Sodium chloride inhibits effective bubbly drag reduction in turbulent bubbly Taylor-Couette flows

Abstract

Using the Taylor--Couette geometry we experimentally investigate the effect of salt on drag reduction caused by bubbles present in the flow. We combine torque measurements with optical high-speed imaging to relate the bubble size to the drag experienced by the flow. Previous findings have shown that a small percentage of air (4%) can lead to dramatic drag reduction (40%). In contrast to previous laboratory experiments, which mainly used fresh water, we will vary the salinity from fresh water to the average salinity of ocean water. We find that the drag reduction is increasingly more inhibited for increasing salt concentration; going from 40% for fresh water to just 15% for sea water. Salts present in the working fluid inhibit coalescence events, resulting in smaller bubbles in the flow and with that, decreasing the drag reduction. Above a critical salinity, increasing the salinity has no further effect on the bubble sizes in the flow and thus the drag experienced by the flow. Our new findings demonstrate the importance of NaCl on the bubbly drag reduction mechanism, and will further challenge the naval architects to implement promising air lubrication systems on marine vessels.