Jet-Driven Mixing Regimes Identified in the Unsteady Isothermal Filling of Rectangular Municipal Water Storage Tanks

Abstract

Poor mixing of old and new water in municipal water storage vessels is a well-documented basis for potentially harmful water quality degradation in drinking water distribution systems. This numerical study investigates the effects of inflow and operational variables on mixing in the jet-driven filling process, with a particular focus on the transition from inadequate to sufficient mixing levels. An isothermal unsteady reynolds-averaged-navier-stokes volume-of-fluid (RANS-VOF) simulation is used to model the variable-volume filling process, accounting for the moving free surface following a draw-down in the stored water volume. A low diffusivity tracer is used to mark the old-water volume, and a coefficient of variation (CoV) quantifying the departure from a uniform tracer distribution is used to monitor the time-dependent mixing. The results indicate that adequate mixing does not necessarily follow refills from common draw-down levels. Three distinct mixing regimes are identified by unique CoV transients. Introducing consideration of the mean-flow kinetic energy, the observed mixing behaviors are readily explained by the jet inlet power and the distribution of the mean-flow kinetic energy in the tank. Extending the simulations to periods after cessation of the inflow and to partial refills, the role of residual mean-flow kinetic energy is further highlighted, especially its limited vertical reach. For cases in which a sufficiently mixed condition is achieved, the time-to-mix results are well described by a mixing-time correlation closely matching previously published results.