Particle concentration measurement and flow regime identification in multiphase pipe flow using a generalised dual-frequency inversion method

An acoustic particle concentration measurement method, originally developed for marine sediment, in which the backscattered energy received by emitter-receiver transducers in the megahertz range is used to construct concentration profiles in suspensions of solid particles in a carrier fluid is applied to suspensions of general engineering interest. Four particle species with range of densities and sizes are used. Concentration profiles in horizontal, turbulent pipe flow at a Reynolds number of 50,000 and three nominal volume fractions are presented for each particle species, using experimentally determined acoustic coefficients, in order to isolate the influence of particle size and density on transport and settling in solid-liquid multiphase flows. It is clear from the results that the method allows the degree of segregation in real suspensions and slurries to be measured, and has a range of potential applications in the nuclear and minerals processing industries, for example. Lastly, the limiting conditions of the method are explored through the concept of an acoustic penetration depth

A CFD-DEM solver to model bubbly flow. Part I: Model development and assessment in upward vertical pipes

[EN] In the computational modeling of two-phase flow, many uncertainties are usually faced in simulations and validations with experiments. This has traditionally made it difficult to provide a general method to predict the two-phase flow characteristics for any geometry and condition, even for bubbly flow regimes. Thus, we focus our research on studying in depth the bubbly flow modeling and validation from a critical point of view. The conditions are intentionally limited to scenarios where coalescence and breakup can be neglected, to concentrate on the study of bubble dynamics and its interaction with the main fluid. This study required the development of a solver for bubbly flow with higher resolution level than TFM and a new methodology to obtain the data from the simulation. Part I shows the development of a solver based on the CFD-DEM formulation. The motion of each bubble is computed individually with this solver and aspects as inhomogeneity, nonlinearity of the interfacial forces, bubble-wall interactions and turbulence effects in interfacial forces are taken into account. To develop the solver, several features that are not usually required for traditional CFD-DEM simulations but are relevant for bubbly flow in pipes, have been included. Models for the assignment of void fraction into the grid, seeding of bubbles at the inlet, pressure change influence on the bubble size and turbulence effects on both phases have been assessed and compared with experiments for an upward vertical pipe scenario. Finally, the bubble path for bubbles of different size have been investigated and the interfacial forces analyzed. (C) 2017 Elsevier Ltd. All rights reserved.The authors sincerely thank the ''Plan Nacional de I + D+ i" for funding the project MODEXFLAT ENE2013-48565-C2-1-P and ENE2013-48565-C2-2-P.Peña-Monferrer, C.; Monrós Andreu, G.; Chiva Vicent, S.; Martinez-Cuenca, R.; Muñoz-Cobo, JL. (2018). A CFD-DEM solver to model bubbly flow. Part I: Model development and assessment in upward vertical pipes. Chemical Engineering Science. 176:524-545. https://doi.org/10.1016/j.ces.2017.11.005S52454517

Large eddy simulation of microbubble dispersion and flow field modulation in vertical channel flows

Turbulent liquid–gas vertical channel flows laden with microbubbles are investigated using large eddy simulation (LES) two-way coupled to a Lagrangian bubble tracking technique. Upward and downward flows at shear Reynolds numbers of Re τ = 150 and 590 are analyzed for three different microbubble diameters of 110, 220, and 330 μm. Predicted results are compared with published direct numerical simulation results although, with respect to comparable studies available in the literature, the range of bubble diameters and shear Reynolds numbers considered herein is extended to larger values. Microbubble concentration profiles are analyzed, with the microbubbles segregating at the wall in upflow conditions and moving toward the channel centre in downflow. The various forces acting on the bubbles, and the effect of the flow turbulence on the bubble concentration, are considered and quantified. Overall, the results suggest that the level of detail achievable with LES is sufficient to predict the fluid structures impacting bubble behavior. Therefore, LES coupled with Lagrangian bubble tracking shows promise for enabling the reliable prediction of bubble-laden flows that are of industrial relevance

Axial mixing in pipe flows: turbulent and transition regions

In the present work, the flow pattern in pipe flow has been simulated using low Reynolds number k-ε model. The CFD model has been extended to simulate the axial dispersion phenomena in both the transition and turbulent regions. An extensive comparison of the predicted axial dispersion coefficient with the experimental data has been presented along with the predictions of various models published in the literature. The proposed CFD model for axial mixing was found to give an excellent agreement with the experimental measurements

CFD simulation of mixing and dispersion in bubble columns

Liquid phase mixing time was measured in 0.2 and 0.4 m i.d. columns over a wide range of superficial gas velocity (0.07-0.295 ms-1) and height-to-diameter ratio (1-10). A CFD model was developed for the prediction of flow pattern in terms of mean velocity and eddy diffusivity profiles. The predictions agree favourably with all the experimental data published in the literature. Complete energy balance was established in all cases. The validated CFD model was extended for the simulation of the macro-mixing process by incorporating the effects of both the bulk motion and the eddy diffusion. Excellent agreement was observed between the CFD predicted and experimental values of the mixing time over the entire range of D, VG and HD/D covered. The model was further extended for the prediction of residence time distribution and hence the axial dispersion coefficient (DL). The predicted values of DL were found to agree very well with all the experimental data published in the literature

Computational fluid dynamics simulations in bubble-column reactors: laminar and transition regimes

In the present work, a computational fluid dynamics (CFD) model was developed to describe both of the extreme regimes (viscous and turbulent), including the transition regime. The second objective was to examine the extent to which CFD models are able to describe quantitatively the variation of εGwith VG as a function of flow regimes. This study helps to underline the distinguishing characteristics of both regimes: homogeneous and heterogeneous. An extensive comparison of predicted mean axial liquid velocity profiles and fractional gas hold-up profiles with the experimental data has been presented. The agreement has been shown to be excellent. The CFD model has also been compared with the simplified analytical solutions. In a fully developed heterogeneous regime and under turbulent conditions, the reversal point for the axial velocity was found to be in the range of 0.7-0.75R. With a decrease in the Reynolds number (DVCρL/μL), the reversal point was found to shift toward the center up to 0.5R. Further, the CFD simulation was found to reveal a large number of characteristic features of flow in viscous and transition regimes

Axial mixing in laminar pipe flows

A computational model has been developed to simulate the axial dispersion phenomena in laminar pipe flows. Peclet number (Pe=au<SUB>0</SUB>ID<SUB>m</SUB>) and dimensionless time (τ=D<SUB>m</SUB>t/a<SUP>2</SUP>) have been covered over a wide range of 1-10<SUP>5</SUP> and 10<SUP>−8</SUP> to 10<SUP>2</SUP>, respectively. An extensive comparison of predicted mean concentration profiles with the experimental data has been presented. The proposed computational model for concentration profiles was found to give an excellent agreement with the experimental measurements. Further, the results compare well with other analytical and experimental studies. The solution has been shown to be valid at both the short and long times and for all values of the Peclet number

CFD simulation of the flow pattern for drag reducing fluids in turbulent pipe flows

In the present work, the flow pattern in pipe flows has been simulated for drag reducing fluids using a low Reynolds number k-ε model. The model uses a non-linear molecular viscosity and damping function to account for near wall effects. The comparison between the predictions and the experimental profiles of axial velocity and kinetic energy are in good agreement. A systematic study has been undertaken to investigate the effect of rheological parameters and to consider the modification to the flow that arises in the presence of a fluid yield stress