# Current Limitations for Predicting Liquid Dispersion in Continuous Flow Bubble Columns Using CFD
## 3 Results
### Liquid-Phase Dispersion in Air-Water Two-Phase System
Therefore, the mesh used, apart from correctly resolving the velocity field for the two fluids, also represented a satisfactory compromise between the speed of calculation and solution of the turbulent cascade and was found adequate for simulations of the two-phase air-water flow. The temporal profiles of the tracer response for the simulations and experiments are shown in Figure 5. The profiles correspond to the centerline of the column at two axial locations (z = 1.05 and 1.84 m) for the two extreme flow scenarios. For the low-flow experiments (Figure 5A,C), the simulated response rose less quickly than the experimental response, and the simulated response peak was sharper than the experimental peak. After the rise, the experimental tracer concentration decreased at a much lower rate compared to simulations. Similar results were observed for the high-flow condition (Figure 5B,D). In this case, the rising part of the experimental tracer response was correctly simulated, but the simulated tracer concentration declined faster than the experimental data. Therefore, the simulation could not capture the level of back-mixing or the presence of fluid dead zones. Differences between pH data obtained from probes located at 1.05 and 1.84 m suggest that the tracer dispersed in an increasingly larger water volume along its path. Under high-flow conditions, the difference between the measured and simulated responses was smaller compared to the difference under low-flow conditions. The patterns shown in Figure 5 are consistent with other observations in batch bubble columns [36]. In prior studies, the LES-based models were shown to better capture the anisotropic bubble plume movement in a batch bubble column compared to some of the other models [36]. Compared to low gas flow rates, simulations better matched the experimental data for higher superficial gas velocities (>9 \(\times\) 10\({}^{-4}\) m s\({}^{-1}\)) [36]. In the present study, the superficial air velocities were much higher (\(\geq\)6 \(\times\) 10\({}^{-3}\) m s\({}^{-1}\)). In Figure 5, the difference between the experimental and simulated tracer data was probably a result of the presence of bubbles in the system that were not adequately accounted for by the selected combination of models. Compared to single-phase water flow, the presence of air in the two-phase system significantly increased the dispersion of the tracer. Bubbles increased the residence time of the tracer in the column, and thus mixing was improved. Gas flow rate is known to have a stronger influence on liquid phase dispersion in a bubble column compared to the flow rate of the liquid [37]. Although the simulations in Figure 5 did not exactly reproduce experimental tracer responses, the reduced local perturbations in both the simulated and experimental concentration profiles were noticeable in Figure 5 compared to the case for the single-phase flow shown in Figure 4. Perhaps, although the mesh size is adequate for resolving turbulence and velocity fields, it may not be adequate for resolving air bubbles with the precision that the VOF model needs. It might not be, therefore, suitable to choose the VOF model to simulate the system of this work on a workstation because the scale of the fluid domain is too large for implementing the significantly finer mesh required. Consequently, the combination of SAS-VOF models can only be used in ANSYS Fluent(r) for small, two-phase systems. In order to quantitatively compare the experimental and simulated data, mean residence times (\(t_{m}\)) and their variances (\({\sigma_{t}}^{2}\), \({\sigma_{\theta}}^{2}\)) were calculated based on the different measurement locations. The results are summarized in Table 4. The mean residence times confirmed the qualitative observations in Figures 4 and 5: the actual experimental residence time of the tracer was substantially and consistently longer than the simulations.