Stefan J. Kowalski

Thermomechanics of Drying Processes

Nomenclature.- 1 Properties of dried materials.- 1.1 Classification of wet materials.- 1.2 Characterization of moisture bounding a solid skeleton.- 1.3 The equilibrium humidity.- 1.4 Mechanisms of moisture movement.- 2 Characterization of drying processes.- 2.1 Drying technique.- 2.2 Kinetics of drying processes.- 2.3 Mechanisms of heat and mass transfer.- 3 The equations of balance.- 3.1 Preliminaries.- 3.2 Balance equations: control volume approach.- 3.3 Second law of thermodynamics.- 4 Thermodynamic foundation of dried materials in elastic range.- 4.1 Postulate of local state.- 4.2 Equations of state.- 4.3 Phenomenological rate equations of heat and mass transfer.- 4.4 Physical relations for elastic materials under drying.- 5 Thermodynamics of viscoelastic materials under drying.- 5.1 Rheological properties of dried materials.- 5.2 Constitutive equations for a viscoelastic body.- 5.3 Analogy between viscoelasticity and elasticity.- 6 Plasticity in drying.- 6.1 Elastoplastic behavior of dried materials.- 6.2 Thermodynamical restrictions.- 6.3 Theory of plastic flow.- 6.4 Identification of the coefficients ? and L in the plastic potential.- 7 Destruction of materials by drying.- 7.1 Preliminary remarks.- 7.2 Cohesion forces in drying processes.- 7.3 Theoretical strength of dried materials.- 7.4 Fracture of brittle dried materials.- 7.5 Fracture of partly ductile dried materials.- 7.6 A measure of damage.- 8 Conditions of stress generation and boundary conditions.- 8.1 Conditions of stress generation.- 8.2 Balance equations for a body with a discontinuity surface.- 8.3 Boundary conditions in the first and second period of drying.- 8.4 Receding of the evaporation zone.- 9 Approach to numerical analysis in drying.- 9.1 Set of governing equations.- 9.2 Galerkins formulation of the numerical problem.- 9.3 Time integration.- 9.4 Numerical solution for two-dimensional problem.- 10 One-dimensional initial-boundary value problem.- 10.1 Distribution of temperature and moisture content in wet porous plates under drying.- 10.2 Drying induced stresses in an elastic saturated plate.- 10.3 Drying induced stresses in a viscoelastic saturated plate.- 11 Drying induced stresses in products of cylindrical and spherical symmetry.- 11.1 The phenomenon of stress reverse in a cylindrical sample.- 11.2 Determination of shrinkage factor on a spherical sample.- 12 Mechanical effects in dried materials examined on the basis of two-dimensional boundary value problems.- 12.1 Deformations and stresses dependent on material shape.- 12.3 Thermal effects in dried materials.- 12.4 Analysis of dried body response on drying conditions the response of dried body to drying conditions.- 12.5 Significance of the wet bulb temperature in drying.- 12.6 Control of drying processes.- 12.7 Mechanical properties dependent on moisture content.- 12.8 Concentration of stresses around grooves.- 12.9 Phase transition inside the dried body.- 13 Dried materials with anisotropic structure.- 13.1 Constitutive equations for anisotropic materials with variable moisture content.- 13.2 Experimental identification of mechano-sorptive strain in wood.- 13.3 Deformation and stresses in dried wood.- 13.4 Identification of wood fracture during drying.- 14 Experimental studies in drying.- 14.1 Determination of coefficients in physical relations.- 14.2 Determination of coefficients in rate equations.- 14.3 Application of acoustic emission in studies of dried materials.- References.

Drying of Wet Materials in Intermittent Conditions

This article presents experimental results of convective drying of capillary-porous material carried out in intermittent and also in stationary conditions for comparison. The aim of these studies is to justify that intermittent drying can be more propitious from the point of view of material quality than the stationary one. The experiments were carried out on cylindrically shaped samples made of kaolin. Intermittent drying was accomplished by periodic changes of the drying medium (air) temperature and humidity. It was shown that drying rate in periodically changing conditions is located between those obtained in stationary drying at the upper and the lower parameters of the periodically changing drying parameters. The quality of the material looks much better when dried in intermittent than in stationary conditions.

