Enhancing thermal mixing in turbulent bubbly flow by adding salt

TL;DR

This study experimentally demonstrates that adding salt to turbulent bubbly flows reduces bubble size, shifts the thermal energy spectrum to higher frequencies, and enhances heat transport, revealing new insights into salty bubbly turbulence.

Contribution

It provides direct experimental evidence that salinity alters bubble coalescence, thermal spectra, and heat transfer in turbulent bubbly flows, a novel investigation in this field.

Findings

Salt decreases bubble size due to inhibited coalescence.

Salinity shifts the spectral transition to higher frequencies.

Heat transport increases with void fraction and salinity.

Abstract

The presence of bubbles in a turbulent flow changes the flow drastically and enhances the mixing. Adding salt to the bubbly aqueous flow changes the bubble coalescence properties as compared to pure water. Here we provide direct experimental evidence that also the turbulent thermal energy spectra are strongly changed. Experiments were performed in the Twente Mass and Heat Transfer water tunnel,in which we can measure the thermal spectra in bubbly turbulence in salty water. We find that the mean bubble diameter decreases with increasing concentration of salt (NaCl), due to the inhibition of bubble coalescence. With increasing salinity, the transition frequency from the classical $-5/3$ scaling of the thermal energy spectrum to the bubble induced $-3$ scaling shifts to higher frequencies, thus enhancing the overall thermal energy. We relate this frequency shift to the smaller size of the bubbles for the salty bubbly flow. Finally we measure the heat transport in the bubbly flow, and show how it varies with changing void fraction and salinity: Increases in both result into increases in the number of extreme events.