1,721,039 research outputs found
Computational models for the simulation of turbulent poly-dispersed flows: Large Eddy Simulation and Quadrature-Based Moment Method
Full text linkThis work focuses on the development of efficient computational tools for the simulation of turbulent multiphase polydispersed flows. In terms of methodologies we focus here on the use of Large Eddy Simulation (LES) and Quadrature-Based Methods of Moments (QBMM). In terms of applications the work is finalised, in order to be applied in the future, to particle production processes (precipitation and crystallisation in particular).
An important part of the work concerns the study of the flow field in a Confined Impinging Jets Reactor (CIJR), frequently used in particle production processes. The first part is limited to the comparison and analysis of micro Particle Image Velocimetry (μPIV) experiments, carried out in a previous work, and Direct Numerical Simulation (DNS), carried out in this thesis. In particular the effects of boundary and operating conditions are studied and the numerical simulations are used to understand the experimental predictions and demonstrate the importance of unavoidable fluctuations in the experimental inlets. This represents a preparatory work for the LES modelling of the CIJR. Before investigating the accuracy of LES predictions for this particular application, the model and the implementation are studied in a more general context, represented by a well-known test case such as the periodic turbulent channel flow: the LES model implementation in
TransAT, the code used in this work, is compared with DNS data and with predictions of other codes. LES simulations for the CIJR, provided with the proper boundary conditions obtained by the previous DNS/μPIV study, are then performed and compared with experiments, validating the model in a more realistic test case. Since particle precipitation and crystallization often result in complex interactions between particles and the continuous phase, in the second part of the work particular attention has been paid in the modelling of the momentum transfer and the resulting velocity of the particles (relative to the fluid). In particular the possibility of describing poly-disperse fluid-solid systems with QBMM together with LES and Equilibrium Eulerian Model (EEM) is assessed. The study is performed by comparing our predictions with DNS Lagrangian data in the turbulent channel flow previously described, seeded with particles corresponding to
a realistic Particle Size Distribution (PSD). The last part of the work deals with particle collisions, extending QBMM to the investigation of non-equilibrium flows governed by the Boltzmann Equation with a hard-sphere collision kernel. The evolution of the particle velocity distribution is predicted and compared with other methods for kinetic equations such as Lattice Boltzmann Method (LBM), Discrete Velocity Method (DVM) and Grad’s Moment Method (GM). The overall results of this thesis can be extended to a broad range of other applications of single-phase, dispersed multiphase and non-equilibrium flows
Macroscopic models for filtration and heterogeneous reactions in porous media
Full text linkDerivation of macroscopic models for advection-diffusion processes in the presence of dominant heterogeneous (e.g., surface) reactions using homogenisation theory or volume averaging is often deemed unfeasible (Valdés-Parada et al., 2011; Battiato and Tartakovsky, 2011a) due to the strong coupling between scales that characterise such systems. In this work, we show how the upscaling can be carried out by applying and extending the methods presented in Allaire and Raphael (2007), Mauri (1991). The approach relies on the decomposition of the microscale concentration into a reactive component, given by the eigenfunction of the advection-diffusion operator, the associated eigenvalue which represents the macroscopic effective reaction rate, and a non-reactive component. The latter can be then upscaled with a two-scale asymptotic expansion and the final macroscopic equation is obtained for the leading order. The same method can also be used to overcome another classical assumption, namely of non solenoidal velocity fields, such as the case of deposition of charged colloidal particles driven by electrostatic potential forces. The whole upscaling procedure, which consists in solving three cell problems, is implemented for arbitrarily complex two- and three-dimensional periodic structures using the open-source finite volume library OpenFOAM®. We provide details on the implementation and test the methodology for two-dimensional periodic arrays of spheres, and we compare the results against fully resolved numerical simulations, demonstrating the accuracy and generality of the upscaling approach. The effective velocity, dispersion and reaction coefficients are obtained for a wide range of Péclet and surface Damköhler numbers, and for Coulomb-like forces to the grains. Noticeably, all the effective transport parameters are significantly different from the non-reactive (conserved scalar) case, as the heterogeneity introduced by the reaction strongly affects the micro-scale profiles
The Immersed Surfaces Technology for Reliable and Fast Setup of Microfluidics Simulation Problems
