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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 aim of this study was to find models for turbulent fragmenting forces in the high-pressure homogeniser from data available in Computational Fluid Dynamics (CFD) simulations with Reynolds Averaged Navier Stokes (RANS) turbulence models. In addition to the more common RANS k–ε turbulence models, a Multi-scale k–ε model was tested since experimental investigations of the geometry imply large differences in behaviour between turbulent eddies of different length-scales. Empiric models for the driving hydrodynamic factors for turbulent fragmentation using the extra information given by multi-scale simulations were developed. These models are shown to give a more reasonable approximation of local fragmentation than models based on the previously used RANS k–ε models when comparing to hydrodynamic measurements in an experimental model. [Copyright &y& Elsevier]

Abstract: Micromixing of viscous systems in a stirred-tank reactor with Rushton turbine is investigated numerically. It is characterized by the product selectivity of parallel competitive reactions. Flow fields inside the reactor are determined by the Reynolds Stress Model (RSM). A computational fluid dynamics (CFD) method combining the standard E model and Finite-Rate/Eddy-Dissipation (FR/ED) model is implemented and is validated using experimental data in the literature. The simulations show that a higher agitation speed, a lower fluid viscosity and/or a feeding location closer to the discharge area of the impeller favor micromixing and the reaction rate. The trajectory of the reaction plume also influences the micromixing performance. The value of the FR/ED model parameter determined by lab-scale experiments decreases with increasing fluid viscosity. However, it is little affected by the agitation speed. These results provide useful guidelines for the scale-up of industrial reactors with complex chemical reactions. [Copyright &y& Elsevier]

Abstract: Large Eddy Simulations (LESs) were performed for a confined rectangular liquid jet with a co-flow and compared in detail with particle image velocimetry (PIV) measurements. A finite-volume CFD library, OpenFOAM, was used to discretize and solve the filtered Navier–Stokes equation. The effects of grid resolution, numerical schemes and subgrid models on the LES results were investigated. The second and fourth order schemes gave similar performance, while the fourth order scheme costs much more computationally. Subgrid model comparison showed that the locally dynamic procedure is necessary for complex flow simulation. Model validation was performed by comparing LES data for the point-wise velocity statistics such as the mean and the root-mean-square velocity, shear stress, correlation coefficient, velocity skewness and flatness with the PIV data. In addition, LES data for the two-point spatial correlations of velocity fluctuations that provide structural information were computed and compared with PIV data. Good agreement was observed leading to the conclusion that the LES velocity field accurately captures the important characteristics of all the turbulent length scales present in the flow, from the fully resolved energy-containing eddies to the subgrid-scale dissipative eddies. [Copyright &y& Elsevier]