Conclusions

JIC(I)dl

3.6 Conclusions

One of the major simplifications in the study was using a one-phase, water-only, model to represent the two-phase system. Since the effect of gas injection on the average experimental RTD was slight, and the gas injection could not be represented directly in the single phase model. it was tried to represent the effect by adjusting the turbulent intensity of the flow entering the contactor through the static mixer.

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Chapter 3 Hydrodynamic model

The turbulent intensity of the incoming flow is a required boundary condition for the turbulence model.

Even without gas injection, its value was not known due to the effect of the static mixer blades. Therefore it made sense to treat the turbulent intensity as an unknown parameter that could be adjusted to match the experimental tracer response as closely as possible.

The predicted overall RTD by the one-phase model agrees remarkably well with the experimental results.

The variation of tracer response along the weir predicted by the model was confirmed by the experimental profiles, with tracer exiting earlier on the left than on the right. From the model, it was shown that the left-right flow distribution was initiated by the contactor geometry; however, the flow asymmetry predicted by the model responded in opposite ways to the experimental observation. Modifying the turbulence intensity ratio at the inlet to fit the experimental tracer data was initially an attempt to model the effect of gas injection. Although it is clearly a very rough approximation, the subsequent simulated results show that the effect from the upstream flow history is more substantial than the presence of the gas phase.

Experimentally the effect of the gas injection neutralises the flow asymmetry. Therefore a decision had to be made as to which to accept to represent the goodness-of-fit to the measurements. For the function of the contactor, it was more important to match the overall RTD. The experimental tracer results also suggested that the gas injection has no distinct effect on the average tracer response cuves. The effect of the gas was manifested in the shift in the left-right distribution of flow in the contactor. Since this was considered of lesser importance to the reaction modelling, it was decided to continue with the single phase, water only model. [t is also evident that, nom the present study, CFD modelling provides a more robust approach than the Ct concept for dealing with systems with unconventional configurations.

It must be noted that in many instances, the effect of individual model parameters could not be differentiated experimentally. The type of model and the parameter values were chosen to represent as closely as possible to the experimental results. The flow details of the base model could not be validated by other experimental means. The model is therefore constrained to fit the predicted RTD to the experimental results. A full two-phase model, besides involving considerably greater computational difficulties, would also introduce many unknown factors and uncertainties concerning aspects such as bubble size, bubble coalescence etc. However, in terms of the main purpose for which the model is required, which is to predict the disinfection performance of the contactor, the very good agreement obtained between the approximate model and the overall experimental RTD. indicated that the model is adequate. It was decided to proceed with the one-phase, water-only model for the ozone reaction modelling.

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