J.A. Gallego-Juárez

Application of high-power ultrasound to enhance fluid/solid particle separation processes

The separation of fine particles from gases or liquids is a topic of permanent industrial attention. The use of ultrasonic energy to assist conventional separation techniques seems to be very promising. The adequate applications of high-intensity ultrasonic fields may contribute to improve the efficiency and capacity of the separation methods presently used. The specific mechanisms to ultrasonically enhance separation processes basically depend on the medium to be treated. In gas suspensions, where very fine particles have to be removed, ultrasonic action involves agglomeration of particles in order to increase their size and, consequently, to improve collection efficiency of conventional filters. In liquid suspensions, agglomeration is, in general, less efficient than in gases. Nevertheless, the ultrasonic energy is useful to dewater fine-particle high-concentration suspensions such as slurries and sludges. This paper deals with the application of acoustic energy to assist fluid/solid separation processes in gas and liquid suspensions and presents some theoretical and experimental results in specific applications.

Power ultrasonic transducers with extensive radiators for industrial processing

High-power ultrasonics (HPU) is a green emerging technology that offers a great potential for a wide range of industrial processes. Nevertheless such potential have remained restricted during many years to a limited number of applications which reached commercial development. The possible major problem for extending the range of HPU industrial applications has been the lack of power ultrasonic transducers for large-scale application, adapted to the requirements of each specific problem with high efficiency and power capacity. A new family of HPU transducers with extensive radiators have been recently introduced. It comprises a variety of transducer types designed with the radiators adapted to different specific uses in fluids and multi-phase media. Such transducers implement high power capacity, high efficiency and radiation pattern control. In addition, their design incorporate strategies to eliminate or reduce modal interactions produced at high power as a consequence of their nonlinear behaviour. The introduction of such new transducers has significantly contributed to the development at semi-industrial and industrial level of a number of processes in the food and beverage industry, in environment and in manufacturing. This paper deals with the basic structure and main characteristics of such transducers as well as their performance in the developed application processes.

High-power ultrasonic system for the enhancement of mass transfer in supercritical CO2 extraction processes

Oil is an important component of almonds and other vegetable substrates that can show an influence on human health. In this work the development and validation of an innovative, robust, stable, reliable and efficient ultrasonic system at pilot scale to assist supercritical CO(2) extraction of oils from different substrates is presented. In the extraction procedure ultrasonic energy represents an efficient way of producing deep agitation enhancing mass transfer processes because of some mechanisms (radiation pressure, streaming, agitation, high amplitude vibrations, etc.). A previous work to this research pointed out the feasibility of integrating an ultrasonic field inside a supercritical extractor without losing a significant volume fraction. This pioneer method enabled to accelerate mass transfer and then, improving supercritical extraction times. To commercially develop the new procedure fulfilling industrial requirements, a new configuration device has been designed, implemented, tested and successfully validated for supercritical fluid extraction of oil from different vegetable substrates.

Automatic system for dynamic control of resonance in high power and high Q ultrasonic transducers

Abstract This paper describes the fundamentals, design criteria and electronic structure of a new frequency control system to keep permanently at resonance high Q ultrasonic transducers whose mechanical resonance band may vary within wide limits under normal operating conditions. The procedure developed is based on keeping constant at its zero value the phase of the motional admittance of the transducer by automatically and instantaneously adjusting the frequency of the driving signal provided by a voltage controlled oscillator. The characteristics of this system, especially the fact that the transducer is not an integral part of the feed-back loop of the oscillatory circuit and the frequency tracking mechanism does not depend directly on the magnitude of the motional variables of the transducer, offer some advantages in construction and performance with respect to the conventional motional positive feed-back systems.

A macrosonic system for industrial processing

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 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 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.

Ultrasonic agglomeration of micron aerosols under standing wave conditions

This paper deals with a study of the dynamic growth of micron and submicron aerosol particles under the action of an ultrasonic high intensity standing wave field. The study has basically consisted of a very wide set of measurements and an analysis of the data obtained. The experiments were carried out with carbon black smoke aerosol under static flow conditions. The ranges of variation of the main physical parameters were as follows: ultrasonic field intensity, I=0·44 to 2·14 W/cm2; irradiation times, t=0 to 5 s; initial particle mean radius, R0=0·10 to 0·57 μm; geometric standard deviation, σg=1·34 to 1·97; aerosol mass concentration, C=3 to 12 g/m3. In all cases the frequency was 20·4 kHz. By processing the experimental data, some correlations were found and the relative influence of ultrasonic field and aerosol parameters was determined. In addition, the mathematical expression established provides a basis for analysis and hence control of the coagulation process and for its possible extension for industrial applications.

