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In this work, the heat and mass transfer characteristics of kiwifruit samples during vacuum freeze-drying process have been investigated numerically and experimentally. The numerical simulation with moving grid method is adopted to simulate the radial and axial moving sublimation interfaces during vacuum freeze-drying. The simulated average moisture content and temperature of kiwifruit during drying are validated with the measured data, with the mean average error (MAE) of 14.7%. The findings demonstrate that the sublimation interface shifts toward the center during the drying process and the sublimation rate and pressure gradient are significant in the early stage of drying. After 10 h of drying process, the sublimation rate dropped by 80% due to the increase of the thickness of the dried region and the decrease of the shelf plate heat transfer. In addition, it is noted that the increasing shelf plate temperature and chamber pressure would cause an increase of sublimation rate, thereby reducing the drying time. Although increasing the sample thickness can improve the sublimation rate, the drying time becomes longer due to the presence of more ice crystals in thicker kiwifruit samples. [ABSTRACT FROM AUTHOR]

Recently, the use of nanoparticles in extraction columns has gained increasing attention due to their potential to enhance mass transfer processes. Most of the reported work, however, has been concerned with mass transfer enhancement in gas–liquid systems in the presence of nanoparticles, while limited studies of liquid–liquid systems have been reported. Liquid–liquid extraction is an extensively employed separation method that has taken on a substantial role in engineering and industrial applications for recovery processes. Therefore, the aim of this review article is to provide an overview of the state-of-the-art from the available literature on the effect of nanoparticles on the hydrodynamic behavior and mass transfer performance of liquid–liquid systems. Attention is also given to providing an understanding of the mechanisms responsible for the improvements in mass transfer with the use of nanoparticles. In the majority of studies, the presence of nanoparticles was found to be beneficial and to lead to improved mass transfer, although a few investigations reported a detrimental impact of nanoparticles. The literature does not clearly describe the conditions under which the addition of nanoparticles would be unfavorable. Moreover, the addition of nanoparticles always introduces potential drawbacks, such as sedimentation, accumulation, or clogging inside extraction columns, as well as the need to separate the nanoparticles at the outlets. These drawbacks have not been considered in the literature, limiting the practical employment of nanoparticles in this industry. [ABSTRACT FROM AUTHOR]

Recently, it has been observed that the use of magnetic nanoparticles and the magnetic field would result in heat and mass transfer intensification. But most of the investigations have been devoted to the enhancement of heat transfer. However, the investigations on the applicability of magnetic nanoparticles for mass transfer improvement are still in the developing stage. In the present study, the effect of MnFe 2 O 4 nanoparticles on the mass transfer performance of a packed extraction column is evaluated in the presence of constant and alternating magnetic fields. The column is filled with Penta-Pak TM PS-500M1 structured packing; while four identical bar electromagnets made from zinc ferrite cores are used to generate the quadrupole magnetic field. The (toluene-acetic acid)-water system is employed. Spinel-type manganese ferrite nanoparticles are added to the organic dispersed phase (toluene + acetic acid) to intensify solute transfer. The quadrupole magnetic field is applied from 100 to 500 G intensity and 100 to 500 Hz frequency, accompanied with different nanoparticle concentrations from 0.001 wt% to 0.005 wt%. According to our observations, mass transfer performance increases remarkably in the presence of magnetic nanoparticles under the influence of magnetic field. Compared with no magnetic field, maximum improvements of 115% and 43% in overall mass transfer coefficient and extraction fraction are observed under alternating magnetic field with 300 Hz frequency and 500 G intensity for 0.003 wt% nanoparticle concentration. That is, by using the magnetic nanoparticles and the magnetic field, one can considerably improve the performance of a liquid-liquid extraction system. [ABSTRACT FROM AUTHOR]

An investigation into the effect of mass flow rate oscillation on the R-134a evaporation heat transportation in a horizontal annular duct was experimentally conducted. The experiments were performed at different amplitudes (10, 20, and 30%) and periods (20, 30, 60, and 120 s) of the mass flux oscillation in a duct with different gap sizes (1.0, 2.0 and 5.0 mm). In this regard, the time variations of evaporation heat transportation coefficient and the heated wall temperature were also analyzed for the thermal characteristics of the oscillatory evaporation heat transfer. Measured results showed that the amplitude and period of the mass flux oscillation insignificantly affected the time-average heat transportation coefficient for the R-134a oscillatory evaporation heat transfer. However, the larger amplitudes and longer periods of the mass flux oscillation led to stronger wall temperature oscillations. [ABSTRACT FROM AUTHOR]

• Plate-type membrane humidifier for PEMFC was analyzed with various channel sizes. • Water flux, water recovery ratio, and pressure loss were investigated. • Both width and depth of the channel must be taken into precise consideration. • Counter flow approach shows better performance rather than the parallel flow approach. In the present study, the performance of a planar membrane humidifier for proton exchange membrane fuel cell is evaluated experimentally for different channel structures. For this purpose, three performance indices including water flux, water recovery ratio (WRR), and pressure loss are employed. The measured results disclose that the humidifier using counter flow approach and the channel with width of 1 mm and depth of 1.5 mm has the best overall humidification performance in this work. Increasing the channel depth increases the water flux and therefore increases the WRR of the humidifier. Moreover, the pressure loss decreases with increasing the channel depth because the friction factor at an identical flow rate decreases with increasing the hydraulic diameter of the flow channel. Increasing the channel width expands the contact area with the membrane, leading to an improvement of heat and water vapor transfer. Thus, a higher WRR and pressure loss can be obtained at elevated channel widths. Both depth and width of the channel must be taken into precise consideration in order to minimizing the pressure loss and maximizing the heat and mass transfer. [ABSTRACT FROM AUTHOR]