具有预制孔隙多孔介质冷冻干燥的多相传递模型

基于局部质量非平衡假设,建立了多相多孔介质热、质耦合传递数学模型,理论验证具有预制孔隙的初始非饱和多孔物料对冷冻干燥过程的强化作用。模型考虑了多孔介质的吸湿效应,构建了3种吸附-解吸平衡关系。模型使用基于有限元法的COMSOL Multiphysics软件平台数值求解,并与实验数据进行了比较。结果表明,初始非饱和冷冻物料能够有效地强化冷冻干燥过程。采用不同函数形式的吸附-解吸平衡关系模拟的干燥曲线均与实验数据非常吻合。通过分析物料内部的饱和度、温度和质量源分布,探讨了初始非饱和物料冷冻干燥过程的传热传质机理。初始非饱和物料的干燥速率控制因素主要是传热。模拟考察环境辐射温度对冷冻干燥过程影响的结果表明,所建模型具有良好的预测能力。     

多孔介质内部传热传质规律的研究进展

多孔介质中的热质交换理论及其实验研究是一个涉及面广、研究难度大而又颇具工程应用价值的课题。本文对多孔介质内部传热传质规律的研究从理论和实验两方面作了简要的综述及展望,并对一些描述热质传递过程的数学模型及方程作了介绍。     

固定床内石灰石煅烧与传递过程的数值模拟

将固态颗粒填料床视为由规则颗粒堆积而成的松散多孔介质,在推导出传热控制机制下微元体综合速率的基础上,采用局部热不平衡假设建立了一维固定床中石灰石热分解反应与传热、传质耦合的数学模型,运用有效容积法对其离散求解,并采用文献中实验数据对模型进行验证. 计算了不同条件下颗粒物料层内气体和固体骨架的温度场、产物气体浓度场以及固体转化率分布,以得到多孔介质体系内固有化学反应时的传热传质规律. 研究结果对具有强吸热反应的固定床反应器的设计和运行具有一定的参考作用.     

非热平衡多孔介质内反应与传热传质耦合过程

采用局部热不平衡假设,对发生强吸热化学反应的多孔介质体系建立了反应与传热、传质耦合问题的数学模型,采用Ergun-Forchheimer-Brinkman方程描述多孔介质中的流体流动.运用交替方向隐式(ADI)方法对模型离散求解,并采用文献中的实验数据对模型进行验证.计算了不同条件下颗粒物料层内气体和固体骨架的温度场、产物气体浓度场以及固体转化率分布,以得到多孔介质体系内固有化学反应时的传热、传质规律.结果表明,不能忽略固体骨架与流体间的温度差.入口渗流速度、入口气体温度以及固体颗粒尺寸是影响系统反应特性的重要参数.研究结果对具有强吸热反应的固定床反应器的设计和运行具有一定的参考作用.     

多孔介质内部热质传递的等效耦合扩散模型的推导及其应用

基于Whitaker的体积平均方程，在不附加任何新的假设的基础上，对多孔介质内部热质传递的等效耦合扩散模型进行推导，得出了多孔介质内部热质传递的等效耦合扩散模型。并应用该模型对瓷质砖体干燥过程进行了数值模拟，模拟结果与实验结果十分吻合。     

降膜结晶分离过程热、质及动量传递的数值模拟(Ⅰ)分区域统一模型

基于混合物理论和实验观察所获得的有关结晶形态及形貌特征 ,建立了描述降膜结晶过程液相区、枝晶状固液两相共存区及固相区的动量、热量和质量传递过程的分区域统一数学模型 .模型的一个重要特点是认为在降膜结晶过程中 ,于熔体液相区和固相区之间存在一薄厚不匀且不断变化的固液两相共存区 .随着结晶过程的进行 ,二相区内固相体积分数不断增加并不断向液相区推移直至完全固化 .二相区的存在为降膜结晶时含杂质低纯度液体提供了在晶层内栖身的场所 ,这可能是产生杂质包藏的实际原因。引入多孔介质传递理论并采用Brinkman扩展的非稳态Darcy方程来描述固液两相区多孔介质内的流体流动 .进一步 ,采用VanDriest修正的Prandtl混合长度模型来描述降膜结晶过程中液体区的湍流流动与传热传质     

多孔介质内复合对流传热传质的数值分析

采用数值方法分析了具有非均匀内热源的竖直套管中复合对流传热传质,考查内热源分布系数M和热质二浮力比N对速度、温度、浓度分布以及Nusselt数和Sherwood数的影响。结果表明：当N〉1时速度V为正,其值随N的增加而增大;当N〈-1.5时,V则先负后正。随着M增大,等温线和等浓度线分布更加密集,传热和传质过程更加明显。     

Technical and economic analysis of solvent-based lithium-ion electrode drying with water and NMP

