Thermal Engineering最新文献

Experimental data on the thermal resistance of thermosiphons designed with different structural junctions (“pipe in pipe,” “flat shelve”) are obtained, and the possibility is shown to decrease it as a result of heat transfer enhancement in the evaporator by using a combined coating in the form of longitudinal grooves 0.1 mm in radius, filled with aluminum oxide nanoparticles 100–200 nm in size. The effect the coating has on the thermosiphon thermal resistance was studied in an experimental section in the form of a 100 mm long pipe 38 × 3 mm in diameter made of grade AISI304 stainless steel. As a result of applying the coating, the thermosiphon thermal resistance is decreased by 2.4–3.0 times at heat fluxes in the range 200–1700 W/m² and evaporator slope angles 0°–30°. Experiments with slope angles equal to 0°, 10°, and 20° were carried out, and it is pointed out that the maximal effect is reached at the evaporator horizontal and slightly inclined positions. For calculating the heat transfer in a thermosiphon, a procedure based on the Yu.A. Kuzma-Kichta formula is proposed. The predicted thermosiphon thermal resistance values agree with the experimental data within 20%, a circumstance that extends the model applicability area.

Loop heat pipes (LHPs) are two-phase heat transfer devices with capillary pumping of the working fluid inside a sealed, closed loop, including an evaporator and a condenser connected by smooth-walled pipelines for separate movement of vapor and liquid, the diameter of which usually varies from 2 to 8 mm. This design feature enables easy integration of LHPs into densely packed systems. The LHP evaporator is equipped with a fine-pored capillary structure (wick) (pore radius from 1 to 20 μm), which creates high capillary pressure. Loop heat pipes are used in energy-efficient systems for utilizing low-potential heat, heating or cooling remote objects, and for uniform heat distribution over a large surface area of ​​heat sinks. Currently, the most popular area of ​​application for LHPs is electronic cooling systems operating under a wide variety of conditions. Various liquids can be used as working fluids in LHPs, depending on their operating temperature range, chemical compatibility with structural elements, and the required operating characteristics of the LHPs. This paper presents the theoretical foundations and operational conditions for LHPs. It also discusses the issue of selecting a working fluid to enhance the heat transfer capacity of LHPs. Quality criteria are proposed for assessing working fluids based on their efficiency in these devices. The results of a comparison of six different working fluids using these criteria are presented, showing that ammonia is the most effective working fluid in the operating temperature range relevant for electronics, from 40 to 70°C. Based on the p, t diagram of the working cycle of the working fluid, the thermal resistance of the “heat source–LHP–heat sink” system is visualized, as well as the thermal resistance of the main structural elements.

The article summarizes the key scientific and technological achievements in the field of applying gas hydrates in the power industry. The modern ideas about the specific features relating to the synthesis, transportation, storage, regasification, and use of hydrates for thermal conversion of low-grade fuels and waste, separation of gaseous and liquid media, as well as sequestration of anthropogenic emissions are analyzed. It is shown that the experimental data and the results of mathematical modeling and various-scale tests that have been obtained for the last years in the world’s scientific society form a basis for the development of technologies for applying natural and man-made hydrates. Concepts for integrated use of hydrates in the power systems at mines and in supply of energy resources to settlements are outlined. The technical and economic constraints connected with the development of gas hydrate technologies, their target indicators, and potential solutions of the formulated problems are determined. For the gas hydrate technologies to be efficiently introduced in the energy sector, the following so-called target business metrics (threshold indicators) should be achieved: gas content is no less than 180 vol/vol.;¹ purification ratio of gas obtained after separation of smoke, natural, casing-head, and synthesized gases is no less than 90%; maximal loss of gas in storing and transporting hydrates is not more than 0.1%; hydrate storage term is up to six months; stable combustion temperature during joint low-emission combustion of hydrates with low-grade fuels (sludges, heavy coal-tar products, cakes, etc.) is not lower than 1100°С. For solving these problems, it is necessary to carry out fundamental and applied studies, as well as various-scale tests for integrating gas hydrates into modern power systems with ensuring environmental cleanliness and economic efficiency. The conditions under which essential advantages are achieved in comparison with alternative technologies in each hydrate application area have been determined.

