ZhangJ.etalIWA-LETManuscript (2).doc

1 to 03 ms and the volume fraction of water was 1 the

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the range of 0.1 to 0.3 m/s and the volume fraction of water was 1. The velocity of gas inlets varied from 0.01 to 0.03 m/s and the volume fraction of air was 1. The outlet was set as a zero pressure gradient boundary condition and the reference pressure was set as 0 atmosphere (atm). The free water surface was set as a degassing outlet which assumed that the surface was flat, frictionless and impervious to liquid to simplify the simulation, however, gas was allowed to leave at the rate at which it arrived at the surface. Numerical Method The numerical solution of the modeling was carried out with a finite-volume based program CFX10 (Ansys, 2005). A second order high-resolution differencing scheme was applied to all flow equations. The time step used varied in range between 100s and 0.001s from the start to the end of a run. All the simulations were done on a Pentium 4 PC with 3.0 GHz CPU and 2G memory. The computational times for obtaining the converged solutions usually were 1 day to 4 days. RESULTS AND DISCUSSION Model Validation The model was validated by comparison of the simulated tracer residence time distribution (RTD) with field tracer test data (El-Baz, 2002). The details of the validation study can be found in Zhang (2006). Good agreement was observed between the CFD modelling and experimental test results at two different water flow rates. The simulated and measured T 10 /T ratios at flow rates of 2.16 m 3 /s were 0.51 and 0.55, respectively; and the simulated and measured T 10 /T ratios at flow rates of 1.47 m 3 /s were 0.50 and 0.51, respectively. The differences are less than 10%. In addition, local ozone residual concentrations were obtained by solving transport equations for ozone mass transfer and reactions. The predicted ozone concentrations at most monitoring points were within 11% of the measured values. It was thus considered that the model could provide sufficient accuracy in predicting mass transfer and reactions of ozone within the contactors. Contactor Performance The CFD model was applied to investigate the contactor hydraulics under various operational conditions. Figure 2 displays a typical flow field inside the contactor. It was observed that large recirculation zones existed in the two cells, causing short-circuiting and dead zones. The existence of dead zones and short-circuiting would reduce the contacting opportunities between ozone and pathogens (or between ozone and contaminants in an oxidation process) and make disinfection/oxidation efficiency significantly less than it otherwise could be. Figure 3 and 4 show the simulated ozone residual and CFD-based CT distribution along
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the length direction of the contactor. Obviously, the flow field (shown in Figure 2) can significantly affect ozone residual distribution and disinfection efficiency. A portion of the water might experience short-circuiting phenomena (for example, at the top surface of cell 2) and have a much smaller CT than other portions of the water exiting the contactor.
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