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

2.2.1 Buoyancy-Driven VentilationVentilation driven by buoyancy is prevalent in many naturally ventilated buildings, with air flowcaused by pressure differences across the building envelope. With buoyancy-driven ventilationthe pressure differences are due to air density differences, which in turn are due to temperaturedifferences. It is the magnitude of these temperature differences and resulting pressuredifferences, as well as the building opening characteristics that determine the magnitude of theairflow due to buoyancy. In stack-driven ventilation, the addition of stack increases the height,and therefore the pressure difference, between an inlet and outlet. A temperature differencebetween the inlet and outlet can enhance the effects of buoyancy-driven ventilation.A neutral pressure level (NPL) is created at the point where the internal pressure is equal to theexternal pressure, resulting in no airflow in or out of an opening at that particular height. Aboveor below the NPL, the airflow and direction can be determined; the direction of the airflow isalways from the region of higher pressure to the area of lower pressure. The NPL can becalculated based on the total inlet and outlet areas and respective resistances, and their relativeheight if more than one floor level exists. Roof openings and chimneys or raised stacks can shiftthe NPL, usually to higher levels. For buoyancy driven flow, the NPL is presented graphically inFigure 7.The Bernoulli equation is used to derive the flow due to buoyancy-driven ventilation, calculatingthe pressure differential due to height, i.e. the hydrostatic head, for both the exterior environmentand the interior environment. The overall pressure difference between the interior and exteriorcan be expressed in terms of the height difference, H, gravitational constant, g, density at areference temperature, ρ o , and the interior and exterior temperatures. The Bernoulli equation isgiven by:v2O2POρ gzO constant(2.1)For the buoyancy-driven case, there is no external velocity so the relationship reduces to:POO gzO constant(2.2)The pressure difference is applied to the outside environment, using subscript E, and the internalenvironment, using subscript I. The resulting pressure differences due to height between anorigin height, z O , and at some height H, z H , for the outside and inside become:EEgzHOP z z(2.3)P g z(2.4)IIHOTo determine the total pressure difference, ΔP T , the pressure difference across the inlet and outletopenings is calculated. Figure 7 illustrates this, with it resulting in:26