1、给水排水工程专业外文翻译密封的建筑排水系统和通气系统外文原文：Sealed building drainage and vent systemsan application of active air pressure transient control and suppressionAbstractThe introduction of sealed building drainage and vent systems is considered a viable proposition for complex buildings due to the use of active press
2、ure transient control and suppression in the form of air admittance valves and positive air pressure attenuators coupled with the interconnection of the networks vertical stacks. This paper presents a simulation based on a four-stack network that illustrates flow mechanisms within the pipework follo
3、wing both appliance discharge generated, and sewer imposed, transients. This simulation identifies the role of the active air pressure control devices in maintaining system pressures at levels that do not deplete trap seals. Further simulation exercises would be necessary to provide proof of concept
4、, and it would be advantageous to parallel these with laboratory, and possibly site, trials for validation purposes. Despite this caution the initial results are highly encouraging and are sufficient to confirm the potential to provide definite benefits in terms of enhanced system security as well a
5、s increased reliability and reduced installation and material costs. Keywords: Active control; Trap retention; Transient propagation NomenclatureC+-characteristic equations cwave speed, m/s Dbranch or stack diameter, m ffriction factor, UK definition via Darcy h=4fLu2/2Dggacceleration due to gravity
6、, m/s2 Kloss coefficient Lpipe length, m pair pressure, N/m2 ttime, s umean air velocity, m/s xdistance, mratio specific heats hhead loss, m ppressure difference, N/m2 ttime step, s xinternodal length, m density, kg/m3Article OutlineNomenclature 1. Introductionair pressure transient control and supp
7、ression2. Mathematical basis for the simulation of transient propagation in multi-stack building drainage networks 3. Role of diversity in system operation 4. Simulation of the operation of a multi-stack sealed building drainage and vent system 5. Simulation sign conventions 6. Water discharge to th
8、e network 7. Surcharge at base of stack 1 8. Sewer imposed transients 9. Trap seal oscillation and retention 10. Conclusionviability of a sealed building drainage and vent system1.Air pressure transients generated within building drainage and vent systems as a natural consequence of system operation
9、 may be responsible for trap seal depletion and cross contamination of habitable space 1. Traditional modes of trap seal protection, based on the Victorian engineers obsession with odour exclusion 2, 3 and 4, depend predominantly on passive solutions where reliance is placed on cross connections and
10、 vertical stacks vented to atmosphere 5 and 6. This approach, while both proven and traditional, has inherent weaknesses, including the remoteness of the vent terminations 7, leading to delays in the arrival of relieving reflections, and the multiplicity of open roof level stack terminations inheren
11、t within complex buildings. The complexity of the vent system required also has significant cost and space implications 8. The development of air admittance valves (AAVs) over the past two decades provides the designer with a means of alleviating negative transients generated as random appliance dis
12、charges contribute to the time dependent water-flow conditions within the system. AAVs represent an active control solution as they respond directly to the local pressure conditions, opening as pressure falls to allow a relief air inflow and hence limit the pressure excursions experienced by the app
13、liance trap seal 9. However, AAVs do not address the problems of positive air pressure transient propagation within building drainage and vent systems as a result of intermittent closure of the free airpath through the network or the arrival of positive transients generated remotely within the sewer
14、 system, possibly by some surcharge event downstreamincluding heavy rainfall in combined sewer applications. The development of variable volume containment attenuators 10 that are designed to absorb airflow driven by positive air pressure transients completes the necessary device provision to allow
15、active air pressure transient control and suppression to be introduced into the design of building drainage and vent systems, for both standard buildings and those requiring particular attention to be paid to the security implications of multiple roof level open stack terminations. The positive air
16、pressure attenuator (PAPA) consists of a variable volume bag that expands under the influence of a positive transient and therefore allows system airflows to attenuate gradually, therefore reducing the level of positive transients generated. Together with the use of AAVs the introduction of the PAPA
