A macrosonic system for industrial processing

Ultrasonics 38 (2000) 331–336 www.elsevier.nl/locate/ultras

A macrosonic system for industrial processing J.A. Gallego-Jua´rez *, G. Rodrı´guez-Corral, E. Riera-Franco de Sarabia, C. Campos-Pozuelo, F. Va´zquez-Martı´nez, V.M. Acosta-Aparicio Instituto de Acu´stica, CSIC, Serrano 144, 28006 Madrid, Spain

Abstract The development of high-power applications of sonic and ultrasonic energy in industrial processing requires a great variety of practical systems with characteristics which are dependent on the effect to be exploited. Nevertheless, the majority of systems are basically constituted of a treatment chamber and one or several transducers coupled to it. Therefore, the feasibility of the application mainly depends on the efficiency of the transducer–chamber system. This paper deals with a macrosonic system which is essentially constituted of a high-power transducer with a double steppedplate radiator coupled to a chamber of square section. The radiator, which has a rectangular shape, is placed on one face of the chamber in order to drive the inside fluid volume. The stepped profile of the radiator allows a piston-like radiation to be obtained. The radiation from the back face of the radiator is also applied to the chamber by using adequate reflectors. Transducer–chamber systems for sonic and ultrasonic frequencies have been developed with power capacities up to about 5 kW for the treatment of fluid volumes of several cubic meters. The characteristics of these systems are presented in this paper. © 2000 Elsevier Science B.V. All rights reserved. Keywords: High-power ultrasonics; Macrosonic systems; Transducers

1. Introduction The wide field of high-power sonic and ultrasonic applications, which is presently known as macrosonics, implies a great variety of practical processing systems with characteristics which are dependent on the effect to be exploited. Thus, liquid processes are generally based on cavitation effects and require reaching a certain sound pressure level (cavitation threshold ) throughout a volume. Gas processes, which are usually based on radiation pressure and particle velocity, may require adequate vibration amplitudes and treatment times. In solid processing, generally carried out directly on the material, high stresses are needed to produce friction, heat and other suitable secondary effects. In general, a system for macrosonic processing in fluids is basically constituted of a treatment chamber and a power transducer coupled to it. Therefore, the feasibility of the applications depends on the efficiency of the transducer–chamber system, bearing in mind that the concept of efficiency has to be considered in relation * Corresponding author. Tel.: +34-91-561-88-06; fax: +34-91-411-76-51.

to the useful yield. As a consequence, a knowledge of the influence of geometry and dimensions of the processing chamber as well as the effect of the excitation transducers on the acoustic field distribution is essential for the development of practical systems. As a general rule, the high-power systems operate in continuous non-linear waves and the chamber dimensions are large compared with wavelength. The environment is usually reverberant and a diffuse or standingwave field is established. In a diffuse field the energy is equally distributed and all directions of the energy flux are equally probable. A diffuse field requires a chamber with irregular shape. Under ideal conditions a diffuse field seems to be optimum for a regular and uniform treatment of all the fluid inside the volume. Nevertheless, this acoustic field configuration demands the delivery of too much power to the system. Frequently, a standing-wave field is more desirable because the pressure or the particle velocity can be amplified at determinate areas (nodes or loops) where the treatment should take place. To set up strong standing-wave fields, rigid-walled regular shape chambers are required. If the two dimensions of the chamber

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cross-section are smaller than the wavelength, a onedimensional standing wave can be obtained by placing the transducer face parallel to the opposite surface of the chamber and at a distance adjusted to a resonance length. In the case in which the three dimensions of the chamber are higher than the wavelength, the standing wave pattern becomes much more complicated and it is determined by the normal modes of the chamber and the characteristics of the transducer. In addition, the finite amplitude waves generated in a real macrosonic processing system introduce non-linearities in the standing-wave pattern which make still more complex the distribution of the acoustic field. Consequently, the design of macrosonic processing systems constitutes a challenging topic which is the object of current research [1–3]. This paper deals with the design and acoustic characteristics of a macrosonic system for flue gas processing which has been developed for the treatment of fluid volumes of several cubic meters with an acoustic power capacity of about 5 kW.

2. Structure of the system The macrosonic processing system basically consists of a high-power transducer with a double stepped-plate radiator coupled to a chamber of square section. The radiator, which has a rectangular shape, is placed on one face of the chamber in order to drive the inside fluid volume. The radiation from the back face of the radiator is also applied to the chamber by using adequate reflectors. The chamber is provided with proper means for circulating and distributing the fluid throughout the

Fig. 1. Schematic drawing of the processing system.

