Two-phase flows
Instructor: H. Djeridi
This course provides fundamental notions for understanding and modelling the dynamics of elementary particles (solids, bubbles, drops...) imbedded in complex flows of a continuous carrier phase. This course aims at delivering key ingredients exploited in modern simulation tools devoted to multiphase flows.
Detailed presentation:
- 0. Multiphase flow characteristics and principal applications, need for a multi-scale approach, presentation of the available strategies to cope with multiphase flow modelling.
- I. Interface mechanical description: a) conservation, state and constitutive equations, b) surface rheology: general concepts (surface tension, surfactants, adsorption isotherm, adsorption kinetics, interfacial molecular transport) and surface rheology c) illustrations: drag on a contaminated inclusion (Marangoni effect) / rheology characterisation using capillary waves / adsorption kinetics by way of the Savart experiment
- II. Inclusion dynamics: permanent and transient movement in an infinite medium (drag, deformation and trajectory instability, added mass, history force) / general equation for a small inclusion in a general flow field (BBOT equation) / Other forces: lift forces, wall interactions... / particule - turbulence interactions, dispersion / Connections with dispersed flow modelling illustrated on elementary systems
- III. Coalescence, break-up and population balance equation: main mechanisms driving coalescence, break-up or agglomeration / introduction to population balance equation / examples
Key-words: interface, interface rheology, particle or inclusion dynamics, two-fluid model, dispersed two-phase flows
Skills: Understanding of the principal mechanisms occuring in dispersed two-phase flows, and how to model them.
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Instructor: F. François (researcher, CEA/Grenoble)
The thermal-hydraulics lecture deals with the snag of two-phase flow modeling in presence of phase change. It is organized into four main parts describing successively the fundamental balance equations and how they are derived, some measuring techniques that are specifically used in the field of two-phase flows, some simple one-dimensional models, and the main regimes of heat transfer in boiling two-phase flows.
Detailed presentation: The subject of the lecture deals with the snag of two-phase flows modeling with a specific focus on boiling two-phase flows. It is divided into four main parts. The first-one is devoted to the drawing up of the fundamental balance equations (mass, momentum and energy) for each phase associated to the interface jump conditions (with or without phase-change). The second part concerns the problem of measurement in two-phase flows. Several measuring techniques are briefly introduced and their main specificities are discussed (optical probe, thermal anemometry, X-ray tomography...). In a third part, some simple one-dimensional models (homogenous, slip ratio, drift flux model...) are presented and their fundamental hypotheses are discussed. The main models of pressure losses in two-phase flows are also given. Finally, the last part is devoted to the boiling phenomena. Some reminders are made about thermodynamics. The main regimes of heat transfer in two-phase flows (from nucleate boiling to film boiling) are illustrated and some heat transfer correlations are given. A specific focus is made on the boiling crisis (departure from nucleate boiling) problem.
Key-words: Two-phase flow, Phase-change, Boiling, Balance equations, Measurement techniques
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