具有电介质核心多孔介质微波冷冻干燥过程的耦合传热传质的数值研究

利用变时间步长的有限体积法对具有电介质核心多孔介质微波冷冻干燥的耦合热质传递过程进行了数值模拟。计算结果表明：在有电介质核的多孔介质内部存在着两个升华界面,该双升华界面模型成功地模拟了该干燥过程：合理的选用电介质核心可以大大缩短干燥时间,对于几个大小分别为1．0,1.5,2．0和2．5mm的电介质核,其单位体积所需干燥时间同无核相比分别减少了8％,19％,33％,48％：在相同的电场强度下,电介质核的损耗系数越大,所需干燥时间越短。     

多孔介质干燥机理在褐煤热力脱水中的应用

我国揭煤资源丰富,但是由于其水分高,导致了揭煤燃烧发电效率低下,不宜远距离运输.通过热力干燥可降低揭煤的水分含量,有利于揭煤运输和贮存,提高锅炉燃烧效率.从揭煤结构出发,提出其热力干燥过程的模型可将之视为多孔介质.分析了多孔介质的干燥过程特点,阐述了目前国内外的多孔介质干燥机理,总结了相关的多孔介质干燥模型,为多孔介质褐煤热力脱水模型化提供理论支持.     

多孔介质干燥过程的格子气自动机法模拟

介绍了格子气自动机模型的特点。应用该模型对多孔介质干燥过程的水分分布和干燥过程的特征进行了模拟,其结果可以给出多孔介质干燥过程液体分布的细节,也可得到表征多孔介质干燥过程的宏观特征曲线。初步研究表明:格子气自动机模型可用于模拟复杂边界条件的多孔介质体的干燥过程。     

高初始含湿多孔介质喷雾干燥过程的热质耦合传递

多孔介质是大量干燥过程的主体,由于实际多孔介质干燥过程的复杂性,建立通用的干燥过程传热传质模型十分困难。通过分析喷雾干燥过程中高初始含湿多孔介质与干燥介质之间的传热传质机理以及各因素对传热传质的影响,根据马歇尔方程探讨了干燥介质与料雾之间的水蒸汽分压差在干燥过程中的变化情况,反映了多孔湿介质在喷雾干燥操作中的传热传质过程的几种特性,为确定实际生产中喷雾干燥器的操作条件指明了新的出路。     

多孔介质喷雾干燥过程的热质传递

多孔介质是大量干燥过程的主体，由于实际多孔介质干燥过程的复杂性，建立通用的干燥过程传热传质模型十分困难。本文通过分析喷雾干燥过程中高初始含湿多孔介质与干燥介质之间的传热传质机理以及各因素对传热传质的影响，根据马歇尔方程探讨了干燥介质与料雾之间的水蒸汽分压差在干燥过程中的变化情况，反映了多孔湿介质在喷雾干燥操作中的传热传质过程的几种特性，为确定实际生产中喷雾干燥器的操作奈件指明了新的出路。     

大规格建陶坯体对流干燥热质耦合传递的数值模拟

以Whitaker的体积平均方程为基础,推导出多孔介质内部热质传递的等效耦合扩散模型,并对瓷质砖坯体干燥过程进行了数值分析和实验测定。探讨了干燥介质参数（湿度、温度和流速）和坯体物性参数（有效导热系数和有效扩散系数）对干燥过程的影响。结果表明,温度高、流速快、湿度小的热空气有利于干燥,其中,改变风速对干燥速率的影响很小,介质的温度和湿度对干燥影响较大,但有可能造成干燥缺陷。有效扩散系数对坯体内湿度的分布影响很大,而有效导热系数对坯体内湿度的分布几乎没有影响。     

