Salts promote or inhibit bubbly drag reduction in turbulent Taylor–Couette flows

Abstract

Bubbly drag reduction is considered as one of the most promising techniques to reduce the energy consumption of marine vessels. With this technique bubbles are injected under the hull where they then lubricate the hull, thus reducing the drag of the vessel. Understanding the effects of salts on bubbly drag reduction is therefore of crucial importance in the application of this technique for salt waters. In this study we investigate the effects of ${\mathrm{MgCl}}_2$, ${\mathrm{Na}}_2{\mathrm{SO}}_4$, substitute sea salt, and ${\mathrm{NaCH}}_3\mathrm{COO}$ on the reduction of drag by bubbles in turbulent Taylor–Couette flow. We find that ${\mathrm{MgCl}}_2$, ${\mathrm{Na}}_2{\mathrm{SO}}_4$, and substitute sea salt inhibit bubble coalescence, leading to smaller bubbles in the flow, which prove to be less effective for bubbly drag reduction. For these salts we find that the ionic strength is a decent indicator for the observed drag reduction and solutions of these salts with an ionic strength higher than $I\ge \mathrm{0.7\; mol/L}$ show little to no drag reduction. In contrast, ${\mathrm{NaCH}}_3\mathrm{COO}$ solutions do not inhibit bubble coalescence and for this salt we even observe an enhanced drag reduction with increasing salt concentration. Finally, for all cases we connect the observed drag reduction to the bubble Weber number and show that bubble deformability is of utmost importance for effective bubbly drag reduction.