The influence of entrainment on acoustically induced interactions between aerosol particles—an experimental study

Abstract An experimental study of particle interactions acoustically induced in monodisperse aerosols is presented in this paper. It is mainly devoted to analyse the influence of the acoustic entrainment experienced by the single particles on their attraction processes and, in particular, on the hydrodynamic mechanisms that govern them. Glass micro-spheres immersed in air as very dilute aerosols were subjected to homogeneous plane standing waves at frequencies ranging from 200 Hz up to 5 kHz . At these variable acoustic conditions the particle experiences different acoustic entrainment coefficients, varying from q p =0.86 down to 0.05 and covering almost its complete range of variability. A collection of experimental particle interactions were filmed, from which the particle behaviours were analysed. Our results experimentally confirm for the first time the presence of the acoustic wake effect (AWE) as a dominant mechanism of attraction at every entrainment ratio. In addition, the mutual radiation pressure is observed as a repulsion mechanism acting on nearby particles (separated at few diameter distances). It produces a disturbing effect on the particle attractions due to the AWE. Its influence becomes negligible for entrainment coefficients below q p =0.1.

Ultrasonic system for continuous washing of textiles in liquid layers

The use of ultrasonic energy for washing of textiles has been tried several times without achieving practical development. In fact, the softness of the fibres makes the cavitation to produce small erosion effect and the reticulate structure of the fabric favours the formation of air bubble layers which obstruct wave penetration. In addition, a high proportion of water with respect to the wash load and a certain water degassing is required to assure efficiency and homogeneity in the wash performance. Such requirements have hindered the commercial development of the ultrasonic washing machines for domestic purposes. For specific industrial applications, a great part of these limitations may be overcome. This article deals with a new process in which the fabric is exposed to the ultrasonic field in a flat format. Such process has been implemented at laboratory and at semi-industrial stage by using specially designed power ultrasonic transducers with rectangular plate radiators. The cleaning effect is produced by the intense cavitation field generated by the plate radiator within a thin layer of liquid where the fabric is introduced. The homogeneity of such effect is achieved by the successive exposure of all the fabric areas to the intense acoustic field. In this paper the structure and performance of the developed system are shown.

Development of industrial models of high-power stepped-plate sonic and ultrasonic transducers for use in fluids

The extension of the high-power applications of sonic and ultrasonic energy in industrial processing requires the development of efficient and powerful transducers. Years ago some of the authors of the present paper proposed a new concept of ultrasonic transducer for use in fluids (more specifically in gases) based on a stepped-plate radiator. This concept was successfully applied to the design and development of a series of circular plate transducers of small and medium power capacities (lower than 1 kW) and radiating surfaces smaller than 0.5 m/sup 2/. Nevertheless, the scale of a great part of industrial applications requires higher powers and larger radiating surfaces. Looking for such applications (i.e. fume precipitation, defoaming, drying and dewatering, etc.) we have designed and developed an industrial model of transducer with a rectangular plate radiator of double-stepped profile. The design of the new model was made with the help of the finite element method (FEM) and the acoustic field was computed by the boundary element method (BEM). In such a way the distribution of displacements and stresses as well as the corresponding radiated field could be known previously to the real construction of the unit. The industrial prototype of macrosonic transducer was designed with a radiating plate of 1.8/spl times/0.9 m/sup 2/ for an estimated power capacity of about 3500 W. Previously a first scale model was developed and tested with a radiating plate of 0.6/spl times/0.3 m/sup 2/ for a frequency of 20 kHz and a power capacity of about 400 W. The industrial prototype was constructed by scaling up the first model in a factor of three. Typical problems faced in the development of the industrial transducers were, the adequate selection and characterisation of the plate material, the excitation of the useful vibration mode without interference of the closer modes of the plate, the decrease of the maximum stress by improving the uniformity of the vibration amplitude, etc. The industrial prototype, constructed initially with an aluminium plate and eventually with a titanium alloy plate, presents an electroacoustic efficiency of 67% and it has been operated in air with an applied power of two kilowatts in continuous wave.

Recent developments in vibrating-plate macrosonic transducers.

As is known, the stepped-plate transducer [Ultrasonics 16 (6) (1978) 267] represents an optimum system for the efficient generation of high-intensity sonic and ultrasonic radiation in fluid media. Nevertheless, the design of this transducer may be difficult to adapt to some specific problems. Such is the case of the treatment of large volumes in industrial installations. A solution is the enlargement of the surface of the radiating plate. However, that means to work at high-order vibration modes which implies numerous practical problems. Another case is the application of the stepped-plate transducer for the generation at sonic frequencies where the height of the steps of the radiating plate, which has to be half a wavelength of the radiation, becomes too high and it makes the transducer construction impractical. To face these specific situations a series of new designs in transducer development have been recently carried out. This paper presents the characteristics of two new transducer devices, one for the treatment of large industrial volumes and the other for low-frequency sonic applications. Both devices are based on vibrating-plate radiators and represent a novel approach to practical existing problems.