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。     

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。     

一种多孔介质有效扩散系数和孔隙率的测定方法

依据拟稳态扩散原理,通过物料衡算,建立了用蒸发管法测定多孔介质有效扩散系数及孔隙率的实验方法。在水—空气系统、乙醇—空气系统拟稳态扩散模型的基础上,分别对水蒸汽、乙醇蒸汽通过玻璃管内自由空气段和多孔介质段的扩散进行物料衡算,得到在常压下液体蒸汽在自由空气中和在多孔介质内的扩散系数,进而计算得到多孔介质的孔隙率。实验结果表明:蒸发管法测定的有效扩散系数及孔隙率与文献值的平均相对误差小于4%,这是一种简单有效地测定多孔介质扩散系数的方法,具有无毒、成本低,无需特殊设备等优点,为后续有效扩散系数的测定奠定了坚实的基础,有很强的实用性。     

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。     

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。     

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。     

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。     

明胶热风干燥特性的实验研究

采用静态法研究了明胶的平衡含湿质量分数,得到了20℃下吸湿和解吸等温线,结果表明,当空气相对湿度在16%—39%,存在吸湿滞后现象。在对流干燥实验台上进行了明胶干燥特性的实验,以不同厚度的明胶块为实验对象研究了热风温度、风速、湿度对干燥过程的影响。实验结果表明:明胶的干燥过程只有降速阶段,提高热风温度、加大风速均可以在前期提高干燥速率,但在干燥后期干燥速率反而降低;明胶块中心温度受胶块厚度、热风温度影响较大,而在实验范围内空气相对湿度的变化对明胶中心温度影响甚微;明胶的相对含湿质量分数随时间呈指数规律下降,提高风温、加大风速后明胶含湿质量分数在开始阶段下降较快,但最终含湿质量分数反而偏高。     

气流干燥含湿氧化铝颗粒传热传质数值模拟

基于二相流理论,提出了一个描述含湿氧化铝颗粒气流干燥过程的一维数学模型。模型考虑了干燥管内气固二相间的传热传质和动量传递、含水质量分数和气固二相温度的变化。根据B ird所提出的努赛尔数经验公式,利用标准四阶龙格-库塔法对由非线性常微分方程组成的气流干燥耦合模型进行计算,得到了含湿氧化铝颗粒在不同气流干燥条件的干燥曲线。整个干燥过程中数值计算结果与试验实测数据吻合得很好,可以用来预测含湿氧化铝颗粒的干燥含水质量分数。     

Multiscale and multilayer structural modeling and simulation on drying of grain packing porous media

ABSTRACT

Aiming at the problem of multilayer physical structure for the skeleton of porous media, a multiscale and multilayer structural model of heat and mass transfer processes for drying of grain packing porous media was established by applying the pore network method and multiscale theory. An experimental study on rice drying was conducted in order to validate this model. The simulation and experimental results indicated that the established model could explain the mechanical properties of rice drying well. The rate of heat transfer was faster than the rate of mass transfer and there was a higher moisture gradient inside the rice grain. The diffusion coefficient of rice embryo played an important role in the drying process, and whose effect on drying was larger than the diffusion coefficient of rice hull and chaff. The moisture was imprisoned effectively inside the rice when the diffusion coefficient of rice embryo was very small.     

BOUND MOISTURE REMOVAL: HEAT AND MASS TRANSFER IN CONTACT DRYING

Coupled heat and mass transfer in short-term contact of the moist material and the heating surface (the physical model of drying with agitation) is examined. Technological characteristics of the drying process: heating rate and drying rate, heat transfer coefficient, etc. have been determined based on solutions of the diffusion and diffusion-filtration heat and mass transfer. The usage of non-field method of determination of mass and heat fluxes on the phase interface allows calculation of the drying equipment efficiency without preliminary determination of the fields of required quantities. The results may be used for estimation of the influence of drying conditions and material properties on the moisture removal process.     

BOUND MOISTURE REMOVAL: HEAT AND MASS TRANSFER IN CONTACT DRYING

ABSTRACT

Coupled heat and mass transfer in short-term contact of the moist material and the heating surface (the physical model of drying with agitation) is examined. Technological characteristics of the drying process: heating rate and drying rate, heat transfer coefficient, etc. have been determined based on solutions of the diffusion and diffusion-filtration heat and mass transfer. The usage of non-field method of determination of mass and heat fluxes on the phase interface allows calculation of the drying equipment efficiency without preliminary determination of the fields of required quantities. The results may be used for estimation of the influence of drying conditions and material properties on the moisture removal process.     