volume. A schematic drawing of the processing system is shown in Fig. 1. The design and development of the transducer–chamber system were carried out by the application of theoretical and numerical methods in connection with a program of experiments in scale models. In the following sections the structure and characteristics of the system will be presented. 2.1. High power stepped-plate transducers For the generation of high-intensity sonic and ultrasonic waves, special high-power transducers are needed covering adequate requirements. Particularly, the gas media present a low specific acoustic impedance and a high acoustic absorption. Therefore, in order to obtain an efficient transmission of energy, it is necessary to achieve good impedance matching between the transducer and the medium and large vibration amplitude. High directional radiation is also required for energy concentration and to establish a good standing-wave field. In addition, for large-scale industrial applications, high-power capacity and an extensive radiating area are needed in order to handle large volumes. At present, commercial transducers have many limitations in covering these requirements. During many years we have been involved in the development of a new type of sonic and ultrasonic power generator for use in fluids, which implements high power capacity, efficiency and directivity [4,5]. This generator is based on the steppedplate transducer, which has a basic structure shown in the scheme of Fig. 2. It consists essentially of an extensive vibrating plate of stepped profile driven at its center by a piezoelectrically activated vibrator. The vibrator itself is constituted of a piezoelectric element of transduction in a sandwich configuration and a solid horn which acts as a vibration amplifier. The extensional vibration generated by the transducer element and amplified by the mechanical amplifier drives the radiating plate which vibrates flexurally in one of its modes. The extensive

Fig. 2. Scheme of the circular stepped-plate transducer.

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surface of the plate increases the radiation resistance and offers the vibrating system good impedance matching with the medium. In addition, the special profile of the plate permits the control of the vibration amplitude and the radiation pattern in such a way that high directional radiation can be obtained in order to produce high-intensity acoustic levels. The idea behind the design of the stepped profile is as follows. A flat plate radiator presents in general a poor directivity pattern due to phase cancellation. Nevertheless, if the surface elements vibrating in counterphase on the two sides of the nodal lines are alternatively shifted along the acoustic axis to half a wavelength of the sound in the propagation medium, the radiation produced will be in phase across the whole beam and a directivity pattern equivalent to that of the theoretical piston will be obtained [6 ]. Following a similar procedure it is possible, with adequate displacements of the different plate zones, to achieve any acoustic field configuration. Focused radiators were also designed and constructed [7]. Different prototypes of stepped-plate directional and focused transducers were developed for the frequency range 10–40 kHz and power capacities up to about 1 kW by using circular plate radiators of diameters up to about 90 cm. The design of these transducers, initially made by analytical methods, was further improved by applying the finite element method (FEM ) [8]. The acoustic field was computed by the boundary element method (BEM ) [6 ]. The main characteristics of the previously developed circular stepped-plate transducers can be summarized in the following data: electroacoustic efficiency directivity (3 dB beamwidth) power capacity frequency range experimented maximum intensity levels

bers is achieved. Finally, there is easier commercial availability of the plate material (special titanium alloy) as rolled rectangular plate than as forged disc. The new radiating plate was designed with steps on both faces to obtain directional radiation also from the back face. The purpose was to use the back radiation in the forward direction by means of adequate reflectors. A first scale model of the transducer was developed with a rectangular plate of 0.6×0.3 m2 ( Fig. 3) for a frequency of about 20 kHz. The plate is vibrating in a resonant mode with 14 nodal lines parallel to its smaller side. The vibration amplitude distribution was calculated by FEM and validated by measuring it with a laser vibrometer. Vibration amplitude distributions along the two plates axes parallel to the width (X-axis) and to the length (Y-axis) are shown in Fig. 4. For comparison, the axial displacement as a function of distance in the radial direction in a circular stepped plate is also shown. Experimental tests were carried out to fully characterize the transducer scale model. They essentially consisted of measuring the electrical impedances in air and in vacuum and the acoustic field. In addition, the limiting strain of the titanium alloy was measured to determine the power capacity [10]. The results are summarized in Table 1. The performance of the transducer scale model, and particularly the values obtained in power capacity and the excellent behavior during the power tests, demonstrated the viability of the construction for a real indu-

75–80% < 2° 1 kW 10–40 kHz 165 dB.

2.2. The double-stepped rectangular plate transducer Looking for industrial applications and in order to increase the power capacity of the transducers, we have recently developed a new model with rectangular plate radiators of double-stepped profile. The use of rectangular plates instead of circular plates is due to several practical reasons. First, the more uniform distribution of vibration displacements, which may be obtained in a rectangular plate [9], increases the power capacity of the transducer. This is because the power capacity depends on the maximum displacement that the radiating plate can reach without breaking off. Second, a more homogeneous distribution of energy may be reached in the acoustic field by using rectangular radiators. Third, a better matching of rectangular plates with the geometry of parallelepipedic processing cham-

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Fig. 3. Scheme of the rectangular plate transducer.

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levels. To that end, the number of piezoelectric ceramics of the sandwich transducer was increased and the ratio between the two diameters of the mechanical amplifier was slightly diminished. Finally, the transducer was placed in a frame where it was supported by small rubber holders at the nodal lines. The measuring procedure was different from that of the scale model transducer because of the difficulties of having a vacuum cavity big enough to measure the losses resistance of such a large transducer. Therefore, the method was to measure the vibration displacements of the radiating plate by laser vibrometry in order to calculate the radiating power and then to compare it with the electrical applied power to determine the efficiency. In addition, the electric impedance was measured by means of an impedance bridge. In this way, the values of Table 2 were obtained.

