A complete analysis of the effects of transfer phenomenons and reaction heats on sono-hydrogen production from reacting bubbles: Impact of ambient bubble size

Abstract

Several experimental and computational works have been focused on the production of hydrogen by using ultrasonic irradiation. However, the effects of the different ultrasonic conditions have been analyzed by considering a single value for the ambient bubble radius R0 (mean value), which is not the true case as the size of active bubbles in sonicating medium is an interval rather than a sole value. In the present paper, the impacts of mass transport, heat exchange and chemical reactions heat on the sono-production of hydrogen are examined over a range of ambient bubble radii. These effects are shown for various ultrasonic frequencies of 355, 500 and 1000 kHz and under a range of acoustic amplitudes, from 1.5 to 3 atm. The numerical simulations results demonstrated that the increase of the production rate of hydrogen (around R0 of the maximal production rate) is amortized (for all models) for the wave frequencies of 355 and 500 kHz at higher amplitude (i.e. 3 atm). On the other hand, the total production rate (around R0 of the maximal response) is increased proportionally with the reduction of ultrasonic frequency or if the acoustic amplitude is increased. The effect of heat exchange mechanism (on H2 and the total production rate) was found to be dominant whatever the acoustic amplitude or the wave frequency (on all the range of R0). It has been demonstrated that at the acoustic amplitudes >1.5 atm (for f = 355 and 500 kHz) and >2 atm (for f = 1000 kHz), the impacts of chemical reactions heat and mass transport are clear compared to the normal model throughout a range of bubble sizes. The ambient bubble size (R0) of the maximal response (maximal production rate) is shifted toward lower values when the ultrasound frequency or the acoustic amplitude is raised. In addition, it is observed that the increase in the wave frequency or the decrease in acoustic amplitude cause a narrowing in the range of active bubbles.