Some aspects of bubbly flows dynamics as revealed by advanced measuring techniques combined with hybrid modeling
Résumé
Buoyancy driven bubbly (or particulate) flows are commonly exploited in industry. Owing to the strong mechanical coupling between the two phases, their structure is still difficult to predict in spite of the numerous contributions that have been dedicated to determine and/or to improve closure laws. Nowadays, direct numerical simulations of a large set of freely moving inclusions are becoming more and more realistic, and it is sometimes argued that such a direct approach offers the best option for predictions. In this lecture, we will focus on simple prototype bubbly flows formed from spherical monodispersed bubbles in a laminar continuous phase. We will first review key behaviours of such flows, as well as what can be learned from axial momentum balances. Then, we will illustrate the diversity and the complexity of the coupling mechanisms that must be accounted for in order to acccurately predict the transverse distributions. The discussion will be grounded on a hybrid formalism combining kinetic theory concepts for the dispersed phase and a continuous approach for the carrier fluid. Measuring techniques issues will also be evoked during the lecture, both in terms of uncertainty associated with usual variables (such as void fraction, bubble velocity...), and also in terms of the determination of new quantities as required by the hybrid model. We will show how the proposed averaged formalism can lead to a closed system of equations when combined with direct simulations involving only a few test inclusions. We will conclude on a possible numerical implementation of that scheme in order to predict simple laminar bubbly (or particulate) flows.