Mukminov I. Intensification of heat and mass transfer processes in dense layers of granular material

The work is aimed at creating theoretical and practical conditions for the effective use of a dense layer of granular material as a nozzle in a regenerative heat exchanger for the accumulation of low-potential heat in an explicit form. In order to obtain the necessary scientific and practical data, a number of tasks were formulated and solved in this work. Based on the analysis of modern achievements in the field of research, the rational areas of use of heat accumulators with granular (bulk) materials and the main physical and geometric parameters that affect the intensity of heat exchange between a dense layer and a through gas flow are determined. A mathematical model of interconnected intercomponent heat and mass transfer in a dense stationary layer is developed, according to which the layer consists of two quasi-homogeneous components - gas and solid. The model includes the equation of heat transfer in the gas component, the equation of heat transfer on the solid component, and the equation of mass transfer in the gas component. In order to obtain a calculated dependence for temperatures, a mathematical model of heat transfer in a heat-insulated channel at a given gas inlet temperature was developed. The solution of the mathematical model involved the development of a method for determining the roots of the characteristic equation and assessing the influence of Bio numbers using the graphical and analytical method. The mathematical model was verified, which proved that it corresponds to the physical picture of the unsteady-state heating process and correctly reflects the influence of the main process parameters (density, intercomponent heat transfer coefficient, porosity, heating time) on the temperature change of the material layer. The calculated data correlate satisfactorily with the experimental data obtained under similar conditions. To refine the calculation, it is necessary to have accurate data on the thermophysical characteristics of the material, its porosity, and the value of the specific surface area of the particles. To expand the theoretical concepts and obtain recommendations for intensifying the heat exchange process in a dense nozzle, a soil regenerator for a greenhouse was designed, a pilot plant was manufactured, and field studies were conducted. The analysis of the temperature distribution in the granular nozzle when heated by a through air flow shows that the layers of material in the channel sequentially absorb the heat of the through air flow, and due to the heat capacity of the selected material, the previous layers absorb heat intensively, which causes a significant decrease in the intensity of the process in the subsequent layers. Over time, the temperature of all layers increases, but thermal equilibrium was not observed. The difference between the temperatures at the channel boundaries decreases. The analysis of temperature curves shows that no abrupt temperature change zones are observed during the heating of a dense layer of material, which confirms the adequacy of the developed mathematical model. To answer the question of how the first and second stages of particle heating are distributed in time, additional studies of the heating of a single particle in the channel were carried out. This is important when assessing the effect of thermal conductivity in a solid material on the distribution of the temperature field and optimising particle sizes. Experiments have shown that the time course of the temperature curves for the surface and centre of the particle repeats each other, i.e. the law of temperature change is the same for all points and the first heating period is extremely short. Thus, the equations for the second stage, which is characterised by the same law of temperature change at all points of the material, can be used in thermal calculations. Another important issue was the study of the aerodynamic drag of the material layer. Based on the results of the analysis of aerodynamic studies, it was determined that the Ergun equation describes the experimental data with acceptable accuracy for calculating the dependence of pressure loss on the length of the channel. To perform thermal design calculations of the soil regenerator, a methodology was developed that allows, based on the given geometric characteristics of the greenhouse, the average solar radiation flux, the absorption rate of the solar radiation heat flux by the soil, the average ambient temperature, the type of granular material and other input data, to determine the volume of the heat exchange area, the weight of the load, the amount of heat accumulated by the granular nozzle, and to estimate the duration of the cooling period from. The results of the implementation of the soil regenerator and the methodology for its thermal calculation were confirmed by a certificate of implementation at the Research and Production Enterprise AGROFARMTECHNIKA LLC.