The development of the structure of water – air bubble grid turbulence

The development of the turbulent flow field generated due to the interaction of grid turbulence with a swarm of bubbles is investigated experimentally, in a vertical channel of rectangular cross section. Void fraction and streamwise mean and rms velocity distributions have been measured at several distances from the grid, with an optical probe and Laser Doppler Velocimetry, in relation to the air flow rate ratio. The obtained results indicate that close to the grid the void fraction and velocity distributions are dictated by the bubble injectors’ location on the grid. Downstream the void fraction distribution changes to a double peak pattern. The velocity distribution is characterized by a shear layer between the wall area and the central area of the channel. The extend of this shear layer is increasing as the distance from the grid and the gas flow rate ratio are increasing, and is associated with a corresponding increase of the turbulence fluctuations. Autocorrelation and spectra measurements at the centre of the channel show a reduction of the flow scales for low void fraction. Consistently, power spectra distributions indicate that bubbles cause a redistribution of energy manifested by the relative enhancement of the intermediate scales’ energy content and a consequent reduction in the larger scales. These trends are gradually alleviated and reversed at large distances from the grid, as the air flow rate is increased.