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The paper describes the results of computer simulation of time-dependent temperature fields arising in polar dielectrics irradiated by focused electron bunches with average electron energy when analyzing with electron microscopy techniques. The mathematical model was based on solving several-dimensional nonstationary heat conduction equation with use of numerical finite element method. The approximation of thermal source was performed taking into account the estimation of initial electron distribution determined by Monte-Carlo simulation of electron trajectories. The simulation program was designed in Matlab. The geometrical modeling and calculation results demonstrated the main features of model sample heating by electron beam were presented at the given experimental parameters as well as source approximation.

The physical and mathematical model of combustion of the polydisperse suspension of coal dust was developed. The formulation of the problem takes into account the evaporation of particle volatile components during the heating, the particle emitting and the gas heat transfer to a surrounding area via the sphere volume side surface, heat transfer coefficient as a function of temperature. The polydisperse of coal-dust is taken into consideration. N — the number of fraction. Fractions are subdivided into inert and reacting particles. The oxidizer mass balance equation takes into consideration the oxidizer consumption per each reaction (heterogeneous on the particle surface and homogenous in the gas). Exothermic chemical reactions in gas are determined by Arrhenius equation with second-order kinetics. The heterogeneous reaction on the particle surface was first-order reaction. The numerical simulation was solved by Runge–Kutta–Merson method. Reliability of the calculations was verified by solving the partial problems. During the numerical calculation the percentage composition of inert and reacting particles in coal-dust and their total mass were changed for each simulation. We have determined the influence of the percentage composition of inert and reacting particles on burning characteristics of polydisperse coal-dust methane-air mixture. The results showed that the percent increase of volatile components in the mixture lead to the increase of total pressure in the volume. The value of total pressure decreases with the increasing of the inert components in the mixture. It has been determined that there is the extremism radius value of coarse particles by which the maximum pressure reaches the highest value.

The work is devoted to the study of the theoretical value of destructive influence on normal tissues of an organism by infrared radiation that goes beyond the treated pathological focus. This situation is possible if the direct laser radiation on the tissues is extremely long-acting. The solution to this problem can be the uniform distribution of heat inside the volume through indirect heating of the liquid, which contributes to minimal damage to the perifocal structures. A non-stationary thermophysical model of the process of heat propagation in biological tissues is presented, allowing to carry out studies of energy transfer from internal liquid contents of Baker's cyst heated by infrared laser radiation of a given specific power through a certain thickness of its wall to surrounding biological tissues. Calculation of the spacetime temperature distribution in the cyst wall and surrounding fat tissue is carried out by the finite-difference method. The time of effective exposure to temperature on the entire thickness of the cyst wall was estimated to be 55 ° C on its outer surface. The safety procedure ensures the exposure duration of this value is not more than 10 seconds.

As a result of the calculations carried out, it is established that there are several operating modes of a surgical laser that meet all the safety requirements with a simultaneous effective procedure. Local one-sided hyperthermia of the synovial membrane and subsequent coagulation of the entire wall thickness due to heat transfer contributes to the elimination of the cavity neoplasm of the popliteal region. With a thickness of 3 mm, the heating mode is satisfactory, under which the exposure time lasts about 200 seconds, and the specific power of the laser radiation in the internal medium of the liquid contents of the Baker cyst is approximately 1.

The article is devoted to the effect of thermal feedback, which occurs during the operation of integrated circuits and electronic systems with their use. Thermal feedback is due to the fact that the power consumed by the functioning of the microchip heats it and, due to the significant dependence of its electrical parameters on temperature, interactive interaction arises between its electrical and thermal processes. The effect of thermal feedback leads to a change in both electrical parameters and temperature levels in microcircuits. Positive thermal feedback is an undesirable phenomenon, because it causes the output of the electrical parameters of the microcircuits beyond the permissible values, the reduction in reliability and, in some cases, burn out. Negative thermal feedback is manifested in stabilizing the electrical and thermal regimes at lower temperature levels. Therefore, when designing microcircuits and electronic systems with their application, it is necessary to achieve the implementation of negative feedback. In this paper, we propose a method for modeling of thermal modes in electronic systems, taking into account the effect of thermal feedback. The method is based on introducing into the thermal model of the electronic system new model circuit elements that are nonlinearly dependent on temperature, the number of which is equal to the number of microcircuits in the electronic system. This approach makes it possible to apply matrix-topological equations of thermal processes to the thermal model with new circuit elements introduced into it and incorporate them into existing thermal design software packages. An example of modeling a thermal process in a real electronic system is presented, taking into account the effect of thermal feedback on the example of a microcircuit installed on a printed circuit board. It is shown that in order to adequately model the electrical and thermal processes of microcircuits and electronic systems, it is necessary to take into account the effects of thermal feedback in order to avoid design errors and create competitive electronic systems.

