8.3 Design Considerations
119
An interesting point of these systems is the possibility to design and optimize the
radiative surface as a function of the density of the medium in order to provide better
radiation impedance improving the overall device (Gallego-Juárez et al. 2010).
• Use a Multifrequency System
The multiplication of the transducers number during the intensification of the process
offers the possibility to associate elements of different operational frequencies and
allows a great versatility of ultrasonic cavitation systems. It has been reported in the
literature that the use of multifrequency reactors decreases the cavitation threshold
and increases the number of cavitation bubbles. Moreover, the combination of low
and high frequencies has positive effects on the nucleation of cavitation bubbles and
increases the efficiency of mass transfer.
The use of a pilot system with several irradiation frequencies may in certain
cases make it possible to improve cavitation. However, it is difficult to generalize
because the positive effects of coupling depend on the frequencies used, the position
of the transducers relative to each other and also the geometry of the reactor (De La
Rochebrochard et al. 2012, Tiong et al. 2017).
• Pulsed Modes
Pulsed mode ultrasound can modify significantly the streaming velocity by sending
a burst of n pulses during a period of time separated by a repetitive silent time.
The residual cavitation bubbles, which occur in silent time between two irradiation
exposures, may act as cavitation nuclei to enhance cavitation effects. However, at
a high power level, the use of pulsing irradiation may cause a quenching effect
due to excessive sound pressure. Moreover, the efficiency of pulse is higher at high
frequencies than at low frequencies (Casadonte et al. 2005).
8.3.3 Measuring the Spatial Distribution of Cavitation
As previously shown, the optimization of a sonoreactor in scaling-up operation
requires to control the spatial distribution of cavitation and the scattering of energy
through the overall of important volume (Mhetre and Gogate 2014). In a synthetic
way, the current investigative techniques measure the sound field, power and chemical activity associated with the cavitation field through optical, calorimetric and
chemical methods, respectively.
The sound pressure field is measured by a hydrophone in order to access at the
profile of sound pressure versus frequency (Leong et al. 2015). In transparent media,
cavitation mapping by high-speed photography can achieve as high as 200 million
frames per second in an exposure time of few nanoseconds to visualize multiplebubble dynamic cavitation. For a bulk assessment of energy input, calorimetric measurements are based on the transformation by absorption of the acoustic energy to
heat and are effective for measuring the mechanical effects of ultrasound. Although
119
An interesting point of these systems is the possibility to design and optimize the
radiative surface as a function of the density of the medium in order to provide better
radiation impedance improving the overall device (Gallego-Juárez et al. 2010).
• Use a Multifrequency System
The multiplication of the transducers number during the intensification of the process
offers the possibility to associate elements of different operational frequencies and
allows a great versatility of ultrasonic cavitation systems. It has been reported in the
literature that the use of multifrequency reactors decreases the cavitation threshold
and increases the number of cavitation bubbles. Moreover, the combination of low
and high frequencies has positive effects on the nucleation of cavitation bubbles and
increases the efficiency of mass transfer.
The use of a pilot system with several irradiation frequencies may in certain
cases make it possible to improve cavitation. However, it is difficult to generalize
because the positive effects of coupling depend on the frequencies used, the position
of the transducers relative to each other and also the geometry of the reactor (De La
Rochebrochard et al. 2012, Tiong et al. 2017).
• Pulsed Modes
Pulsed mode ultrasound can modify significantly the streaming velocity by sending
a burst of n pulses during a period of time separated by a repetitive silent time.
The residual cavitation bubbles, which occur in silent time between two irradiation
exposures, may act as cavitation nuclei to enhance cavitation effects. However, at
a high power level, the use of pulsing irradiation may cause a quenching effect
due to excessive sound pressure. Moreover, the efficiency of pulse is higher at high
frequencies than at low frequencies (Casadonte et al. 2005).
8.3.3 Measuring the Spatial Distribution of Cavitation
As previously shown, the optimization of a sonoreactor in scaling-up operation
requires to control the spatial distribution of cavitation and the scattering of energy
through the overall of important volume (Mhetre and Gogate 2014). In a synthetic
way, the current investigative techniques measure the sound field, power and chemical activity associated with the cavitation field through optical, calorimetric and
chemical methods, respectively.
The sound pressure field is measured by a hydrophone in order to access at the
profile of sound pressure versus frequency (Leong et al. 2015). In transparent media,
cavitation mapping by high-speed photography can achieve as high as 200 million
frames per second in an exposure time of few nanoseconds to visualize multiplebubble dynamic cavitation. For a bulk assessment of energy input, calorimetric measurements are based on the transformation by absorption of the acoustic energy to
heat and are effective for measuring the mechanical effects of ultrasound. Although