The effects of dynamic factors inside the bubble on sono-hydrogen yield: A numerical study

The formation of H2 by introducing ultrasonic waves to liquid has been widely recognized as a way to provide a clean, efficient, and reliable source of H2, known as Sono-Hydro-Gen. H2 comes from the chemical effects of ultrasonic waves (sonochemistry) caused by the growth and collapse of acoustic cavitation bubbles. In this work, the effects of dynamic parameters (i.e., bubble temperature, the amount of water vapor trapped inside the bubble, and collapse time) in the evolution of cavitation bubbles on H2 production are studied numerically. For an oxygen bubble, computational simulations are performed for the wide range of acoustic amplitudes (1.5–3 atm), ultrasonic frequencies (140–515 kHz), and ambient radii (0.25–20 μm), considering 22 reversible chemical reactions and 10 chemical species inside the bubble. The numerical results show that the amount of water vapor has a significant effect on the bubble collapse temperature. At low excitation amplitudes, the amount of water vapor is not enough to cause the bubble to form a strong collapse. Nevertheless, at high excitation amplitudes, the amount of water vapor is too much to reduce the bubble temperature. There exist optimal values of bubble temperature and amount of water vapor for H2 production. The optimal bubble temperatures are 5267, 4813, 4626, and 3856 K, corresponding to H2 productions of 4.21 × 10−18, 1.29 × 10−18, 2.61 × 10−19, and 8.48 × 10−20 mol, respectively, at ultrasonic frequencies of 140, 213, 355, and 515 kHz. No matter what the excitation parameters are, the optimal water vapor fraction is 0.78 ± 0.04 for H2 production. The obtained results of the present work can provide guidelines for H2 production in acoustic cavitation.