具有预制孔隙多孔物料的冷冻干燥

实验探究了具有初始预制孔隙多孔物料对冷冻干燥过程的强化作用。以注射用抗生素药剂--头孢曲松钠为主要溶质,采用“液氮制作冰激凌法”制备了具有不同初始孔隙率的冷冻物料,在相同的条件下进行冷冻干燥实验。结果表明,初始饱和度为0.3的冷冻物料（初始孔隙率为0.67）干燥时间比饱和物料（初始孔隙率为0）缩短了21.3%。干燥产品的SEM图显示,初始非饱和冷冻物料的固体骨架和孔隙结构连续而均匀,初始饱和度越低,骨架越纤细,可大大地降低传质阻力。对冷冻速率和退火处理的研究表明,冷冻速率对于两种物料干燥过程的影响甚微;退火处理能够提高冷冻干燥速率。适当提高操作温度可以明显缩短两种物料的干燥时间;操作压力对冷冻干燥过程几乎没有影响。     

多孔介质干燥的孔道网络试验及模拟

结合孔道网络及侵入渗流理论。试验并模拟研究了刚性多孔介质的干燥过程。观察了孔道网络内部湿组份的分布状态，孔道网络干燥前沿的形成及发展过程。验证了多孔介质在干燥过程出现的蒸发前沿及其具有的不规则特征。模拟研究了多孔介质在等温条件下不考虑重力时的干燥过程。通过模拟给出了多孔介质在干燥过程内部相的动态分布。模拟结果显示，干燥前沿具有非常不规则的特征。通过相图分析。干燥中内部涅分的迁移不仅是一个从里到外的过程。也同时存在逆向扩散。     

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。     

逾渗多孔介质对固体颗粒吸附过程的影响

研究了悬浮液中的颗粒在通过逾渗多孔介质时的被吸附特性。采用数值计算的方法;通过求解描述低速流体流动的Stokes方程以及简化的颗粒运动方程;初步得到颗粒在逾渗多孔介质中的运动轨迹;并在此基础上;求得颗粒与多孔介质内表面的碰撞概率;进而研究颗粒的被吸附特性。数值结果表明均匀多孔介质和分形多孔介质对颗粒的吸附存在本质差异。颗粒流出概率（实际中常表示为出口悬浮液中的颗粒浓度）与多孔床深度间的指数关系仅对均匀多孔介质成立;而对分形多孔介质并不成立。     

Conjugate heat and mass transfer process within porous media with dielectric cores in microwave freeze drying

A conjugate heat and mass transfer model was developed for microwave freeze drying within porous media with dielectric cores. The set of transient governing equations developed was solved numerically using finite volume method with variable time-steps. Analysis of numerical results may lead to following main conclusions: (1) by carefully choosing the loss factor and size of the dielectric cores could dramatically reduce the drying time, (2) the size and the loss factor ε″ of the dielectric core are the two important parameters influencing the drying process, (3) there may be two sublimation fronts in existence within the porous media.     

初始饱和度对微波冷冻干燥过程传热传质的影响

以非饱和含湿牛肉为例,进行了微波冷冻干燥实验研究。获得了干燥时间与物料初始饱和度近似成正比的结论。与升华面模型的计算结果比较表明,升华冷凝模型更符合实验规律,证实了非饱和含湿多孔介质微波冷冻干燥时升华冷凝区的存在。     

冷冻干燥法制备层状多孔羟基磷灰石支架过程中的溶剂升华行为

采用水基羟基磷灰石(hydroxyapatite,HA)浆料,经冷冻干燥和烧结工艺(1 250℃烧结3 h)制备了层状多孔HA支架.研究了冷冻温度、干燥压力和干燥温度对水基HA浆料中溶剂升华行为的影响.结果表明:随着冷冻温度的降低,多孔HA支架的层间距逐渐减小,支架的升华时间增加;由于样品的干燥过程同时受到传热和传质的...     

