Turbulence in the Liquid Phase of a Bubbly Air-Water Flow

Abstract

This paper is devoted to the study of the turbulence in the liquid phase of a vertical, co-current air-water bubbly flow. The air bubbles are blown uniformly into a hydrodynamic tunnel, through a grid which generates a quasi-isotropic turbulent field. The turbulent characteristics have been obtained by hot-film and Laser Doppler Anemometry. The longitudinal turbulent kinetic energy increases strongly with the void fraction α. Roughly speaking, there exist two distinct regimes : the first one corresponds to low values of α, where hydrodynamic interactions between bubbles are negligible and the second one to higher values, for which, due to their mutual interactions, the bubbles transfer a significant amount of kinetic energy. The critical value of α at which the transition occurs proves to be a decreasing function of the level of the turbulence generated by the grid. The one-dimensional spectra exhibit a progressive disappearance of the classical — 5/3 law, and a large range of high frequencies associated with the wakes of the bubbles. The Reynolds stress tensor shows that the quasi-isotropy is not altered. If the turbulent kinetic energy is assumed to be the sum of the energy created by the grid and that due to the bubbles, it proves to be a linear function of a whose slope depends on the regime under consideration.