Acoustic Analysis of Photothermal Boiling in Nanofluids

Solar energy harvesting and utilization are critical components of the green energy transition, though the fundamental understanding of the solar boiling process in nanofluids is lacking. In this study, auditory acoustic-emission spectra from photothermal boiling in carbon black nanofluids up to 2 wt % were measured. Frequency peaks and boiling modalities were investigated to determine the effects of nanoparticle addition on bubble number and size distribution. Steam flow rates were maximally enhanced by 90% (at 0.5 wt %) compared to surface boiling in water due to improved heat and mass transfer and incident radiation. As concentration rose, flow rates decreased due to shielding and thermal losses. Moreover, acoustic spectra for photothermal boiling exhibited higher amplitudes and more high-frequency peaks, indicating more small bubbles and bubbles in total for the nanofluid. Using a coherence analysis, two new peaks were found for the highest flow rate concentrations: 17 (new bubbles) and 14.5 (new bubble coalescence) kHz. The behavior of these new peaks correlated with steam flow rates. Finally, experimental results were compared with existing boiling models for theory validation, with volumetric boiling theory matching closely. Bubble size predictions from theory overlapped with the new peaks, lending theoretical backing to the finding. Therefore, auditory acoustic spectra can be a reliable tool for investigating mechanistic changes in bubble formation in optically occluded systems.