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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 regular demineralized
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.
