Thermal Engineering最新文献

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.

Due to the constant increase in energy consumption, remoteness from industrial centers, the need to import organic fuel for economic activities and livelihoods of the population, a wider deployment of wind power plants (WPPs) in hard-to-reach areas of the Arctic is required. The key element influencing the efficiency of a WPP is the wind wheel, the design of which is associated with certain difficulties due to the extreme climatic conditions in the Arctic. The presented work describes an approach to digital design of the aerodynamic shape of a wind turbine blade based on parametric optimization technology. The target indicator is the wind energy efficiency (WEE), which is calculated by direct numerical modeling of the aerodynamics problem using modern computational methods, as well as high-performance supercomputer technologies. The introduction of digital design and modeling principles has enabled the integration of geometric models and associated engineering modeling tools into the computer environment. The main concept of the approach is to describe the geometric characteristics of the blade with a finite number of parameters, changing which one can obtain the shape of the wind wheel blade in a wide range of possible configurations. For each shape, a computational CFD model is created, which, based on the numerical solution of the Navier–Stokes viscous flow equations, makes it possible to determine target indicators in the form of aerodynamic characteristics of the blade at a given wind speed. An example of the use of parametric optimization technology for the design of a wind wheel intended for operation in the Arctic regions is considered. The functionality of the concept for determining the shape of the blade in single-mode and multimode options for using a wind wheel has been demonstrated. It is shown that the obtained geometric characteristics make it possible to improve the target performance of a typical blade used in practice. For a WPP with a power of 100 kW with a wind wheel with a diameter of 24 m, solutions were obtained that provide a WEE value of 0.45 in the range of design wind speeds from 6 to 9 m/s.

Heat exchangers in a gas turbine circuit for advanced space power facilities are innovative developments in the field of elaboration of heat exchangers meeting a set of technical and operating requirements. A design of a heat-transfer matrix consisting of a set of biconvex stamped plates with a specific surface relief is proposed. It enables construction of heat exchangers meeting the requirements for strength, stiffness, weight, and size under the specified operating conditions. The results of testing of heat exchangers made of 288 and 450 plates are presented. A nonuniformity of the coolant distribution among heat-transfer panels was found. Under the same operating conditions, it makes the thermal efficiency of a heat exchanger made of 450 plates lower than that of a heat exchanger made of 288 plates. Based on the analysis of test results, a mathematical model has been developed for the thermal and gas-dynamic design calculation of a heat exchanger with an arbitrary number of plates, which takes into account the effect of nonuniformity of the coolant flow through the channels between the plates. The calculation calls for determination of a temperature distribution along the length of each channel considering variable thermophysical properties of the coolant. For the mathematical model, a dimensionless dependence of the resistance coefficient and the Nusselt number on the Reynolds number was found in the Reynolds number range from 500 to 2000. Universal dependences enable simulation in both hot coolant paths consisting of identical interplate channels and cold coolant paths consisting of the same complete and half channels. The correlations for the hydraulic resistance coefficient and the Nusselt number versus the Reynolds number agree well with the known dependences obtained for corrugated heat-transfer surfaces of the same class. The mathematical model has been verified against experimental data. The effect of the scheme of connection of heat exchangers with different number of plates to a gas circuit is examined. The connection schemes are analyzed, and one of them is recommended as the most suitable for practice.

This report examines the issue of economic feasibility of constructing small nuclear power plants (SNPPs). The preparation of this commentary is due to a number of controversial theses presented in the article by V.O. Kindra, I.A. Maksimov, I.I. Komarov, S.K. Osipov, and O.V. Zlyvko “Small-Power Nuclear Power Plants: Technical Level and Prospects for Commercialization (Review)”, published in the journal Teploenergetika no. 4, 2024. The article notes the need to use new circuit solutions and working fluids for SNPPs. However, an analysis of the prerequisites for the creation of SNPPs allows for the conclusion that the use of complex thermal cycle circuits for such plants is inappropriate. The article rightly notes that SNPPs can only be competitive in remote areas that are difficult to access for the import of fossil fuels. But the inaccessibility of the territories is a determining and largely limiting factor for the construction and operation of SNPPs themselves on these sites.

The article considers assessment methods and criteria of damage inflicted to turbine blades under the effect of static loads in carrying out 3D structural analyses of modern foreign and domestically produced high-capacity power units. Factors that should be considered in performing strength and lifetime analyses of the rotor blades of high-capacity turbines when subjected to short- and long-term static loading are pointed out. The article also describes 3D techniques for carrying out elastoplastic assessment of short-term static strength using a procedure for determining the limit rotation speed to blade fracture, airfoil residual displacements and strains, shank ultimate strength and displacement, root tearing-off, shear, flexural strength, etc. The article presents mutually complementary techniques for determining the bearing capacity as well as global and local long-term strength with using cumulative strain predictions by creep curves. Criteria used in different lifetime assessment methods are described, including those applied at different design stages and in using thermal protection coatings. Cases are considered in which creep strains are determined in the absence of data on creep curves by carrying out elastoplastic analyses by isochronous curves and lifetime analysis using the Larson–Miller curves. The need to take multiaxiality into account in estimating local creep in places of stress concentration is shown, and the applicability limits and criteria of such assessment that make it possible to increase the predicted lifetime by up to two times are described. Examples of tensile and compressive stress relaxation in estimating cumulative creep strain are given. Matters of creep interaction with other types of damage, including high-cycle and low-cycle (thermal cycling) fatigue, and various turbine loading kinds are considered.

The district heating system is an important heating mode in the northern cities of China. In recent years, the scale of the district heating system is expanding day by day, the pipe network structure is more and more complex. The problem of hydraulic imbalance of the pipe network is gradually emerging, therefore, it is urgent to establish an accurate and perfect hydraulic simulation model of heating network to assist operation management. Pipe network simulation modeling is one of the important prerequisites to solve the hydraulic imbalance problem of heating pipe network. However, with the increase of service time, the actual resistance coefficient of heating network becomes difficult to obtain, which is one of the key reasons for the low accuracy of pipe network simulation model. In order to overcome this difficulty, this paper proposes to use the resistance coefficient identification model based on the differential evolution algorithm (DEA) to identify the resistance coefficient of the heating pipe network. Based on graph theory, network matrix and the law of conservation of mass, the hydraulic model of the heating pipe network is built, and the nodal pressure method is used to solve the model. On the basis of comprehensive consideration of the mainstream intelligent algorithm, the differential evolution method is selected as the algorithm to identify the resistance coefficient of pipeline. In order to verify the identification effect, the feasibility of the model was verified by calculating the data of three different operating conditions of the practical engineering named “K district heating system”. The results demonstrated that the relative errors of the identified resistance coefficients are all within 10, and 98% of the identified values are less than 5%.