颗粒堆积型多孔介质内部液体形态实验研究及机理分析

实验研究了不同含湿率时堆积颗粒聚集状态和试样表观体积的变化,并分析了内在影响机理。含湿率较低时液体在试样内部的主要存在形式为液桥,含湿率较高时液体主要以联通液的形式存在,两种形态水分发生转换的临界含湿率由水在颗粒表面的表观接触角决定。试样内部液体形态及分布的演化,导致液体对颗粒的作用力发生变化,进而影响颗粒的聚集状态以及试样的宏观表观体积变化。利用建立的模型推导了液体形态发生转换的临界含湿率与表观接触角之间的函数关系,计算结果与实验具有较好的一致性。     

基于毛细理论的含湿多孔介质内水分迁移过程

基于含湿多孔介质的水分蒸发过程及其内部毛细管水分的蒸发特性和毛细孔隙的分布规律，讨论了含湿多孔介质中孔径分布对水分蒸发过程的影响，加深了对含湿多孔介质水分蒸发过程的物理机制的认识：在多孔介质的降速干燥阶段，蒸发过程只在介质内部的某一蒸发区间内进行。     

独立探头3ω法表征甲烷水合物热导率和热扩散率

甲烷水合物热物性参数的测量一般是基于时域信号测量,测量方法没有考虑探测器与试样之间的接触热阻。基于频域信号测量原理,研发的3ω独立探头大大拓展了该方法的应用范围。建立了低温高压甲烷水合物合成测量系统。利用独立探头3ω法实时测量甲烷水合物热导率、热扩散率、探头和甲烷水合物之间的接触热阻。分析了甲烷水合物热导率、热扩散率随温度的变化规律;比较了测量值与国内外学者测量数据的不同;发现接触热阻对甲烷水合物热导率有显著影响。     

独立探头3ω法表征甲烷水合物热导率和热扩散率

甲烷水合物热物性参数的测量一般是基于时域信号测量,测量方法没有考虑探测器与试样之间的接触热阻。基于频域信号测量原理,研发的3ω独立探头大大拓展了该方法的应用范围。建立了低温高压甲烷水合物合成测量系统。利用独立探头3ω法实时测量甲烷水合物热导率、热扩散率、探头和甲烷水合物之间的接触热阻。分析了甲烷水合物热导率、热扩散率随温度的变化规律;比较了测量值与国内外学者测量数据的不同;发现接触热阻对甲烷水合物热导率有显著影响。     

大规格抛光砖干燥过程及控温要点分析

1砖坯的干燥过程及特征 所谓干燥就是指固体物料受热后蒸发出所含水分的过程。湿坯进入干燥窑后，通过与干燥介质交换，坯体表面水分首先向周围介质扩散，并借助干燥介质的不断流动和扩散作用，使蒸发过程连续进行。     

稀土电解槽石墨内衬氧化扩散与孔隙结构演化模型

基于Fick定律和质量守恒定律,考虑石墨内衬中氧化反应、气体扩散和孔隙结构演化之间耦合作用,建立稀土熔盐电解环境下的石墨内衬试样的氧化扩散与孔隙结构演化数学模型,研究石墨内衬氧化扩散和孔隙结构演化过程。在二氧化碳环境下进行石墨高温氧化实验,采用CT显微技术获取试样孔隙结构图像,对CT图像进行二值化分割处理,统计计算孔隙率,将圆柱试样模型孔隙率、失重率的计算值与实验测试值进行对比验证,并对氧化腐蚀过程中圆柱试样二氧化碳浓度、孔隙率、失重率、氧化速率变化规律进行数值分析。结果表明:反应扩散与孔隙结构演化密切相关,氧化扩散会显著改变孔隙结构,孔隙结构演化会反过来影响氧化扩散进程;石墨内衬氧化扩散与孔隙结构演化受温度影响显著,温度越高,反应越快,孔隙率越大,槽体寿命越短。模型计算结果与实测结果基本一致,表明运用该模型对稀土电解槽石墨内衬在电解环境下氧化扩散与孔隙结构演化过程进行计算分析可行、有效。     

