Modeling of Dispersion Effect for Intermittent Flow in Home Plumbing Systems
Citation:
Woo, H., J. Burkhardt, R. Murray, J. Mason, AND L. Rossman. Modeling of Dispersion Effect for Intermittent Flow in Home Plumbing Systems. 1st International WDSA / CCWI 2018 Joint Conference, Kingston, Ontario, CANADA, July 23 - 25, 2018.
Impact/Purpose:
The purpose of the work described in the abstract is to improve the water quality simulations related to EPANET and associated software. This focuses on better characterization of dispersion effects under flowing conditions, specifically for home plumbing systems. This work aims to improve the water quality modeling capability of EPANET and will be of interest to anyone interested in hydraulic and water quality modeling of distribution systems.
Description:
Modeling tools for water distribution systems have been developed and used for many years, however, only limited attention has been given to home or premise plumbing modeling. Recent issues in Flint, MI highlighted the need for modeling premise plumbing systems (PPSs) and their unique characteristics and contaminants (e.g., lead from lead service lines). As a result of their intermittent operation and wide range of flow rates, modeling PPSs requires the inclusion of axial dispersion to accurately predict water quality variations. Flow in PPSs is typically stagnant with intermittent uses ranging from laminar to turbulent flow rates depending on the water uses in the fixtures. Specifically, low-velocity flows happen sporadically at times in a PPS (e.g., the uses from the ice machine and water line in the refrigerator, brushing the teeth, a cup of water, and various low flow devices). Flow under laminar conditions with low velocity and high dispersion can move contaminants at twice the speed of the average velocity. EPANET which has been widely used for the analysis of water distribution systems with reasonable accuracy does not calculate axial dispersion of low-velocity flow for the analysis of PPSs. The impact of dispersion in PPSs has not been thoroughly investigated. The object of this study is to investigate how dispersion will impact the predicted water quality when modeling PPSs. This study tested various approaches of including dispersion for PPS modeling and were compared to data collected in a pilot scale home plumbing system and analytical solutions. In this study, the solute transport mechanism in PPSs was investigated by solving the advection-dispersion-reaction (ADR) equations numerically. For this approach, the method of characteristics (MOC) using the Lagrangian method was used for advection-reaction step. Three approaches were compared for modeling dispersion using Eulerian methods; an explicit finite difference method (e.g., forward-time-central-space or FTCS), an implicit finite difference method (e.g., backward-time-central-space or BTCS), and the Green's function method. The results of the 1D simulation were also verified with an analytical solution and that of 2D simulation since these 1D numerical schemes are limited to calculate the condition after steady state advection using the average velocity effect. Dispersion coefficients were calculated for steady state using the Aris-Taylor equation (1956). The calculation of dispersion coefficient was extended to the unsteady state case using Lee's approximation equation (2004). Overall, the dispersion effect in the PPS was compared using various numerical methods for solving the ADR equation under various flow states.