Thermal Engineering最新文献

Advances in computer technology have significantly expanded the possibilities for studying heat and mass transfer processes using Computational Fluid Dynamics (CFD) methods and, in particular, vapor condensation in pipes. One of the promising methods of numerical research is Volume of Fluid (VOF), which allows direct modeling of the behavior of the interphase surface in complex unsteady flows with mass transfer. Currently, the main efforts of researchers are aimed at the active development and testing of effective VOF models and algorithms and the selection of optimal characteristics of the grids used that are necessary for modeling a moving interphase surface and modes in which the vapor flow can be turbulent and the flow in the condensate film can consistently change from laminar (laminar-wave) to turbulent. An important issue remains the influence of taking into account real three-dimensionality in problems traditionally considered as two-dimensional: condensation of vapor on the surface of a horizontal cylinder, bundles of horizontal tubes, or in a vertical cooled tube. For this purpose, the authors previously performed methodological calculations, including verification of models and VOF algorithms as applied to condensation processes in pipes. Based on the results obtained in a two-dimensional (2D) formulation when modeling condensation in a vertical pipe of turbulent vapor flow, the optimal sizes of grid cells in the liquid film and vapor in the radial and longitudinal directions were selected, various turbulence models were tested, and the method for determining the constant in the Lee model was verified. When comparing the calculated values and data obtained experimentally at the Department of Engineering Thermal Physics of the National Research University MPEI, their good agreement was observed (arithmetic mean deviation 14.4%). This paper examines the results of modeling the specified problem in a three-dimensional (3D) formulation. Based on the performed calculations, the operability of the proposed algorithms, methods, and grid parameters was confirmed when transferring them from a two-dimensional to a three-dimensional problem statement. The values obtained from 3D modeling are in better agreement with the experimental data (average arithmetic deviation 10.2%); the accuracy of calculations relating to the laminar-wave mode of condensate film movement is significantly increased.

Implementation of modern methods for the design and upgrading of low-emission combustion chambers for gas turbine engines requires performance of a wide variety of computational experiments with appropriate fuel surrogates, which are hydrocarbon compositions capable of simulating the essential physical and chemical characteristics of the fuel. Complex multicomponent surrogates of commercial aviation kerosene fuels have been developed in this work. Surrogates consist of hydrocarbons from the main structural classes of compounds specific for aviation kerosene fuels and reproduce the key physical and chemical characteristics of fuels, such as the H/C ratio, molecular weight, density, lower heating value, and heat of evaporation. The surrogates were tested against temperature-independent and temperature-dependent characteristics of Jet A, Jet A-1, and TS-1 fuels, including their distillation curves. Surrogates have been identified, which offer the best agreement with the published data on temperature-dependent and temperature-independent characteristics of Jet A, Jet A-1, and TS-1 fuels.

Stacks with steel barrels installed inside of a reinforced concrete shell are the most widely used type of multibarrel smoke stacks in Russia. The reinforced concrete shell of such stacks is protected against the effect of flue gases, and with correctly conducted operation, it can perform its functions for an almost unlimited period of time. The smoke stack main damages occur as a consequence of steel barrel corrosion, especially if flue gases contain sulfur oxides. The possibility to decrease corrosion of the smoke stack steel barrels by changing their operation conditions is considered. A new method for achieving more reliable and efficient operation of multibarrel smoke stacks is proposed, according to which the barrel shutdown corrosion is eliminated, and measures are taken for the smoke stack to help unload the exhaust fans to a greater extent through optimal use of the barrel stack effect. The main methods applied to decrease corrosion of the metal gas removing barrels used in multibarrel smoke stacks are considered. A new design of the metal barrel base part is proposed, which in the course of boiler equipment operation makes it possible to redistribute the gas flows among the smoke stack barrels. In addition, the smoke stack operation modes are proposed, in which the steel barrel shutdown corrosion is eliminated, and the exhaust fans are unloaded due to an increased total stack effect of the barrels. A computer program for calculating such operation modes has been developed. Variants calculations of the smoke stack operation mode performance parameters have been carried out with taking into account the real operation conditions. It has been determined that in the case of using the stack effect of the barrel from which the boilers are disconnected, the electricity consumption for driving the exhaust fans of operating boilers decreases by 5–10%; in addition, with such operation, the possibility of shutdown corrosion to occur in the smoke stack steel barrels is excluded.

