Thermal Engineering最新文献

Based on literature data, an analysis of the modes of bubble formation at orifices immersed into the working environment of bubblers was carried out. The limits of applicability of calculated dependencies for determining the average bubble diameter between different areas of bubble formation on single orifices under conditions of constant gas flow into the bubble and constant gas pressure in the resulting bubble are given. When comparing calculated and experimental data on the sizes of the formed bubbles, it was found that, for the jet mode, there is no single calculation dependence that can at the same time quite accurately reflect the influence of both the orifice diameter and the physical properties of the two-phase medium on the bubble sizes. Despite the fact that a number of studies have shown experimentally and theoretically that the movement of liquid caused by bubbles floating above has a significant effect on the size of the bubbles, there are no verified calculation dependencies at present in the literature that take into account this effect over the entire range of gas flow through orifices of different diameters under different physical properties of the working environment. Based on the balance of forces acting at the moment of bubble separation, a model is proposed that also takes into account the dependence of the size of bubbles formed at the orifice by the movement of liquid caused by bubbles floating above. As a result of generalizing a large amount of experimental data available in the literature, a generalized dependence of the dimensionless average diameter of bubbles on the Bond, Froude, and Reynolds numbers was obtained for constant flow conditions for bubble and jet modes. The derived relationship is valid for orifices with different inner diameters and a wide range of physical properties of the working medium. The lower and upper limits of applicability of the formula for bubble and jet modes of bubble formation have been established.

CO₂ emissions into the atmosphere in the electricity sector in 2022 exceeded 12.4 billion t, which is 1.8 times more than in 2000. The reasons for this growth are analyzed. It is noted that a significant contribution to these emissions (75%) is made by electricity generation using coal as fuel. It has been shown that it cannot be expected that CO₂ emissions will decrease in the near future as a result of the reduction in coal capacity; there is a steady increase in the world. In the 21st century, the total capacity of coal-fired thermal power plants increased approximately 1.9 times. Alternative ways to reduce greenhouse gas emissions are being considered, primarily through the construction of new, highly efficient power units with increased steam parameters and the decommissioning of obsolete equipment. Thanks to this, the structure of coal generation in the world is changing significantly: Thermal power plants with power units for super-supercritical (SSCP) steam parameters and supercritical pressure (SCP) already account for more than 47% of the total capacity of coal-fired thermal power plants. Such changes contributed to a reduction in specific greenhouse gas emissions from 466 g CO₂/(kW h) in 2000 to 436 g CO₂/(kW h) in 2022. In the Russian electricity sector, CO₂ emissions in 2022 amounted to approximately 410 million t. Since 2000, they have grown by only 22%. The share of CO₂ emissions from coal thermal power plants in Russia are estimated at 35–45% of the total amount of greenhouse gases associated with electricity production and does not exceed 0.5% of the global total due to the use of fossil fuels. Due to the low contribution of CO₂ emissions by Russian coal-fired thermal power plants, reducing greenhouse gas emissions from coal-fired power generation is not so relevant in the global problem and are solved mainly by replacing coal with natural gas. The need to introduce highly efficient but expensive equipment (for example, SSCP power units) at coal-fired thermal power plants to reduce emissions greenhouse gases is not as obvious as abroad, and its implementation requires a detailed feasibility study.

In the present-day conditions under which the nuclear power industry is developed, a need arises to diversify the designs of new nuclear power plant units, which should differ from the previously constructed ones by featuring flexibility to the customer requirements and by using safety systems based on fully passive safety assurance principles. In 2022, specialists of Experimental and Design Organization (OKB) Gidropress commenced activities on elaborating the draft design of a new integral pressurized water-cooled reactor plant VVER-I with natural circulation of coolant for a basic thermal capacity of 250 MW. The design incorporates passive safety systems able to provide reliable heat removal from the core under the conditions of a long-term NPP blackout and without the operator’s participation. The article presents the results obtained from thermal and fluid dynamic computations of the new reactor plant carried out using the KORSAR/GP code that has been certified for safety analyses. A reactor plant thermal-hydraulic model, which can be used for computations of stationary normal operation conditions and, subsequently, also for simulating the accident scenarios evolvement dynamics, has been developed and tested. Computations carried out using the system code have confirmed a correct choice of the reactor’s main geometric parameters and the steam generator’s heat-transfer surface for operation at the nominal power. Based on the computation results for optimizing the design, it is proposed to use a jacketed steam generator, which will make it possible to exclude stray coolant leaks in bypass of the heat-transfer surface. It is shown that the newly developed reactor plant has a significant potential for increasing the thermal power capacity up to 400 MW without introducing fundamental changes in the design. The study results can be used in designing new VVER reactors with natural coolant circulation, and also in the development of passive safety systems.

