ELECTROMAGNETIC DRYING OF WET MATERIALS WITH A SMALL DEPTH OF PENETRATION OF MICROWAVE RADIATION IN THE CONDITIONS OF HEAT REMOVAL BY RADIATION AND CONVECTION. II. STAGE OF CONSTANT DRYING SPEED

Abstract

The relevance of the research is dictated by the need to develop mathematical models of microwave heating and MW-drying of wet materials to obtain technologically optimal and cost-effective modes. This publication is a continuation of the article by the same authors in «Bulletin of the Tomsk Polytechnic University. Geo Assets Engineering», in which the authors using mathematical modeling, studied in detail the process at the first stage of drying –the heating stage, when heat exchange between the surface of a moist body and the environment is due to radiation and convection – energy is absorbed by the surface layer due to its small depth of penetration. The authors constructed the asymptotic solutions of this nonlinear problem for small and large values ​​of dimensionless time demanded by engineering practice, both for parametric analysis, and for carrying out operational calculations. The approach associated with the consideration of the second stage, the stage of constant drying rate, is based on a more detailed study of heat transfer, and the calculation of the rate of drying is carried out using the approximation dependence of A.V. Lykov. The paper proposes a condition for cross-linking of thermal modes of I and II stages of drying. The aim of the research is the state of the problem of the second stage of microwave drying of wet material – a stage of constant drying rate – and implementation of a theoretical solution to determine the distribution of the temperature field across the layer thickness and the magnitude of the drying rate. The object of the research is a flat layer of wet material – coal, sand, wood, and other capillary-porous arrays, which are affected by microwave radiation. Such materials have a high dielectric constant and, as a result, very effectively absorb microwave radiation, which is almost 100 % converted to thermal energy. The research methods are associated with mathematical modeling, which are based on Maxwell's electrodynamics equations and A.V. Lykov heat and moisture transfer. In this article, the Maxwell problem is solved independently of the problem of heat and mass transfer; therefore, the flux density absorbed by microwave radiation is known. Also, one of the features of this problem is the consideration of materials with a small absorption depth, that is why the source term in the system of equations for heating is in the boundary condition. As a result of the research, the authors involving the stationary temperature field conditions and the constancy of the moisture content flux density over time stated and solved the problem of temperature field distribution across the thickness of the wet plate, which strictly follows the square parabolic law. The drying speed of the II stage and the critical moisture content at the end of the II stage were determined from A,V. Lykov correlation dependencies. The paper introduces the stitching solutions for the I and II stages and the analysis of the constructed solutions.

Keywords

• microwave energy
• electromagnetic drying
• capillary-porous massif
• heat radiation
• convection
• a.v. lykov heat and moisture transfer equations
• drying speed