2.3. The processing chamber

Fig. 4. Displacement distribution along the two axes of a rectangular double-stepped plate with 14 nodal lines (19.5 kHz) and along the diameter of a circular stepped plate (20 kHz).

strial prototype. The objective was to reach a power capacity of about 5 kW. To that purpose the transducer was scaled up by applying the classical acoustic linear scaling method, consisting of multiplying all the dimensions by the scale factor K. As a result, the power capacity should be increased by a factor K2, and the frequency reduced by 1/K. The distribution of the vibration displacements, the impedance and the efficiency should remain unchanged [11,12]. In this way, an industrial prototype of a macrosonic transducer was designed with a double-stepped rectangular plate of 1.8×0.9 m2. Nevertheless, in the development of the real transducer some modifications were introduced in order to keep the final impedance as low as possible. The reason for this was to avoid very high voltages while driving the transducer at higher power

As shown in Fig. 1, the chamber consists of a rectangular or square section enclosure provided with two additional input and output tapered terminals to facilitate the homogeneous distribution of the gas flow. The transducer is placed on one face of the chamber in such a way that it will radiate perpendicularly to the flow direction. Due to the back reflectors the real radiating surface of the transducer is double the radiating plate surface. Therefore, the dimensions of the chamber crosssection (X–Y ) in the flow direction are determined by the radiating surface. To fix the length (L) of the chamber in the perpendicular direction to the radiating plate (Zaxis), a practical efficient criterion had to be adopted. This was to optimize the quality factor (Q value) of the chamber as a function of L for L values corresponding to resonant modes of the chamber. By applying a simple analytical model it was deduced that the quality factor increases up to a certain L value where the dissipation of energy balances the increase of energy stored. This length was calculated and tested for a scale model chamber with the 19.5 kHz transducer and proved to be about the same as the side length of the radiating face. As a consequence, we designed and constructed processing chambers with cubic volumes. The wall parallel to the radiating plate was kept mobile in order to adjust it for resonance conditions in the chamber.

Table 1 Characteristics of the scale model transducer Radiation resistance (V)

517

Losses resistance (V)

172

Resonant frequency (Hz)

19 563

Bandwidth (Hz)

2.5

3 dB Beamwidth (°) XZ plane

YZ plane

3.4

3.8

Efficiency (%)

Power capacity (W)

75

500

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Table 2 Characteristics of the industrial prototype macrosonic transducer Radiation resistance (V)

352

Losses resistance (V)

173

Resonant frequency (Hz)

7388

Bandwidth (Hz)

1

3 dB Beamwidth (°) XZ plane

YZ plane

3.4

3.8

Efficiency (%)

Power capacity (W)

67

5000

Fig. 5. Acoustic field distribution in two parallel YZ planes (x=0, x= 30 cm) in the scale model processing chamber (applied power 1 W ).

3. Acoustic field in the processing chamber The acoustic field was initially studied in a scale model chamber with the 19.5 kHz transducer. The chamber was a cube of about 0.22 m3. Fig. 5 shows the sound pressure levels, averaged along the Y-axis, in two YZ parallel planes which are perpendicular to the radiating plate surface. The planes are located at the center (x=0) and at the edge of the radiating surface (x=30 cm). The similarity of the values obtained in both distributions can be noticed. The power applied to the transducer was only 1 W. By averaging all the values contained in the distribution curves obtained for each plane, a single sound pressure level (SPL) is determined which can be considered as a representative value of this plane. Fig. 6 shows the variation of these SPL values for several YZ planes located along the X-axis. The small difference between the acoustic pressure values at the central and at the lateral planes is observed. The same kind of measurements were carried out in the industrial prototype of a macrosonic chamber: a cube of about 6 m3. The results were qualitatively very similar. As an example, Fig. 7 shows the acoustic field on several Y–Z parallel planes for an applied power on the transducer of 500 W. The high sound pressure levels obtained in all the volume guarantee the efficiency of

Fig. 6. Averaged values of the acoustic pressure on several YZ planes in the scale mode processing chamber (applied power to the transducer 1 W ).

Fig. 7. Averaged values of the acoustic pressure on several YZ planes. Industrial model macrosonic chamber (applied power to the transducer 500 W ).

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the chamber for the production of the non-linear effects needed for high-power sonic and ultrasonic applications. 4. Conclusions In this work a powerful new system for macrosonic applications in gases has been presented. The main feature of the system is its capacity for homogeneous treatment of great volumes at industrial scale. The new double-stepped rectangular plate transducer with a power capacity of about 5 kW is, as far as we know, the most powerful macrosonic generator presently existing for use in gases. The new macrosonic system has been specifically developed for the agglomeration of very fine suspended particles in power plant emissions in order to precondition this particulate matter to be removed by conventional electrostatic filters. However, the general conception of the system may be extended to many other applications, even in liquid media. Acknowledgement This work has been supported by the research project PIE-131.095 ( ENDESA/OCIDE.)

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