New ultrasonic technologies for food dehydration process intensification

Transcription

1 PROCEEDINGS of the 22 nd International Congress on Acoustics Sonochemistry and Sonoprocessing: Paper ICA New ultrasonic technologies for food dehydration process intensification Roque R. Andrés (a), Alfonso Blanco (a), Enrique Riera (a), Ángel Guinot (a) (a) Departamento de Sensores y Sistemas Ultrasónicos (DSSU), ITEFI, CSIC, Serrano 144 E28006, Madrid, Spain. roque.andres@csic.es Abstract Food dehydration processes assisted by power ultrasound constitute a new, 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, the energy propagation and absorption in the samples. Ultrasonic waves produce different effects when propagating through a medium, like an increase in mass transport kinetics by accelerating these kind of processes, and others related to the so called sponge effect and cavitation. This kind of processes needs the whole system to work in a high-power regime. This may imply the appearance of non-linear effects in the transducer behaviour and in the acoustic field generated inside the dehydration chamber. The aim of this work is to describe this technology, capable of producing permanent changes in the samples treated, considering the design and characterization of high-power ultrasonic transducers, the generation and performance of the high-intensity ultrasonic field and the ultrasonic effects produced in the samples, and paying especial attention to those non-linear effects that may appear and its influence on the global behaviour of the whole system. Keywords: high-power ultrasound, mass transfer processes, ultrasonic processing, dehydration processes.

2 1 Introduction Industrial processes assisted by high-power ultrasound (HPU) have become a new, green and efficient technology with a great potential in its implementation. Previous researches like [1, 2] show that these HPU technologies provide a good performance in processes like particle agglomeration, ultrasonic cleaning or defoaming, among others. In the particular case of food dehydration at low temperature, it has been proved by [3] that HPU provides a faster and more economic performance, improving also the quality of the final product. The massive industrial application for this green technology has been studied since the 1950s, but there are some issues that have prevented its implantation, and that are being solved even today. Those difficulties lie on the need of generating a stable high level ultrasonic field, covering a wide volume in gas media. Previous researches indicated that the introduction of high-power ultrasonic transducers with extensive radiators may get us close to solve those issues. Anyway, the development of this kind of devices has to deal with the need of a high level ultrasonic field in a gas media, with a good impedance matching between the radiator and the gas and a high amplitude of vibration in the radiating plate [4]. Because of these special requirements, the transducers have to be driven to its desired resonance frequency and with a high-power level, provoking the appearance of some nonlinear effects like hysteretic response in the piezoelectric ceramics, frequency shifts and drops, or modal interactions, saturation, and harmonics among others [5, 6]. The developing process of a high-power ultrasonic transducer with extensive radiator can be divided in several stages: design of the transducer using numerical methods [7], stability tests and nonlinear characterization of the devices [8, 9] and determination of the ultrasonic field generated by the radiator by numerical and/or experimental methods [7, 10]. According to these needs, there have been designed different types of high-power ultrasonic transducers for food dehydration processes with the samples in direct contact with the vibrating plate of the transducer [11] or with ultrasonic radiation in air, where the samples are located [12]. The aim of this work is to present the latest progresses in the development of a high-power ultrasonic transducer with circular radiator vibrating at a high flexural mode at 25 khz and with seven nodal circles (7NC), designed for food dehydration processes. 2 Description of the transducer The structure of this type of transducer follows the general configuration of this kind of transducers. Basically consists on a piezoelectric Langevin-type transducer as an element of transduction on which a pre-stress is applied for a better performance [13], followed by a ½ wavelength metallic horn vibrating in an extensional mode and whose objective is to amplify the vibration of the sandwich. This horn is also known as mechanical amplifier. The tip of this vibrator is the place with the highest displacement and where the circular plate is bolted. The 2

3 extensional vibration obtained in the tuned Langevin transducer and horn excites, in this case, the seven nodal circles mode of the circular plate. This transducer is shown in figure 1. Figure1: High-power ultrasonic plate transducer with circular radiator: a) FEM Model; b) Transducer 3 Design of the circular plate transducer The goal was to develop a circular plate transducer vibrating in a flexural mode with sevennodal-circle at around 25 khz. The first step was the numerical analysis with a finite element model of this transducer, following the indications of [7]. There are three main issues to deal with when developing the transducer. First of all, the plate radiator shall provide a good impedance matching when radiating in air. This goal is achieved by increasing the radiating surface of the transducer because the radiation impedance is proportional to de surface of the radiator. Then, in order to produce the desired effects in the food samples, and considering that the ultrasonic propagation in air has a high absorption level, it is necessary to generate a coherent radiation in air. For this reason, the radiating surface of the plate is designed with a stepped profile according to its vibration mode. This enables the compensation for the vibrating zones of the radiator in counter phase on the two sides of nodal circles of the excited flexural mode. By introducing steps into the profile of the radiator it is possible to modify the radiation beam by shifting the initiation points of neighbouring waves. In [10], the ultrasonic field generated by a circular plate transducer vibrating in a high flexural mode with 5 nodal circles was studied, showing that the stepped plate transducer generates a coherent ultrasonic field. Figure 2 shows the model, obtained by numerical methods, of the vibration mode with 7 NC, as well as the displacement contribution obtained, also by numerical methods (fig 2.b) and experimentally with laser vibrometry techniques (fig 2.c)[14]: 3

