Study On Modeling And Simulation Of Vacuum Sewerage Systems

Vacuum sewerage system (VSS) is a promising ecological wastewater collection system. However, its basic theories and design methods at home and abroad have still remain in the level of qualitative analysis and empirical calculation yet. With the extensive applications, scale enlargement and interest in the energy consumption of VSSs, a deeper insight into their transfer mechanisms, design methods and operating characteristics is needed. The modeling and simulation methods of user terminals, pipeline networks and vacuum stations in VSSs were investigated in this paper. A system-level dynamic simulation was achieved and the system energy consumptions were analyzed. A two-fluid gas-liquid model and its solution method dedicated to vacuum pipelines were presented. Some common scenarios in VSSs were chosen as examples here to illuminate the numerical simulation of unsteady two-phase flows in vacuum pipelines with a more delicate scale. This study would be a foundation for the follow-up study on optimization design, operation and management.Subsystem mathematic models of VSSs were established:1. Mathematical models of two types of user terminal in VSSs were built.2. A hydraulic model of vacuum pipeline networks based on continuity equations, pressure loss equations of one-dimension gas-liquid two phase flows, and point head equations was introduced. The physical meaning of the dynamic lift loss coefficient was deduced, of which an optimized calculation method was developed based on the improvement to the correlation of Dix made by Woldesemayat and Ghajar. Four theoretic or semi-empirical calculation methods of friction loss (e.g. modified Hazen-Williams formula, Averil-Heinke method, Duklerâ…¡method and Duan method) were reviewed and compared. To make up for their deficiencies, an experimental two-phase coefficient method for friction loss calculation based on vacuum pipeline experiments was introduced.3. The mathematic model of vacuum station was also built, which comprises three governing equations, e.g. the gas phase mass equation, liquid phase mass equation and volume equation.Based on the system mathematic model, a physical domain of one-dimension isothermal gas-liquid two-phase flow for VSS modeling was defined and a simulation block library was established in Matlab\Simulink\Simscape software environment for the first time. A typical outdoor VSS was taken as an example here to illuminate the modeling process. A system-level system-level dynamic simulation was achieved. Some important information, e.g. the sewage flow rates at each collection unit, hydraulic characteristics of the pipeline network and operation parameters of the vacuum station were obtained. This simulation platform is especially suitable for future large-scale VSSs.The concerned problem of energy consumption in VSSs was solved for the first time. Three energy consumption models (e.g., pressure-coupled intermittent model, uncoupled intermittent model and uncoupled continuous model) for VSSs based on different hypotheses were proposed and discussed. The specific energy demand in outdoor systems and energy demand per flush in indoor systems were predicted and compared. Among the three models, the pressure-coupled intermittent model turns out to be closest to the real system and can be used for equipment selection and control strategy optimization. However, its solution requires in the Monte Carlo simulation in computer platform.A four-equation two-fluid model for transient gas-liquid two-phase flows in vacuum pipelines based on an isothermal hypothesis was put forward. The treatment methods of common boundary conditions for the numerical simulation of vacuum pipelines were firstly presented. The steady pressure loss of horizontal pipe in this numerical simulation was compared with existing theoretic or semi-empirical calculation methods. The results verified the reliability and accuracy of the numerical simulation to a certain extent. Some typical scenarios were numerically investigated for the first time, e.g. saw-tooth pipelines in outdoor systems, U-shape transport-pocket pipelines and uphill-type risers in indoor systems. The transient phenomena of gas-liquid two-phase flows were observed, of which the mechanisms were discussed.This study is an active exploration of VSSs from the qualitative analysis to the quantitative calculation, which will provide a reference to system optimization design, operation and management.