Surfactant effects on bubble-induced turbulence

We experimentally explore the surfactant effect on bubble-induced turbulence (BIT) at different scales. To this end, high-resolution Particle Shadow Velocimetry measurements are carried out in a bubble column in which the flow is generated by a homogeneous distributed bubble swarm rising in water for two different bubble diameters (3 mm & 4 mm) and moderate gas volume fractions. To contaminate the flow, different amounts of 1-Pentanol were added to the bubble column, leading to different bubble shapes and surface boundary conditions. Concerning the surfactant effect on the flow kinetic energy at different scales, the velocity structure functions are found to be influenced differently in the vertical and horizontal directions of the flow. Furthermore, we considered the issue of modeling BIT with respect of surfactant effect. The experimental results reveal that with increasing concentration of surfactants, the generated BIT increases as well, although the bubbles rise slower. We show that this phenomenon has not be considered so far in the standard BIT modeling using the ansatz with a source term, reading C_IF_D·u_r added to the transport equation of turbulent kinetic energy. Here, C_I, F_D and u_r are a coefficient, drag and slip velocity, respectively.