For the Omsk combined heat and power plant no. 5 (Omsk CHPP-5), which is located in the central part of a large city, there is an acute need to reduce the atmospheric emissions of pollutants. Grade KSN (coking low-caking low-metamorphized) high-ash Ekibastuz coal is the design fuel for this power plant. For reducing the solid fuel fly ash and nitrogen oxide NO_x emissions, it was proposed to make a shift for combustion of other coals having a higher heating value and lower ash content. To see whether or not off-design coals can be used, experimental combustion of such coals as grade DR (as-received, jet) Vinogradov coal, grade DR Shubarkol coal, and grade DR Maikubensk coal was carried out; the furnace process mathematical simulation was performed out in the ANSYS Fluent software, and the thermal balance was compiled in the Boiler Designer software. As a result of the studies performed, a conclusion has been drawn that for securing efficient use of off-design fuel, it is necessary to modernize the existing combustion system with installing new low emission vortex burners. Based on the data of a computational study carried out in the ANSYS Fluent software, the design of a new burner was developed. The simulation results have shown that by using the new burners it will be possible to achieve a significantly lower amount of nitrogen oxide emissions produced. After the design documentation had been developed, the new burners were manufactured and installed in BKZ-420-140 boiler No. 7 at the Omsk CHPP-5. The tests of the boiler equipped with the new burners carried out with combustion of design grade KSN Ekibastuz coal have confirmed the effectiveness of the adopted technical solutions: the concentration of nitrogen oxides in the boiler flue gases is a factor 1.7 lower in comparison with the values that were before the boiler refurbishment. A lower level of carbon-in-ash losses and the corresponding increase in the boiler efficiency have been recorded. Preliminary experiments on combusting off-design grade DR Vinogradov coal confirm the possibility of using it on a regular basis jointly with grade KSN Ekibastuz coal provided that the project has been implemented in a full scope.

The heat-transmitting capability, i.e., the ability to transfer heat flux with minimal losses, of thermosyphons and the prospects for using passive systems based on them for heat exchangers of various purposes, such as those for the utilization of heat from renewable energy sources and secondary energy resources (water basins, soil, groundwater, waste water and steam from industrial production, etc.) are considered. In small-scale power engineering, thermosyphons can be used to increase the potential of heat pumps that use heat from alternative sources. Passive heating/cooling systems ensure savings in the electricity required to power electric motors. Thermosyphons are easy to operate, do not require constant maintenance and can be effective intermediate links between heat sources and consumers, and are capable of maintaining a constant temperature of cooled objects. They can be used to organize the removal and transfer of heat outside a high-temperature environment. The design of a loop thermosyphon with a porous evaporator (LTSPE) and a condenser installed horizontally is presented. The article presents the results of experimental studies of a thermosyphon with two working fluids (freon R245fa and water). The temperature distribution and thermal resistances of the evaporator, condenser and thermosyphon as a whole are determined under different thermal loads. The effect of the cooling medium temperature on the heat-transmitting capability of the thermosyphon heated by a constant heat flux is analyzed. With an increase in the cooling medium temperature, the thermal resistance of the thermosyphon monotonically decreases. The studied device has a high heat-transmitting capability (up to 1.5 kW), a short start-up time, and a dynamic attainment of a steady-state mode when the load changes. The developed loop thermosyphons can be recommended for use in energy-saving systems, in particular in solar power engineering (thermal control of PV and PVT panels¹); in combination with heat pumps – in trigeneration plants generating electricity, heat and cold; in thermostatic equipment for electric transport, electronic equipment and in other areas

Microchannel heat sinks (MCHS) belong to one of the most prominent methods of passive cooling of microelectronics. In this work, a circular microchannel-based MCHS was installed over a microelectronic mimicking heated surface, which was subjected to 50 to 125 kW/m², and the convective cooling of MCHS was studied using nanofluids of copper (Cu) and carbon nanotube (CNT) [both at 0.05 wt % concentration in de-ionized (DI) water] as coolant, along with DI water. The experimental results suggest that the nanofluid-cooled MCHS, especially the CNT one, outperformed the pure water-cooled system, with significantly higher heat transfer coefficient (HTC), and lower pumping power, rendering the former system more energetically favorable. At a flow rate of 60 ml/min and heat flux of 100 kW/m², the HTC enhancements in water + CNT and water + Cu were 15.7 and 6.2% more than water, respectively. Due to addition of surfactant in DI water for suspending CNT, an apparent slip flow became prevalent in the microchannel, leading to a significant pressure drop reduction while pumping water + CNT. This observation helped in gauging the total power saving that can be accessed using water + CNT, if one follows periodic heating/cooling between an upper critical temperature and safe temperature range rather than continuous cooling of the electronic surface.

