Thermal modelling of the high temperature treatment of wood based on Luikov's approach

A 3D, unsteady‐state mathematical model was used to simulate the behaviour of wood during high temperature treatment. The model is based on Luikov's approach and solves a set of coupled heat and mass transfer equations. Using the model, the temperature and moisture content profiles of wood were predicted as a function of time for different heating rates. Parallel to the modelling study, an experimental study was carried out using small birch samples. The samples were subjected to high temperature treatment in a thermogravimetric system under different operating conditions. The experimental results and the model predictions were found to be in good agreement. The results show that the distributions of temperature and moisture content are influenced appreciably by the heating rate and the initial moisture content. Copyright © 2005 John Wiley & Sons, Ltd.     

Study of Drying of a Porous Medium Using Gamma-Ray Attenuation Technique

ABSTRACT

In this work the drying of porous media is considered and modeled according to Luikov equations. The experimental apparatus consists of a cylindrical porous medium positioned vertically, with its bottom closed and maintained at a constant temperature. The top surface of this porous medium is in contact with a known flow rate of dry air at room temperature. Under these conditions, the transient axial temperature and moisture concentration profiles are measured. A noninvasive technique has been used to measure the moisture content, which is based on the attenuation of a horizontal beam of gamma-rays penetrating the porous medium. The measurement of this attenuation allows determining the water content of the sample. The mathematical model has been developed using two different approaches. In the first one, the one dimensional Luikov equations are applied considering all the physical parameters as constant. In the second approach, heat and mass balance equations of this process are the simple one dimensional transient diffusion equations, considering both diffusion coefficients as adjustable function of the moisture content. As shown, this second approach is superior to the first one.

     

On the solution of non-linear drying problems in capillary porous media through integral transformation of Luikov equations

The generalized integral transform technique is employed to provide hybrid numerical–analytical solutions to the non-linear Luikov equations, that govern drying within capillary porous media. The coupled equations for moisture and temperature distributions are integral transformed to eliminate the spatial co-ordinates, yielding an ordinary differential system for the time variation of the transformed potentials, which is readily solved through scientific subroutine libraries with automatic error control schemes. The analytical inversion formula is then recalled to provide explicit expressions for the original potentials at any desired position. The convergence behaviour of the proposed eigenfunction expansions is illustrated, and a parametric study is performed, for the drying of a slab subjected to non-linear radiative boundary conditions.     

Periodic solution of Luikov equations for heat and mass transfer in capillary porous bodies

Closed‐form solutions for the heat and moisture distribution given by the Luikov system of partial differential equations have been obtained by using periodic solution approach. Boundary conditions of the third type valid for capillary porous structure have been used to find the unknown constants. Calculations have been performed for Spruce wood, the properties of which were known. The results show that moisture reduces the temperature distribution between outside and inside by 2–3°C which is in conformity with the earlier results of Hall. Copyright © 1999 John Wiley & Sons, Ltd.     

SIMULATION OF INFRARED DRYING PROCESS

A control volume formulation for the solution of a set of two-way coupled heat and diffusive moisture transfer equations with an infrared source term is presented in three dimensions. The solution procedure developed uses a fully implicit time stepping scheme for the solution of the coupled set of equations and is applied to study the infrared drying of potato using two different drying systems. The maximum deviation between experimental and simulation results were 9.0°C in temperature for potato drying, and 10% (d.b.) for moisture predictions. The overall predictions agreed well with the available experimental data from the two systems and show a good potential for application in infrared drying of food grains and biomaterials.     

SIMULATION OF INFRARED DRYING PROCESS

ABSTRACT

A control volume formulation for the solution of a set of two-way coupled heat and diffusive moisture transfer equations with an infrared source term is presented in three dimensions. The solution procedure developed uses a fully implicit time stepping scheme for the solution of the coupled set of equations and is applied to study the infrared drying of potato using two different drying systems. The maximum deviation between experimental and simulation results were 9.0°C in temperature for potato drying, and 10% (d.b.) for moisture predictions. The overall predictions agreed well with the available experimental data from the two systems and show a good potential for application in infrared drying of food grains and biomaterials.     

基于Luikov理论的烟丝贮丝过程热湿特性研究

通过对烟草热湿传递机理的分析,基于Luikov理论,建立了烟丝贮丝过程热湿传递方程,并利用量纲分析和Laplace转换对方程求解,得到烟丝堆积床温度场和湿含量分布的解析解,其结论为深入研究贮丝过程热湿迁移规律提供了理论依据.     

Study of Drying of a Porous Medium Using Gamma-Ray Attenuation Technique

In this work the drying of porous media is considered and modeled according to Luikov equations. The experimental apparatus consists of a cylindrical porous medium positioned vertically, with its bottom closed and maintained at a constant temperature. The top surface of this porous medium is in contact with a known flow rate of dry air at room temperature. Under these conditions, the transient axial temperature and moisture concentration profiles are measured. A noninvasive technique has been used to measure the moisture content, which is based on the attenuation of a horizontal beam of gamma-rays penetrating the porous medium. The measurement of this attenuation allows determining the water content of the sample. The mathematical model has been developed using two different approaches. In the first one, the one dimensional Luikov equations are applied considering all the physical parameters as constant. In the second approach, heat and mass balance equations of this process are the simple one dimensional transient diffusion equations, considering both diffusion coefficients as adjustable function of the moisture content. As shown, this second approach is superior to the first one.     

Lumped- Differential Formulations for Drying in Capillary Porous Media

ABSTRACT

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.