Thermal Engineering最新文献

Reducing emissions of harmful substances in the production of electricity at thermal power plants is an intensive task, which can be solved by switching to semiclosed cycles with oxygen combination of fuel and carbon dioxide working fluid. The main advantage of the promising technology, which is the absence of the danger of the formation of toxic substances and the ease of separation of excess carbon dioxide, is provided by burning hydrocarbons in high purity oxygen. This paper presents the results of developing the new power equipment preliminary design: an oxy-fuel combustion chamber, a cooled carbon dioxide turbine, and a high-temperature regenerator. The results of mathematical simulation of physical processes that occurred in the main power equipment are presented. Special attention is paid to calculations of methane--oxygen mixture combustion kinetics in excess carbon dioxide medium and numerical analysis of the processes in the oxy-fuel combustion chamber with taking into account the need to cool the flame tube. The optimal mass ratio of carbon dioxide in the mixture of solvent with oxygen equal to 0.872 is determined, at which the minimal incomplete combustion of fuel can be achieved. The temperature state of the carbon dioxide turbine first-stage nozzle vane was mathematically simulated. The simulation results confirmed the possibility to ensure a sufficient uniformity degree of the temperature field due to making a number of small-radius cooled channels near the airfoil outer surface. A few configurations of heat transfer surfaces for the plate regenerator of the oxy-fuel power unit are proposed. Flow turbulizers with a cylindrical shape and with a fin-type airfoil, and also both of their design versions with ribs were used. The obtained mathematical simulation results have shown that of all channels considered, at the Reynolds number values up to 78 000, the ones having turbulizers with ribs show the best thermal-hydraulic efficiency, whereas at higher Re number values, channels having turbulizers without ribs show the best performance.

The results of full-scale studies of the corrosion and abrasive resistance of the surfaces of experimental samples of tubular steel 20 with a protective ion-plasma coating based on Cr–CrN and without it during their exposure for 1000 h in the furnace of a boiler with a nominal capacity of 200 kW operating on granulated biofuels are presented. Samples were held at operating mode for 220 h, and at idle mode for 780 h. The temperature inside the rear smoke box, where the experimental samples were placed, was approximately 700°C. Chemical analysis of the composition of ash obtained during the burning of sunflower husks showed the presence of potassium, phosphorus, sulfur and chlorine, i.e., those elements whose presence can lead to the formation of highly aggressive corrosion-hazardous compounds when moisture is condensed during periods of boiler downtime. It was revealed that the studied coating was not subjected to corrosion destruction, its adhesive strength did not change during the holding time, the initial thickness of the coating decreased by an average of 10–15%, and the surface roughness remained the same. There was a slight deviation in the morphology due to oxidation of the surface layer of the coating at the maximum holding time, no oxidation of the material under the coating on the transverse splines of the coated samples was detected. Tests conducted at the incidence angles of the air-abrasive flow of 30°, 60° and 90° tests of steel samples 20 with a protective ion-plasma coating based on Cr–CrN after holding in the furnace of a biofuel boiler for 1000 h showed that the abrasive resistance at a steady speed increased by at least 2.5 times compared to samples without coating. According to the results of metallographic, corrosion, and abrasive studies, Cr–CrN-based ion-plasma coating is promising for the protection of pipe surfaces of biofuel boilers exposed to corrosion and abrasive effects of alkali metal salts and ash particles at high temperatures and variable load.

The article considers problems relating to reduction of pollutant emissions at Russian thermal power plants (TPPs) in connection with the introduction of new technological indicators proposed in the new edition of the Reference Document on Best Available Techniques (ITS) ITS 38-2022. The results obtained from investigating more than 40% of boiler units that operate at Russian TPPs in regard to technical and environmental characteristics are summarized. It is shown that boilers commissioned before December 31, 2000, pose the main problem in adapting the existing TPPs to the new technological indicators, because they were designed without the use of air-protection measures and technologies except for ash collectors. The article estimates the scales of replacing and modernizing the main and auxiliary equipment of these Russian large fuel combustion power plants (LCPs) for reducing the emissions of marker pollutants to a level not higher than the technological indicators specified for this group. The introduction of new technological indicators for solid fuel ash emissions will require a serious change in the existing structure of ash-collecting plants by replacing or modernizing them. Currently, Russian TPPs are not equipped with operating flue gas desulfurization systems, as a result of which the sulfur dioxide emissions from more than 40 coal-fired boilers do not comply with the established technological indicators. The nitrogen oxide emissions from gas-and-fuel oil fired boilers are in the main in compliance with the environmental requirements in contrast to 25% of coal-fired boilers, at which these requirements are not complied with. For the oldest and numerous group of boilers that were commissioned before December 31, 2000, the article considers ways of introducing the best available techniques (BAT) recommended in the ITS 38-2022 and proposes specific low-cost and quickly introduced air-protection measures for reducing the marker pollutant emissions into atmospheric air to a level not higher than the technological indicators (BAT-associated Emission Levels, BAT-AELs) with taking into account the existing technical and economic constraints.

