Thermal Engineering最新文献

The article presents the results of studying the wet torrefaction (or accelerated hydrothermal carbonization) process in a fluidized bed in superheated steam medium. The aim of such experiment is to obtain charcoal from biomass the characteristics of which are at least as good as those of biochar obtained using the classic hydrothermal carbonization method, but with a drastically reduced the process duration and abandoning the use of reactors operating at high pressure. Previously, the processes of torrefaction in superheated steam medium of a mixture of chicken manure and wood sawdust, wastewater settlement, hazelnut shell, already used tea leaves, and sunflower husk were studied. Samples of biochar that can be used as biofuel, sorbents, or soil improvers were obtained. The duration of the wet torrefaction process in a fluidized bed was not more than 30 min instead of 5–12 h in the case of using the classic hydrothermal carbonization, and the steam gage pressure was not higher than 0.07 MPa. In the framework of the project that is being implemented jointly with the universities of the BRICS member states, the thermochemical reprocessing of potato peel into biofuel with the process temperature varied from 200 to 300°С and duration from 10 to 30 min was studied. As a result of potato peel torrefaction, the biomass heating value (in ash-free dry state) increases by a factor of 1.35 (from 20.68 to 28.0 MJ/kg). It has also been found that an increase in the torrefaction process temperature and duration facilitates an increase in the potato peel heating value with the activation energy ranging from 16.8 to 23.9 MJ/kg.

The photovoltaic parameters of a perovskite solar cell with the ITO/ZnO/CH₃NH₃PbI₃/NiO/Ag heterostructure are calculated and optimized. Perovskite CH₃NH₃PbI₃, which is the most promising material for photovoltaic converters, was used as the solar energy absorber. Zinc oxide ZnO with n-type conductivity, which ensures high mobility of electrons, and nickel oxide NiO, which features high performance stability and optimal electrical characteristics for use as a buffer layer with p-type conductivity, were used as electrodes at the junctions with perovskite. Such buffer layers ensure stable, reliable, and long-term operation of a solar cell. The modeling was carried out by solving the system of Poisson equations and continuity equations for electrons and holes in 1D stationary approximation with taking into account defects in the perovskite volume (10¹³ cm^–3) and at the ZnO/CH₃NH₃PbI₃ (10⁸ cm⁻³) and CH₃NH₃PbI₃/NiO (10¹⁰ cm^–3) junctions. Numerical calculations were carried out using the SCAPS-1D software package, which implements the diffusion-drift model of charge transfer in semiconductors. It has been determined that the perovskite layer thickness has an essential influence on the cell key parameters: open-circuit voltage, short-circuit current density, efficiency, and fill factor of current-voltage characteristic. The maximal efficiency (24.3%) is reached at a perovskite layer thickness of around 1.55 μm. In that case, the open-circuit voltage is 1.15 V, the short-circuit current density is 24.6 mA/cm², and the fill factor is 86.6%. The current-voltage characteristic shows that the current density remains stable to the voltage value of around 0.9 V.

Bulk condensation is a fairly common phenomenon in science and technology, so there is a need to determine the characteristics of a flow with condensing impurities, including the mass flow rate, quantity, and size of the formed and growing particles. To solve the problem of separating these particles from a gas–droplet flow, the type of distribution function, as well as the determination of the mass flow rate of droplets depending on their size, are the main initial data on which the quality of the design of the separation device depends. In the process of solving the kinetic equation using a direct numerical solution, the moments, as well as the particle size distribution function, are available at each step and do not require their reconstruction, whereas only the corresponding moments are available when using the moment method, but the distribution function is not. It is known that it is possible to reconstruct the form of the distribution function using the values of its moments, but the implementation of standard methods requires a large number of moments, and they have low stability or low accuracy. In this regard, the author’s modification of the well-known method for restoring the distribution function using the gamma distribution was developed, tested by comparison with known solutions. It is shown that the maximum difference between the known distribution function and the function reconstructed through a modification of the gamma distribution approximation, when solving the problem of bulk condensation during supersonic outflow of a vapor-gas mixture through a nozzle in a wide range of initial pressures, temperatures, and mass fractions of impurities for various components of the mixture, does not exceed 5%. In a subsequent comparison of the distribution function, reconstructed from the moments determined from the solution of the kinetic equation by the method of moments, with a direct numerical solution, the difference was at the level of 10%, while the probability of particles falling into a predetermined interval when ranking the distribution function by intervals was no more than 5%, which is acceptable for taking these data into account in flow separation problems. A method is proposed for determining the mass flow rate of droplets (particles) whose radius falls within a predetermined interval for the subsequent formulation of the problem of separating a gas–droplet or gas–dust flow using the reconstructed distribution function.