Toward a thermodynamics and mechanics of drying processes

Abstract The paper is concerned with deformations and drying-induced stresses of moistened capillary-porous medium during intensive drying. A non-linear theory of drying is constructed based on the concept of mixture theory and the thermodynamics of irreversible processes. The porous material is considered for a “container” interacting with the mixture of liquid and gas filling the pores. The coupled system of differential equations describing the mechanical behaviour of moist material during drying as well as the distributions of temperature and moisture content is derived in the paper. Diversified constituent temperatures are considered, what makes possible to apply the presented model to intensive drying processes.

Drying-induced stresses in elastic and viscoelastic saturated materials

The paper presents a theoretical analysis of stresses generated during convective drying of kaolin, based on elastic and viscoelastic models. The equations of these models were solved analytically for a cylindrically shaped sample; the distribution and evolution of the radial and circumferential stresses are illustrated in diagrams. The acoustic emission method was used in experimental tests for identification on line of the time period during which the stresses reach their maximal values. A better correlation has been found between the experimental tests and the theoretical predictions obtained on the basis of the viscoelastic model.

Drying induced stresses estimated on the base of elastic and viscoelastic models

The main subject of this paper is to study how the chosen constitutive model of a saturated porous body undergoing drying process influences the numerically estimated drying induced stresses and deformations. To compare the results one assumes the material of a convectively dried cylinder to be both elastic and viscoelastic. One states a significant difference between the results obtained for both these models, particularly for the drying induced stresses.

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.

Analysis of Effectiveness and Stress Development during Convective and Microwave Drying

The aim of the article is to study the effectiveness of convective and microwave drying with respect to drying time and stress generation in clay-like dried bodies. The theoretical analysis of stresses was confined to the constant drying rate period because clay shrinks and the stresses are generated mainly in this period. The theoretical prediction of stress development was validated using an acoustic emission method by monitoring the micro- and macrocracks formation caused by the stresses. Apart from the stress analysis, the combined convective-microwave drying was examined experimentally during whole process in order to show that the volumetric heat supply due to microwave heating enhances convective drying and, apart from this, such a combined drying process develops less stress in dried material. The theoretical and experimental studies were performed on a kaolin sample in the form of a thin plate.

The identification of fracture in dried wood based on theoretical modelling and acoustic emission

Physical interpretation of the three characteristic groups of acoustic signals emitted during convective drying of wood is the main subject of this paper. The acoustic emission (AE) was to evidence the fracture intensity during drying of a birch wood sample of cylindrical shape. To explain the three characteristic groups of acoustic signals, and particularly the last one, a mechanistic model of drying was applied to analyse the drying induced stresses in the tested sample. One can conclude from this analysis that the third group of acoustic signals arises when the surface stop to shrink and the wet core (initially in compression) begins to dry. The shrinkage of the core causes compression of the boundary layer and tension of the core. Thus, the reverse of the stress signs in the cylinder cross-section takes place and this possibly involves the destruction of wood structure in the tensed core.

Convective Drying Enhanced with Microwave and Infrared Radiation

The aim of this article is to examine the effectiveness of convective drying enhanced with microwave and infrared radiation. Eight programs of combined convective-microwave, convective-infrared, and convective-microwave-infrared drying processes were realized experimentally to compare the drying time and the quality of dried products. The experimental tests enabled determination of the drying curves, the temperatures of dried bodies, and the drying rates for the individual drying programs. The dried samples were photographed for visualization of the sample quality. The experiments were carried out on kaolin samples in the form of cylinders. It was stated that a proper combination of these three drying methods may result in a very high drying rate and at the same time preserving a very good quality of the dried product.

Experimental Validation of the Heat and Mass Transfer Model for Convective Drying

The aim of this article is to present a self-consistent mathematical model describing the heat and mass transfer phenomena during the convective drying both in the constant and in the falling drying rate periods. This general model is developed on the basis of the theory of mixtures and the thermodynamics of irreversible processes. The boundary conditions are formulated and the numerical algorithm enabling calculation of the temperature and the drying curves in the two mentioned periods of drying is constructed. In this paper much effort is devoted to the experimental validation of the model. The convective drying of a cylindrical sample made of kaolin was examined both experimentally and numerically for comparison and the distribution of temperature and the drying curves were determined. A very good agreement of the experimental and theoretical results is stated.