Theoretical Study on Microwave Freeze-Drying of an Aqueous Pharmaceutical Excipient with the Aid of Dielectric Material

Abstract

A mathematic model of simultaneous heat and mass transfer for the dielectric material assisted microwave freeze-drying was derived and solved numerically using the finite-deference technique with two moving boundaries. Lactose, a typical pharmaceutical excipient, was used as the representative solid material in the aqueous solution to be freeze-dried. Silicon carbide (SiC) was selected as the dielectric material. Numerical results show that the dielectric material can significantly enhance the microwave freeze-drying process. Under typical operating conditions, the drying time is 43% shorter than that of ordinary microwave freeze-drying. Temperature variations at sublimation fronts were examined in order to determine the appropriate microwave power input. Profiles of temperature, ice saturation, vapor concentration, and pressure during freeze-drying are presented, and rate-controlling mechanisms are discussed.     

CFD Model of Apple Atmospheric Freeze Drying at Low Temperature

An Internet-controlled atmospheric freeze dryer was designed, built, and tested in the Department of Heat and Mass Transfer, Technical University of ?ód?. A film sublimation-based CFD model was developed and verified using Fluent 6.1 commercial CFD software. The model enables the simulation of phase change and water vapor diffusion process within porous media. Transport of non-condensable species can be calculated with species transport inbuilt model of Fluent 6.1 and used to predict sublimation rate under given conditions. Results were compared with AFD experimental data of 10-mm Idared apple cubes. The viability of applying the film sublimation model to atmospheric freeze-drying process was demonstrated. Higher mass flux was found on the leading edge, relatively uniform mass flux within the porous zone illustrates that vapor diffusion dominates atmospheric freeze drying process at low temperature (below 0°C). CFD results for apple cubes show a predomination of diffusional resistance of porous tissue.     

Neutron Radiography Applications in Studies of Drying of Capillary-Porous Systems

The application of modern digital neutron radiography in the study of the drying process in porous media is demonstrated with two simple examples of cylindrical and rectangular samples made of different materials. The statistical method of image analysis is presented and the standard deviation of grey scale values (referred to brightness) is shown to be a useful measure of receding drying front emergence. The motion of the drying front was analyzed and found neither linear nor proportional to the square root of time and was delineated with nonlinear functions different for each sample shape.     

Ceramic Bodies with Complex Geometries and Ceramic Shells by Freeze Casting Using Ice as Mold Material

Ceramic bodies with a complex shape and closed ceramic shells encapsulating other components like steel parts were fabricated by the freeze-casting technique using ice as a mold and as core material. The ice molds and cores were simultaneously removed with the frozen suspension liquid of the ceramic slip by subsequent freeze drying. In this way, cores can be eliminated from a closed shell by sublimation through the porosity of the shell. Moreover, the ice cores allowed to transfect other components into porous ceramic bodies. Complex dental parts such as steel rods encapsulated in porous zirconia shells achieved by this ice mold freeze casting are represented in this article.     

非饱和含湿多孔介质微波冷冻干燥过程传热传质分析

基于升华冷凝模型,对非饱和含湿多孔介质微波冷冻干燥过程作了数值计算．结果表明,干燥过程中不饱和含冰区内的冰饱和度有较大变化．通过与不考虑升华冷凝区相比较,表明升华冷凝区的存在不可忽略．     

OSMOTIC DRYING KINETICS OF CYLINDRICAL APPLE SLICES OF DIFFERENT SIZES

The objective of this study was to evaluate the osmotic drying kinetics of cylindrical slices of apples as influenced by particle size at different concentrations of sucrose solutions and different temperatures. Osmotic drying was carried out, with cut apple cylinders of three different sizes (12, 17 and 20mm diameter), all with a length to diameter ratio of 1:1, in a well agitated large tank containing the osmotic solution at the desired temperature. The solution to fruit volume ratio was kept greater than 60. A central composite rotatable design (CCRD) was used with dive levels of sucrose concentrations (34-63°Brix) and five temperatures (34-66°C). Kinetic parameters included weight loss, moisture loss, solids gain, rates of moisture loss and solids gain generally increased with increasing treatment time, temperature and concentration of osmotic solution, and decreased with an increase in sample size. The parameter “osmotic drying time to achieve a given moisture loss” obviously showed the opposite. Composite models were developed to describe the effect of process variables and particle size on the drying behavior of apple slices.