基于泡沫金属的复合毛细芯的物性测试

为改进毛细芯的传热传质性能,以泡沫金属铜或镍为骨架,在其内部填充树形金属铜粉或镍粉,通过树形金属粉末调控泡沫金属内的孔隙结构及孔径分布,制备出一种以金属泡沫为基底的复合毛细芯,并对制备的复合毛细芯的孔隙率、抽吸性能、有效热导率及蒸发率进行研究。结果表明,这种结构的复合毛细芯孔隙率较高,有效热导率为4.1?9.8 W/(m?K)。从毛细芯毛细抽吸、有效热导率和蒸发率综合来看,以金属泡沫镍为骨架、树形镍粉末与造孔剂质量比为5:5的复合毛细芯性能最好。     

含湿多孔介质的干燥特性

基于含湿多孔介质的水分蒸发过程及其内部毛细管水分的蒸发特性，分析了含湿多孔介质在干燥过程中发生体积收缩的原因．讨论了多孔介质的物性和外部干燥条件对其体积收缩特性的影响。     

碳纤维复合材料孔隙分布及形貌分析研究

通过改变固化工艺制备了含孔隙的碳纤维复合材料试样,采用超声检测对试样进行了初步的孔隙检测与筛选,采用金相显微法对典型区域的孔隙率、孔隙分布和形貌特征进行统计。实验结果表明,固化压力不足和袋内真空不合适会引起复合材料内部孔隙的产生,且孔隙的分布存在必然性和随机性,孔隙形貌与孔隙率存在一定的联系。     

光电毛细管探头法的改进及搅拌槽内的气含率分布

用光电毛细管探头法测定搅拌槽中的气含率,毛细管喇叭口外气泡和周围液体之间存在一定的相对速度会导致喇叭口内外的局部气含率之间有不容忽视的偏差。本文提出了由毛细管内的气体和液体流量计算搅拌槽中取样微元内的气含率的新方法。改进后的光电毛细管探头法能成功地用于局部气含率的测定。本文利用改进后的测量方法,测定了搅拌槽不同区域内的气含率,给出了气合率空间分布的初步规律。     

Study of Drying of a Porous Medium Using Gamma-Ray Attenuation Technique

ABSTRACT

In this work the drying of porous media is considered and modeled according to Luikov equations. The experimental apparatus consists of a cylindrical porous medium positioned vertically, with its bottom closed and maintained at a constant temperature. The top surface of this porous medium is in contact with a known flow rate of dry air at room temperature. Under these conditions, the transient axial temperature and moisture concentration profiles are measured. A noninvasive technique has been used to measure the moisture content, which is based on the attenuation of a horizontal beam of gamma-rays penetrating the porous medium. The measurement of this attenuation allows determining the water content of the sample. The mathematical model has been developed using two different approaches. In the first one, the one dimensional Luikov equations are applied considering all the physical parameters as constant. In the second approach, heat and mass balance equations of this process are the simple one dimensional transient diffusion equations, considering both diffusion coefficients as adjustable function of the moisture content. As shown, this second approach is superior to the first one.

     

HEAT AND MOISTURE TRANSFER IN BAKING OF POTATO SLABS

Heal and moss transfer in a slab of potato potato during baking were explained using a multiphase porous media model that includes internal evaporation and capillary, diffusive, and pressure driven transport of water and vapor. The model also includes the effect of the formation of a surface “skin” during baking. Model predictions were validated using experimental data. Most of the temperature drop in the material occurs in a relatively thin and dry surface region, while much of the interior stays at somewhat uniform temperature. Moisture content inside the material stays uniform except near the surface region where it drops to very low values. Evaporation occurs over a significant region, as opposed to at a sharp front. Baking time reduces significantly with thickness, however at d slower rate compared to conduction-only heating (no moisture transport). In addition, the surface temperature also reaches a higher value for a thinner material, which may help develop the often     

