Hydraulic Efficiency in RANS of the Flow in Multichambered Contactors

Abstract

Accurate and practical models are in demand for optimization of ozone contactor designs. This paper reports results from Reynolds-averaged Navier-Stokes simulation (RANS) of the flow and passive nonreactive tracer transport inside a multichambered, laboratory-scale ozone contactor using a structured, collocated, finite volume discretization. Simulations are posed following previously published laboratory experiments. Results are presented in terms of velocity distributions, tracer concentration distributions, and tracer residence times. The flow is characterized by short-circuiting and dead zone regions that reduce the hydraulic (disinfection) efficiency or baffling performance of the contactor. RANS-predicted cumulative residence time distribution (RTD) of the tracer (released at the inflow as a pulse) is shown to be in excellent agreement with published experimental data despite the under resolution of the RANS methodology compared with better-resolved methodologies, such as large-eddy simulation (LES). The authors also compare the baffling performance and friction energy loss (molecular and turbulent) of three ozone contactor configurations by RANS simulation. A trade-off between baffling performance and energy loss is identified for the first time, as previous works have focused on baffling performance only. However, it is seen that the overall energy saving afforded by increasing the hydraulic efficiency (thus requiring less energy for ozone generation) offsets the energy increase required for driving the flow through a more hydraulically efficient contactor (characterized by more baffles). Overall, it is seen that energy considerations associated with contactor hydraulic efficiency (i.e., energy loss due to friction, and thus energy to drive the flow, and energy required for ozone generation) are important for determining the operational costs of a water treatment plant.