The paper presents a physico-mathematical model of the perturbed region formed in the lower D-layer of the ionosphere under the action of directed radio emission flux from a terrestrial stand of the megahertz frequency range, obtained as a result of comprehensive theoretical studies. The model is based on the consideration of a wide range of kinetic processes taking into account their nonequilibrium and in the two-temperature approximation for describing the transformation of the radio beam energy absorbed by electrons. The initial data on radio emission achieved by the most powerful radio-heating stands are taken in the paper. Their basic characteristics and principles of functioning, and features of the altitude distribution of the absorbed electromagnetic energy of the radio beam are briefly described. The paper presents the decisive role of the D-layer of the ionosphere in the absorption of the energy of the radio beam. On the basis of theoretical analysis, analytical expressions are obtained for the contribution of various inelastic processes to the distribution of the absorbed energy, which makes it possible to correctly describe the contribution of each of the processes considered. The work considers more than 60 components. The change of the component concentration describe about 160 reactions. All the reactions are divided into five groups according to their physical content: ionization-chemical block, excitation block of metastable electronic states, cluster block, excitation block of vibrational states and block of impurities. Blocks are interrelated and can be calculated both jointly and separately. The paper presents the behavior of the parameters of the perturbed region in daytime and nighttime conditions is significantly different at the same radio flux density: under day conditions, the maximum electron concentration and temperature are at an altitude of ~45–55 km; in night ~80 km, with the temperature of heavy particles rapidly increasing, which leads to the occurrence of a gas-dynamic flow. Therefore, a special numerical algorithm are developed to solve two basic problems: kinetic and gas dynamic. Based on the altitude and temporal behavior of concentrations and temperatures, the algorithm makes it possible to determine the ionization and emission of the ionosphere in the visible and infrared spectral range, which makes it possible to evaluate the influence of the perturbed region on radio engineering and optoelectronic devices used in space technology.

The development of structured molecular systems based on a nucleic acid framework takes into account the ability of single-stranded DNA to form a stable double-stranded structure due to stacking interactions and hydrogen bonds of complementary pairs of nucleotides. To increase the stability of the DNA double helix and to expand the temperature range in the hybridization protocols, it was proposed to use more stable metal-mediated complexes of nucleotide pairs as an alternative to Watson-Crick hydrogen bonds. One of the most frequently considered options is the use of silver ions to stabilize a pair of cytosines from opposite DNA strands. Silver ions specifically bind to N3 cytosines along the helix axis to form, as is believed, a strong N3–Ag⁺–N3 bond, relative to which, two rotational isomers, the cis- and trans-configurations of C–Ag⁺–C can be formed. In present work, a theoretical study and a comparative analysis of the free energy profile of the dissociation of two С–Ag⁺–C isomers were carried out using the combined method of molecular mechanics and quantum chemistry (QM/MM). As a result, it was shown that the cis-configuration is more favorable in energy than the trans- for a single pair of cytosines, and the geometry of the global minimum at free energy profile for both isomers differs from the equilibrium geometries obtained previously by quantum chemistry methods. Apparently, the silver ion stabilization model of the DNA duplex should take into account not only the direct binding of silver ions to cytosines, but also the presence of related factors, such as stacking interaction in extended DNA, interplanar hydrogen bonds, and metallophilic interaction of neighboring silver ions.

The paper presents a mathematical model that describes the process of obtaining biogas from livestock waste. This model describes the processes occurring in a biogas plant for mesophilic and thermophilic media, as well as for continuous and periodic modes of substrate inflow. The values of the coefficients of this model found earlier for the periodic mode, obtained by solving the problem of model identification from experimental data using a genetic algorithm, are given.

For the model of methanogenesis, an optimal control problem is formulated in the form of a Lagrange problem, whose criterial functionality is the output of biogas over a certain period of time. The controlling parameter of the task is the rate of substrate entry into the biogas plant. An algorithm for solving this problem is proposed, based on the numerical implementation of the Pontryagin maximum principle. In this case, a hybrid genetic algorithm with an additional search in the vicinity of the best solution using the method of conjugate gradients was used as an optimization method. This numerical method for solving an optimal control problem is universal and applicable to a wide class of mathematical models.

