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Abstract
A model is presented which allows steady-state pressure profiles in high-rise wastewater drainage networks to be related to intake air flowrates and discharge water flowrates. This model is developed using data taken from academic literature, and is based on experimental observations which suggest that a vertical annular downflow develops over distance such that the pressure gradient in the wet stack may be expressed as the sum of junction components and developed flow components. The model is used to analyse a simplified ‘medium rise’ primary vented system of height 40 m, hosting two inflow junctions, crossvents and Air Admittance Valves (AAVs). The model illustrates how the air supply configuration affects the airflow rates within the stack and the vents, and how the configuration affects the steady-state hydraulic pressure profile. The model offers the possibility of an alternative approach to the design of high-rise wastewater drainage networks, compared to existing design codes. These codes generally do not explain the role that the air admitted into the network has upon its performance.
Highlights
Tall-building wastewater drainage systems (WDSs) are comprised of wet stacks and networks of air vents
Pressure gradients which arise due bubble, slugbe and churn flow patterns are the to occur in vertical drainage systems, thetostack should large enough to encourage typically an order of magnitude greater than pressure gradients arising due to an annular annular flow pattern and to discourage the bubble, churn or slug flow patterns as indiflow [19,20]
The orientation of the transition boundary suggests that it easier to surges to occur in vertical drainage systems, the stack should be large enough to encourage satisfy this requirement with a large, normalized air velocity U ; that is to say, if a strong the annular flow pattern and to discourage the bubble, churn or slug flow patterns as airindicated current by canFigure be drawn through the vertical stack
Summary
Tall-building wastewater drainage systems (WDSs) are comprised of wet stacks and networks of air vents. Empty trap seals provide a path for contaminated air to spread from the stack into the habitable space of a building This spread is enhanced if local pressure in the stack is higher than the exterior pressure; under these conditions the stack actively expels contaminated air rather than removing it as intended. Steady progress has been made in recent decades in the identification, qualification and mitigation of pressure surges [9] This progress has largely been restricted to lowrise WDSs. Significant gaps remain in understanding of behaviour in high-rise WDSs [10]. The design codes relegate a potentially very important aspect of WDS design, having potentially very significant impact for highrise structures, to a position of triviality This carries the risk that design might not be optimised, or, possibly, a novel design solution might be overlooked. It is these limitations which have prompt the development of a novel two-phase flow model which is described in this article
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