Does macroscopic mass transfer affect sonochemical reaction rate in an ultrasonic bath?

In the present study, a planar laser induced fluorescence (P-LIF) measurement, reaction rate measurement, the sonochemical luminescence (SCL) observation, and the particle image velocimetry (PIV) measurement were conducted to clarify the effect of macroscopic mass transfer on sonochemical reaction rate in an ultrasonic bath. The concentration distribution was measured by the fluorescence intensity of Rhodamine 6G (Rh6G), which was illuminated by a CW-YAG laser sheet. The concentration of Rh6G decreases first in the high reaction zone measured by the SCL observation, and the resulting low-concentration zone expands to the low reaction zone through macroscopic convective mass transfer, which can be observed as solute plumes. Therefore, it is concluded that the mass transfer rate can slightly affect the chemical reaction rate due to the nonuniform concentration distribution in the early stage of sonochemical reaction. The reaction rate is slightly underestimated due to the spatial variation of the concentration in the early stage of ultrasonic degradation. The effect of macroscopic mass transfer on the sonochemical reaction rate was evaluated by first Damköhler number, which was calculated based on the flow velocity obtained by the PIV measurement and the reaction rate constant obtained by the decomposition experiment. Finally, it could be concluded that the first Damköhler number evaluates the effect of macroscopic mass transfer on the sonochemical reaction rate quantitatively and this dimensionless number can be applied to other ultrasonic bath with different condition.