Enhancement of convective drying by application of airborne ultrasound – A response surface approach

Drying is one of the oldest and most commonly used processes in the food manufacturing industry. The conventional way of drying is by forced convection at elevated temperatures. However, this process step often requires a very long treatment time, is highly energy consuming and detrimental to the product quality. Therefore, an investigation of whether the drying time and temperature can be reduced with the assistance of an airborne ultrasound intervention is of interest.Previous studies have shown that contact ultrasound can accelerate the drying process. It is assumed that mechanical vibrations, creating micro channels in the food matrix or keeping these channels from collapsing upon drying, are responsible for the faster water removal. In food samples, due to their natural origin, drying is also influenced by fluctuations in tissue structure, varying between different trials. For this reason, a model food system with thermo-physical properties and composition (water, cellulose, starch, fructose) similar to those of plant-based foods has been used in this study.The main objective was, therefore, to investigate the influence of airborne ultrasound conditions on the drying behaviour of the model food. The impact of airborne ultrasound at various power levels, drying temperature, relative humidity of the drying air, and the air speed was analysed. To examine possible interactions between these parameters, the experiments were designed with a Response Surface Method using Minitab 16 Statistical Software (Minitab Inc., State College, PA, USA). In addition, a first attempt at improving the process conditions and performance for better suitability and applicability in industrial scale processing was undertaken by non-continuous/intermittent sonication.     

通讯波段禁带胶体光子晶体的制备及实验参数研究

采用直径为700nm的二氧化硅微球,通过垂直沉积法在玻璃基片上制备了位于光通讯波段的禁带胶体光子晶体。研究了不同蒸发温度、湿度条件、干燥过程对样品晶体质量的影响。用扫描电子显微镜(SEM)对其形貌进行了观测。实验结果表明,在温度为65℃,相对湿度为70%,缓慢干燥条件下制备出的样品具有较好的晶体质量,晶体在大范围内保持了面心立方(FCC)单晶结构,其(111)面平行于生长基片。     

Control water molecules across carbon-based nanochannels

It is important to know the mechanisms of water molecules across carbon-based nanochannels, which is not only beneficial for understanding biological activities but also for designing various smart devices. Here we review the recent progress of research for water transfer across carbon-based nanochannels. In this review, we summarize the recent methods which can affect water molecules across these nanochannels. The methods include exterior factors(i.e., dipolar molecules and gradient electric fields) and interior factors(namely, cone-shaped structures, nonstraight nanochannels, and channel defects). These factors can control water permeation across nanochannels efficiently.     

A spectroscopic study of the effect of humidity on the atmospheric pressure helium plasma jets

Atmospheric-pressure plasma has a great potential in many applications due to its simplicity rather than low pressure plasmas. In material processing, biomedical applications, and many other applications, the input power, gas flow rate, and the geometry of electrode have been mainly considered and studied as important external parameters of atmospheric-pressure plasma control. Besides, since the atmospheric-pressure plasmas are typically generated in an open air, the relative humidity is difficult to control and can change day by day. Therefore, the relative humidity cannot be ignored for plasmas. Thus, in this work, the atmospheric-pressure plasma jet was characterized by changing relative humidity, and it was found that the increase in electron density and OH radicals are due to Penning ionization between helium metastable and water vapors at higher humidity condition.     

反常吸收与高质量的远红外光谱

在水汽含量恒定但相对湿度较大的情况下,远红外光谱仪得不到理想的100%线。这表明过高的水汽含量将会导致反常吸收现象。反常吸收的出现将严重影响远红外光谱的质量。研究了光谱仪内部空气相对湿度和光谱分辨率对反常吸收的影响,发现降低水汽含量和采用适当光谱分辨率可以有效抑制反常吸收现象,对获得高质量的远红外光谱很有益处。在无反常吸收现象发生的实验条件下,采用"湿度中和法"在空气湿度波动的情况下也可以得到理想的100%线,这为快速获得高质量的远红外光谱提供了新测量方法。     

干燥物料水分状态研究

物料在太阳能干燥房中干燥以后 ,用水分测定仪测定其含水量进行再干燥。测定再干燥前后物料的红外光谱 ,研究其水分状态表明 ,在干燥过程后期 ,将干燥温度控制在结合水开始逸出的温度 ,可得到最好的干燥效果     