Processing lithium-ion battery (LIB) electrode dispersions with water as the solvent during primary drying offers many advantages over N-methylpyrrolidone (NMP). An in-depth analysis of the comparative drying costs of LIB electrodes is discussed for both NMP- and water-based dispersion processing in terms of battery pack $/kWh. Electrode coating manufacturing and capital equipment cost savings are compared for water vs. conventional NMP organic solvent processing. A major finding of this work is that the total electrode manufacturing costs, whether water- or NMP-based, contribute about 8–9% of the total pack cost. However, it was found that up to a 2?×?reduction in electrode processing (drying and solvent recovery) cost can be expected along with a $3–6?M savings in associated plant capital equipment (for a plant producing 100,000 10-kWh Plug-in Hybrid Electric Vehicle (PHEV) batteries) using water as the electrode solvent. This paper shows a different perspective in that the most important benefits of aqueous electrode processing actually revolve around capital equipment savings and environmental stewardship and not processing cost savings.     

Microwave drying process of corns based on double-porous model

Current microwave drying of food product models have generally neglected mass transfer physics, which have resulted in higher predicted temperatures within the food matrix. It is necessary to include mass transfer physics in modeling microwave drying of food products, especially when they are dry for longer durations. In the actual drying process, most of the dry material is a double-porous media formed by the mutual accumulation of porous media, and there is no systematic theory to analyze the drying process of double-porous media. This work presents a new theoretical model based on the ractive pellet bed to achieve the simulation of microwave drying in situation of stacked corns. Compared with the applicable to porous model and analyzed the difference, finally through the change of temperature and moisture was measured by experiments to verify the rationality and accuracy of the double-porous model.     

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。     

干燥过程热质传递的简化模型

对多孔介质体积平均理论进行简化,得到简化的热质传递干燥模型,用一个理论式表达模型中有效扩散系数。对香蕉片千燥的研究表明,模型的预测值与实验结果相     

多孔介质对流干燥机理及其模型

在对现有的多孔介质对流干燥传热、传质模型归类分析的基础上,从介质内部热湿迁移机制出发,建立了能较完善、较准确地描述多孔介质在恒速段及降速段热质传递规律的“三耦合-六场量”混合理论模型.同时针对干燥问题数值模拟中的移动边界问题,提出了一种迭代修正的思想,并发展了相应的数值计算方法.对砖的干燥模拟计算结果表明,本文的模型较其他模型具有更好的精确性.     

具有初始孔隙的多孔物料冷冻干燥

实验研究了具有一定孔隙的非饱和多孔物料对液体物料冷冻干燥过程的影响。以甘露醇为主要溶质的待干料液采用“液氮制冰激凌法”制备非饱和物料进行冷冻干燥,并与常规饱和的冷冻物料相比较。结果表明,非饱和冷冻物料确实能够显著地强化液体物料的冷冻干燥过程。干燥产品SEM形貌分析显示,初始非饱和冷冻物料具有连续均匀的固体骨架和孔隙,固体基质更加纤细,孔隙空间更大,可以大大减小传质阻力。考察物料内部各点的温度变化发现,初始非饱和物料内部冰晶确实发生整体升华,但仍然存在主要升华区域;非饱和多孔物料的冷冻干燥过程主要是传热控制,而常规饱和物料冷冻干燥主要是传质控制。操作压力对过程的影响可以忽略。采用辐射/导热组合加热方式可改善初始非饱和多孔物料冷冻干燥过程的传热,进一步缩短干燥时间。     

Thermophysical Characteristics of Potatoes,Vegetables and Fruits

ABSTRACT

The main objective in this work is to study and deduce a governing equation for net mass transfer in moist air and turbulent flow. Development of simple and reliable steady state models for turbulent moist air-drying has been considered to be quite well covered in literature. However, the lack of necessary background information concerning classical drying models is now being rectified through research carried out with new approaches, which are initiated by advancement in laboratory equipment.

The known and trusted models are combined with coupled momentum, heat and mass transfer equations creating a reliable governing mass transfer equation for use in turbulent moist air drying processes, i.e. the advanced drying model (ADM). The ADM is a relatively user friendly and robust model, and it is well-suited for identifying transfer coefficients from boundary layer measurements, for example in modem high intensity paper drying machines.

The advanced drying model is analysed and verified with the specially designed experimental apparatus described in this article. The deduced mass transfer equation is then presented and experimentally verified to clarify why the use of Stefan's diffusion equation should be avoided when calculating high drying intensities in turbulent flow.

Finally, when applied to a wide drying range, the classical drying models require parameters which have been experimentally verified. Therefore, a comprehensive knowledge of governing mass transfer mechanisms will also reduce the large number of necessary drying experiments. The advanced drying model, which includes variable physical properties and transport coefficients, allows the simulation of many geometrical shapes and drying configurations and therefore provides a tool for optimising drying processes in a new manner.     