Coal, a fossil fuel, has been one of the most prominent sources of energy throughout the globe. Alongside its many blessings of being a reliable energy source, it has some curses, including global warming and air, water pollution, and environmental impacts. Born from ancient flora, decaying through epochs past, carbon-laden, fuelling eons in a vast contrast. Anthracite, bituminous, a trove of diverse grades, a worldwide energy titan, but with environmental shades. This study explores into the intricate impacts of coal-fired power plants, navigating the intersection of energy demand, environmental responsibility, and the historical legacy of this carbon-rich resource. In doing so, it employs the “cradle-to-gate” method of life cycle assessments (LCA), a well-researched approach that scrutinizes the entire life cycle of coal-fired power generation. During all three stages, fuel extraction, fuel transportation, and plant operation, basic hotspots of pollution are identified and their adverse effects on the environment are looked into. An analysis of a 530 MW power plant in China has been considered. This report uses both CML (Centrum voor Milieukunde Leiden) 2001 (Baseline) and ReCiPe Midpoint (H) analyses to conduct a detailed comparative examination of the environmental implications of the plant’s operation in addition to only the electricity generation. Climate change, freshwater aquatic ecotoxicity, acidification potential, marine aquatic ecotoxicity, etc are some of the hazards identified during the study. A better scientific approach following standard guidance and efficient management can help to mitigate the pollution caused. The article presents the results of studies of the diverse impact of coal generation on the environment and discusses the most environmentally friendly methods of using this type of fuel to generate electricity.

The article presents substantiation of the possibility to extend the operation of the SGT5-2000E series gas turbine units beyond the period specified by the manufacturer after which the “hot” parts and, primarily, the cooled nozzle vanes and rotor blades of the turbine’s first stages should be replaced. Each gas turbine unit is provided, along with the operation manual, with a maintenance program proceeding from the assigned fleet service life, in accordance with which the time of operation with one set of cooled blades of the turbine’s first stages is determined. A gas turbine cannot operate reliably unless its worn “hot parts” are checked and, if necessary, are subjected to restorative repair. As a rule, this can be done in the course of appropriately long outages (e.g., minor inspections, overhauls, and hot gas path visual examinations). All time-dependent wear coefficients are calculated simultaneously, and the calculation result is expressed in equivalent hours of operation (equiv. h), which vary depending on the pattern and number of working cycles, operational mode, used fuel, and water injection availability. A service life reduction is determined and expressed as an equivalent number of operation at the base load. The total number of equivalent hours of operation is the sum of hours calculated under the specific operation conditions. The article presents scientifically substantiated recommendations for a limited extension of the interval between maintenances obtained from mathematical modeling of the wear processes of cooled nozzle vanes and rotor blades in the first stages, and from an analysis of a change in the longevity characteristics of the alloy they are made of.

The requirement for heat sinks to better reject excess thermal energy is ever increasing due to the recent improvements in output power capacity in the Information and Communications Technology (ICT) industry. Current ICT thermal management strategies rely on single phase heat transfer techniques which have attained their upper limit. The present work aims to demonstrate that two-phase thermal system strategies can decrease heat sink size. A comparison of the heat dissipation capacity of a natural convection heat sink with and without the thermal transport mechanism of vaporization are measured and discussed. A discussion relating to the mathematical analysis of the heat transfer mechanisms leads to quantified results showing the efficiency gains of a two phase micro-porous heat sink. It is shown that the presence of evaporation from the holes on the front surface of the radiator makes it possible to reduce its size by 37.6% compared to a radiator in which heat removal is carried out only by natural convection.

The construction and operation of nuclear power plants are characterized by significant capital costs associated with ensuring compliance with stringent nuclear safety requirements. To ensure a low estimated cost of electricity generated at nuclear power plants, it is especially important to increase their efficiency, which depends on the thermal efficiency of the turbine unit. Based on the criterion of economic efficiency, directions for increasing the thermal efficiency of nuclear power plants with pressurized water reactors (PWR) have been studied: increasing the fresh steam pressure, reducing the steam pressure in the condenser, optimizing the structure and parameters of the second circuit, and improving the efficiency of the turbine flow parts. Significant economic losses are caused by the use of a circulating technical water-supply system provided for at all designed nuclear power plants (according to Article 60 of the Water Code of the Russian Federation). It is noted that the ban on the use of direct-flow water supply systems contradicts the world experience of creating nuclear power plants. The difference in the efficiency of cooling systems of two types is clearly shown by the example of the design indicators of the Leningrad NPP-2 (LNPP-2) and the Tianwan NPP (power unit nos. 7, 8), which use identical reactor systems (RS) but different turbine units and technical water supply systems, which determines the difference in electrical power (up to 66 MW). Using data from the PRIS (power reactor information system) information system on power reactors around the world and the results of calculations by turbine construction companies, the level of thermal efficiency of low-speed turbines abroad, achieved through comprehensive optimization of technical solutions, was assessed. The reserves for increasing the economic efficiency of domestic nuclear power plants with PWR have been identified. It was noted that foreign companies do not stop working to improve the performance of the flow part of low-speed turbine units: models have been created with a last-stage blade length of 1905 mm. According to estimates, the total economic effect from increasing the efficiency of nuclear power plants when implementing all of the above measures, expressed through allowable additional investments, is 14 billion rubles, which is comparable to the cost of supplying all the key equipment of the power unit’s turbine room.