The article presents the results of experimental studies aimed at determining the dimensionless heat transfer coefficient (the Nusselt criterion) in a round tube during forced upward motion of salt melt. The relevance of this problem is substantiated by the presented review of literature sources, which has shown that there is an insufficient scope of experimental data on heat transfer for the flow of salt melts against the gravitation forces in laminar ant transition regimes in the temperature range 230–350°C. The experiments were carried out in transition and turbulent flow regimes in the range of Reynolds numbers 2600–23 000 with the applied heat fluxes 70–670 kW/m² and Prandtl numbers 8.9–14.7. A melt of 50% NaNO₃–50% KNO₃ (in molar fractions) salt was used as working medium. The experimental values of heat transfer coefficients are in good agreement with the data obtained using the well-known relationships, such as the correlations proposed by Petukhov–Kirillov, Sieder–Tate, Gnielinsky, and Hausen. Based on the authors’ own data and an analysis of existing correlations, a new empirical formula is proposed for calculating the Nusselt number in the range of Reynolds numbers from 4000 to 10 000. For the developed turbulent flow regime, the Hausen correlation has shown the best agreement with the experimental values: the maximal mismatch does not exceed 7%, due to which this correlation can be recommended for carrying out calculations in the studied ranges of the parameters.

The article presents an analysis of the effect the aspect number A (the ratio of the melt layer height to the reactor pressure vessel inner diameter) and the temperature conditions at the boundaries of the cylindrical steel melt layer heated at its lower surface have on the heat flux focusing on its lateral surface. Such layer corresponds to the metallic layer in the stratified corium melt pool that is produced in the nuclear reactor pressure vessel during a severe accident, and the heat flux of which affects the pressure vessel, causing its heating and partial melting. The pressure vessel ability to retain high-temperature melt inside of it in the course of a severe accident in using external cooling of the vessel depends on the heat fluxes acting on it from the high-temperature corium side. If the metallic melt is stratified, a heat flux focusing effect emerges in the area of melt upper layer, which determines the most dangerous and thermally stressed place, the heat flux in which may exceed 1.5 MW/m². The possibility to promptly and adequately estimate the influence of heat fluxes on the reactor pressure vessel is an important objective in elaborating severe accident management measures. The article proposes a procedure for determining the heat flux focusing factor on the steel melt layer lateral surface, which is based on applying the previously obtained modified formulas for calculating the Nusselt number and the melt layer bulk temperature. The newly developed procedure was used to perform a parametric analysis of the influence of the layer geometrical parameters and the temperature conditions at its boundary surfaces on the focusing effect. The analysis results have shown that the heat flux focusing effect depends on the temperature conditions at the layer boundaries and on the layer thickness. Thus, variations of differences between the temperatures of layer lower and upper surfaces, and also between the temperatures of its lateral and lower surfaces may entail a several-time increase/decrease of heat flux intensities acting at the melt layer surface and on the reactor pressure vessel during a severe accident. The heat flux focusing factor depends most essentially on the temperature conditions at the layer boundaries at the aspect ratio A values not larger than 0.4, which should be taken into account in elaborating severe accident management strategies. In the subsequent, it is worthwhile to perform experimental studies to confirm the validity of the newly developed procedure for determining the heat flux focusing effect in the relevant scenarios and under real severe accident progression conditions, as well as with model media (melts) having properties close to those of the materials forming the corium melt bath in nuclear power facilities during accidents of a similar class.

Steam-water injectors are widely utilized in various industrial applications and production processes, particularly for their ability to enhance energy efficiency, reduce emissions, lower energy consumption, and facilitate energy reuse. In this study, computational fluid dynamics (CFD) is employed to investigate the impact of internal geometric parameters on the performance of steam injectors. Key internal flow characteristics are analyzed, focusing on the effects of the orifice injection angle, dilatation inclination, throat size, and the presence of a porous medium within the mixing chamber. The results reveal that an injection angle of 45°, a dilatation inclination between 0.9° and 1.1°, and a throat diameter of 14–16 mm significantly enhance steam-water phase mixing, leading to superior heat exchange efficiency. While including a porous medium does not improve heating performance, it substantially reduces noise levels within the system. By optimizing the full set of internal geometric parameters, the study demonstrates a marked improvement in the injector’s entrainment capacity. These findings provide a valuable reference for the continued research, development, and optimization of steam–water mixing heaters, offering practical insights for enhancing performance in industrial applications.