During normal operation, the upper section of the injection pipeline of the pressurizer system in a reactor plants (RP) with VVER reactors contains steam and subcooled water induced by prescribed operating coolant leakages. To substantiate the safe operation of the RP, experimental studies of the conditions for the initiation of the condensation-induced water hammer (CIWH) are required. Such studies are needed to confirm the absence of CIWH events under steady-state conditions and to estimate potential impact loads during transients. The problem was formulated to perform preliminary calculations of the experiments to substantiate that the results obtained in a downscaled model made to 1 : 2 scale for all dimensions may be transferred to the conditions of a full-scale pipeline. The developed computational model, implemented in the Russian code KORSAR, is described. The paper presents the results of a computational study of thermohydraulic processes in a full-scale injection pipeline after a decrease in the water level in the pressurizer below the spray nozzles during heating of the reactor plant in the presence of the prescribed permanent coolant leakage through the pipeline. Critical flow rates at which steam does not enter the injection pipeline or fills only the section located in the pressurizer have been determined. Variant calculations were performed of a scaled pipeline with different values of the permanent leakage flowrate; flow rates were set as required to ensure similarity in terms of Reynolds, Mach, Froude, and homochronicity numbers. When comparing predictions for the full-scale and downscaled pipelines, the similarity was demonstrated of thermohydraulic processes at a model flowrate of permanent leakages, selected to obtain the same of Froude numbers, and the full-scale values of the steam pressure and the incoming water temperature.

So far, there has been a little imperfection in the analytical theory of the radiation heat transfer. Although there is the calculation formula for non-transparent bodies, but more universal calculation formulae for partially transparent bodies have not yet been found. Up to now, the radiation heat transfer in solar greenhouses, solar collectors and general houses and buildings with windows has been analyzed using the repeated reflection method. However, the repeated reflection method should make a great effort to calculate compared to the effective radiation method, that is to say, the radiosity method. Therefore, we derived formulae for the calculation of radiation heat flux by using the effective radiation method (namely, the radiosity method) when two gray bodies randomly placed in space are partially transparent bodies. As a result, we ensured the universality of calculation theory in the radiation heat transfer.

Morphology and thermophysiscal properties of deposits of construction material corrosion products (CMCPs) in steam-generating systems at nuclear power plants (NPPs) and thermal power plants (TPPs) are analyzed. Modern power units utilize high-quality water, and construction materials are essentially steels. Therefore, the main impurities in water are various iron compounds. The deposits of corrosion products usually consist of iron oxides (mainly magnetite). These deposits have a specific capillary-porous structure and considerably affect heat-and-mass transfer during phase transformations. Capillary hydrodynamics also has a pronounced effect on transfer processes. In recent decades, many publications have appeared about the effect of deposit morphology on the concentration of impurities and reagents that control the water chemistry in reactors and steam generators of nuclear power plants. Growing attention has also focused on the temperature conditions of steam-generating surfaces with deposits during heat transfer. The governing effect is produced by the structural features of the deposit layer, such as the diameters of steam and liquid microchannels, density of their distribution over the layer surface, internal characteristics of the boiling process, thermophysical properties of the deposits, thermodynamic and transport properties of water and steam. An analysis of the published data yielded approximating correlations for the diameter of steam microchannels, their distribution density, effective thermal conductivity of the deposit layer, and the maximum water superheating temperature in the deposit layer. Predictions by these correlations satisfactory agree with experimental data. A classification of deposits by their thickness based on the maximum temperature during steam generation at nuclear and thermal power plants is recommended.

Lack of a unified and commonly accepted standard for describing flaws in performing equipment diagnostics results in that different authors use essentially different terminology for denoting them. In view of this circumstance, difficulties are encountered in attempts to generalize the signs presented in various literature sources, as well as flaw revealing methods. In addition, in many publications on diagnostics, their authors use, at a first glance, similar terminology: “defect,” “damage,” and “flaw.” The term “defect” means each individual nonconformity of products to established requirements; i.e., this term is more relevant to errors in the technology of manufacture, assembling, and repair of a product by the corresponding specialized enterprises, at which the product quality control is performed, so that this term is not quite relevant to equipment damages occurred during its operation. The term “damage” denotes an event which consists in a loss of an object’s perfect state with retaining its up state, and the term “flaw” denotes an object’s state in which it does not meet at least one of the requirements stated by the regulatory technical and/or engineering (design) documentation. The term “flaw” is the most exhaustive one and includes both the cause (the actual damage of parts) and effect (deviation of the parameters monitored and manifestation of this damage). For the purpose of further development of diagnostic systems, it is necessary to elaborate a group of standards regulating the developments in the field of power equipment diagnostics. The article presents—taking a steam turbine as an example—proposals on general classification of damage kinds and on standardization of terms and definitions of main damages.