In the course of the study, various modes of submission of the substrate to the digesters, temperature environments and types of raw materials were analyzed. It is shown that the rate of biogas production in the continuous feed mode is 1.4–1.9 times higher in the mesophilic medium (1.9–3.2 in the thermophilic medium) than in the periodic mode over the period of complete fermentation, which is associated with a higher feed rate of the substrate and a greater concentration of nutrients in the substrate. However, the yield of biogas during the period of complete fermentation with a periodic mode is twice as high as the output over the period of a complete change of the substrate in the methane tank at a continuous mode, which means incomplete processing of the substrate in the second case. The rate of biogas formation for a thermophilic medium in continuous mode and the optimal rate of supply of raw materials is three times higher than for a mesophilic medium. Comparison of biogas output for various types of raw materials shows that the highest biogas output is observed for waste poultry farms, the least — for cattle farms waste, which is associated with the nutrient content in a unit of substrate of each type.

A cluster method of mathematical modeling of interval-stochastic thermal processes in complex electronic systems (ES), is developed. In the cluster method, the construction of a complex ES is represented in the form of a thermal model, which is a system of clusters, each of which contains a core that combines the heat-generating elements falling into a given cluster, the cluster shell and a medium flow through the cluster. The state of the thermal process in each cluster and every moment of time is characterized by three interval-stochastic state variables, namely, the temperatures of the core, shell, and medium flow. The elements of each cluster, namely, the core, shell, and medium flow, are in thermal interaction between themselves and elements of neighboring clusters. In contrast to existing methods, the cluster method allows you to simulate thermal processes in complex ESs, taking into account the uneven distribution of temperature in the medium flow pumped into the ES, the conjugate nature of heat exchange between the medium flow in the ES, core and shells of clusters, and the intervalstochastic nature of thermal processes in the ES, caused by statistical technological variation in the manufacture and installation of electronic elements in ES and random fluctuations in the thermal parameters of the environment. The mathematical model describing the state of thermal processes in a cluster thermal model is a system of interval-stochastic matrix-block equations with matrix and vector blocks corresponding to the clusters of the thermal model. The solution to the interval-stochastic equations are statistical measures of the state variables of thermal processes in clusters - mathematical expectations, covariances between state variables and variance. The methodology for applying the cluster method is shown on the example of a real ES.

The second part presents numerical studies of the parameters of the lower ionosphere at altitudes of 40–90 km when heated by powerful high-frequency radio waves of various frequencies and powers. The problem statement is considered in the first part of the article. The main attention is paid to the interrelation between the energy and kinetic parameters of the disturbed $D$-region of the ionosphere in the processes that determine the absorption and transformation of the radio beam energy flux in space and time. The possibility of a significant difference in the behavior of the parameters of the disturbed region in the daytime and at nighttime, both in magnitude and in space-time distribution, is shown. In the absence of sufficiently reliable values of the rate constants for a number of important kinetic processes, numerical studies were carried out in stages with the gradual addition of individual processes and kinetic blocks corresponding at the same time to a certain physical content. It is shown that the energy thresholds for inelastic collisions of electrons with air molecules are the main ones. This approach made it possible to detect the effect of the emergence of a self-oscillating mode of changing parameters if the main channel for energy losses in inelastic processes is the most energy-intensive process — ionization. This effect may play a role in plasma studies using high-frequency inductive and capacitive discharges. The results of calculations of the ionization and optical parameters of the disturbed $D$-region for daytime conditions are presented. The electron temperature, density, emission coefficients in the visible and infrared ranges of the spectrum are obtained for various values of the power of the radio beam and its frequency in the lower ionosphere. The height-time distribution of the absorbed radiation power is calculated, which is necessary in studies of higher layers of the ionosphere. The influence on the electron temperature and on the general behavior of the parameters of energy losses by electrons on the excitation of vibrational and metastable states of molecules has been studied in detail. It is shown that under nighttime conditions, when the electron concentration begins at altitudes of about 80 km, and the concentration of heavy particles decreases by two orders of magnitude compared to the average $D$-region, large-scale gas-dynamic motion can develop with sufficient radio emission power The algorithm was developed based on the McCormack method and two-dimensional gas-dynamic calculations of the behavior of the parameters of the perturbed region were performed with some simplifications of the kinetics.

The most part of biological tasks coincide with temperature areas, where quantum and classical effects are equivalent, or the classical one is dominating. The extent of influence of quantum or classical effect was considered in the work in application to one of the most significant problems of biological activity: particle transition through non-stationary barriers. It is interesting that the results obtained using different approaches, quantum and classical, are the same. It seems that the particle dynamics is characterized by non-coherent relaxation with rate of diffusion.