For designing facilities operating on organic working fluid according to the Rankine cycle, reliable methods for calculating organic coolant condensation processes in channels, including inclined ones, are required. In this regard, the problem of special importance is to determine condensation heat transfer in the cases when the heat transfer intensity on the coolant side is commensurable with that on the condensation side. The article describes the results of the activity aimed at studying the condensation of R-245fa (considered as a promising coolant) in horizontal and inclined tubes using the Volume of Fluid (VOF) method. By using this method, supplemented with the model of heat and mass transfer at the phase interface, one can visualize the flow structure and obtain data on the local heat transfer characteristics. The prediction results are compared with the experimental data obtained on the experimental setup at the CJSC Turbokon, intended for studying heat and mass transfer processes during condensation of various promising working media. The predicted data have been compared with the experimental data obtained in condensation regimes in a 2 m long copper tube 32 × 2 mm in diameter at mass fluxes up to 27 kg/(m² s) and tube inclination angles up to ‒24.3°. For simulating the processes at the phase interface, the Lee model was used with automatically calculating the relaxation constant based on the working fluid properties and computation mesh parameters. All calculations were carried out using the in-house CFD code ANES. The article presents data on the distribution of heat transfer coefficients along the tube inner surface. The prediction results are in good agreement with the experimental data, which have shown that the heat transfer coefficient increases significantly even at a small tube inclination. The accomplished study helps gain deeper insight into the condensation heat transfer processes in horizontal and inclined tubes, which is of importance for designing heat transfer apparatuses, such as air and water cooled condensers of the vapors of refrigerants, petroleum products, and other chemical substances. The obtained results can be used for improving the accuracy of engineering calculations and elaborating new design solutions for condensers with taking into account local flow features.

The development of small modular reactors (SMRs) attracts increasingly growing interest both in Russia and abroad in connection with the trend toward decreasing of greenhouse gas emissions and diversification of energy sources. Small nuclear power plants (SNPPs) could occupy their niche in the field of electricity production in remote regions, in which there is no demand for large power capacity. It is exactly for this reason that great attention is presently paid to the development of SMRs and SNPPs as a whole. Given that there is no demand for large capacity NPPs, and with more stringent requirements for safety systems based on fully passive principles of their operation, and also in view of the fact that there has been no assessment of decreasing the prime cost of electricity in putting the SNPP equipment in mass scale manufacture, the problem of their development is becoming increasingly more relevant. The article presents the results of calculations of thermohydrodynamic processes in the new integral modular small capacity reactor plant (RP) with natural coolant circulation called VVER-I as applied to a beyond design basis accident involving the power unit long-term blackout. For the VVER-I RP, all safety systems relating to defense-in-depth levels two and three are based on the passive principle of their operation, also with the use of natural circulation, without the need of power supply and operator’s intervention for quite a long period of time. The calculations were carried out using the KORSAR/GP computer code certified for analyzing the safety of NPPs with VVERs. The possibility of reliable decay heat removal from the core in the natural circulation mode with the reactor pressure vessel externally cooled with water has been confirmed. The passive safety systems incorporated in the design ensure reliable heat removal from the core under the conditions of all NPP power supply sources lost for a long period of time without intervention of the operational personnel. The study results can be used in elaborating new designs of VVER type RPs with natural coolant circulation, including those with passive safety systems.

The problem of improving the corrosion conditions during operation of a nuclear power facility (NPF) is directly connected with increasing the corrosion resistance of structural steels to general (uniform) corrosion, chemical transformation and mass transfer of corrosion products in water coolant in conducting different water chemistry (WC) versions. The article gives a description of the generation processes and chemical transformations of the initial ionic forms of structural steel corrosion products at the initial stage of the general corrosion process followed by the generation of the precursors of solid phase oxide-hydroxide particles (sludge and crud) dispersed in coolant and generation of final solid phase forms of corrosion products such as suspensions, deposits, and corrosion oxide films in the nuclear power facility circuits under the conditions of reduction and oxidative water chemistries. The article proposes a physicochemical model of mass exchange and mass transfer of corrosion products in the steel–water coolant system with taking into account the chemical transformations of corrosion product ionic forms up to the generation of solid phase products in water coolant and on the corroding steel surface, on which a two-layer (with topotaxic and epitaxic layers) oxide film of a local origin is produced. On this film, dense corrosion product deposits from the coolant are formed along with loose (dissipative, weakly fixed) corrosion product deposits that are in equilibrium between the circuit surfaces and coolant. The chemical composition of all product existence forms is determined by the compounds of iron with admixtures of structural steel alloying elements, and the radionuclide composition is determined by the activation products of reactor materials (⁵⁸Co, ⁶⁰Co, ⁵⁴Mn, ⁵⁹Fe, and ⁵¹Cr) and other, more short-lived radionuclides. The proposed body of mathematics can be used for determining the numerical values of the indicators characterizing different corrosion process stages – from metal ionization to generation of final corrosion products on the surfaces and in the coolant of the nuclear power facility circuits under the conditions of different water chemistries.