A phenomenological investigation into the opposing effects of fluid flow on sonochemical activity at different frequency and power settings. 1. Overhead stirring

Abstract

The effect of flow in an ultrasonic reactor is an important consideration for practical applications and for the scale-up of ultrasonic processing. Previous literature on the influence of flow on sonochemical activity has reported conflicting results. Therefore, this work examined the effect of overhead stirring at four different frequencies, 40, 376, 995 and 1179 kHz, in two different reactor configurations. Comparable power settings were utilised to elucidate the underlying mechanisms of interactions between the flow and sonochemical activity. The sonochemical activity was determined by the yield of hydrogen peroxide, measured by iodide dosimetry, and the active region was visualised with sonochemiluminescence imaging. The overhead stirring in the low frequency reactor altered the yield of hydrogen peroxide so it produced the maximum yield out of the four frequencies. The increase in hydrogen peroxide yield was attributed to a reduction in coalescence at 40 kHz. However at the higher frequencies, coalescence was not found to be the main reason behind the observed reductions in sonochemical yield. Rather the prevention of wave propagation and the reduction of the standing wave portion of the field were considered.

A parametric review of sonochemistry: Control and augmentation of sonochemical activity in aqueous solutions

“…This is due to reduction in the mobility of the bubble surface and a surface concentration gradient that exists as two bubbles coalesce [40,41]. Coalescence, in addition to rectified diffusion, represents another means of bubble growth and thus at high frequencies can be beneficial in producing bubbles of active size [42]. Rectified diffusion and coalescence may occur in approximately equal proportions and are important to consider when evaluating overall parametric effects in an ultrasonic system [43].…”

“…All these things are dependent on the reactor geometry, ambient pressure, and temperature, viscosity and gas composition of the liquid [42]. In general, higher frequencies will result in increased nucleation and production of bubbles that are comparatively small in size [106,[151][152][153].…”

“…As frequency increases, the amount of vapour entering the bubble decreases due to the reduction in its expansion phase [32]. This increased rate of oscillation and reduction in growth per cycle allows small bubbles in steps to reach resonance size more efficiently than lower frequencies (under 100 kHz) where the bubbles have relatively long growth periods [42,69]. It should be considered however that the cavitation threshold is increased at higher frequencies and as such higher powers are required to induce cavitation [154,155].…”

“…A sound field consists of both standing and travelling wave components, with optimisation of sonochemical yields observed in the presence of travelling waves [42,44,156]. These waves are often referred to as a (quasi) acoustic streaming phenomenon with flow associated with the travelling part of the sound field [145,157].…”

“…This is discussed further in Section 3.3.1. Since sonochemical yields have been seen to increase during transient cavitation, the presence of travelling waves can increase sonochemical activity [42]. The effect of frequency on coalescence can also be considered in terms of its effect on bubble size.…”