Highly Turbulent Couette-Taylor Bubbly Flow Patterns

We present the results of experimental study of a Couette{Taylor system with superimposed axial flow and an upper free surface, in the high Taylor number regime. At large Taylor numbers, when the rotational speed of the inner cylinder increases, bubbles created near the free surface are distributed throughout the test section and permit the study of the spatial and temporal properties of turbulent flows using visualization techniques. In addition to classic travelling Taylor vortices, intermittent pulses of vortices with higher phase velocities are also observed. These patterns are described in terms of the rotational speed and the intensity of the throughflow. Couette{Taylor instabilities are prototypes for general studies in hydrodynamic stability and transition to turbulence. Less known are various industrial applications of these characteristic patterns coupled with an axial flow, such as cooling of rotating machinery or in continuous chemical reactors. Each pair of vortices can be idealized, in a rst approximation, as a well-mixed batch reactor advected by the axial flow and without exchange of liquid between the adjacent vortices (gure 1a). This property can be useful in liquid{liquid contact devices used in nuclear fuel reprocessing because it may allow accurate control of the fluid residence time. In the nuclear fuel cycle, the main steps of the reprocessing are carried out by liquid extraction cycles (Treybal 1981). Liquid extraction is the separation of the constituents of an aqueous solution by contact with another insoluble organic liquid. The purpose of the equipment used for the liquid{liquid operations is to provide intimate contact between the two fluids for a sucient time to permit interphase mass transfer of the energetic elements (uranium and plutonium). The rate of mass transfer is directly dependent upon the interface surface area between the phases. Intense agitation is thus required to generate an emulsion composed of small droplets (with a diameter smaller than 500m), and consequently to increase the rate of mass transfer; this involves flows characterized by very high Taylor number, Ta. Moreover, the typical flow rates in a reprocessing plant induce a signicant axial flow in such liquid extraction devices, which implies a relatively high axial Reynolds number, Re. Such innovative applications to chemical and nuclear engineering follow up original fundamental problems related to the Couette{Taylor instability for multi-phase flows and highly turbulent situations. y Present address: CEA Valrh ^ o, DTE, B.P. 111, F-26702 Pierrelatte, France.