**5. Conclusion**

234 Acoustic Waves – From Microdevices to Helioseismology

The curves show the temporal response and the frequency range of the sensor following a stress induced by a mechanical shock of short duration. The experiments carried out on the overall system (sensor & exchanger) in real configuration show that the temporal response is maximum 4 ms and its frequency response is around a central frequency of approximately 4

Heat exchanger fouling is a dynamic process. The phenomenon continues to evolve, generally until equilibrium is reached or cleaning is required. The period of fouling can vary

Müller (Müller-Steinhagen & Middis, 1989) looked at five stages in the process of the

• The initiation, which corresponds to the time necessary before fouling, can be observed on a clean surface. The duration depends on the nature of the deposit, the initial state of

• The denaturing of the product (protein, organic matter...) under the effect of heat and the surrounding parameters (pH...), their aggregation and transport within the vicinity

• The adhesion of the particles transported to the wall, controlled by surface adhesion forces (Van der Waals, electrostatic...) and cohesion of the deposit. It has been shown that the particles can adhere to a clean surface or adhere to other particles already

• The dislodging of deposited particles, caused by hydrodynamic forces which exert

• The aging of the deposit over time results in changes in its structure which can either

Generally, the initiation phase is rarely taken into account in particulate fouling models. The mechanisms that govern the deposit of particles are generally presented as being the transport of the particles to the surface, then the "adhesion" to the wall and finally the

Before studying the phenomenon of fouling, the metrological variation of the measurement

This phase is essential in order to separate the interferences of acoustic values generated by the fouling phenomenon from those linked to the technological conditions of the exchanger

The curves in figure 21 show the evolution of the energy of the acoustic signals as well as

The "Power" curve shows the damping effect linked to the load on the plate caused by

system was taken into account according to the main technological parameters:

the surface (material, roughness) and the temperature of the wall.

kHz.

**4.2 Application** 

of the wall.

deposited.

**4.2.2 Results** 

and its environment.

fouling.

**4.2.1 Fouling mechanism** 

from a few hours to several months.

shear stress on the deposit.

weaken or consolidate it.

possible dislodging of the particles.

• Variation in temperature at constant flow. • Variation in flow at constant temperature.

• Variation in viscosity at constant temperature and flow.

the pressure drop in the system as a function of the process time.

appearance and development of particulate fouling:

This chapter has proposed a synopsis of all the work that has led to the development of novel low frequency sensors. By using structural resonance modes excited by a transducer, these sensors present the advantage of having small sized sources with regard to the acoustic wavelength generated. These sensors are omni-directional but can nevertheless present significant contact areas with the medium to be characterised. This is the case for sensors developed for the characterisation of gels. The close contact of the elements set in resonance with the medium enables phenomena linked to changes in state to be monitored easily. Various applications have led us to develop sensors with very different geometries and which are optimised with the application in mind.

Indeed, for each need expressed, the approach consisted in optimising not only the geometry of the sensors but also their optimum position according to the problem posed. Three different cases were thus studied:

• identical near-field coupled sensors, through the medium to be characterised. They were used for monitoring the evolution of the ultrasonic values to characterise a sol-gel transition or the cohesion kinetics of a medium. For certain applications, the sensors are immersed in the medium. This direct immersion is essential for characterising fragile media.

Low Frequency Acoustic Devices for Viscoelastic Complex Media Characterization 237

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The work carried out provides a solid base of knowledge on ultrasound-complex media interactions. This knowledge could be put to good use in the development of sensors and integrated ultrasonic methods and their applications in the analysis and monitoring of local properties.