周期性边界条件下埋地热油管道传热影响因素的分析

建立土壤多孔介质模型,采用有限容积法对地表温度周期性波动条件下埋地热油管道非稳态传热过程进行数值计算。考虑了土壤中水相、气相迁移对管道传热的影响,对比分析了有、无保温层及保温层厚度、保温层导热系数、土壤导热系数、土壤含水率、管径、埋深等因素对埋地管道非稳态传热规律的影响。研究表明：保温层厚度、导热系数、土壤导热系数对埋地热油管道非稳态传热的影响相对较大。管径、埋深对管道传热的影响相对次之,且埋深对管道的影响冬季远要大于夏季,而土壤含水率对管道传热的影响相对较小。     

Study of Drying of a Porous Medium Using Gamma-Ray Attenuation Technique

In this work the drying of porous media is considered and modeled according to Luikov equations. The experimental apparatus consists of a cylindrical porous medium positioned vertically, with its bottom closed and maintained at a constant temperature. The top surface of this porous medium is in contact with a known flow rate of dry air at room temperature. Under these conditions, the transient axial temperature and moisture concentration profiles are measured. A noninvasive technique has been used to measure the moisture content, which is based on the attenuation of a horizontal beam of gamma-rays penetrating the porous medium. The measurement of this attenuation allows determining the water content of the sample. The mathematical model has been developed using two different approaches. In the first one, the one dimensional Luikov equations are applied considering all the physical parameters as constant. In the second approach, heat and mass balance equations of this process are the simple one dimensional transient diffusion equations, considering both diffusion coefficients as adjustable function of the moisture content. As shown, this second approach is superior to the first one.     

HEAT AND MOISTURE TRANSFER IN BAKING OF POTATO SLABS

ABSTRACT

Heal and moss transfer in a slab of potato potato during baking were explained using a multiphase porous media model that includes internal evaporation and capillary, diffusive, and pressure driven transport of water and vapor. The model also includes the effect of the formation of a surface “skin” during baking. Model predictions were validated using experimental data. Most of the temperature drop in the material occurs in a relatively thin and dry surface region, while much of the interior stays at somewhat uniform temperature. Moisture content inside the material stays uniform except near the surface region where it drops to very low values. Evaporation occurs over a significant region, as opposed to at a sharp front. Baking time reduces significantly with thickness, however at d slower rate compared to conduction-only heating (no moisture transport). In addition, the surface temperature also reaches a higher value for a thinner material, which may help develop the often     

埋地输油管道泄漏污染物热力迁移实验技术研究进展

总结了污染物热力迁移实验中主要参数的测量方法。在温度测量方面重点对热电阻、热电偶以及红外热像仪测量温度的原理及应用进展加以总结,论述了各方法的优缺点及应用现状;在含水率测量方面主要介绍了传统的称重法与发展快速的介电常数法。还总结了石油类污染物在多孔介质热力迁移具有代表性的实验研究,指出了现有实验研究中存在的问题及发展趋势。     

Modeling of the limiting step of water sorption by composite sorbents of the “calcium chloride in porous matrix” type

A nonstationary model is proposed to describe the water sorption from air by composite sorbents of the “salt in a porous matrix” type via the interaction of water with the salt to form a salt solution in pores. It is shown that the dynamics of the moisture content at the adsorber outlet cannot be described under the assumption of constancy of the effective sorption rate constant. The limiting step of water sorption is analyzed, and it is supposed that the limiting step is the water diffusion through the solution layer forming near the external surface of a granule. An expression for the sorption rate constant as a function of the moisture content of the sorbent is proposed that takes into account the monotonic decrease in this constant with an increase in the amount of water sorbed. The effective diffusion coefficient at the limiting step of sorption is determined, which turns out to be close to the effective diffusion coefficient in an aqueous calcium chloride solution.     

