Improvement of Convective Drying of Carrot by Applying Power Ultrasound—Influence of Mass Load Density

Power ultrasound is considered to be a novel and promising technology with which to improve heat and mass transfer phenomena in drying processes. The aim of this work was to contribute to the knowledge of ultrasound application to air drying by addressing the influence of mass load density on the ultrasonically assisted air drying of carrot. Drying kinetics of carrot cubes were carried out (in triplicate) with or without power ultrasound application (75 W, 21.7 kHz) at 40°C, 1 m/s, and several mass load densities: 12, 24, 36, 42, 48, 60, 72, 84, 96, 108, and 120 kg/m³. The experimental results showed a significant (p < 0.05) influence of both factors, mass load density and power ultrasound application, on drying kinetics. As expected, the increase of mass load density did not affect the effective moisture diffusivity (D_e, m²/s) but produced a reduction of the mass transfer coefficient (k, kg water/m²/s). This was explained by considering perturbations in the air flow through the drying chamber thus creating preferential pathways and, as a consequence, increasing external mass transfer resistance. On the other hand, it was found that the power ultrasound application increased the mass transfer coefficient and the effective moisture diffusivity regardless of the mass load density used. However, the influence of power ultrasound was not significant at the highest mass load densities tested (108 and 120 kg/m³), which may be explained from the high ratio (acoustic energy/sample mass) found under those experimental conditions. Therefore, the application of ultrasound was considered as a useful technology with which to improve the convective drying, although its effects may be reduced at high mass load densities.     

Influence of High-Intensity Ultrasound on Drying Kinetics of Persimmon

Drying persimmon pieces is recognized as a way to preserve and add value to the excess production of the fruit in Spain. To this end, air drying kinetics of persimmon cylinders (30 mm height and 13 mm diameter) were determined under different drying conditions: 8 air drying velocities (0.5, 1, 2, 4, 6, 8, 10, and 12 m/s) with and without application of high-intensity ultrasound (21.8 kHz and 154.3 dB). The drying process was modeled using two diffusion models with and without the influence of external resistance to drying. From the effective diffusivity and the mass transfer coefficient identified from the data it was concluded that high-intensity ultrasound increased the drying rate at the lowest air velocities tested, affecting both external and internal resistances.     

Performance improvement of a PEMFC system controlling the cathode outlet air flow

This paper presents a stationary and dynamic study of the advantages of using a regulating valve for the cathode outlet flow in combination with the compressor motor voltage as manipulated variables in a fuel cell system. At a given load current, the cathode input and output flowrate determine the cathode pressure and stoichiometry, and consequently determine the oxygen partial pressure, the generated voltage and the compressor power consumption. In order to maintain a high efficiency during operation, the cathode output regulating valve has to be adjusted to the operating conditions, specially marked by the current drawn from the stack. Besides, the appropriate valve manipulation produces an improvement in the transient response of the system. The influence of this input variable is exploited by implementing a predictive control strategy based on dynamic matrix control (DMC), using the compressor voltage and the cathode output regulating valve as manipulated variables. The objectives of this control strategy are to regulate both the fuel cell voltage and oxygen excess ratio in the cathode, and thus, to improve the system performance. All the simulation results have been obtained using the MATLAB-Simulink environment.     

Review of generative models for the narrowband land mobile satellite propagation channel

The land mobile satellite (LMS) propagation channel is frequently described using statistical models. These models usually make different assumptions regarding the behavior of the direct signal, the diffuse multipath component and the shadowing effects. This paper analyzes the theoretical formulation and implementation of time‐series synthesizers based on three typical statistical models: Loo, Corazza–Vatalaro and Suzuki, describing their similarities and differences. The discussion is not limited to the amplitude of the complex envelope but also to the phase variations and Doppler spectra. Finally, guidelines are also provided for comparing model parameters supplied by different authors. Copyright © 2008 John Wiley & Sons, Ltd.     

Discrete elements model for evaluating impact and impulsive response of reinforced concrete plates and shells subjected to impulsive loading

The application of a discrete element representation of solids to the analysis of reinforced concrete plates and shells is discussed. Yielding of steel as well as fracture of concrete are duly accounted for by means of constitutive criteria that quantifies coupling between both effects. Comparison with experimental results show excellent correlation.     

Influence of support friction on three-point bend test

One of the most commonly used tests for the experimental determination of the stress intensity factor is the so-called three-point bend test which has been adopted as a standard test by the American Society for Testing and Materials. It has been verified that the actual boundary conditions, i.e. support friction, play an important role in the stress intensity factor in the three-point bend specimen.

In this paper the stress intensity factor considering friction at the supports is determined by a novel procedure, which may be of interest to researchers working on the numerical evaluation of stress concentration factors.     

PM and FM generation with DDS: effects of sampling the modulatingsignal

Direct digital synthesisers (DDSs) can generate phase- or frequency-modulated signals, with high linearity. The spurs that appear because of the sampling of the modulating signal are analysed. Analytical expressions are given to predict their level. Empirical results are provided to show the accuracy of the theoretical expressions