Abstract
Mixing characteristics of multi-chambered contact tank are analyzed employing the validated three-dimensional numerical model developed in the companion paper. Based on the flow characterization, novel volumetric mixing efficiency definitions are proposed for the assessment of the hydrodynamic and chemical transport properties of the contact tank and its chambers. Residence time distribution functions are analyzed not only at the outlet of each chamber but also inside the chambers using the efficiency definitions for both Reynolds averaged Navier–Stokes (RANS) and large eddy simulation (LES) results. A novel tracer mixing index is defined to characterize short circuiting and mixing effects of the contact system. Comparisons of the results of these indexes for RANS and LES solutions indicate that mixing characteristics are stronger in LES due to the unsteady turbulent eddy mixing even though short circuiting effects are also more prominent in LES results. This result indicates that the mixing analysis based on the LES results simulates the mixing characteristics instantaneously, which is more realistic than that in RANS. Since LES analysis can capture turbulent eddy mixing better than RANS analysis, the interaction of recirculation and jet zones are captured more effectively in LES, which tends to predict higher turbulent mixing in the contact system. The analysis also shows that the mixing efficiency of each chamber of the contact tank is different, thus it is necessary to consider distinct chemical release and volumetric designs for each chamber in order to maximize the mixing efficiency of the overall process in a contact tank system.
Highlights
In a companion paper [1], recirculation zones and jet zones were separated using the definitions of vorticity gradient and the flexion product
It is important to note that mixing of the chemical within the recirculation zone where the tracer is retained for a period and released, and the interaction of the recirculation zone with the jet zone where the tracer is transported through the tank, both contribute to the overall mixing process in the contactor
Cinit = 1, τ is theoretical mean residence time of the tank (τ = V/Q, where V is the volume of the contact tank and Q is the volumetric flow rate), which is calculated as 109.2 s for the present contact tank volume and discharge based on the assumption of plug flow in the contact system, and θ = τt is the dimensionless time
Summary
In a companion paper [1], recirculation zones and jet zones were separated using the definitions of vorticity gradient and the flexion product. In this paper, mixing efficiency indexes are analyzed for the assessment of mixing performance of the contact tank system This analysis is based on the residence time distribution (RTD) functions, which are obtained from tracer transport analysis. It is important to note that mixing of the chemical within the recirculation zone where the tracer is retained for a period and released, and the interaction of the recirculation zone with the jet zone where the tracer is transported through the tank, both contribute to the overall mixing process in the contactor The interactions between these two zones are not well known or analyzed since the mixing indexes that are used in the literature are based on monitoring the tracer concentration only at specified outlet locations. It is important to emphasize again that the mixing indexes need to be defined for each chamber separately in order to design the mixing process more effectively for the complete contact tank system since the mixing characteristics of each chamber are distinct as pointed out in [1]
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