The Role of Nonuniformity in Convective Heat and Mass Transfer through Porous Media,Part 2

Nonuniformities in porous materials can play a significant role in the convective and diffusive transport of fluid, heat, and mass. This study provides experimental results and corresponding numerical simulation results. The experiments report continuous data in transient test runs with measurements of temperature and velocity at distributed locations in the domain immediately downstream of the porous materials. An increasing degree of nonuniformity was found to produce a lower drying rate as well as an earlier onset of falling rate drying. The numerical model provides results of the effects of different nonhomogeneities such as distributed holes or distributed regions of varying permeability and moisture content. Comparisons between numerical and experimental test results indicate general agreement with differences with regard to details of the drying curve features. This provides a tool for studying the role of nonuniformity in fluid flow and heat and mass transfer in porous media.     

SUPERHEATED STEAM DRYING: A BIBLIOGRAPHY

An experimental setup was developed to study the through air–drying characteristics of permeable grades such as tissue and towel under commercially relevant conditions of basis weight, airflow rate, temperature, and humidity conditions. The experimental setup is capable of evaluating the transient fluid flow, heat, and mass transfer characteristics of relatively larger samples (TAPPI standard hand sheets; 0.1524 m) and is capable of studying the effect of local heterogeneity and structure on convective heat and mass transfer. The system is capable of airflow rates of 0.5–10 m/s with corresponding high-speed data collection and acquisition for measuring important variables such as exhaust air humidity. To study the effect of nonuniformity, local temperature and velocity profiles can also be measured using grid of thermocouples and hot wire anemometers. The instantaneous drying rate and airflow characteristics during through air drying was measured and dry permeability, wet permeability, and convective heat and mass transfer characteristics were then calculated. The experimental results were verified by comparing with the results from literature. Typical experimental results were presented to show the effect of sheet basis weight, initial moisture content, and airflow rates on the drying characteristics for two different types of paper samples.     

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.     

Through Air Drying

Process of through-air-drying is becoming increasingly popular in the manufacture of textiles, non-wovens, tissue, and towel. Very high drying rates, enhanced product properties, i.e., softness, bulk, absorbency, unique 3D structure are the driving forces behind its increasing popularity. In this article, experimental results on convective heat and mass transfer and fluid flow characteristics of tissue and towel products using commercially realistic structures are presented. Comparison with literature data using wet pressed, dried, rewetted sheets indicate significant differences in drying and permeability characteristics confirming that the internal structure of the material does indeed play a significant role in through-air-drying and should be taken into account in modeling, optimization, and control of commercial systems.     

GRANULATION AND DRYING OF POWDERY OR LIQUID MATERIALS BY FLUIDIZED-BED TECHNOLOGY

Abstract

Process of through-air-drying is becoming increasingly popular in the manufacture of textiles, non-wovens, tissue, and towel. Very high drying rates, enhanced product properties, i.e., softness, bulk, absorbency, unique 3D structure are the driving forces behind its increasing popularity. In this article, experimental results on convective heat and mass transfer and fluid flow characteristics of tissue and towel products using commercially realistic structures are presented. Comparison with literature data using wet pressed, dried, rewetted sheets indicate significant differences in drying and permeability characteristics confirming that the internal structure of the material does indeed play a significant role in through-air-drying and should be taken into account in modeling, optimization, and control of commercial systems.     

Mass Transfer Coefficients and Drying Rates in Through Air Drying: Part II

In this study experimental results and analysis are given of convective heat and mass transfer in the drying of tissue and towel products (25 and 50 g/m² basis weight) under commercially realistic conditions under through flow. Parameter effects on drying rate include process variables such as refining, shaping, commercial forming, initial moisture content, and drying temperature. Drying rate curves display initial increase, plateau, and final decrease as the paper dries. Nondimensional Sherwood numbers (versus Peclet number) generally agreed with and augment the available dataset on through drying.     

Integral Flow Parameters and Material Characteristics Analysis in Through Air Drying: Part I

The extension of the Darcy law (the Forchheimer flow equation) relating second-order nonlinear pressure drop with flow velocity is studied during fast transient through air drying of sheets of porous biobased materials such as paper. A range of the paper materials with open structure consistent with tissue and towel products (basis weights 25 and 50 g/m²) made using different production processes are analyzed for the factor-specific influences with regard to changes in the fluid resistance from the removal of moisture from the material interstices. A characteristic dimension suitable for the drying process is applied from viscous and inertial momentum transport analysis.     

COMBINED MICROWAVE AND CONVECTIVE DRYING OF A POROUS MATERIAL

A model is formulated to describe the drying of a slab of porous material in a combined microwave and convective environment. The model describes the evolution of temperature, pressure, moisture and power distributions that occur during the drying process. The microwave internal heat source is calculated from electromagnetic theory with varying dielectric properties. The inclusion of pressure in the model allows the physical phenomena of “water pumping”, often observed in microwave drying systems, to be accounted for. The influence of sample size; on the drying kinetics 1s examined and found to be an important parameter during the drying process. In particular the effect of resonance on the moisture and temperature profiles and the need for careful consideration of surface mass transfer coefficients are investigated. Simulation results are presented for the combined microwave and convective drying of a homogeneous, isotropic porous material.     