低维限域结构中水与物质的输运

低维限域结构中水与物质的输运研究,对于解决界面化学和流体力学中的遗留问题十分关键.近年来,研究人员采用分子动力学模拟和实验手段研究低维限域结构中水与物质的输运,并将其应用于物质输运、纳米限域化学反应、纳米材料制备等领域.本文从理论和实验的角度总结一维和二维纳米通道的水与物质输运,介绍了本研究组提出的"量子限域超流体"概念,并用于解释纳米通道中超快物质的输运现象;在此基础上概述了一维纳米通道中的分子动力学模拟和水浸润性,以及外部环境(如温度和电压)对限域结构中水浸润性的调控,同时阐述了低维限域结构中的液体输运;对二维纳米通道中的分子动力学模拟、液体浸润性以及液体输运进行了综述;讨论了纳米通道限域结构在物质输运、纳米限域化学反应和纳米材料制备等领域的应用;对低维限域结构中水与物质输运面临的挑战和前景进行了展望.     

红外光谱差减技术扣除水汽吸收干扰的局限性

红外光谱差减技术在扣除背景组份干扰方面得到广泛应用,但其扣除空气中水汽效果却不尽如人意。研究了不同湿度的水汽光谱与光谱差减效果的关系,以探究光谱差减技术在水汽扣除领域局限性的原因。结果表明：(1)相对湿度改变,水汽的红外光谱也发生变化,不管如何小心地选择比例系数f,从相对湿度x%的水汽光谱A_x,也不能完全扣除相对湿度为y%的水汽光谱A_y,即fA_x≠A_y。(2)相对湿度改变,水分子团簇(H₂O)_n的相对组成也会发生变化,这是导致光谱差减技术局限性的主要原因。(3)将湿度为x%和y%的两水汽光谱A_x和A_y进行线性组合,则可以高度近似地模拟出介于两者之间的湿度的水汽光谱。比如用40%水汽谱和30%水汽谱,可以模拟得到32%或35%或37%的水汽谱。实验结果表明这是扣除水汽干扰效果更好的路径。(4)论证了水汽补偿湿度滴定法具有高效性的原因。     

新型复合固体除湿材料的吸附和解吸性能研究

为实现固体除湿系统吸附材料的低温再生及提高吸附材料的除湿效率,制备了新型复合固体除湿材料。新型复合固体除湿材料是以陶瓷纤维为基质,运用MgCl2浸渍改性硅胶及CaCl2二次强化方法而形成的复合物,复合固体除湿材料中MgCl2和CaCl2浓度均为25%。通过恒温恒湿空调室实验研究温度、风速、湿度对MgCl2/CaCl2改性复合材料除湿性能及再生特性。实验表明:在温度20℃、湿度70%条件下,复合固体除湿材料除湿量可达161 mg/g,是未改性除湿材料的3.2倍,是单一改性除湿材料的1.3倍;当系统平衡时,其除湿速率分别是未改性除湿材料的6.1倍,是单一改性除湿材料的2.6倍。除湿材料的平衡吸附量和吸附速率均与相对湿度成正比,与温度成反比;且风速在0.5 m/s条件时具有最大的除湿量。同时,在约70℃较低的脱附温度,8 min可脱附≥90%的总吸附水量,每1 g吸附剂可脱附水量高达145 mg;脱附再生6次后,除湿量依然较高为138 mg/g,且基本不再变化。     

MPM—An atmospheric millimeter-wave propagation model

A broadband model for complex refractivity is presented to predict propagation effects of loss and delay for the neutral atmosphere at frequencies up to 1000 GHz. Contributions from dry air, water vapor, suspended water droplets (haze, fog, cloud), and rain are addressed. For clear air, the local line base (44 O₂ plus 30 H₂O lines) is complemented by an empirical water-vapor continuum. Input variables are barometric pressure, temperature, relative humidity, suspended water droplet concentration, and rainfall rate.     

Anomalous flow behavior in closed and open thin walled nanochannels

Molecular dynamics simulations have been carried out to examine water flow in symmetric and asymmetric open and closed ends nanochannels with hydrophilic surfaces. The results are counterintuitive and the opposite of what is observed in macro-systems-closed channels fill faster with fluid than do their open counterparts. In addition, hybrid closed-open asymmetric channels fill up even faster. These results can be explained on the basis of the fluid-structure interaction that arises through the different vibrational behaviors of the surface molecules that are part of the wall forming these channels. Such effects are not expected to be of significance in macro-channels, and point to an important case where macro and nanochannels exhibit contrary behavior. Since these effects results from strong interactions between the fluid molecules and solid surface, one would not expect them with hydrophobic walls, and our simulations confirm such behavior.     

