E. Riera-Franco de Sarabia

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.

Investigation of the influence of humidity on the ultrasonic agglomeration of submicron particles in diesel exhausts.

Removing very fine particles in the 0.01-1 micro m range generated in diesel combustion is important for air pollution abatement because of the impact such particles have on the environment. By forming larger particles, acoustic agglomeration of submicron particles is presented as a promising process for enhancing the efficiency of the current filtration systems for particle removal. Nevertheless, some authors have pointed out that acoustic agglomeration is much more efficient for larger particles than for smaller particles. This paper studies the effect of humidity on the acoustic agglomeration of diesel exhausts particles in the nanometer size range at 21 kHz. For the agglomeration tests, the experimental facility basically consists of a pilot scale plant with a diesel engine, an ultrasonic agglomeration chamber a dilution system, a nozzle atomizer, and an aerosol sampling and measuring station. The effect of the ultrasonic treatment, generated by a linear array of four high-power stepped-plate transducers on fumes at flow rates of 900 Nm(3)/h, was a small reduction in the number concentration of particles at the outlet of the chamber. However, the presence of humidity raised the agglomeration rate by decreasing the number particle concentration by up to 56%. A numerical study of the agglomeration process as a linear combination of the orthokinetic and hydrodynamic agglomeration coefficients resulting from mutual radiation pressure also found that acoustic agglomeration was enhanced by humidity. Both results confirm the benefit of using high-power ultrasound together with humidity to enhance the agglomeration of particles much smaller than 1 micro m.

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.

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.

Frequency dependence of the acoustic agglomeration rate of a glycolfog

The concentration of liquid aerosol particles subjected to high intensity sound has been measured as a function of the acoustic irradiation time. Experiments were performed at sound wave frequencies of 9.4 and 21 kHz on aerosol with geometric mean particle diameter 0.8 μm standard geometric deviation o = 1.35, and initial particle concentrations in the range of 104 − 107 cm3. The results are only partially consistent with the orthokinetic mechanism of agglomeration

Temporal evolution of transient transverse beam profiles in near-field zones

Abstract The transmission through transversal planes of short ultrasonic pulses, generated in water by circular planar and focussed broadband piezoelectric transducers, is analysed in detail by means of a thin temporal chopping of the transverse beam profiles. This is carried out with a quasi-punctual microprobe in the MHz frequency range using a detection procedure based on shifting narrow temporal gates (widths of the order of 1 [10f 0 ] ). In this way, the resulting series of temporal chops enables certain sequences of acoustic events, normally hidden in conventional plots, to be visualized. This fact facilitates the analysis of some significant near-field transient effects which must be taken into account for high resolution pulse—echo applications. The influence of some kinds of radiation irregularities from the transducer vibrating surface is also analysed in field-planes parallel to the emitting aperture. Theoretical results, obtained through approaches based on the impulse response method for particular near-field zones, are compared with experimental data revealing some non-ideal effects.

Characteristics of an electro-acoustic precipitator (EAP)

Abstract Results of a laboratory scale investigation to remove sub-micron particles from a gas stream, using a combined acoustic module and electrostatic precipitator (ESP), are presented. In these experiments, 1kW of electrical power is required per 1000m 3 of gas treated to increase the particle separation efficiency of the ESP from 90 to 95%.

Ultrasonic evaluation of creep damage in steel

Abstract Creep processes are the main processes responsible for the failure of structural materials in prolonged service at high temperatures and pressures. Failure starts with the formation of cavities in the material. This paper deals with an ultrasonic method to evaluate creep damage. Measurements of both ultrasonic attenuation and velocity, together with improved theoretical models, allow the porosity of the material and the size and concentration of cavities to be determined. Good agreement between the results and data obtained from a metallographic study indicates the usefulness of the method for in-service evaluation of creep damage.

Observation of a very slow ultrasonic bulk compressional wave in an inhomogeneous porous material

Abstract The propagation of an ultrasonic wave through a porous material, when the wavelength is large compared with the size of the inhomogeneities, can be studied in terms of Biots theory. The fundamental prediction of this theory is the existence of a second bulk compressional wave which propagates with a velocity lower than the velocity of propagation of the wave in the fluid. This second bulk compressional wave has only been observed in a few experimental cases. In this paper the observation of a second bulk compressional wave, in a quite different experimental situation, which can be related to Biots predictions, will be described. Samples are not fully water-saturated, as was required, but include small air bubbles. The influence of air bubbles in the generation of the second bulk compressional wave will be analysed. It is also possible to make some hypotheses on the influence of material properties on slow-wave generation. Since the measurements include strong dispersive phenomena, specific spectral techniques to measure the phase velocity of both propagating modes have been employed.