COMBINED MICROWAVE AND CONVECTIVE DRYING OF A POROUS MATERIAL

A model is formulated to describe the drying of a slab of porous material in a combined microwave and convective environment. The model describes the evolution of temperature, pressure, moisture and power distributions that occur during the drying process. The microwave internal heat source is calculated from electromagnetic theory with varying dielectric properties. The inclusion of pressure in the model allows the physical phenomena of “water pumping”, often observed in microwave drying systems, to be accounted for. The influence of sample size; on the drying kinetics 1s examined and found to be an important parameter during the drying process. In particular the effect of resonance on the moisture and temperature profiles and the need for careful consideration of surface mass transfer coefficients are investigated. Simulation results are presented for the combined microwave and convective drying of a homogeneous, isotropic porous material.     

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     

Two Dimensional Heat and Mass Transfer During Convective Drying of Porous Media

In this paper, we study numerically two-dimensional heat and mass transfer during convective drying in porous media. The set of macroscopic equations is very comprehensive and takes into account the effect of gaseous pressure. A numerical code has been established. This code has allowed us to determine t.he space-time evolution of the temperature, the total pressure and the moisture content.     

纸张干燥中湿分扩散系数的估算

纸张干燥过程涉及到多孔介质的热质传递,如何确定质量扩散系数是所建立的多孔物料湿分扩散模型能否进行数值计算的关键。按Liukov公式将湿分扩散系数视为含湿质量分数的非线性函数,在恒温下进行纸张干燥实验,通过比较湿分蒸发质量的测量值与理论计算值,采用多变量寻优的方法对多孔介质一维情况下的湿分扩散系数进行估算,得到了实验条件下的纸张湿分扩散系数的计算公式。并进一步确定了纸张中的湿分扩散系数与含湿质量分数和温度之间的函数关系式。     

A Moisture Transmembrane Transfer Model for Pore Network Simulation of Plant Materials Drying

The drying of plant materials with cellular tissue is often viewed as drying of porous media that is assumed to consist of cell cytoskeleton and intercellular space. Various approaches have been reported in the literature to describe heat and mass transfer during drying of such porous materials. However, the fact remains that the water in a cellular tissue is mostly intracellular and it should be driven out of the cells across cell membranes before transporting in cell gaps, as in a general porous media. In the present study, the transport process of moisture in a cellular tissue was analyzed. A mathematical model for moisture transport across the cell membrane was established, which was correlated to a self-developed, dual-scale pore network model (cell and pore network) for drying of plant materials. The relationship between mass volumetric flux and average intracellular moisture content was developed based on the microscopic images and the drying experiments.     

DISCONTINUOUS VACUUM DRYING OF OAK WOOD: MODELLING AND EXPERIMENTAL INVESTIGATIONS

Abstract

Vacuum drying and especially discontinuous vacuum drying is a very attractive process for such a wood as oak, because of a reduced drying time and a high quality of the final product. In this paper, a model describing heat, mass and momentum transfer in a capillary porous and hygroscopic medium under vacuum drying with no external heating agent (the material is pre-heated during a convective phase), is presented. The choice of the following three independent unknowns, moisture content, enthalpy and dry air density, leads to a fully well described problem where the effect of gaseous pressure inside the material is taken into account. A new formulation of the boundary conditions and its numerical resolution are validated by experimental results.     

Heat Transfer Between a Porous Layer and a Forced Flow: Influence of Layer Thickness

The heat transfer between a forced flow of a pure fluid phase and a porous medium is theoretically studied. A unified enthalpy model is used to investigate the transient temperature field in the pure fluid phase and in the porous medium. Possible transition from saturated to nonsaturated porous medium is considered. From the temperature field obtained, the time variation of the local temperature and convective heat transfer coefficient at the interface porous-pure fluid regions are studied. Numerical results provide also the parametric information concerning effects of the thickness of the porous layer on theses parameters at the transient and the steady regimes.     

表面辐射对部分填充吸湿性多孔介质的封闭腔体内热湿耦合传递的影响

根据多孔介质热质传递原理, 基于有限元的方法数值分析了具有表面热辐射的部分填充吸湿性多孔介质的封闭腔体内部自然对流流动及热湿耦合传递过程, 探讨了表面发射率、Rayleigh数和Darcy数等参数对封闭腔体内部自然对流流动及热湿耦合传递过程的影响, 研究结果表明, 壁面热辐射的作用可以提高多孔介质内部的温度, 而且随着表面发射率的增大, 多孔介质内部的水分逐步向其右上角迁移和聚集。另外, Darcy数、多孔介质与空气的热导率比对方腔内部多孔介质的热量传递和水分迁移影响较小。     

Finite Element Analysis of a Moving Boundary Value Problem

A finite element formulation and the solution of a set of nonlinear coupled heat and mass transfer equations for a two-phase system with a moving evaporation interface is presented. The interface condition takes into account the moisture transfer balance at the moving boundary. The finite element results were compared with existing results for a single phase system for model validation. In the two-phase system, the movement of evaporation front has an appreciable effect on the temperature and moisture distribution inside the porous medium during drying. The effect of the nondimensional heat of vapourization parameter γ on the evaporation front, temperature and moisture distribution in porous medium was studied. The higher the value of γ, the slower is the movement of the evaporation front. The temperature decreased and the moisture content increased as the nondimensional vapourization parameter γ increased. This model has potential applications in studying the heat and mass transfer characteristics in food and biomaterials.