No full textPore-scale simulation and hydrodynamic dispersion estimation in realistic porous media
No full textThe mathematical modelling and numerical simulation of flow and transport in porous media is traditionally approached from a macroscopic point of view, by using macro-scale averaged equations such as the Darcy law and the Advection-Dispersion-Reaction (ADR) equation with macroscopic parameters that are the results of the averaging procedure on the microscopic structures of the medium (e.g., permeability, porosity) or of the flow (e.g., tortuosity, dispersivity). This classical description however cannot be rigorously applied when dealing with problems that involve high heterogeneity, sharp transitions and discontinuities in the physical parameters, two-phase flows, non-linearities due to high flow velocities. Furthermore the estimation of the macroscopic parameters is often impossible or can be affected by large uncertainties. Moreover the non-linear dependence of these parameters on medium and flow properties is not completely understood. Therefore the simulation of micro-scale flows (i.e., pore-scale simulations) in porous media is becoming an important topic for a better understanding of complex problems arising in different fields including contaminant transport, reservoir simulation, CO storage, colloidal transport. The present work, that is motivated by an important application in the field of porous media modelling, namely the remediation of contaminated aquifers by means of nanoscopic zerovalent iron particles (NZVI) injections, deals with the development of multi-scale simulation tools and numerical methods to derive adequate macroscopic models and estimate accurately their macroscopic parameters. This is achieved by solving the full Navier-Stokes equations in a realistic pore-scale sample of a three-dimensional porous media together with a transport equation for passive tracer particles. Transport coefficients and parameters of the macro-scale equations are then extracted from the micro-scale results by using non-linear fitting and method of moments for a simple one-dimensional mode
EFFICIENT SIMULATION OF A TWO-PHASE VERTICAL PIPE FLOW WITH POPULATION BALANCE METHOD
No full textThe population balance equation for bubble size distribution in a vertical turbulent pipe flow is solved with a Direct Quadrature Method of Moment (DQMOM) comparing the results with a classical approach where bubbles are characterized by their mean size. The turbulent two-phase flow field, solved within a RANS formulation, is assumed to be in local equilibrium and the relative gas and liquid velocities are therefore calculated with an algebraic slip model, considering drag and lift forces. The non-linear relation between the bubble size and the resulting forces is accurately described through the DQMOM method, in which each quadrature node represents a dynamic class of particles with a characteristic size. Results are compared to experimental results (Szalinski et al., 2010), demonstrating that fast and accurate predictions are obtained for the void fraction and the bubble size distribution in the case of moderate bubble Stokes number and void fraction
Towards multi-scale modelling and simulation of colloid transport in porous media
No full textThe simulation of flow and transport and porous media is of pivotal importance in many fields. Our reference application is the remediation of contaminated aquifers through the injections of nano-scale zero-valent iron particles. The efficiency of this process is influenced by phenomena occurring at three different scales: - Micro-scale: molecular description for forces and surface interactions between particles and solid grains, Lagrangian description - Pore-scale: continuum models for fluid and particle concentration in the pore geometry. Stokes or Navier-Stokes flow coupled with advection-diffusion-reaction problem - Field-scale: porous media equations for flow and particle transport, Darcy law coupled with advection-diffusion-reaction problem The objective of our work is to derive a unified kinetic model to take into account the relevant phenomena occurring at the different scales and propose efficient numerical techniques to approximate the solution
Microscale Simulation of Nanoparticles Transport in Porous Media for Groundwater Remediation
No full textMicro and Nanoscale zerovalent iron (MZVI and NZVI) is one of the most promising reagent for the remediation of contaminated groundwater; these particles, in fact, can efficiently degrade, through redox reactions, recalcitrant and carcinogenic compounds. The aim of this study is to simulate at the microscale the transport of iron nanoparticles, their interaction with the porous media and their deposition on the aquifer material. The simulations have been carried out with a Langrangian approach implemented in COMSOL Multiphysics 4.2a. The model under study includes the relevant forces acting on the single particles such as drag, Brownian, gravity, Van der Waals and electric double layer force. The simulation results can deliver, thanks to this microscale description, an estimation of the attachement efficiency that can be used for macroscale simulations and compared with the relationships obtained by the clean bed filtration approac
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