An Improved,Easily Implementable,Porous Media Based Model for Deep-Fat Frying: Part II: Results,Validation and Sensitivity Analysis

The multiphase porous media model, developed in the companion paper, has been applied to frying of a restructured potato slice to obtain temperature, pressure, moisture, oil content, acrylamide content and evaporation rate profiles, providing valuable insight into the frying process. The model is validated by comparing temperature, moisture content and crust thickness profiles from literature experimental results. A novel non-equilibrium formulation, different from the existing food literature, is able to describe well the evaporation process. Post-frying cooling is included through appropriate changes in boundary conditions. It is seen that the oil pickup mostly takes place during post-frying cooling and is due to capillary suction created by the negative pressures from condensation of water–vapour. Acrylamide is formed primarily in the crust region where temperature exceeds 100°C. Sensitivity analyses of the process to surface mass transfer coefficient, evaporation rate constant and oil diffusivity show that they all have significant effects on the process. Development of this mechanistic model that is also more easily implementable than previous models should make computer-aided design and optimization of frying processes closer to reality.     

Effect of moisture content on mechanical properties,thermal and structural stability and extrudate texture of poly(vinyl chloride)/wood sawdust composites

Mechanical properties and thermal and structural changes of poly(vinyl chloride) (PVC)/wood sawdust composites were assessed with respect to the effect of moisture content, varying from 0.33 to 3.00 % by weight in the composite, for three different wood sawdust contents. The swell ratio and texture characteristics of the composite extrudates were also evaluated. Unique explanations were given to describe changes in the composite properties in terms of molecular interactions between PVC, cellulosic sawdust and moisture, such as dipole–dipole interactions, interfacial defects and bonding, fibre swelling, and moisture evaporation. The results suggest that at low moisture content the tensile modulus decreased and elongation at break of the composites increased with moisture content, the effect being reversed for high moisture content. Tensile strength decreased with increasing moisture content up to 1–2 %, and then unexpectedly increased at higher moisture contents. The effect of moisture content on flexural properties of the composite was similar to that on tensile properties. Impact strength of the composites was considerably improved with moisture content at low sawdust contents (16.7 wt%), and was independent of the moisture content at higher sawdust contents (28.6 and 37.5 wt%). A decrease in decomposition temperature with an increase in polyene content was evidenced with increasing moisture content, while the glass transition temperature did not change with varying moisture content. The extrudate swell ratio increased with the shear rate but remained unaffected by moisture content. The bubbling and peeling‐off in the composite extrudate occurred as a result of the evaporation of water molecules and the application of a high shear rate. Copyright © 2004 Society of Chemical Industry     

Online measurement and control of solids moisture in fluidised bed dryers

Currently, two main methods are used to take online measurement of the solids moisture in fluidised bed dryers, namely microwave resonance and near infrared spectroscopy. In this paper, a new online approach to solids moisture measurement of batch fluidised bed dryers by electrical capacitance tomography (ECT) is presented for the first time. Based on online measurement of solids moisture, it is possible to implement feedback control and process optimisation of batch fluidised bed drying processes, aiming to increase the operation efficiency and to improve product quality. A twin-plane ECT sensor with eight electrodes in each plane is mounted in the bottom of a glass fluidisation chamber. From the adjacent electrode pairs, the water content of the solids is estimated based on the correlation between the moisture content and the permittivity value. To reduce measurement error, the effect of temperature on moisture measurement is compensated. The fluidisation velocity is estimated by a semi-empirical function based on the measured water content. The acquired information is sent to a controller to adjust the air flow rate of the fluidised bed dryer. To validate the moisture measurement by ECT, a mathematical model has been developed, based on the measured temperature and relative humidity of the outlet air. The Landweber iteration method is applied to reconstruct images. The averaged solids concentration along the radial direction at different fluidisation conditions is given and compared with results by the linear back-projection (LBP) method. Results from batch drying processes with online measurement and feedback control are given and compared with no feedback control. To compare the operation efficiency, the thermal efficiency is considered and the results show the possibility of online control and optimisation of the fluidised bed drying processes, based on online measurement of solids moisture by ECT. Some challenges and future work are discussed.