Marchisio / Fox Multiphase reacting flows: modelling and simulation

1. Auflage 2007
ISBN: 978-3-211-72464-4
Verlag: Springer Wien
Format: PDF
Kopierschutz: 1 - PDF Watermark

E-Book, Englisch, Band 492, 269 Seiten, eBook

Reihe: CISM International Centre for Mechanical Sciences

This book entitled ''Multiphase reacting flows: modelling and simulation'' contains the lecture notes of the CISM (International Centre for Mechanical Sciences) course held in Udine, Italy, on July 3-7, 2006, and it describes various modelling approaches for dealing with polydisperse multiphase reacting flows. A multiphase reacting system is characterized by the presence of multiple phases and in this book we focus on disperse multiphase systems, where one phase can be considered as a continuum, whereas the additional phases are dispersed in the continuous one. In other words, in this book we deal with multiphase systems constituted by particles, droplets or bubbles (i.e., solid particles suspended in a continuous liquid phase, liquid droplets in a gaseous phase, or gas bubbles in liquid.) The other important characteristic elements of the systems discussed in this book are the presence of one or more chemical reactions and the turbulent nature of the flow. The chemical reactions usually involve all the phases present in the system and might be responsible for the formation or disappearance of the disperse and/or continuous phases. The evolution of the different phases is not only governed by chemical reactions, but also by other fluid-dynamical interactions between the continuous and the disperse phases, and by interactions among elements of the disperse phases, such as coalescence, aggregation, agglomeration and break-up.

Zielgruppe

Research

Autoren/Hrsg.

Marchisio, Daniele L.

Fox, Rodney O.

Inhaltsverzeichnis

and Fundamentals of Modeling Approaches for Polydisperse Multiphase Flows.- Quadrature Method of Moments for Poly-Disperse Flows.- Eulerian Multi-Fluid Models for Polydisperse Evaporating Sprays.- Multi-fluid CFD Analysis of Chemical Reactors.- The Lattice-Boltzmann Method for Multiphase Fluid Flow Simulations and Euler-Lagrange Large-Eddy Simulations.- Direct Numerical Simulation of Sprays: Turbulent Dispersion, Evaporation and Combustion.

Leseproben

Eulerian Multi-Fluid Models for Polydisperse Evaporating Sprays (p. 78-79)

Marc Massot

Laboratoire EM2C - CNRS UPR 288, Ecole Centrale Paris, France

Abstract In this contribution we propose a presentation of Eulerian multi-fluid models for polydisperse evaporating sprays. The purpose of such a model is to obtain a Eulerian-type description with three main criteria: to take into account accurately the polydispersity of the spray as well as size-conditioned dynamics and evaporation, to keep a rigorous link with the Williams spray equation at the kinetic, also called mesoscopic, level of description, where elementary phenomena such as coalescence can be described properly, to have an extension to take into account non-resolved but modeled fluctuating quantities in turbulent flows. We aim at presenting the fundamentals of the model, the associated precise set of related assumptions as well as its implication on the mathematical structure of solutions, robust numerical methods able to cope with the potential presence of singularities and finally a set of validations showing the efficiency and the limits of the model.

1 Introduction

In many industrial combustion applications such as Diesel engines, fuel is stocked in condensed form and burned as a disperse liquid phase carried by a gaseous flow. Tw^o-phase effects as well as the polydisperse character of the droplet size distribution (since the droplet dynamics depend on their inertia and are conditioned by size) can significantly influence flame structure. Size distribution effects are also encountered in a crucial way in solid propellant rocket boosters, where the cloud of alumina particles experiences coalescence and become polydisperse in size, thus determining their global dynamical behavior (Hylkema, 1999, Hylkema and Villedieu, 1998). The coupling of dynamics, conditioned on particle size, with coalescence or aggregation as well as with evaporation can also be found in the study of fluidized beds (Tsuji et al., 1998) and planet formation in solar nebulae (Bracco et al., 1999, Chavanis, 2000). Consequently, it is important to have reliable models and numerical methods in order to be able to describe precisely the physics of two-phase flows where the disperse phase is constituted of a cloud of particles of various sizes that can evaporate, coalesce or aggregate, break-up and also have their own inertia and size-conditioned dynamics. Since our main area of interest is combustion, we will work with sprays throughout this paper, keeping in mind the broad application fields related to this study.

By spray, we denote a disperse liquid phase constituted of droplets carried by a gaseous phase. Even with this seemingly precise definition, two approaches corresponding to two levels of description can be distinguished. The first, associated with a full direct numerical simulation (DNS) of the process, provides a model for the dynamics of the interface between the gas and liquid, as well as the exchanges of heat and mass between the two phases using various techniques such as the Volume of Fluids (VOF) or Level Set methods (see, for example, Aulisa et al, 2003, Herrmann, 2005, Josserand et al., 2005, Tanguy and Berlemont, 2005). This "microscopic" point of view is very rich in information on the detailed properties at the single-droplet level concerning, for example, the resulting drag exerted on a droplet depending on its surroundings flow or the details of one event of droplet break-up following the geometry of the interface between the phases. The second approach, based on a more global point of view (thus called "mesoscopic") describes the droplets as a cloud of point particles for which the exchanges of mass, momentum and heat are described using a statistical point of view, with eventual correlations, and the details of the interface behavior, angular momentum of droplets, detailed internal temperature distribution inside the droplet, etc., are not predicted. Instead, a finite set of global properties such as size of spherical droplets, velocity of the center of mass, and temperature are modeled. Because it is the only one for which numerical simulations at the scale of a combustion chamber or in a free jet can be conducted, this "mesoscopic" point of view will be adopted in the present work.