Bubbles and microbubbles for hydrodynamic test facilities

Abstract

Bubbles are known to have important effects in hydrodynamic facilities such as cavitation tunnels, hydraulic flow loops, pumps, valves and turbines test facilities, towing tanks etc. They are also used to simulate real two-phase flows with various void fractions or bubble size distributions under different pressure and velocity conditions. In order to manage safely these problems or to simulate at best the prototype conditions, an arsenal of theoretical and experimental techniques is necessary, including: - The control of dissolved air, which is under the form of independent molecules moving in the water. - The control of nuclei which are generally microbubbles of some tens of micrometers. - The injection of emulsions. Emulsions are a mixture of water and bubbles of millimetric size. - The separation of free air. Free air concerns bubbles of any size. - The resorption and the control of bubbles growing or shrinking by diffusion of air in water. The production and control of bubbles also implies the use of dedicated measuring techniques. Among these are devices to measure the void fraction and size of bubbles in emulsions as well as dissolved gas: - Local optical and electrical probes [14], - Optical techniques, - Venturi measurements (susceptibility meters). Susceptibility is the pressure at which a nucleus must be submitted to explode and create a vapor bubble. - Optical and polarographic gages for dissolved O2 analysis. Finally, microbubbles can be utilized as non-polluting tracers for optical velocity measurements in water and transparent fluids, such as LDV or PIV. Many of these techniques have been developed by YLEC Consultants. Some of these can be utilized in fluids other than water, such as liquid metals.