Surfactant-induced suppression of bubble coalescence in turbulent duct flow at high Reynolds numbers

In this talk, we present the characterization of coalescence-dominated turbulent bubbly flow at high Reynolds number by systematically introducing surfactants to probe interfacial effects under extreme turbulence. Experiments are performed in a vertical square duct flow generated downstream of a regenerative pump at bulk Reynolds number Re_D ~ 10^5, with void fractions \phi ~ 1% to 50%. Bubble size distribution and velocity field are measured via backlit shadowgraph and particle shadow velocimetry. With the addition of sodium dodecyl sulfate surfactant from 0.5% to 50% CMC, we observe a marked suppression of coalescence (compared to previous studies on surfactant-free flows): bubble growth is attenuated, the bubble size distribution remains narrow, and the transition to equilibrium size distribution is delayed. Surfactant slows film drainage between bubbles, reducing coalescence rates even as turbulent collisions persist. Bubble size distributions follow unimodal or truncated distributions. Radial migration and core-peaking of void fraction profiles are diminished, consistent with reduced deformability and lift forces. These results establish that even low surfactant concentrations fundamentally alter bubble population dynamics and radial transport in highly turbulent, developing multiphase flows.