Kinga Rajewska

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.

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.

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.

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.

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.

Acoustic Emission in Drying Materials

Drying of wet materials is one of the oldest and most common unit operation found in di‐ verse processes such as those used in the agricultural, ceramic, chemical, food, pharmaceuti‐ cal, pulp and paper, mineral, polymer, and textile industries. It is also one of the most complex and least understood operations because of the difficulties and deficiencies in mathematical descriptions of the phenomena of simultaneous – and often coupled and mul‐ tiphase – transport of heat, mass, and momentum in saturated porous materials. Drying is therefore an amalgam of science, technology, and art, or know-how based on extensive ex‐ perimental observations and operating experience [Strumillo, 1983; Mujumdar (Ed.), 2007].

Hybrid and Non-stationary Drying—Process Effectiveness and Products Quality

During last decades a lot of new drying techniques have been developed. Some of them are focused on breaking the limits of convective drying usually by applying intermittent conditions or utilization of few drying techniques in one process (hybrid drying). The purpose of the chapter is to discuss these new opportunities. Hence the convective non-stationary drying and various hybrid drying techniques (convective–microwave, convective–microwave–infrared, convective–microwave–ultrasonic and microwave-vacuum drying) are discussed. Many examples are provided in this chapter to illustrate the impact of the applied drying conditions and techniques on time consumption, process energy consumption, and on the quality of the product obtained. In particular, the drying of kaolin clay, oak, pine and walnut wood, apple, carrots, kale, potatoes, raspberries and red pepper are presented. The results of the studies indicate that both variable drying conditions and hybrid techniques may result in improved drying kinetics, reduced process energy consumption, and increased product quality.