Conformal mapping for the Helmholtz equation: acoustic wave scattering by a two dimensional inclusion with irregular shape in an ideal fluid

The complex variables method with mapping function was extended to solve the linear acoustic wave scattering by an inclusion with sharp/smooth corners in an infinite ideal fluid domain. The improved solutions of Helmholtz equation, shown as Bessel function with mapping function as the argument and fractional order Bessel function, were analytically obtained. Based on the mapping function, the initial geometry as well as the original physical vector can be transformed into the corresponding expressions inside the mapping plane. As all the physical vectors are calculated in the mapping plane (η,η), this method can lead to potential vast savings of computational resources and memory. In this work, the results are validated against several published works in the literature. The different geometries of the inclusion with sharp corners based on the proposed mapping functions for irregular polygons are studied and discussed. The findings show that the variation of angles and frequencies of the incident waves have significant influence on the bistatic scattering pattern and the far-field form factor for the pressure in the fluid.     

聚(偏氟乙烯-三氟乙烯)纳米薄膜极化反转与疲劳特性

采用旋涂法制备了厚度为140 nm的聚(偏氟乙烯-三氟乙烯)[P(VDF-TrFE)]纳米薄膜, 研究了不同退火温度以及环境相对湿度对薄膜的极化反转和疲劳性能的影响. 运用X射线衍射仪、扫描电子显微镜和傅里叶变换红外光谱仪等测试技术对薄膜的微结构进行了表征. 实验结果表明, 通过不同温度的退火处理, P(VDF-TrFE)铁电薄膜的结晶度随着退火温度的升高而不断提高, 并且一定的温度范围内的退火处理可以提高薄膜的极化性能; 此外, P(VDF-TrFE) 铁电薄膜性能还表现出一定的环境湿度的敏感特性, 这与薄膜的物理性能和结构特点密切相关; P(VDF-TrFE)铁电薄膜在不同的环境湿度条件下 表现出较好的电学特性, 其漏电流均保持在10 ^-7A/cm² 的较低水平. 本工作揭示了再退火过程对薄膜的极化反转速度和疲劳恢复特性的影响, 并结合薄膜二次疲劳结果, 探讨了薄膜可逆的内部疲劳恢复特性机理.     

Experimental investigation on the flow over normal flat plates with various corner shapes

The influence of corner modification on the flow over normal flat plates is experimentally investigated in a water tunnel. Particle image velocimetry measurement is performed at Re = 2240 based on the width of the plate. Besides the flat plate with sharp edges, others with chamfered corners, rounded corners and stepped corners at the windward side are studied. Although the flat plate is very thin compared with square cylinders, corner modification can also achieve a significant drag reduction. The mechanism for the drag reduction is explored based on the solid analysis of the vortical structures and the statistical parameters. With corner modification, the strength and the scale of the vortex are decreased, while the formation of the vortex is postponed, resulting in a weakened vortex-interaction process. Thus, the fluctuation intensity of the wake is weakened, the wake width is narrowed down with the recirculation region elongated, and the vortex shedding frequency becomes higher. These modifications interpret well how the drag of a normal plate could be reduced, which is confirmed by the drag estimation from the wake profiles.     

Temporal-spatial diversities of long-range correlation for relative humidity over China

Long-range correlations of daily relative humidity anomaly records from 191 weather stations over China during 1951-2000 are analyzed by means of fluctuation analysis (FA) and detrended fluctuation analysis (DFA). The information about trends in the relative humidity records can be obtained by comparing the FA curve with DFA curves. The daily relative humidity fluctuations are found to be power-law correlated and their average scaling exponent is higher than that of the temperature fluctuations, indicating that the relative humidity fluctuations take different statistical behavior from other meteorological quantities and there exists a stronger persistence in the relative humidity fluctuations. Furthermore, it is also found that these power-law scaling properties vary from station to station and show both spatial and temporal diversities, which may be explained by a proposed mechanism.     

Electric and magnetic losses modeled by a stable hybrid with explicit–implicit time-stepping for Maxwell’s equations

A stable hybridization of the finite-element method (FEM) and the finite-difference time-domain (FDTD) scheme for Maxwell’s equations with electric and magnetic losses is presented for two-dimensional problems. The hybrid method combines the flexibility of the FEM with the efficiency of the FDTD scheme and it is based directly on Ampère’s and Faraday’s law. The electric and magnetic losses can be treated implicitly by the FEM on an unstructured mesh, which allows for local mesh refinement in order to resolve rapid variations in the material parameters and/or the electromagnetic field. It is also feasible to handle larger homogeneous regions with losses by the explicit FDTD scheme connected to an implicitly time-stepped and lossy FEM region. The hybrid method shows second-order convergence for smooth scatterers. The bistatic radar cross section (RCS) for a circular metal cylinder with a lossy coating converges to the analytical solution and an accuracy of 2% is achieved for about 20 points per wavelength. The monostatic RCS for an airfoil that features sharp corners yields a lower order of convergence and it is found to agree well with what can be expected for singular fields at the sharp corners. A careful convergence study with resolutions from 20 to 140 points per wavelength provides accurate extrapolated results for this non-trivial test case, which makes it possible to use as a reference problem for scattering codes that model both electric and magnetic losses.     

