window rotates about a central axis. Asummary of the window types and their characteristics is presented in Table 2 and Table 3.32