The recent development of a novel family of power ultrasonic transducers with extensive radiating surfaces represents a step forward in the implementation of new airborne power ultrasonic (APU) technologies for drying process intensification at low temperature. These systems have been successfully used to assist drying of different products at laboratory level. This work deals with the development, integration, and testing of a new APU transducer with a stepped-grooved circular radiator as an initial step in scaling up the process and the results achieved in experiments of airborne atmospheric freeze-drying (AFD) of apples. The results confirm that ultrasound application leads to a significant intensification of the processes. The modeling shows that the increase of kinetic parameters was lower than those obtained with cylindrical transducers having the same electrical input. This fact is attributed to the treatment of greater volume.

Acoustophoretic blood plasma separation is based on cell enrichment processes driven by acoustic radiation forces. The combined influence of hematocrit and hydrodynamics has not yet been quantified in the literature for these processes acoustically induced on blood. In this paper, we present an experimental study of blood samples exposed to ultrasonic standing waves at different hematocrit percentages and hydrodynamic conditions, in order to enlighten their individual influence on the acoustic response of the samples. The experiments were performed in a glass capillary (700 µm-square cross section) actuated by a piezoelectric ceramic at a frequency of 1.153 MHz, hosting 2D orthogonal half-wavelength resonances transverse to the channel length, with a single-pressure-node along its central axis. Different hematocrit percentages Hct = 2.25%, 4.50%, 9.00%, and 22.50%, were tested at eight flow rate conditions of Q = 0:80 µL/min. Cells were collected along the central axis driven by the acoustic radiation force, releasing plasma progressively free of cells. The study shows an optimal performance in a flow rate interval between 20 and 80 µL/min for low hematocrit percentages Hct ≤ 9.0%, which required very short times close to 10 s to achieve cell-free plasma in percentages over 90%. This study opens new lines for low-cost personalized blood diagnosis.

Food dehydration processes assisted by power ultrasound constitute an efficient and green technology. In order to obtain good results with this technology it is necessary to take into account several aspects regarding the ultrasonic generation by a power ultrasonic transducer, the energy propagation in the fluid media and absorption in the samples. Ultrasonic waves produce different effects when propagating through a medium, like an increase in mass transport kinetics, and others related to the so called sponge effect and cavitation. This kind of process needs the whole system to work in a power regime. This may imply the appearance of non-linear effects in the transducer behavior and in the acoustic field generated inside the dehydration chamber. This paper describes this technology, paying special attention to the numerical design and the dynamic and modal characterization of the power ultrasonic transducer. As a conclusion, neither modal interaction nor any other non-linear effect, apart from a slight softening behavior, is expected when applying high power.

Airborne power ultrasound is a green technology with a great potential for food and environmental applications, among others. This technology aims at producing permanent changes in objects and substances by means of the propagation of high-intensity waves through air and multiphase media. Specifically, the nonlinear effects produced in such media are responsible for the beneficial repercussions of ultrasound in airborne applications. Processing enhancement is achieved through minimizing the impedance mismatch between the ultrasonic radiator source and the medium by the generation of large vibration displacements and the concentration of energy radiation thus overcoming the high acoustic absorption of fluids, and in particular of gases such as air. Within this work the enhancing effects of airborne power ultrasound in various solid/liquid/gas applications including drying of solid and semi-solid substances, and the agglomeration of tiny particles in air cleaning processes are presented. Moreover, the design of new ultrasonic devices capable of generating these effects are described along with practical methods aimed at maintaining a stable performance of the tuned systems at operational powers. Hence, design strategies based on finite element modelling (FEM) and experimental methods consolidated through the years for material and tuned assembly characterizations are highlighted.