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.     

Conjugate heat and mass transfer model for predicting thin-layer drying uniformity in a compact,crossflow dehydrator

Abstract

A conjugate heat and mass transfer model was implemented into a commercial CFD code to analyze the convective drying of corn. The Navier–Stokes equations for drying air flow were coupled to diffusion equations for heat and moisture transport in a corn kernel during drying. Model formulation and implementation in the commercial software is discussed. Validation simulations were conducted to compare numerical results to experimental, thin-layer drying data. The model was then used to analyze drying performance for a compact, crossflow dehydrator. At low inlet air temperatures, the drying rate in the compact dehydrator matched the thin-layer drying rate. At higher temperatures, heat losses through the external walls resulted in temperature and moisture variations across the dehydrator.     

A Theoretical Model for the Nongray Radiation Drying of Polyvinylalcohol/Water Solutions

A theoretical analysis of heat transfer and moisture variation was performed while a PVA solution was exposed to high-intensity nongray irradiation and/or air flow convection. Effective absorption coefficients were incorporated in the radiative transfer analysis. The influence of various radiation and convection parameters on the transfer of heat and moisture variation in the coated layers on an optically thick substrate was investigated. The effects of radiation and convection parameters on the transfer process were presented in terms of the rate of water content removal, heat transfer, and moisture distribution. Results were compared to those of drying when using convective heat. It is evident that the use of thermal radiation combined with convective heat will help in improving the drying rate. Numerical results show that both the radiative energy absorbed by the solution and the substrate and the distribution of water mass fraction in the solution are closely related to the rate of water removal from the solution during the process.     

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。     

A Theoretical Model for the Nongray Radiation Drying of Polyvinylalcohol/Water Solutions

Abstract

A theoretical analysis of heat transfer and moisture variation was performed while a PVA solution was exposed to high-intensity nongray irradiation and/or air flow convection. Effective absorption coefficients were incorporated in the radiative transfer analysis. The influence of various radiation and convection parameters on the transfer of heat and moisture variation in the coated layers on an optically thick substrate was investigated. The effects of radiation and convection parameters on the transfer process were presented in terms of the rate of water content removal, heat transfer, and moisture distribution. Results were compared to those of drying when using convective heat. It is evident that the use of thermal radiation combined with convective heat will help in improving the drying rate. Numerical results show that both the radiative energy absorbed by the solution and the substrate and the distribution of water mass fraction in the solution are closely related to the rate of water removal from the solution during the process.     

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.     

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.     

封闭腔内水自然对流换热数值模拟

为了揭示封闭腔内非Boussinesq流体在浮力驱动下所特有的流动换热现象和形成机理,采用CFD软件Fluent对封闭腔内水的自然对流进行数值模拟,得到矩形封闭腔高宽比、Rayleigh数、倾斜角度、壁面温度差对流动和传热的影响规律。研究结果表明：由于水的密度在3.98℃达到最大,两竖壁面温度跨越这一点时会引起流动图像反转;具有流动反转的双涡结构降低了对流换热平均Nusselt数;相同Rayleigh数下,高宽比为1对应对流换热平均Nusselt数最大值;倾斜角度对平均Nusselt数影响与Rayleigh数和温度边界条件有关。     

Numerical Study on Moisture Transfer in Ultrasound-Assisted Convective Drying Process of Sludge

A coupled heat and moisture transfer model for ultrasound-assisted convective drying process of sludge was established. In this model, the permeable flow caused by acoustic pressure gradient in sludge was considered. The pore structure variety in sludge with ultrasonic irradiation was microscopically studied, and the pore size distribution of sludge was described by fractal geometry. Based on the fractal characterization, the physical properties of sludge including permeability, porosity, and tortuosity factor were determined, and the effective moisture diffusion coefficient of sludge under ultrasonic irradiation was also derived considering the effects of ultrasonic excitation energy and thermal effect on migration rate of water molecule. The effects of ultrasonic energy density and convective air temperature on convective drying process of sludge were numerically analyzed. The results showed that the ultrasonic irradiation changes the pore size distribution in sludge, the sludge flocs are dispersed, and the connectivity of pore structure is improved. Ultrasonic treatment is favorable to accelerating the moisture transport in the convective drying process of sludge, and the ultrasonic influence on moisture transport in sludge intensifies gradually with the increase of acoustic energy density from 0.2 to 0.6 W/ml. Furthermore, it can be also found that the enhancement effect of ultrasound on the average drying rate of sludge is more obvious at the connective air temperature of 65°C than that at 40°C under the uniform acoustic energy density and air velocity of 1.5 m/s.     

