TRANSIENTS PROCESSES UNDER DRYING WITH CONVECTION AND INFRARED RADIATION

Abstract

Based on the theory of the A.V. Lykov for heat and mass transfer the mathematical model of propagation of heat and moisture in a flat sample, which is blown by the air flow and is exposed to infrared radiation. The model is based on the following views: the density of heat loss is determined by the heat transfer by convection and heat exchange by radiation, and the intensity of the mass exchange surface with the environment depends on the difference in partial pressure of water vapor through the thickness of the boundary layer (the boundary condition of mass transfer in the form of Dalton); temperature field inside the material is determined by the heat transfer due to the phenomenon of thermal conductivity, and the presence of internal heat sources caused by the absorption of penetrating electromagnetic radiation and the processes of evaporation (condensation); the transfer of moisture inside the material is partly liquid and partly in vapor form, and is caused by moisture content gradients (diffusion) and temperature (thermal diffusion). The results of analytical calculation of steady-state fields of temperature and moisture content for cases of convective drying and infrared drying, and the results of numerical calculation of the same field in transient conditions are presented. Numerical experiment allows us to estimate the duration of the transition process, as well as the behavior of the differential moisture content between the borders of the plate, with the increase which increases the probability of failure of the sample from mechanical deformation. It is shown that for convective drying of such a danger occurs in the transitional regime, and for infrared drying – mode steady-state. To reduce internal mechanical stresses when drying by convection, avoid sudden temperature changes of the air flow; the time during which there is an increase in air temperature, should be around the time of the transition process. When drying with infrared rays, if the differences of moisture content in the steady state are invalid for their reduction can be used or drying in the oscillating radiation, or drying under the combined effect of the sample electromagnetic waves with large and small penetration depth. The distinction in the nature of transients, infrared drying and drying with hot air can be explained using the formula of Dalton, which is part of the used mathematical model and determining the intensity of the problem through the boundary layer. Due to the great inertia of thermal processes, even in the case where the intensity of the infrared radiation changes rapidly in time (for example at the initial time gap of the first kind), the surface temperature of the material, and with it the intensity of drying, continue to be continuous functions; on the contrary, an abrupt change in air temperature in convective drier automatically leads to a gap function of the flux density of moisture on the surface.Forcitation:Afanas'ev A.M., Nikishova A.V., Siplivy B.N. Transients processes under drying with convection and infrared radiation. Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol. 2017. V. 60. N 10. P. 94-101