PEMFC气体扩散层干燥过程孔隙网络模拟

本文采用三维孔隙网络模型从孔隙尺度上对质子交换膜燃料电池气体扩散层中水分蒸发的干燥过程进行了模拟,并考虑了阴极流道的影响.计算结果表明毛细力在气体扩散层的干燥过程中起主要作用,气相从阴极流道底部开始,以沿气体扩散层厚度方向侵入为先,直到到达气体扩散层底部,随后气相才向流道肩部水平延伸侵入.蒸发速率随气体扩散层中水饱和度...     

Shape effect of nanochannels on water mobility

Confinement can induce unusual behaviors of water. Inspired by the fabrication of carbon nanotubes with noncircular cross sections, we performed molecular dynamics simulations to investigate the mobilities of water confined in carbon nanochannels with circular, square, and equilateral triangular cross sections over a variety of dimensions. We find that water exhibits disparate mobilities across different types of channels below 0.796 nm². Notably, compared with the other two channels, water in equilateral triangular channels displays the greatest mobilities. Moreover, at 0.425 nm², different ordered structures are found in the three channels, and water inside the square channel exhibits an extremely low mobility. It is also found that above 0.796 nm², the mobilities along the tube axis of water converge to that of the bulk. These phenomena are understood by analyzing the structure, dynamics, and hydrogen bonding of water. Our work explores the mobilities of water across noncircular carbon nanochannels, which may expand the prospect of noncircular nanochannels in scientific studies and practical applications, such as desalination and drug delivery.     

Systematic parametric investigation on the CVD process of polysiloxane nano- and microstructures

Amorphous polysiloxane nano- and microstructures with different shapes can be synthesized from trifunctional organosilane precursors. In the present study, various polysiloxane nano- and microstructures have been produced via a chemical vapor deposition process using ethyltrichlorosilane as precursor. The structure formation and shape are the result of a delicate interplay between temperature, absolute amount of water, and relative humidity. The impact of these reaction parameters during a chemical vapor deposition process has been examined. Experiments have been performed to find a correlation between the reaction conditions and the final shape. Scanning electron microscopy data show that different structures like polysiloxane microrings, microrods, sprouts, nanofilaments, and mixtures of them can be synthesized depending on the reaction conditions. Furthermore, the in-depth comparison of the nanofilament diameters illustrates the dominating influence of relative humidity on structure formation. There is a general trend that at a higher value of relative humidity, structures with a larger diameter are formed independent from the temperature. Here, we clearly differentiate between relative humidity as major and absolute amount of water and temperature as minor important adjusting screws defining the thickness and shape of the resulting nano- and microstructures. Based on these observations, we proof the mechanism of the initial step of structure formation. It is shown that nano- and micro-sized water droplets formed on the substrate surface are likely to act as starting points for structure formation. All results described here strongly confirm the recently published droplet assisted growth and shaping mechanism.     

A stray-field imaging,CRAMPS and MQMAS study of the drying process of precasting materials used in a steel-making converter

Optimization of the drying process conditions in a steel-making converter in a steel works is very difficult since the process is off-line and time-consuming. However, it is important to optimize drying process conditions (temperature, surface active agents, etc.), as steam explosion can readily occur with insufficient drying time. To help understanding, we have demonstrated that we can monitor the drying of real refractory mortar with stray-field imaging. We chose, this method because of the possibility of detecting shortT ₂ components. This paper shows the effect of varying water content in different materials on the drying rate. In particular, we find that the free-water loss rate is relatively independent of water content. However the bound-water loss rate is more affected. Also, solid-state nuclear magnetic resonance (NMR) (¹H-CRAMPS and²⁷Al M(3 and 5)QMAS) studies are performed to clarify the change of chemical structure by drying treatment. It is clear that imaging and solid-state NMR give useful information to optimize drying conditions. With this data, we can adjust and optimize the drying process and time in steel works.