4 a) b) c) Figure2: a) Seven nodal circle vibration mode. b) Displacement contribution along the diameter of the plate obtained by numerical methods. c) Displacement contribution obtained by laser vibrometry. As mentioned before, when applying these steps we can achieve a coherent acoustic field. A comparison between the measured directivity diagrams of a flat-plate radiator and a steppedplate radiator of the same diameter and frequency is shown in figure 3 [15]: Figure 3: a) Directivity of a plate without steps. b) Directivity of a circular plate with steps. Finally, it is important to avoid any modal interaction when applying high power at the operational frequency. The excitation of close undesired modes may provoke a dramatic 4

5 decrease in the efficiency of the system and other problems related to ultrasonic fatigue, noise, heating, instabilities, etc. Some physical changes may be applied in the radiator in order to avoid this undesired modal interaction by adjusting the mass balances between different parts of the radiator and, thus, strengthening the operational mode and weakening or separating the other close modes [4]. Grooving one side of the plate may provide this adjustment and minimize the modal interactions when applying high power levels. As a conclusion, this new stepped-grooved circular plate transducer for food drying process intensification, has been designed to work in a high flexural mode with seven nodal circles at a frequency around 25 khz. 4 Modal analysis After building the transducer, there are some tests to perform. The modal analysis tests allow us to know the real operational frequency and if there are some other close vibration modes. The test has been done with a 3D laser vibrometer driven by a Modal Analysis Software, with three measurement channels and capable of measure the vibration velocity in the three orthogonal directions. The measurement setup is shown in figure 4: COMPUTER POWER AMPLIFIER I/O DATA ACQUISITION CARD VIBRATION VELOCITY IMPEDANCE MATCHING UNIT ULTRASONIC TRANSDUCER 3D LASER VIBROMETER Figure 4: a) Modal analysis measurement setup. b) Modal analysis of the circular plate transducer. The results obtained after the measurements done over the whole surface of the circular plate are shown in figure 5, where a high density of modal frequencies can be observed (fig 5.a), and a zoom around the operational mode at Hz, where the nature of the closest modes can be observed (fig 5.b): 5

6 Figure 5: a) Frequency modes up to 90kHz. b) Zoom around 25.8 khz. In figure 5.b, three main modes at Hz, Hz, Hz are clearly observed, but also smaller vibration modes that is necessary to check. The closest mode appears at Hz, 400 Hz away from the operational mode at Hz. This separation may be high enough to avoid modal interactions when applying high power. In figure 6, the appearance of these modes is shown: a) Hz b) Hz 6

7 c) Hz d) Hz Figure 6: 3D vibration shapes for the main frequency modes. The seven nodal circles in the operational mode are perfectly clear at Hz, and the shape of the nearest mode corresponds to the seven nodal circles and a diametric mode. 5 Dynamic behaviour tests As mentioned before, to produce the desired effects in the food samples, it is necessary to drive this transducer into that 25.8 khz mode and with a high power, that may lead the transducer to work into a nonlinear regime, provoking some effects like tuned frequency drops, hysteretic response; or modal non-linear effects like multimodal response, the appearance of harmonics or combination of resonance. This dynamic behaviour tests can be performed with the non-linear characterization system presented in [8], and shown in figure 7: Figure 7: Nonlinear characterization system setup. 7

8 For this reason, a series of experiments have been taken, with the laser vibrometer pointing at the chosen point in the plate, corresponding to an area with maximum displacement. Among these experiments, it is interesting to know how efficient is the transducer, by determining the electric power obtained when applying different voltages. In figure 8.a, the evolution of power with voltage, between 220 V and 360 V, is shown. The maximum power obtained in this case is 225 W. Other interesting experiment is to determine how the transducer heats up when working for a long time in a high power regime. In figure 8.2, this evolution of the temperature with time is shown, for three different values for the excitation power (50 W, 100 W and 200 W). The temperature sensor points at the union point between the plate and the mechanical amplifier, one of the most critical points in the whole system. The results obtained after these experiments indicate that, after two hours, the temperature doesn t increase to any dangerous level. Figure 8: Dynamic behaviour tests. a) Relation between voltage and power. b) Temperature evolution with time. Once the operational mode has been identified by numerical and experimental means, there are some interesting experiments to carry out in order to determine if there is any non-linear behaviour when working at that operational frequency with high power. Among those experiments are the 2D sweeps around the resonance frequency in both directions, upwards and downwards, and for different values of excitation. In figure 9, the results of these sweeps are shown for two different configurations. figure 9.a shows the behaviour of the transducer for a 2D sweeps with a continuous excitation voltage values between 150 V and 300 V. In addition, figure 9.b shows the results obtained after the same experiment but with a burst excitation signal, in order to minimize the effects of heating in the ceramics: 8