This study investigated multi-layered thermal barrier coatings deposited by atmospheric plasma spraying for improved performance compared to a single-layered Sm₂Zr₂O₇ (SZO) coating. Two novel designs were evaluated: a yttria-stabilized zirconia (YSZ) intermediate layer and a functionally graded coating with variable YSZ/SZO ratios. Vickers microhardness revealed a uniform 22% increase for all coatings after heat treatment. Functionally graded YSZ/SZO coatings displayed a significant 23% reduction in coefficient of friction compared to the YSZ/SZO double layer coating at room temperature. While heat treatment slightly increased wear depth in functionally graded coatings, they emerged as the most promising candidate due to their improved mechanical strength and low coefficient of friction, suggesting their potential for enhanced durability and thermal barrier performance.

Experimental studies of blading from cooled gas turbines involve difficulties in simulating actual shape and operating conditions of the blades. Therefore, studies of linear turbine blade or vane cascades composed of blades that usually correspond to plane sections of real spatial turbine rows at the hub, at the middle diameter, and at the tip have received wide acceptance. When investigating linear blade cascades, the spatial effects that are crucial for the formation of the overall flow structure in the blade rows cannot be examined. Application of actual spatial turbine rows enables us to determine more reliably the causes and value of energy losses in turbine blade assemblies even under simulated operating conditions used in an experimental facility. Naturally, a study of a complete annular blade row seems most preferable. However, such studies require high costs associated not only with the manufacture of the turbine blading but also with provision of the required flowrate of the working fluid to conduct tests under conditions simulating the real operating conditions of the experimental object. In this case, the study of a sector cascade composed of full-scale cooled nozzle vanes is an acceptable alternative to testing a full-scale complete annular cascade. A sector cascade was tested at the Central Institute of Aviation Motors in a wide range of the reduced adiabatic velocity at the outlet (0.6–1.3) with cooling air ejection through perforation holes on the airfoil and end surfaces as well as through the trailing edge. The tests were performed under isothermal conditions when the temperatures of the working fluid and cooling air were almost the same. The total pressure fields upstream and downstream of the sector cascade were determined in the tests. The numerical study of the spatial structure of the flow and losses was carried out using the 3D NS and ANSYS CFX codes, which solve the 3D Reynolds-averaged Navier–Stokes (RANS) equations using various turbulence models.

The article presents the results of current problem-oriented studies of thermal-hydraulic processes to substantiate the characteristics and safety of sodium-cooled fast neutron reactors across a wide range of thermophysical problems, including hydrodynamics and heat exchange in channels and fuel assemblies (FA) of the core, including under conditions of fuel element assemblies’ deformation during the campaign, intravessel circulation and coolant stratification in the tank of a fast neutron reactor with an integral equipment layout in the nominal mode with forced convection and natural convection of the coolant in the emergency cooldown mode, in the intermediate heat exchanger, and in the large-modular steam generator. The data of experimental hydrodynamic studies of flow parts of reactor installations, heat exchangers, and reactors of nuclear power plants are presented. The article analyzes the patterns of hydrodynamic processes occurring in the flow sections of cylindrical collector systems in reactors and heat exchangers identified during experimental studies on an aerodynamic stand and a hydraulic flume. These patterns are registered as scientific discoveries as previously unknown patterns and phenomena related to the atomic, space, metallurgical, and chemical fields of science and technology. The results of the computational studies performed using the version of the channel-by-channel code implementing a two-fluid model of a two-phase flow of liquid metal in the approximation of equal pressures in the vapor and liquid phases reproduce the development of the flow regimes of a two-phase flow, the pulsations of the liquid metal flow obtained in experimental studies, and also demonstrate antiphase pulsations of the flow of a two-phase coolant in a system of parallel fuel assemblies and interchannel instability characterized by a significant increase in the amplitude of the pulsations of the coolant flow in parallel FA.

It is known that dry spots formed under vapor bubbles during the boiling process have a huge impact on both local heat transfer and the development of crisis phenomena. In this study, new experimental information on the evolution of dry spots under vapor bubbles during liquid boiling was obtained using high-speed reflected light imaging, and an algorithm for automatic processing of experimental data based on U-Net convolutional neural networks was developed. It is shown that it is possible using machine learning models and high-precision optical high-speed methods to determine a wide range of characteristics of dry spots during liquid boiling in a short period of time and with high accuracy, including the evolution of the total area and size of dry spots, total number, and the growth rate and lifetimes of dry spots in a wide range of heat fluxes. Based on the analysis of the collected data, it was established that the average total area of dry spots and the nucleation site density during boiling of water increase linearly with increasing heat flux in the studied range. It has been demonstrated that the growth rate of dry spots is constant in the period before the onset of the bubble detachment stage, with the average value of this rate increasing with increasing heat flux. The characteristic maximum size of dry spots turns out to be almost half the capillary length. The results obtained, presented in the article, indicate that there is a huge potential for using artificial intelligence methods, which open up new prospects for studying two-phase systems, modeling heat transfer during boiling, and predicting crisis phenomena associated with uncontrolled growth of dry spots.