A complex for studying the structure of the wet-steam flow for a stand with a model turbine of the NPO TsKTI is presented. On the stand, the flow parts of steam turbines have been tested for more than 20 years. The experimental turbine is a model of the flow part of a low-pressure cylinder at a scale of 1 : 3. It is equipped with an extensive measuring system for the study of the vibration reliability of the elements of the flow part and the structural and kinematic characteristics of the flow. One of the most important parts of the measuring system is a complex for studying the dispersed structure of the wet-steam flow, which allows measurements to be carried out simultaneously in three control sections of the flow part and in two sampling pipes. It includes optical probes, secondary hardware units, a traversion system, and software. Measurements made using the optical spectral transparency method allow one to set the size and volume concentration of droplets in the wet-steam stream. A new method for determining droplet size distribution based on optical measurements is described. The droplet size distributions obtained by this method in two control sections are presented. It is shown that the calculation of the degree of humidity according to optical measurements is possible with the involvement of the results of independent measurements of both the vapor pressure and its temperature recorded by the optical probe. A comparison of the calculations of the degree of humidity based on pressure and temperature measurements for model and full-scale turbines was made. The result of the complex is the distribution of the degree of humidity, the size of the droplets, and the temperature of the flow along the height of the blade. The distribution of moisture parameters by the height of the blade in three control sections is given.

The major part of electric energy is presently generated by fossil fuel-fired thermal power plants operating according to the Rankine cycle. In the last decades, power technologies on the basis of solar concentrators (SCs) are becoming increasingly more attractive. The article shows the possibility of using heat obtained from solar energy (referred to henceforth as solar energy heat) at existing steam turbine thermal power plants (TPPs). A scheme for connecting an SC to the PVK-150 steam turbine power unit at the Tashkent TPP has been developed, due to which solar energy heat can be used instead of the heat obtained in the low- and high-pressure regenerative heaters (LPH and HPH) and also for partially replacing the heat from the economizer and evaporative heating surfaces of the existing steam generator. Calculations were carried out for different values of the solar energy heat share: in the range from 20 to 80%. Parabolocylindrical concentrators (PCCs) are used as SCs. A formula is proposed for calculating the solar energy into electricity conversion efficiency at hybrid solar and fossil fuel-fired TPPs constructed on the basis of existing steam turbine TPPs. The results obtained from modernizing the PVK-150 power unit by connecting an SC to it are presented. It has been determined in the course of investigations that, in using solar energy heat in the PVK-150 power unit for replacing the heat obtained in the regenerative feed water heaters, the solar energy into electricity conversion efficiency reaches 27.06 and 34.4% in the case of partial replacement of the economizer and evaporative surfaces of the existing steam generator with a solar steam generator.

Results of numerical and experimental investigations into the thermohydraulic processes in cooling channels of a blade in a carbon dioxide gas turbine are presented. Based on the results of a review of design solutions and geometry of cooled blades of gas turbines, a design of cooling channels, which includes radial round channels for the leading edge of the blade and slotted channels with heat-transfer intensifiers for the trailing edge of the airfoil, and the geometric parameters of the channels proper are proposed. The ANSYS CFX software package was used to study thermal and hydraulic characteristics of a slotted channel with fan-shaped pins in the Reynolds number range of Re = 9000–27 000; experimental studies of hydraulic characteristics and heat transfer were also carried out in this range. The difference between the predictions and the experiment was less than 5% for the flow characteristic and less than 10% for the Nusselt number, thereby demonstrating adequate accuracy of the selected settings of the computational grid and the turbulence models. These settings were used to study the thermohydraulic processes in the cooling channels of the blades of a high-temperature carbon dioxide gas turbine in the Reynolds number range of Re = 20 000–100 000. In particular, in addition to slotted channels with fan-shaped pins for cooling the trailing edge of the airfoil, advanced channels with round pins, which are easier to manufacture, were examined. The leading edge of the blade is cooled using smooth radial channels or channels with ring fins. The predictions have demonstrated that the use of fan-shaped pins does not enhance heat transfer compared to round ones, and finned channels are more than 100% efficient compared to smooth channels.