Natural convection heat and moisture transfer with thermal radiation in a cavity partially filled with hygroscopic porous medium

This article deals with natural convection heat and moisture transfer with thermal radiation in a cavity partially filled with hygroscopic porous medium. The governing equations for the momentum and heat transfer in both free fluid and hygroscopic porous medium and moisture content transfer in hygroscopic porous medium were solved by the finite element method. The radiative heat transfer is calculated by making use of the radiosity of the surfaces that are assumed to be grey. Comparisons with experimental and numerical results in the literature have been carried out. Effects of thermal radiation and Rayleigh number on natural convection and heat transfer in both free fluid and porous medium and moisture content transfer in porous medium were analyzed. It was found that surface thermal radiation can significantly change the temperature and moisture content fields in the regions of free flow and porous medium. The mean temperature at the interface decreases, the temperature and moisture content gradients are created on the upper two corners of the porous medium region, and the moisture content in the porous medium decreases in the porous medium as Ra increases.     

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     

Low intensity convective drying of non-hygroscopic porous materials of high initial moisture content

The convective air drying of non-hygroscopic porous materials under low intensity and high moisture conditions is analysed. The variation of the moisture diffusivity with local moisture content is taken into account. The effects of the “dry” patches that develop on the product surface during the falling rate period are included. The concept of “enthalpy potential” is used to derive the coupled heat and mass transfer boundary condition at the product surface. Goodman's integral method is used to obtain the solution for the constant rate period. An iterative finite difference scheme is written to solve the heat and moisture transfer equations for the falling rate period. The results are presented in terms of seven dimensionless parameters for a particular case of linear variation of liquid diffusivity. The results show the trend that could be expected in drying practice.     

AN INTEGRAL MODEL FOR DRYING OF HYGROSCOPIC AND NONHYGROSCOPIC MATERIALS WITH DIELECTRIC HEATING

Heat and mass transport phenomena in drying assisted by microwave or radio-frequency dielectric heating are analyzed. When drying at temperatures near boiling point or with high temperature gradients, the effect of the gas phase pressure gradient on moisture transfer within the solid can be important. The governing heat and mass transfer equations, including consideration of internal heat generation and the effect of the gas phase pressure gradient, are derived and solved in a one-dimensional system using an integral method. The integral model has been used to simulate dielectrically-enhanced convective drying of beds of polymer pellets, glass beads and alumina spheres with flow over the bed surface. Model predictions of drying rates and temperatures agree well with experimental data for these cases.

The model provides a relatively fast and efficient way to simulate drying behavior with dielectric heating, and may be useful in design and optimization of dielectrically-enhanced convective drying processes.     

AN INTEGRAL MODEL FOR DRYING OF HYGROSCOPIC AND NONHYGROSCOPIC MATERIALS WITH DIELECTRIC HEATING

Heat and mass transport phenomena in drying assisted by microwave or radio-frequency dielectric heating are analyzed. When drying at temperatures near boiling point or with high temperature gradients, the effect of the gas phase pressure gradient on moisture transfer within the solid can be important. The governing heat and mass transfer equations, including consideration of internal heat generation and the effect of the gas phase pressure gradient, are derived and solved in a one-dimensional system using an integral method. The integral model has been used to simulate dielectrically-enhanced convective drying of beds of polymer pellets, glass beads and alumina spheres with flow over the bed surface. Model predictions of drying rates and temperatures agree well with experimental data for these cases.

The model provides a relatively fast and efficient way to simulate drying behavior with dielectric heating, and may be useful in design and optimization of dielectrically-enhanced convective drying processes.     

Drying Kinetics and Antioxidant Phytochemicals Retention of Salak Fruit under Different Drying and Pretreatment Conditions

Kinetics of hot air drying and heat pump drying were studied by performing various drying trials on salak slices. Isothermal drying trials were conducted in hot air drying and heat pump drying at a temperature range of 40–90°C and 26–37°C, respectively. Intermittent drying trials were carried out in heat pump drying with two different modes: periodic heat air flow supply and step-up air temperature. It was observed that the effects of relative humidity and air velocity on drying rate were significant when moisture content in salak slices was high, whereas the effects of temperature prevailed when the moisture content was low. As such, it was proposed that drying conditions should be manipulated according to the moisture transport mechanisms at different stages of drying in order to optimize the intermittent drying and improve the product quality. Generally, loss of ascorbic acid during drying was attributed to thermal degradation and enzymatic oxidation, whereas the loss of phenolic compounds was mainly due to thermal degradation. Experimental results showed that heat pump drying with low-temperature dehumidified air not only enhanced the drying kinetics but produced a stable final product. Heat pump–dried samples retained a high concentration of ascorbic acid and total phenolic compounds when an appropriate drying mode was selected.     

