Ultrasonic Power Applied

Therefore, the main factor responsible for ultrasound effects is the actual ultrasonic energy received by the sample. As a consequence, it is interesting to establish the relationship between the actual ultrasonic power applied and the intensification effects induced by ultrasound. 

The research field of ultrasound in chemistry is summarized in numerous reviews [21]. Ultrasonic waves cannot couple directly with molecular energy levels. The influence of ultrasound on a chemical reaction is attributed to the formation of cavitations, which are induced by the ultrasonic waves and whose collapse can release very high local temperatures and pressures. The formation of cavitation depends not only on the ultrasonic power applied but also on physical properties of the irradiated liquid, such as vapor pressure, viscosity, surface tension, or dissolved gases, rather than on chemical properties such as acidity and polarity. The course of a chemical reaction can be influenced in two ways by ultrasonic treatment ... 

Figure 48.1(b) shows the population density of crystals filmed under increasing ultrasonic power output levels. A single 0.1-sec pulse of ultrasound was applied at — 3.8°C in a 15wt% sucrose solution. The growing crystals are shovm 0.40 sec later. These results highlight that a commercial ultrasonic horn can be used to control the nucleation and the size distribution of ice crystals produced within the supercooled liquid. Further results by the first author have shown that as the power output level is increased, the level of transient cavitation increases (Chow-McGarva, 2004). This increase in transient cavitation could be correlated to an increase in the primary nucleation events and thus explain the increase in the observed number of crystals. 

The literature demonstrates that ultrasonic extraction is more efficient with shorter extraction time than the classical method and provides higher yield (Table 1) [71]. This was confirmed by the extraction of polysaccharides from mulberry leaves using ultrasonic method [87]. UAE is therefore a promising alternative technique for the extraction of natural polysaccharides from botanical sources [87,89]. Correct use of UAE method preserves the structural elements of the extracted polysaccharides. If the applied ultrasonic power, irradiation time, and reaction temperature are too high, the extracted polysaccharides are likely to degrade and their structures may not be preserved which leads to changes in their biological properties [89]. 

Power is the total energy irradiated by an ultrasonic system per unit of time (J s or W). Power is very important in ultrasound applications, due to the fact that the greater the power applied in a medium, the higher are the observed ultrasound effects. In general, it is convenient to relate the level of applied energy with the size of the considered system, thereby reporting the ultrasonic energy in a relative form. Thus, the ultrasonic intensity is defined by the power applied per unit of treated area (W m ). The ultrasonic intensity, I, is related to the acoustic pressure. Pa, by... 

The parameters that best characterize an ultrasonic field are the wave frequency and the ultrasonic energy applied, with the latter being expressed as power, intensity, or acoustic pressure. The frequency does not represent a critical point because the variation of the applied frequency is usually less than 5-10% of the nominal frequency provided by the manufacturer. In contrast, the actual applied acoustic energy could be very different from the electrical consumption, and its direct measurement is difficult. In the literature, it is possible to identify a wide variety of methods for that purpose, but the results provided may be difficult to compare one with another. The majority of these methods are based on the measurement of physical or chemical changes produced by ultrasound, and a classification was proposed by Berlan and Mason (1996) ... 

In order to identify the influence of air velocity on the ultrasonic field and, therefore, on the ultrasound effects, the acoustic field inside the drying chamber was measured using a microphone (Riera et al, 2011). When measurements were taken without an air flow, an average sound pressure level of 154.3 dB was obtained for an electrical power applied to the transducer of 75W (30kWm ). However,... 

Fig. 8.12 Influence of applied ultrasonic power density on the identified effective diffusivity for the drying of (a) carrot and (b) lemon peel. The bars indicate intervals with a confidence level of 95% (Garcia-Perez et al., 2009).

Fig. 8.13 Linear relationship between effective diffusivity and applied ultrasonic power density for the drying of different products.

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