The effects of high-frequency ultrasound on turbulent liquid mixing with a rapid chemical reaction

The effects of high-frequency ultrasound and mean fluid shear on turbulent mixing with a rapid chemical reaction were experimentally investigated in three types of liquid mixing-layer flow downstream of a turbulence-generating grid; pure grid-generated turbulence, grid-generated turbulence with high-frequency ultrasonic irradiation, and grid-generated turbulence with mean fluid shear. Instantaneous velocity and concentration were simultaneously measured using the combination of a laser-Doppler velocimeter and a laser-induced fluorescence method. The results show that turbulent mixing and chemical reaction are promoted by ultrasonic irradiation and mean fluid shear. The amount of chemical product in grid-generated turbulence with high-frequency ultrasonic irradiation is much larger than that in grid-generated turbulence with mean fluid shear, despite turbulent mass transport being enhanced at an equivalent level in both flows. This is attributed to the difference in turbulent mass transport at small scales. Ultrasonic irradiation more enhances the mass transport at smaller scales than by mean fluid shear, whereas mean shear can promote the mass transport only at larger scales. As a result, high-frequency ultrasound can be regarded as a better tool for promoting turbulent mixing and chemical reaction than mean fluid shear. (C) 2002 American Institute of Physics.