Analysis of Effectiveness and Stress Development during Convective and Microwave Drying

Abstract

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. INTRODUCTIONThe negative feature of drying processes of capillaryporous materials is the problem of drying-induced stresses responsible for the material destruction. The tendency to stress generation increases with the rate of drying and the size of the drying body. Therefore, the efforts are made to find methods of stress minimizing and thus avoiding the drying body destruction when the drying rate increases.The main aim of this article is to prove the hypothesis that a combined convective-microwave drying shortens the drying time and also diminishes the tendency to stress generation. The hypothesis is based on the statement that the volumetric heat supply, as it takes place in microwave heating, increases the temperature inside the body and thus causes a coincidence of the thermodiffusional and the diffusional moisture fluxes. Such a flux coincidence raises the moisture outflow and moderates its distribution

Microwave Drying of Corn (Zea maysL. ssp.) for the Seed Industry

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Evaporation-induced evolution of the capillary force between two grains

“…The capillary forces between partially wet grains are thought to impart an apparent macroscopic or "sand-castle" strength to the moist granular materials, even in the absence of the intrinsic cohesion or confining stress (see e.g. [1][2][3][4][5][6][7]). Two-grain systems are important not only because they are the most elementary but also because more complex multi-grain liquid bridges will eventually break down to a number of two-grain systems (see e.g.…”

Permanent Strains in Clay-Like Material During Drying

“…[1,2] The process usually influences the quality of the resulting product. High temperature and long drying time can significantly decrease the quality of dried materials; e.g., by change of color, [3][4][5] permanent shape change (shrinkage), [6][7][8] inner structure damage, [9] surface hardening, chemical changes, [10,11] etc. For instance, color characteristics are major quality attributes associated with dried food products.…”