9 Vibration velocity (mm/s) Vibration velocity (mm/s) Frequency (Hz) x Frequency (Hz) x 10 4 Figure 9: Nonlinear behaviour tests. a) For a continuous excitation signal. b) For a burst excitation signal. Both graphics show a softening behaviour when applying high power, but clearer in the case of the burst excitation. However, the frequency displacement is small, not higher than 15 Hz between the lowest and the highest excitation voltage. On the other side, although no frequency jumps are found, there is a slight hysteretic behaviour that can be noticed in both cases. 6 Conclusions The present publication tried to introduce a new transducer for food drying process intensification. This transducer has a stepped-grooved circular plate with a high flexural vibration mode at Hz, with a seven nodal circles shape. After the modal and dynamic experiments, it has been obtained that, apparently, no modal interaction is expected and as well as any non-linear effects, apart from a slight softening behaviour, are willing to appear when applying high power. Although some results have been obtained, there are future tasks to be performed, like the experimental determination of the generated ultrasonic field. The next step consists in the food dehydration tests, initially in the very near field (about few ¼ wavelengths) to continue in dehydration chambers of larger dimensions. Acknowledgments This work has been supported by the project DPI C03-01 funded by the Spanish Ministry of Economy and Competitiveness. References [1] E. Riera, I. González-Gomez, G. Rodríguez, and J. A. Gallego-Juárez, "Ultrasonic agglomeration and preconditioning of aerosol particles for environmental and other applications," in Power Ultrasonics, J. A. Gallego-Juárez and K. F. Graff, Eds., ed Oxford: Woodhead Publishing, 2015, pp

10 [2] G. Rodríguez, E. Riera, J. A. Gallego-Juárez, V. M. Acosta, A. Pinto, I. Martínez, et al., "Experimental study of defoaming by air-borne power ultrasonic technology," Physics Procedia, vol. 3, pp , 1/1/ [3] J. V. Garcia-Perez, J. A. Carcel, E. Riera, C. Rosselló, and A. Mulet, "Intensification of low-temperature drying by using ultrasound," Drying Technology, vol. 30, pp , 2012/09/ [4] J. A. Gallego-Juarez, G. Rodriguez, V. M. Acosta-Aparicio, and E. Riera, "Power ultrasonic transducers with extensive radiators for industrial processing," Ultrasonics Sonochemistry, vol. 17, pp , Aug [5] M. Umeda, K. Nakamura, and S. Ueha, "Effects of vibration stress and temperature on the characteristics of piezoelectric ceramics under high vibration amplitude levels measured by electrical transient responses," Japanese Journal of Applied Physics, vol. 38, p. 5581, [6] J. A. Gallego Juárez, E. Riera, and V. M. Acosta Aparicio, "Modal interactions in highpower ultrasonic processing transducers," AIP Conf. Proc., vol. 1022, pp , [7] E. Riera, J. V. García-Pérez, J. A. Cárcel, V. M. Acosta-Aparicio, and J. A. Gallego- Juárez, "Computational study of ultrasound-assisted drying of food materials," in Innovative Food Processing Technologies: Advances in Multiphysics Simulation, ed: Blackwell Publishing Ltd., 2011, pp [8] R. R. Andrés, A. Blanco, V. M. Acosta-Aparicio, E. Riera, I. Martínez, and A. Pinto, "New ultrasonic controller and characterization system for low temperature drying process intensification," Physics Procedia (in press), [9] E. Riera, A. Cardoni, V. M. Acosta-Aparicio, J. A. Gallego-Juárez, B. B. Linde, J. Paczkowski, et al., "Nonlinear behaviour of power ultrasonic transducers for food processing," in AIP Conf. Proc., 2012, p [10] L. E. Segura, E. R. F. D. E. Sarabia, and J. A. G. Juarez, "Study of the Nearfield Radiated by Stepped Plate Ultrasonic Transducers in Air," in Ultrasonics International 93, ed: Butterworth-Heinemann, 1993, pp [11] V. M. Acosta-Aparicio, E. Andrés-Gallego, J. A. Gallego-Juárez, E. Riera, and G. Rodríguez-Corral, "Application of high-power ultrasound for drying vegetables," in Forum Acusticum, Sevilla, [12] S. de la Fuente-Blanco, E. Riera-Franco de Sarabia, V. M. Acosta-Aparicio, A. Blanco- Blanco, and J. A. Gallego-Juárez, "Food drying process by power ultrasound," Ultrasonics, vol. 44, pp , 12/22/ [13] E. Neppiras, "The pre-stressed piezoelectric sandwich transducer," Ultrasonics international 1973, pp , [14] A. Cardoni, E. Riera, and J. A. Gallego-Juárez, "Nonlinear response in airborne piezoelectric transducer for power ultrasonics," in Proceedings of the 2013 International Congress on Ultrasonics (ICU 2013), Singapore, 2013, pp [15] J. A. Gallego-Juárez, G. Rodríguez, V. M. Acosta-Aparicio, E. Riera, and A. Cardoni, "7 - Power ultrasonic transducers with vibrating plate radiators*," in Power Ultrasonics, J. A. Gallego-Juárez and K. F. Graff, Eds., ed Oxford: Woodhead Publishing, 2015, pp