Effects of salt concentration on top wall bubble injection in a turbulent channel flow: bubble dynamics and wall drag reduction

Abstract

Most applications related to bubble drag reduction (BDR) occur in contaminated environments where the presence of different surface active agents modify bubble coalescence and hence, affect flow drag. Although there have been studies on bubble drag modifications with salt/surfactant, the effects of systematic variation in salt/surfactant concentration on bubble dynamics and drag remain relatively unexplored. Driven by this motivation, in the present work, we experimentally investigate the effects of salt concentration on the bubble dynamics and drag modification in a fully developed horizontal turbulent channel flow for top wall bubble injection, over a wide range of salt concentrations ($0<M<0.08$, moles/litre), channel Reynolds number ($22{,}500<\text{Re}<65{,}000$), and injected bubble void fraction ($0<\alpha <0.16$). The injected bubbles interact with the flow in the turbulent channel and as they move downstream reach an equilibrium state between the bubbly phase and the fully developed carrier phase that persists further downstream. The equilibrium state of the bubble dynamics is captured by high-speed visualizations and the corresponding drag is obtained from stream-wise pressure drop measurements within the channel. Increasing salt concentration levels is seen to lead to reduction in bubble coalescence and consequently in bubble size that modifies bubble deformability, migration, and distribution near the top wall, with the changes being dependent on the Re and $\alpha$ values. At low $\text{Re} \approx 22{,}500$, the addition of salt leads to a dramatic reduction in bubble sizes ($\sim 100$ microns) from the very large coalesced bubbles ($\sim$ cm) seen in the no-salt cases, with consequent changes in the bubble dynamics and increased drag (up to $\approx 70\%$). The reduction in bubble sizes with salt addition leads to an increase in drag with salt concentration, which saturates beyond a critical salt concentration ($M_\text{Critical}$). With increasing Re, due to the smaller time available for coalescence, the bubbles are relatively small even in the absence of salt and hence, the bubble dynamics is found to be less susceptible to modifications by the addition of salt, unlike at low Re, and hence, results in a smaller increase in drag and a lower $M_\text{Critical}$. At the largest Re of about 65,000, both the bubble dynamics and drag are found to be very similar in both the no-salt and salt cases indicating that $M_\text{Critical}\approx 0$. The present observations suggest that at low Re, the bubble dynamics aspects and flow modifications would be very different between salt water and fresh water conditions, whereas at large Re, the differences would be minimal. The present results can thus help deepen our understanding of bubbly flow applications in contaminated environments, such as those that occur in bubble-induced drag reductions in ships and pipelines.