Velocity fields measured in the vicinity of the perforated leading edge of a turbine nozzle vane using the particle image visualization technique are presented. Noncontact measurements were performed in a plane segment consisting of three nozzle vanes and having an optically transparent inlet section offering visual access to the region of the leading edge of the central vane for a high-speed camera and to the laser sheet. The experimental investigations were performed at a fixed incoming flow velocity of 33 m/s, and the relative air flowrate through the cooling holes varied from 1.6 to 6.4%. The cooling film flow near the leading edge was visualized for three models of vanes differing in the air supply method to the holes, hole diameter, and number. Supply of the coolant to the cooling holes from one cavity resulted in a high degree of nonuniformity in the distribution of the film over the leading edge, which was caused by a high blowing ratio for the jets injected through holes located closer to the suction side. The experimental results have revealed that separate supply of cooling air to the holes on the pressure side, leading edge, and suction size minimizes sensitivity of the formed film thickness to the relative flow rate of the coolant and provides a more uniform distribution of the coolant over the vane surface in a wide range of the blowing ratio for the jets that varies from 0.5 to 2.5. Visualization has demonstrated extensive unsteadiness of the film flow along the vane airfoil. In this case, the cooling jet fed through the central hole oscillates, thereby leading to periodic formation of a film on either the pressure side or the suction side.

The work is devoted to an experimental study of individual poorly studied stages of vapor explosion triggering (a dangerous destructive phenomenon that occurs during certain emergency situations in nuclear energy, metallurgical, pulp and paper, and other industries). Experiments were carried out to study the propagation of the detonation front after spontaneous explosive boiling (triggering) of water on a molten drop of salt (NaCl) and a vapor explosion stimulated by it on closely spaced neighboring drops of salt and tin. The temperature of the melted drops in the experiments was 850–1100°C and water temperature was room temperature (22–24°C). The main research tool was high-speed video recording of the process (recording frequency up to 50 kHz, exposure up to 5 μs). In order to study the initial stage of triggering associated with local contact of the cooler with a hot substance, experiments were carried out using high-speed video footage of the process of the vapor film coming off on a hot solid sphere, synchronized with fixing the sphere-cooler contact electrically. The footage of the instantaneous (precipitous) mode of vapor film disappearance with a duration of 200–500 μs and gradual (progressive) mode lasting approximately 100 ms on spheres under similar experimental conditions. It is shown that the main influence on the regime of film melting and vapor explosion on molten tin drops is exerted by the pressure pulse from the vapor explosion on a nearby NaCl drop. The characteristic times of the triggering process have been determined: tens to hundreds of microseconds. The value of the primary pressure pulse in the liquid has been established. The decisive role in triggering fine fragmentation of centimeter-long drops of hot liquid by the first contact of cold liquid with their surface has been confirmed.

Statistical data on the level of use of ash and slag from thermal power plants is provided. The results of an analysis of regulatory and technical documentation in the field of ash and slag management in the energy sector are presented. Laws, regulations, and other documents regulating the management of by-products of coal combustion are considered. It was noted that it is necessary to refine the existing documentation, introduce legally defined terms and definitions for a number of ash and slag processing products, and also legally consider ash and slag as mineral raw materials and not waste. Definitions of ash and slag are given in accordance with the current industry regulatory document, in which ash and slag are тще called waste but mineral residues of solid fuel. It is shown that various government agencies are preparing regulatory documents related to the involvement of ash and slag into economic circulation. However, the existing regulatory framework does not meet the goals of achieving the level of low-waste and waste-free production. It was noted that regional programs have currently been approved to increase the level of ash and slag utilization from thermal power plants in the constituent entities of the Russian Federation. Activities are presented that will make it possible to achieve the indicators for the level of utilization of ash and slag from thermal power plants in accordance with the Energy Development Strategy of the Russian Federation until 2035. It is explained how legally correct regulatory and technical documentation will make it possible to increase the level of ash and slag utilization in Russia and will help eliminate accumulated harm to the environment. It has been shown that the most large-scale, high-tech, environmentally friendly and economical use of fly ash is its replacement of up to 40% of cement in the construction of buildings and structures. Definitions of fly ash used in cements and concretes are given in the standards of different countries (EU countries, United States, Australia, India, China, Japan, Russia) depending on the type of ash formed. A comparative analysis of Russian and foreign national standards for the use of ash in cements and concretes was carried out in terms of physical and chemical characteristics, which are significant limiting factors when choosing directions and projects for the use of ash.