17、 device allows consideration of a fully sealed building drainage and vent system. Fig. 1 illustrates both AAV and PAPA devices, note that the waterless sheath trap acts as an AAV under negative line pressure.Fig. 1. Active air pressure transient suppression devices to control both positive and negat
18、ive surges.Active air pressure transient suppression and control therefore allows for localized intervention to protect trap seals from both positive and negative pressure excursions. This has distinct advantages over the traditional passive approach. The time delay inherent in awaiting the return o
19、f a relieving reflection from a vent open to atmosphere is removed and the effect of the transient on all the other system traps passed during its propagation is avoided. 2.Mathematical basis for the simulation of transient propagation in multi-stack building drainage networks.The propagation of air
20、 pressure transients within building drainage and vent systems belongs to a well understood family of unsteady flow conditions defined by the St Venant equations of continuity and momentum, and solvable via a finite difference scheme utilizing the method of characteristics technique. Air pressure tr
21、ansient generation and propagation within the system as a result of air entrainment by the falling annular water in the system vertical stacks and the reflection and transmission of these transients at the system boundaries, including open terminations, connections to the sewer, appliance trap seals
22、 and both AAV and PAPA active control devices, may be simulated with proven accuracy. The simulation 11 provides local air pressure, velocity and wave speed information throughout a network at time and distance intervals as short as 0.001s and 300mm. In addition, the simulation replicates local appl
23、iance trap seal oscillations and the operation of active control devices, thereby yielding data on network airflows and identifying system failures and consequences. While the simulation has been extensively validated 10, its use to independently confirm the mechanism of SARS virus spread within the
24、 Amoy Gardens outbreak in 2003 has provided further confidence in its predictions 12. Air pressure transient propagation depends upon the rate of change of the system conditions. Increasing annular downflow generates an enhanced entrained airflow and lowers the system pressure. Retarding the entrain
25、ed airflow generates positive transients. External events may also propagate both positive and negative transients into the network. The annular water flow in the wet stack entrains an airflow due to the condition of no slip established between the annular water and air core surfaces and generates t
26、he expected pressure variation down a vertical stack. Pressure falls from atmospheric above the stack entry due to friction and the effects of drawing air through the water curtains formed at discharging branch junctions. In the lower wet stack the pressure recovers to above atmospheric due to the t
27、raction forces exerted on the airflow prior to falling across the water curtain at the stack base. The application of the method of characteristics to the modelling of unsteady flows was first recognized in the 1960s 13. The relationships defined by Jack 14 allows the simulation to model the tractio
28、n force exerted on the entrained air. Extensive experimental data allowed the definition of a pseudo-friction factor applicable in the wet stack and operable across the water annular flow/entrained air core interface to allow combined discharge flows and their effect on air entrainment to be modelle
29、d. The propagation of air pressure transients in building drainage and vent systems is defined by the St Venant equations of continuity and momentum 9,(1)(2)These quasi-linear hyperbolic partial differential equations are amenable to finite difference solution once transformed via the Method of Char
30、acteristics into finite difference relationships, Eqs. (3)(6), that link conditions at a node one time step in the future to current conditions at adjacent upstream and downstream nodes, Fig. 2.Fig.2. St Venant equations of continuity and momentum allow airflow velocity and wave speed to be predicte
31、d on an x-t grid as shown. Note , . For the C+ characteristic:(3)when(4)and the C- characteristic:(5)when(6)where the wave speed c is given byc=(p/)0.5.(7)These equations involve the air mean flow velocity, u, and the local wave speed, c, due to the interdependence of air pressure and density. Local
32、 pressure is calculated as(8)Suitable equations link local pressure to airflow or to the interface oscillation of trap seals.The case of the appliance trap seal is of particular importance. The trap seal water column oscillates under the action of the applied pressure differential between the transients in the network and the room air pressure. The equation of motion for the U-bend trap seal water