A review of modern desalination technologies is conducted with an analysis of their advantages and disadvantages. The processes of heat and mass transfer in a spray heat exchanger are considered to determine the optimal operating parameters that ensure the minimum possible heat-exchange surface area. Mathematical modeling of heat and mass transfer processes in a spray heat exchanger has been performed. The influence of pressure in the irrigation chamber, the amount of salt deposits, and the flow rate of the heating coolant on the thermal and design characteristics of the desalination plant is analyzed. As a result of the numerical experiment, the dependence of the heat-transfer coefficient and heat-exchange surface area on the studied parameters was established. It was found that, with a decrease in pressure in the irrigation chamber and an increase in temperature pressure, the boiling process intensifies, which leads to an increase in the heat-transfer coefficient. This effect allows for significantly reducing the required surface area and dimensions of the irrigation heat exchanger. Based on the calculation studies, the optimal operating parameters of the heat and mass exchange apparatus were determined: the permissible thickness of the carbonate deposit layer (the content of CaCO₃ and MgCO₃ is over 50%, the thermal conductivity coefficient is approximately 1 W/(m² K)) should not exceed 0.1 mm; the pressure in the steam generation chamber is recommended to be maintained at 10 kPa; the heat-exchange surface area (required and possible) under the given design conditions (0.04 and 0.05 m internal and external pipe diameter, respectively; brass material, 29 independent circuits) is 5 m². With the specified parameters, the heat-transfer coefficient reaches 2500 W/(m² K) for the considered design of the device.

The article presents the results of full scale abrasion and corrosion resistance tests carried out on the experimental specimens of grade St20 and 09G2S steels with protective Cr–CrС and Cr–CrN ion-plasma coatings (IPCs) after they have been held for 576 h in a 25 kW fluidized bed cyclone stoker operated on three kinds of pelletized biofuel: flax shives, straw, and sunflower husk. The temperature in the furnace and in the gas flue, in which the experimental specimens made of grade 09G2S and St20 steels were placed, was from 750 to 650°С. A chemical analysis of the ash produced after combusting these three kinds of pelletized fuel has shown that potassium and chlorine were available in the ash composition, the presence of which may give rise to the occurrence of corrosive compounds in the case of moisture condensation during the equipment outage periods. It has been found that the coatings studied did not experience abrasion or corrosion destruction; the coating thickness changed by a factor of 1.2–1.5 on the average. A deviation in the morphology was noted as a consequence of coating surface layer oxidation at the maximal exposure time; however, no material oxidation under the coating was found on the transverse metallographic sections of coated specimens. The bench tests of grade 09G2S steel specimens with protective Cr–CrN IPC carried out after holding them in the installation’s furnace at an abrasive air flow incidence angle of 90° have shown that the abrasive resistance in terms of steady wear rate has increased by no less than a factor of two in comparison with that of noncoated specimens. In testing the corrosion resistance of specimens made of grade St20 steel with Cr–CrC coating after holding them in the installation’s gas flue followed by holding them for 480 h in a water extract from the ash of each of combusted biofuels, it has been noted that the corrosion rate determined using the gravimetric method has been decreased by 1.2–2.3 times in comparison with the same indicator of noncoated specimens. According to the study results obtained, multilayer Cr–CrС and Cr–CrN ion-plasma coatings are promising ones for ensuring protection against corrosion and abrasion wear for the furnace and gas flue materials in fluidized bed boilers that use agricultural waste (flax shives, straw, or sunflower husk) as biofuel.

The application of artificial neural networks (ANN) based on various types of the recurrent architecture for simulation of the dynamics of development of complex process power-generating facilities, in particular gas-turbine units (GTUs), is examined. The challenges are outlined that arise in constructing classical mathematical models of such units, including a need to have reliable information on physical regularities, weight and size of equipment, correction factors, and to perform labor-intensive verification. The proposed approach eliminates these restrictions since the model uses only archived data on signals from the automatic process-control system. As a result, direct specification of physical parameters is no longer required. The author focuses on the application of ANNs to construct models that can reproduce nonlinear relationships among control actions, ambient conditions, and technical state of equipment. The proposed neural network models were trained and validated against actual operating data acquired at a 6-MW gas-turbine unit. Archival time series longer than 2 years were employed. These models have been demonstrated to offer good generalization service. A studied probabilistic extension of the model can form confidence intervals of predictions, thereby improving the reliability of diagnostics and detection of abnormal behavior. The architectural features of the applied solutions, including long short-term memory and sequential encoder-decoder models with an “attention” mechanism are analyzed. Besides numerical metrics, the model response is studied during individual characteristic regimes, including startups, shutdowns, and transients. The presented results confirm the practical significance of the proposed approach in solving the problems of developing digital twins, monitoring systems, and training simulators for operational personnel.