Relationship between liquid depth and the acoustic generation of hydrogen: design aspect for large cavitational reactors with special focus on the role of the wave attenuation

Understanding the effect of the liquid depth (z) on the acoustic generation of hydrogen is highly required for designing large-scale sonoreactors for hydrogen production because acoustic cavitation is the central event that initiates sonochemical reactions. In this paper, we present a computational analysis of the liquid-depth effect on the generation of H₂ from a reactive acoustic bubble trapped in water irradiated with an attenuating sinusoidal ultrasound wave. The computations were made for different operating conditions of frequency (355–1000 kHz), acoustic intensity (1–5 W/cm²), and liquid temperature (10–30°C). The contribution of the acoustic wave attenuation on the overall effect of depth was appreciated for the different conditions. It was found that the acoustic generation of hydrogen diminished hardly with increasing depth up to z = 8 m, and the depth effect was strongly operating parameter-dependent. The sound wave attenuation played a crucial role in quenching H₂ yield, particularly at higher z. The reduction of the H₂ yield with depth was more pronounced at higher frequency (1000 kHz) and lower temperature (10°C) and acoustic intensity (1 W/cm²). The attenuation of the sound wave may contribute up to 100% in the overall reductive effect of depth toward H₂ production rate. This parameter could be imperatively included when studying all aspects of underwater acoustic cavitation.