Abstract
Multi-chamber contact tanks have been extensively used in industry for water treatment to provide potable water to communities, which is essential for human health. To evaluate the efficiency of this treatment process, flow and tracer transport analysis have been used in the literature using Reynolds averaged Navier–Stokes (RANS) and large-eddy simulations (LES). The purpose of this study is two-fold. First a unifying analysis of the flow field is presented and similarities and differences in the numerical results that were reported in the literature are discussed. Second, the vorticity field is identified as the key parameter to use in separating the mean flow (jet zone) and the recirculating zones. Based on the concepts of vorticity gradient and flexion product, it is demonstrated that the separation of the recirculation zone and the jet zone, fluid-fluid flow separation, is possible. The separation of the recirculation zones and vortex core lines are characterized using the definition of the Lamb vector. The separated regions are used to characterize the mixing efficiency in the chambers of the contact tank. This analysis indicates that the recirculation zone and jet zone formation are three-dimensional and require simulations over a long period of time to reach stability. It is recognized that the characteristics of the jet zones and the recirculation zones are distinct for each chamber and they follow a particular pattern and symmetry between the alternating chambers. Hydraulic efficiency coefficients calculated for each chamber show that the chambers having an inlet adjacent to the free surface may be designed to have larger volumes than the chambers having wall bounded inlets to improve the efficiency of the contact tank. This is a simple design alternative that would increase the efficiency of the system. Other observations made through the chamber analysis are also informative in redefining the characteristics of the efficiency of the contact tank system.
Highlights
In recent years multi-chamber contact tanks have gained wide applications in water treatment facilities using either chlorine or ozone treatment
The relationship between the baffling performance and energy loss for the definition of “hydraulic efficiency” of the contact tank system was investigated by Zhang et al [4], where the authors stated that the energy saving afforded by using more baffles in the contactor was offset by the extra energy necessary to drive the flow through the contact tank system
The numerical simulations conducted in this study reveal that the flow field in adjacent chambers was not identical or symmetrical due to the inlet conditions not being the same
Summary
In recent years multi-chamber contact tanks have gained wide applications in water treatment facilities using either chlorine or ozone treatment. An efficient contact tank mixing system is required for the ozonation process since ozone cannot dissolve in water as readily as chlorine. Energy requirements of the mixing process and the amount of ozone required during the process are proportional to the efficiency of mixing in the contact tank. Short-circuiting of mixing due to the development of recirculating flows and friction-induced energy losses are important problems that may reduce the performance of the contact tank. Modifications of the contact tank design were proposed using horizontal baffles or turning vanes to increase the mixing efficiency of the contactor. The relationship between the baffling performance and energy loss for the definition of “hydraulic efficiency” of the contact tank system was investigated by Zhang et al [4], where the authors stated that the energy saving afforded by using more baffles in the contactor was offset by the extra energy necessary to drive the flow through the contact tank system
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