Thermal Engineering最新文献

A unique substance or material that releases or absorbs enough energy during a phase shift is known as a phase change material (PCM). Usually, one of the first two fundamental states of matter—solid or liquid—will change into the other. Phase change materials for thermal energy storage (TES) have excellent capability for providing thermal comfort in building’s occupant by decreasing heating and cooling energy demands. Because of its latent heat property, a PCM has a high energy density. The building uses PCMs mainly for space heating or cooling, control of building material temperature and increase in building durability, solar water heating, and waste heat recovery from high heat loss locations. Phase change materials for thermal energy storage has been proven to be useful for reducing peak electricity demand or increasing energy efficiency in heating, ventilation, and air-conditioning systems. The primary grid benefit of PCM based thermal energy storage system is load shifting and shedding, which is accomplished by recharging the storage system during off-peak times and substituting heating, ventilation, and air-conditioning system operation during peak times. This study examines PCM based thermal energy storage systems in building applications and benefits, focusing on their substantial limitations, and closes with recommendations for further improvement of design for use.

The results are presented of the search for and systematization of information on typical design solutions for the main heat exchangers of installations with low-boiling working fluids. The organic Rankine cycle (ORC) has been widely accepted as a way for converting waste (exhaust) heat into electrical energy. An increase in the installed capacity of operating commercial ORC power plants and their total capacity is noted in the world every year. At the same time, design options for the main heat exchangers (heater, evaporator-superheater, condenser, regenerative heat exchanger) are not available in open access and presented in catalogues: information about them is not disclosed by the manufacturers and information available in publications is limited and disembodied. An attempt is made in this paper to systematize the available information and, based on an analysis of world and domestic experience in industrial production, formulate an idea of potential engineering solutions for heat and mass transfer installations, which can be offered as prototypes of the considered apparatuses. At the same time, the search for such solutions was focused primarily on apparatuses used in the refrigeration industry, conventional steam turbine power units, and modern ventilation and air-conditioning systems. The advantages and disadvantages of such apparatuses are examined. The results are presented of a comparative analysis of their design, power range, operational features, and the potential effect of these factors on the operation of the overall ORC installation. Approaches to the selection of heat-exchange equipment for ORC installations given in the available publications and proven in practice have been investigated and described.

The article discusses the potential problems that have to be solved in the framework of development and justification of the water chemistry (WC) conditions required to ensure corrosion resistance of the structural materials used in the core and coolant circuit of the power-generating reactor used in the supercritical water cooled VVER-SCW nuclear power plant (NPP). In reactors cooled with water at supercritical temperature and pressure, the integrity of their physical barriers (fuel-rod claddings and reactor coolant circuit boundaries) depends in many respects on the possibility of maintaining the necessary water chemistry conditions that will guarantee the corrosion resistance of equipment and pipeline structural materials for the power unit’s entire service life. The most complex challenge in this regard is to inhibit corrosion and flow-accelerated corrosion processes and to minimize the formation of deposits on the surface of equipment operating in the domain of near-critical and supercritical conditions. The article formulates the limitations that are suggested to be considered in transferring the experience gained from the standardization of water chemistry in supercritical pressure (SCP) power units at thermal and nuclear power plants to the VVER-SCW NPPs. An analysis is carried out that makes it possible to estimate the effect the chemical composition of a supercritical water coolant has on the corrosion state of candidate structural materials for fuel-rod claddings with the aim to get better insight in the main processes occurring in aqueous solutions and for developing (elaborating) a WC conduction technology as applied to ensuring the integrity of the VVER-SCW NPP physical safety barriers.

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.