The degradation of phenol in aqueous solution by means of ultrasound was performed at two frequencies: 20 and 487 kHz. Using the same acoustical power (30 W) determined by the calorimetric method, the treatment appears more efficient for the higher frequency. The initial rates were found to be dependent on the initial phenol concentration, reaching limit values k20wz = 1.84 x M min-'. Identification of the first intermediates of the reaction (hydroquinone, catechol, benzoquinone) indicates that 'OH is involved in the degradation pathways. Correlation with hydrogen peroxide formation in water saturated with air has shown that the rate of H202 formation is more elevated at 487 kHz (k = 4.9 x M min-') than at 20 kHz (k = 0.75 x M min-I). It has been shown that the rate of sonochemical degradation is directly linked to the 'OH availability in the solution. Using luminol as a probe to visualize the region where 'OH radicals are produced, it was shown that there is a great difference between the ultrasonic field at the two frequencies.

This paper summarizes some applications of ultrasonic vibrations regarding heat transfer enhancement techniques. Research literature is reviewed, with special attention to examples for which ultrasonic technology was used alongside a conventional heat transfer process in order to enhance it. In several industrial applications, the use of ultrasound is often a way to increase productivity in the process itself, but also to take advantage of various subsequent phenomena. The relevant example brought forward here concerns heat exchangers, where it was found that ultrasound not only increases heat transfer rates, but might also be a solution to fouling reduction.