1,721,331 research outputs found
Quadrature-based moment methods for the simulation of fluid-fluid multiphase systems
No full textFluid-fluid multiphase systems are complicated by the fact that the disperse phase is constituted by deformable bubbles or droplets, that have a high tendency to coalesce and break, generating complex phase coupling for mass, heat and momentum. Different techniques can be used to simulate these systems, but Eulerian approaches are nowadays the ones directly applicable to industrial problems. Historically the problem was studied with simplified assumptions for the fluid dynamics, and focusing of bubble/droplet coalescence and break up, or viceversa, by assuming constant size and composition for bubbles/droplets and investigating the fluid dynamics. This has changed since the introduction of quadrature-based moment methods (QBMM), where the state of the poly-disperse population of bubbles/droplets is tracked by solving a specific equation (called generalized population balance equation) that dictates the evolution of a number density function (NDF). The above mentioned methods are based on the simple idea of solving transport equations for the moments of this NDF and by overcoming the closure problem by adopting a quadrature approximation. In this talk three of these methods are discussed in details: the Quadrature Method of Moments (QMOM), the Direct Quadrature Method of Moments (DQMOM) and the Conditional Quadrature Method of Moments (CQMOM). Particular attention is paid to the specific theoretical and numerical issues encountered in the simulation of gas-liquid systems. The talk will focus on the problem of correctly predicting momentum and mass phase coupling, as well as bubble coalescence and break up. Numerical issues related to spatial discretization, to the use of conservative (i.e., moments) versus primitive (i.e., quadrature nodes and weights) and to the problem of moment realizability, will also be thoroughly analyzed. Moreover, by discussing different systems operating under different operating conditions, issues related to the number of internal coordinates to be tracked and the number of nodes of the quadrature approximation to be used, will be discussed and addressed. Comparison of QBMM with other more sophisticated techniques, such as Direct Simulation Monte Carlo (DSMC), will also be discussed and eventually some guidelines for the use of QBMM will be presented. The talk will conclude with the presentation of some industrially relevant case studies conducted in stirred tanks and bubble columns, together with their validation with experiments in terms of bubble size distributions and mass transfer rates. Finally the main limitations of these methods and the need of better physics for specific interfacial phenomena (accounting for example for the presence of surfactants) will be discusse
Precipitation in turbulent fluids
Full text linkThis thesis concerns the simulation with computational fluid dynamics and population balances of a precipitation proces
Moment-based methods to solve PBEs and their contribution to the growth of population balance modeling
No full textExperimental and Modeling Strategies to Design and Optimize Micro-devices for Continuous Crystallization
No full textOn the use of bi-variate population balance equations for modelling barium titanate nanoparticle precipitation
No full textOn the use of bi-variate population balance equations for modelling barium titanate nanoparticle precipitation
No full textExperimental and Modeling Strategies to Design and Optimize Micro-Devices for the Continuous Production of Micro- and Nano-Particles
No full text
Quadrature-based moment methods for the simulation of turbulent polydisperse multiphase systems
Full text linkThis talk will present and discuss a class of approaches for the simulation of turbulent multiphase flows, called quadrature-based moment methods (QBMM). These methods are based on an Eulerian description of the multiphase system, where polydispersity is modeled through a number density function (NDF). Since only transport equations for the (pure and mixed) moments of the NDF are solved, the NDF must be reconstructed from the moments. This is done by using basis functions (such as Dirac delta functions) that result in the use of quadrature approximations for overcoming the so-called closure problem. The talk is divided in two parts. In the first part the Generalized Population Balance Equation (GPBE), that dictates the evolution of the NDF, will be presented and its relationship with similar balance equations (e.g., Boltzmann, Williams, particle dynamics and population balance equations) will be highlighted. Moreover, the derivation from the GPBE of the characteristic equations of the Eulerian-Eulerian multifluid models, widely adopted in many commercial computational fluid dynamics (CFD) codes will be analyzed and its limitations discussed. In the second part of the talk some practical cases will be presented. In particular, the use of our implementations of the quadrature method of moment (QMOM), conditional quadrature method of moments (DQMOM) and direct quadrature method of moments (DQMOM) in Ansys/Fluent, Openfoam and TransAT will be illustrated. Particular attention will be devoted to the coupling with turbulence models, both with the Reynolds-average Navier-Stokes equations (RANS) and large-eddy simulation (LES) approaches. The issues related to the implementation of QMOM, CQMOM and DQMOM in CFD codes based on the finite-volume method will also be considered. The problems of moment corruption, realizability and conservation and the relative strategies to overcome them will be presented and discussed. The potentials of QBMM will be eventually demonstrated by presenting data concerning the simulation of mixing and segregation in fluidized beds, bubble coalescence, break-up and mass transfer in bubble columns and gas-liquid stirred tanks and of turbophoresis in turbulent particle-laden flow
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