Three-Dimensional Numerical Modeling of Convective Heat Transfer During Shallow-Depth Forced-Air Drying of Brown Rice Grains

We describe here a three-dimensional (3D) numerical study of the convective thermal transfer during forced-air drying of brown rice grains with the inclusion of moisture evaporation. Three levels of temperature were tested across a range from 40 to 60°C. The objective of the study was to determine the temperature distributions in the drying chamber as well as the surface and intra-kernel temperature gradients during convective drying of rice grains. The numerical model was based on the Navier-Stokes equation for fluid flow and the Fourier's equation for heat transfer. Results of the numerical solution showed that temperature distribution in the air and brown rice grains during the early minutes of drying were significantly influenced by the direction of flow of the heated air. Air gaps between grains and at the periphery of the drying chamber affected the rate and direction of airflow. The temperature history of the rice grains showed variations at different levels within the layers of grains. Moreover, the stream of heated air showed regions with airflows eight times greater than the inlet air velocity.     

Particle Shape and Aspect Ratio Effect of Al2O3–Water Nanofluid on Natural Convective Heat Transfer Enhancement in Differentially Heated Square Enclosures

The present numerical investigation, based on the finite volume method, deals with the characterization of flow and thermal fields inside differentially heated square enclosures filled with Al₂O₃–water nanofluid. The study focuses on the effect of shapes and aspect ratios of nanoparticles (NPs), depicted by Rayleigh number (Ra), solid volume fraction (?), and enclosure on both flow and heat transfer enhancement. Streamlines, isotherms contours, and velocity profiles as well as the average Nusselt number are considered. Results found show that the heat transfer rate increases with Rayleigh number as well as with nanofluid volume fraction. For the six different examined cases of NPs’ aspect ratios, nanofluid with oblate spheroids NPs (d_p = 0.13) was found to engender a significant enhancement in the overall heat transfer. In addition, heat transfer rate was more pronounced at great values of aspect ratios of NPs for prolate spheroids. Results also showed that heat transfer enhancement decreases as the Rayleigh number increases independently of the considered enclosure, shapes, and aspect ratios of NPs.     

Modelling the Bulk Cooling of Alfalfa Pellets

ABSTRACT

Cooling of alfalfa pellets after being made is one o f the unit operations in the pelleting of alfalfa. Using the thermal properties and drying diffusion coeficient together with other properties reported in refereed journals. a combined heat and mass transfer model was developed for the cooling of alfalfa pellets in deep beds. The model utilized the distributed heat and mass transfer equations to describe the temperature and moisture of the pellets. The distributed model interacts with the cooling air through a convective boundary condition for the temperature and a time–varying exponential surface condition for the moisture. Coupling o f the heat and mass transfer processes was carried out at the surface of the pellet using evaporative surface condition. The model was validated with field experimental data from a double–deck crossflow cooler. The simulated pellet and air temperatures were within 5⁰C of the collected field data while predicted moisture from the model was within 0.3% o f the experimental data.     

Modeling Superheated Steam Vacuum Drying of Wood

A two-dimensional mathematical model developed for vacuum-contact drying of wood was adapted to simulate superheated steam vacuum drying. The moisture and heat equations are based on the water potential concept whereas the pressure equation is formulated considering unsteady-state mass conservation of dry air. A drying test conducted on sugar maple sapwood in a laboratory vacuum kiln was used to infer the convective mass and heat transfer coefficients through a curve fitting technique. The average air velocity was 2.5 m s^-1 and the dry-bulb temperature varied between 60 and 66°C. The ambient pressure varied from 15 to 11 kPa. Simulation results indicate that heat and mass transfer coefficients are moisture content dependent. The simulated drying curve based on transfer coefficients calculated from boundary layer theory poorly fits experimental results. The functional relation for the relative permeability of wood to air is a key parameter in predicting the pressure evolution in wood in the course of drying. In the case of small vacuum kilns, radiant heat can contribute substantially to the total heat transfer to the evaporative surface at the early stages of drying. As for conventional drying, the air velocity could be reduced at the latter stage of drying with little or no change to the drying rate.