Generation and control of monodisperse bubble suspensions in microgravity

Abstract

A new experimental setup for the generation of homogeneous, monodisperse bubble suspensions in turbulent duct flows in microgravity has been designed and tested in drop tower experiments. The setup provides independent control of bubble size, void fraction and degree of turbulence. The device combines several slug-flow injectors that produce monodisperse bubble jets, with a turbulent co-flow that ensures homogeneous spatial spreading. Bubble separation in the scale of the most energetic eddies of the flow, and bubble size sufficiently smaller, ensure that turbulence is most efficient as a mechanism for spatial spreading of bubbles while preventing coalescence, thus optimizing the homogeneous and monodisperse character of the suspension. The setup works in a regime for which bubbles are spherical, but sufficiently large compared to the turbulent dissipative scales to allow for two-way coupling between bubbles and carrying flow. The volume fraction is kept relatively small to facilitate particle tracking techniques. To illustrate the potential uses of the method we characterize the statistics of bubble velocity fluctuations in steady regimes and we characterize the transient relaxation of the buoyancy-driven pseudo-turbulence when gravity is switched-off.

Controlled Generation of Vapor/Liquid Slug Flows by Local Boiling in Microgravity

“…Specifically, new heat exchange correlations could be developed in optimized conditions. From a fundamental point of view, this method could be used to generate novel configurations of bubble suspensions in larger containers, to study the statistics of bubble coalescence, and in general the properties of turbulent bubble dispersions in microgravity, for instance injecting such vapor-liquid slug flows into controled turbulent flows, such as in [13].…”

“…In this respect, previous work on the non-thermal case has shown the importance of controlling the generation of the two-phase flow in order to gain insight into the physical mechanisms involved in the resulting flow. In particular the possibility of generating monodisperse bubble suspensions of prescribed size and volume fraction in capillary tubes [7][8][9][10] has allowed to have access to a collection of high quality microgravity data about the interaction between bubbles and turbulence in different conditions [11][12][13][14]. In this way, a fundamental problem in fluid physics and transport, namely that of turbulent dispersed multiphase flow [15], in the particularly elusive case of non-buoyant dispersed bubbles [16], was brought to the same footing as other more accessible and common two-phase suspensions.…”