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Abstract: This paper presents the state of the art regarding the understanding of axial mixing in pulsed columns. Residence Time Distribution (RTD) experiments carried out mainly between 1986 and 2002 are reviewed in connection with fluid mechanics studies and CFD simulations. Based on prior work by Buratti (1988), a new model is established to correlate the continuous phase axial dispersion coefficient with the operating conditions in a wide range of operating (pulse intensity up to 7.5cms−1) and geometric conditions (0.17≤H/D≤1.33), thereby reducing the estimation error from 143% to 21% in the case of small-aspect ratio columns (H/D<1), such as those used in nuclear research, and from 95% to 27% in the case of (possibly annular) industrial size columns. Experimental results are relatively well reproduced by RANS-type computational fluid dynamics (CFD) simulations, provided a low-Reynolds model is used for the turbulent viscosity estimation. [Copyright &y& Elsevier]

Abstract: The present paper demonstrates how 2D, dynamic simulations of a flat bubble column are feasible, applying state-of-the-art dynamic turbulence models, when an appropriate turbulent dispersion term is applied in the conservation equation for the gas volume fraction. The turbulence model yielded a better qualitative prediction of the bubble plume than the model, due to the low-Reynolds number treatment of the former model. The simple mixing length turbulence model gave the best prediction of the meandering plume, without any dispersion term. The mixing length model is, however, almost identical to a Large-Eddy simulation when run time-dependent on a fine mesh, and should be applied with care due to the use of a constant turbulence length scale and the inherent 3D nature of turbulence. By refining the mesh to the extreme end, it was shown that an apparently grid independent numerical solution was really grid-dependent, even when dynamic turbulence models were applied. The apparently grid independent solution was computed with an increment in the computational mesh that was of the same size as an equilibrium Kolmogorov length scale. [Copyright &y& Elsevier]