分形多孔介质传热传质过程的格子Boltzmann模拟

针对自然界中实际多孔介质具有的分形特性和随机性,利用中点替代算法和二值化处理构造统计上具有分形特性的随机多孔介质。分析了所构造的多孔介质盒维数与Hurst指数之间的关系。基于随机分形构造的原理,对二维实际多孔介质图像进行了重构。利用两点相关函数,分析了重构图像的结构相关性, 并与实际目标多孔介质的结构特征进行比较。在与解析解对比验证的基础上,将基于二元混合理论的格子Boltzmann模型（LBM）用于模拟多孔介质内流体扩散过程。通过计算不同分形特性的二维多孔介质的有效扩散系数,研究了重构多孔介质的分形维数与有效扩散系数的关系。利用热耦合LBM模型计算多孔介质内传热过程,分析了不同的分形特性对多孔介质蓄热过程的影响。     

Mathematical Modeling of Drying Processes Using Methods of the Nonequilibrium Thermodynamics and Percolation Theory

A twofold refinement of the basic mathematical model for describing a coupled heat and mass transfer taking place in porous media is presented. The common application of irreversible thermodynamics and fluctuation theory of phase transitions is proposed for calculating the moisture level and temperature. Instead of parabolic partial differential equations, hyperbolic type partial differential equations are used. The relaxation time constants, whose percolation state-dependence is also taken into account, are incorporated into this formalism. Some possible new research domains in mathematical and statistical physics are also indicated.     

颗粒填充液冷式散热器的传热及流阻性能

通过在液冷式CPU散热器蛇形流道内填充不同粒径的玻璃珠和碳钢珠,对改进前后液冷式CPU散热器进行实验研究,分析了各因素对液冷式CPU散热器的传热和流阻性能的影响规律。结果表明:散热器填充玻璃珠后的对流换热系数为填充前的1.4—2.5倍,阻力系数f是填充前的4—10倍;散热器填充碳钢珠的散热效果和流阻性能均优于填充玻璃珠的,其Nusselt数是填充玻璃珠的1.5—2倍,而流动阻力仅是填充玻璃珠的0.5—0.7倍。     

Water loss and crust formation during bread baking,Part I: Interpretation aided by mathematical models with highlights on the role of local porosity

This study aimed to achieve greater understanding of the contributions of different mechanisms of water transport to the overall amount of water lost by bread loaves during baking. The analysis was based on simulations from a previously published baking model which takes into account heat and mass transport as well as the expansion of the gaseous phase leading to oven rise. Liquid and gaseous diffusion, convective transport through the gaseous phase (Darcy’s law), and the diffusive transport of vapor (evaporation–condensation–diffusion) were considered. When only permeation (Darcy’s law) prevailed, the greater the porosity of the top dough surface, the lower and slower the water loss (WL): in fact, the greater the porosity, the thicker the top area. This in turn limited the rate at which heat penetrated the top layers down to the vaporization front and slowed WL. At earlier baking times, when the ebullition temperature was not reached uniformly through the crumb, the evaporation–condensation–diffusion was involved at the same time as permeation. The water flux by evaporation–condensation–diffusion was orientated toward the dough core; it accelerated the crust thickening and slowed WL. It appeared that the higher the porosity in the crumb beneath the crust, the higher the flux by evaporation–condensation–diffusion, and the lower the WL. Local porosity variations in the crust (top surface) and the crumb beneath the crust could be reproduced experimentally by constraining the total expansion of the loaf with a permeable fabric cover. Experimental and simulated trends in WL were consistent, supporting the understanding of WL proposed.     

流化床氛围下多孔物料干燥传热传质的数值模拟

用有限差分法数值求解一个热、质传递耦合模型,理论研究多孔物料流化床干燥过程。方程离散采用全隐格式的控制容积方法,三对角矩阵法（TDMA）用来求解线性方程组。选用球形的苹果丁作为多孔物料。在典型操作条件下,通过分析温度、饱和度和压力的分布侧形,讨论了物料内部的热、质传递机理。在对比条件下,考察了气体入口温度、气速和床面积因子对干燥过程的影响。结果表明：干燥过程受气、固相间的耦合传热传质的影响十分明显,干燥时间随气体入口温度和气速的提高而减少;随床面积因子的增大而增加。     

非均质移动颗粒床的渗流换热模型与数值模拟

针对非均质移动颗粒床的换热过程,建立了其渗流换热的数学模型,提出了COADI-GS算法,采用此算法对移动颗粒床的气固换热过程进行流热耦合数值模拟。通过对均质与非均质这两种不同移动颗粒床换热模型的仿真结果进行比较,发现两者的床内温度分布有较大差异,非均质移动颗粒床模型能更为准确地描述床层内的渗流换热过程。计算结果表明,供风压力的增加虽有利于熟料颗粒的冷却,但会降低回收风的温度;颗粒温度随床层移动速度的增加而升高,近似为线性关系。