CROSS-EFFECT OF HEAT AND MASS TRANSFER OF LU1KOV EQUATION: MEASUREMENT AND ANALYSIS

This paper mainly focuses on cross-effect of heat and mass transfer of capillary porous media which A.B.Luikov set up on irreversible thermodynamics principle. On the basis of perfecting the equations of heat and mass transfer, the heat and mass transfer parameters are determined during drying processes, and thermal gradient coefficient δ and moisture gradient coefficient ξ are obtained which show the cross-effect of heat and mass transfer. Thus the fundamentals are provided for quantitative analysis of cross-effect of heat and mass transfer. The convective drying mathematical model under the first unsteady boundary condition is therefore proposed. By the application of Henry transform, the theoretical solution of unsteady drying process is given and its validity is verified     

CROSS-EFFECT OF HEAT AND MASS TRANSFER OF LU1KOV EQUATION: MEASUREMENT AND ANALYSIS

Abstract

This paper mainly focuses on cross-effect of heat and mass transfer of capillary porous media which A.B.Luikov set up on irreversible thermodynamics principle. On the basis of perfecting the equations of heat and mass transfer, the heat and mass transfer parameters are determined during drying processes, and thermal gradient coefficient δ and moisture gradient coefficient ξ are obtained which show the cross-effect of heat and mass transfer. Thus the fundamentals are provided for quantitative analysis of cross-effect of heat and mass transfer. The convective drying mathematical model under the first unsteady boundary condition is therefore proposed. By the application of Henry transform, the theoretical solution of unsteady drying process is given and its validity is verified     

NONLINEAR FINITE ELEMENT ANALYSIS OF COUPLED HEAT AND MASS TRANSFER PROBLEMS WITH AN APPLICATION TO TIMBER DRYING*

A finite element formulation and solution of a set of nonlinear coupled heat and mass transfer equations for porous capillary media is presented. The model considers temperature and moisture dependent material properties and can accomodate diffusion of moisture as either a liquid or a vapor. Application was made to drying of timber and predicted results agreed well with the experimental data.     

NONLINEAR FINITE ELEMENT ANALYSIS OF COUPLED HEAT AND MASS TRANSFER PROBLEMS WITH AN APPLICATION TO TIMBER DRYING*

A finite element formulation and solution of a set of nonlinear coupled heat and mass transfer equations for porous capillary media is presented. The model considers temperature and moisture dependent material properties and can accomodate diffusion of moisture as either a liquid or a vapor. Application was made to drying of timber and predicted results agreed well with the experimental data.     

The Role of Nonuniformity in Convective Heat and Mass Transfer through Porous Media,Part 1

Through-air drying is commonly used in the drying of high-quality tissue and towel products. A representative elementary volume method was used to model the fluid flow and heat and mass transfer during through drying in heterogeneous porous biobased materials such as tissue and towel products. Results of flow both upstream and downstream of a modeled porous sheet allowed visualization of the effects of mixing at the top and bottom of the porous medium. The effect of initial nonuniformity on fluid flow and convective heat and mass transfer in heterogeneous porous media was studied. The effect of material nonhomogeneity and associated transport properties on moisture content of the porous material as a function of drying time was studied. Modeling results indicate that for the first time it is possible to simulate the effect of nonuniformity on fluid flow and convective heat and mass transfer in porous media during through-air drying of paper. Moisture and structural nonuniformity contributing to nonuniformity in air flow might contribute significantly to drying nonuniformity. Depending on the moisture regimes and degree of saturation of the convective medium, heat and mass transfer coefficients may have varying effects on the overall drying.     

Lumped- Differential Formulations for Drying in Capillary Porous Media

Simultaneous heat and mass transfer, during drying of moist capillary porous media, is modeled through different lumped-differential formulations, obtained from spatial integration of the original set of Luikov's equations for temperature and moisture potential. Besides the classical lumped system analysis, improved formulations are proposed, of same mathematicai simplicity, based on approximate evaluations of both temperature and moisture gradients within the medium. The relative merits of such improved zero-dimensional formulations are established, through critical comparisons against analytical solutions for the full partial diflerential system, over a wide range of the governing parameters.     

Free Heat and Mass Transfer in a Porous Enclosure with Side Vents

Free heat and mass transfer during drying in a porous enclosure with free vents has been investigated numerically. Enclosed moist air interacts with the surrounding air through freely vented ports situated on both sides perpendicular to the heated wall. Air, heat, and moisture transport structures are visualized respectively by streamlines, heat lines, and mass lines. Effects of thermal Rayleigh number, Darcy number, vent location, and enclosure inclination on the convective heat/moisture transfer rate and volume flow rate across this enclosure are discussed. For each case, partially enclosed fluid flow undergoes different phases, increasing with buoyancy ratio; that is, heat transfer–driven flow, heat- and moisture-aided flow, and moisture transfer–dominated flow. Numerical results demonstrate that the convective heat and moisture transport patterns and transport rates greatly depend on thermal Rayleigh number, properties of porous medium, and enclosure inclination. Practices for enhancing heat and moisture transfer have been suggested for drying processes.