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Abstract: Liquid-phase turbulent transport in a confined rectangular wake was investigated for a Reynolds number of 37,500 based on bulk velocity and the hydraulic diameter of the test section and a Schmidt number of 1250 using particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF). The velocity and concentration field data were analyzed for flow statistics such as the mean velocity, Reynolds stresses, turbulent kinetic energy, turbulent dissipation rate, mixture-fraction mean, mixture-fraction variance and one-point composition probability density functions (PDF). Computational fluid dynamics (CFD) models, including a two-layer – turbulence model, a scalar gradient-diffusion model and a scalar dissipation rate model were validated against PIV and PLIF data collected at six downstream locations. Low-Reynolds-number effects on turbulent transport were taken into consideration through the mechanical-to-scalar time-scale ratio. The experimental and computational results were found to be in satisfactory agreement. [Copyright &y& Elsevier]

Abstract: A study was performed that utilizes the quadrature method of moments (QMOM) to model the transient spatial evolution of the floc size in a heterogeneous turbulent stirred reactor. The QMOM approach was combined with a commercial computational fluid dynamics (CFD) code (PHOENICS), which was used to simulate the turbulent flow and transport of these aggregates in the reactor. The CFD/QMOM model was applied to a 28l square reactor containing an axial flow impeller and 100mg/l concentration of 1 m nominal clay particles. Simulations were performed for different average characteristic velocity gradients (, and 150s). The average floc size and growth rate were compared with experimental measurements performed in the bulk region and the impeller discharge region. The CFD/QMOM results confirmed the experimentally measured spatial heterogeneity in the floc size and growth rate. In addition, the model predicts spatial variations in the aggregation and breakup rates. Finally, the model also predicts that the transport of flocs into the high shear impeller discharge zone was responsible for the transient evolution of the average floc size curve displaying a maximum before decreasing to a steady-state floc size. [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]

Abstract: Computational fluid dynamics as a simulation tool allows obtaining a more detailed view of the fluid flow and heat transfer mechanisms in fixed-bed reactors, through the resolution of 3D Reynolds averaged transport equations, together with a turbulence model when needed. In this way, this tool permits obtaining of mean and fluctuating flow and temperature values in any point of the bed. An important problem when modeling a turbulent flow fixed-bed reactor is to decide which turbulence model is the most accurate for this situation. To gain insight into this subject, this study presents a comparison between the performance in flow and heat transfer estimation of five different RANS turbulence models in a fixed bed composed of 44 homogeneous stacked spheres in a maximum space-occupying arrangement in a cylindrical container by solving the 3D Navier–Stokes and energy equations by means of a commercial finite volume code, Fluent 6.0. Air is chosen as flowing fluid. Numerical pressure drop, velocity and thermal fields within the bed are obtained. In order to judge the capabilities of these turbulence models, heat transfer parameters (Nu , k /k ) are estimated from numerical data and together with the pressure drop are compared to commonly used correlations for parameter estimations in fixed-bed reactors. [Copyright &y& Elsevier]