Andrzej Rybicki

Stresses Generated During Convective and Microwave Drying

Abstract An outline of the mechanistic model of convective and microwave drying of saturated capillary-porous materials is presented. The model was derived in the framework of irreversible thermodynamics. Particular attention is devoted to construction of the term describing the power of microwave radiation absorbed per unit volume, which is converted into internal heat source. The qualitative difference in distribution of temperature, moisture content, and the drying-induced stresses in materials under convective and microwave drying is illustrated in the examples of cylindrical kaolin samples. The diagrams of acoustic emission are taken off on-line from these samples in order to illustrate the development of material destruction caused by the stresses induced during both convective and microwave drying.

Mechanical Effects in Saturated Capillary-Porous Materials during Convective and Microwave Drying

Abstract The differences are analyzed in distribution and time evolution of the temperature, moisture content, and drying-induced stresses generated by convective and microwave drying. The theoretical analysis of the drying induced stresses and the deformations of dried materials is based on the elastic and viscoelastic constitutive models. The theoretical predictions are confronted with the experimental data obtained by the acoustic emission (AE) method, which enable monitoring on line the development of the drying induced stresses. The system of double coupled differential equations of the thermomechanical drying model is solved numerically using the finite element (FEM) and the finite difference (FDM) methods. A cylindrical sample made of kaolin was chosen to compare experimental data with the model solution. Essential differences were identified in the analyzed items for convective and microwave drying as well as a significant difference in stress distribution was noted for elastic and viscoelastic constitutive models.

Qualitative Aspects of Convective and Microwave Drying of Saturated Porous Materials

Abstract The differences in distribution and temporal evolution of temperature, moisture content, and drying stresses in saturated capillary-porous materials by convective and microwave drying are analyzed. The analysis is based on the mechanistic model of drying taking into account the coupling effects in the heat and mass transfer. The results of numerical simulation allow better understanding of the difference in thermal and mechanical behavior of dried materials to which the energy necessary for drying is supplied volumetrically (microwave drying) or through the material surface (convective drying). The study is carried out on an isotropic cylinder as a model material.

Destruction of wet materials by drying

Self-fracturing of fluid-saturated porous materials during drying processes is considered, and a physical model of this widespread phenomenon in drying problems is proposed. Main analysis deals with dispergate systems like ceramics, which after drying and calcinating become porous bodies with relatively high strength. The emphasis is placed on two problems: an increase of the cohesive force at the initial stage of drying and decohesion of a structure caused by drying-induced stresses.

Cohesive Strength of Materials during Drying Processes

This article is a contribution to modeling and analysis of the cohesion strength of saturated porous bodies during drying processes. The dependence of the drying-induced state of stress, the overall (reduced) stress, and the admissible stress on moisture content is discussed. The strength criterion is proposed stating that possible cracking of the material during drying takes place in regions where the reduced stress exceeds the critical admissible value for the given moisture content. Numerical calculations of the drying-induced stresses were carried out for a convectively dried kaolin cylinder, and the regions of possible material cracking within the cylinder were determined. This theoretical prediction of the possible cracking place in the material is consistent with the experimental data previously presented by the authors.

Residual Stresses in Dried Bodies

The problem of residual stresses in porous materials as a result of drying and the presumable influence of these stresses on the swelling effect by rehydration are discussed. The drying model comprising the specific mechanisms suitable for the first and second periods of drying is applied, and the numerical algorithm enabling calculation of the drying-induced stresses is constructed. The numerical calculations exemplifying the history of stresses leading finally to the residual stresses are carried out. The outcome of these stresses on the swelling strains during rehydration is illustrated using an example of a finite dimensions kaolin cylinder dried convectively.

RATE OF DRYING AND STRESSES IN THE FIRST PERIOD OF DRYING

ABSTRACT The paper presents a computer simulated processes and illustrate how the drying induced stresses are influenced by the rate of drying. It is shown that the moisture transport coefficient, and thus the rate of drying, depends on the thermal state of the drying material, defined by the wet-bulb temperature. Through these simulated processes one can observe the evolution of the moisture content and stress distributions during drying at constant, but in each process different, wet-bulb temperatures. A convective drying process of a bar with rectangular cross-section is considered as example, and a two-dimensional initial-boundary value problem is solved numerically with the use of the finite element method. The numerical results are visualised in spatial diagrams.

Computer Simulation of Drying Optimal Control

The subject of this paper is the control of computer-simulated drying processes for the purpose of their optimization. The simulated processes are carried out in such a way that the drying-induced stresses never exceed the stress limit. This paper presents results of a mechanically admissible drying process on the example of a prismatic bar dried convectively, using the theory for drying of capillary-porous materials. The procedure of designing the safe drying processes, in which the destruction of dried materials no longer proceeds, is presented.

Stresses and Strains in Elastic, Viscoelastic, and Plastic Materials during Drying

The mechanistic theory of drying is developed for the purpose of stress and strain analysis in wet clay-like materials subjected to convective drying. The aim of these studies is to show the difference in stress and strain patterns for cases when the material is considered to be elastic, viscoelastic, or elastic-plastic. Such an analysis is justified by the fact that many materials under drying, e.g., clay-like or woody materials, reveal different mechanical properties and safer, mostly inelastic deformations. In addition, inelastic models are able to describe some additional phenomena as, for example, stress reverse that often appears in dried materials. It is displayed through comparison of the experimental measurements with the numerical results obtained on the basis of inelastic constitutive equations applied to drying materials. These results show that drying models that include inelastic deformations are more realistic and present more natural stress and strain patterns than the purely elastic model. The analysis is presented on an example of kaolin-clay plate subjected to convective drying. The numerical results concerning inelastic deformations are visualized on plots and photographs taken from the samples tested experimentally.