The energy transition for “green” hydrogen is supposed to be carried out through a widescale use of wind energy. The area of wind farms required for this purpose may reach 38.5% of the European Union’s territory. In the scientific literature, it is pointed out that the use of wind turbines for meeting 10% of the world demand for energy can result in that the land surface temperature will increase by more than 1°C by 2100. A change in the temperature is observed immediately after commissioning of wind farms, whereas the climatic gain from reduction of greenhouse gas emissions is a matter of the future. Currently, no attention is paid to the influence of wind and solar power plants (WPPs and SPPs) on the growth of greenhouse gas emissions as a consequence of changes in the structure and loading conditions of generating capacities in the power system. The aim of this work is to determine the possibility of reducing the amount of greenhouse gas emissions in the power system by changing the fuel-balance structure, achieving more efficient generation of electricity, developing WPPs and SPPs, and consider methods for implementing it. Assessments of the reduction of greenhouse gas emissions by power plants in changing the structure of generating capacities and also in replacing one fuel kind by another are given. It is shown that nowadays, the balance of demand and offer in the power system, e.g., of Germany, is maintained owing to electric energy export. For the period from 2000 to 2018, the amount of electricity generated by WPPs and SPPs increased by 146 TW h, whereas that by nuclear power plants (NPPs) dropped by 94 TW h, while the export of electric energy to the power systems of neighboring countries increased by 52 TW h. The decrease in the amount of electricity generated by coal-fired thermal power plants (TPPs) by 69 TW h was compensated by increasing the amount of electricity generated by natural gas fired thermal power plants by 34 TW h, and by 47 TW h owing to power plants that use biogas, solid and liquid biofuel, and solid municipal waste as fuel. Study results have shown that, in the absence of energy storage devices, the development of wind and solar power plants cannot be regarded as an efficient way of reducing greenhouse gas emissions in the power system, the more so that WPPs and SPPs are significantly inferior to various combined electricity- and heat-generation versions.

Investigations were performed of heat transfer to a forced upward flow of mercury in a tube inserted into a heated channel with a rectangular cross-section under the effect of a transverse magnetic field. The outer channel is filled with mercury and connected to a natural circulation loop. Liquid metal heat transfer is simulated in a cell of the cooling system of the channel-type liquid metal blanket for a Tokamak fusion reactor. Experimental data on temperature fields and heat-transfer performance in the inner tube and the outer channel were obtained in the mercury magnetohydrodynamic test rig using microthermocouple probes. Three different cases of natural circulation loop operation are examined: (I) the loop is off, convective flow can occur only in the space between the tube and the channel wall; (II) the loop is open and operates under adiabatic conditions; (III) the loop is open, water cooling is on. The results of measurement in the inner tube demonstrate that heat transfer in the tube-in-channel system is enhanced compared to the heat transfer in a separate tube both with and without a magnetic field. Under the experimental conditions, natural convection is induced by the buoyancy and electromagnetic forces in the gap between the tube and the channel wall. The configuration and structure of the flow in the gap change drastically in a transverse magnetic field, and the heat-transfer rate depends on the operating conditions in the natural circulation loop. Convection reduces temperature nonuniformities in the gap, and the heat transfer in the investigated “tube-in-channel” enhances greater when the natural circulation loop is activated and, especially, when it is additionally cooled. Low-frequency high-amplitude fluctuations induced by the instability of the natural convection and magnetohydrodynamic flows are observed in the gap.

A numerical study of the mixing processes of multicomponent gas flows with the help of static mixers was carried out to reduce the temperature and gas mixture composition inhomogeneities in the fuel pipeline. The literary sources of interest for this work are analyzed. Two types of static mixer are selected: a series of elements from a twisted band and a leaf mixer. For these designs, numerical calculations are made at the specified parameters of mixing gas flows containing methane, hydrogen, and nitrogen. Turbulent flows of the mixture were modeled in a stationary formulation using the equations of conservation of mass, momentum, and energy averaged by Reynolds. Two-parameter models with wall-side functions were used to determine turbulent viscosity. As boundary conditions at the entrance to the static mixer, the fields of the desired variables, obtained earlier by the authors of this article at the exit from the T-shaped mixer with Reynolds numbers (4–6) × 10⁶ in the main and adjacent pipes for the supply of fuel mixture components, were set. The analysis of the efficiency of the mixing process using stationary mixers of various modifications was carried out. The fields of the components of speed, temperature, and mass fractions of the mixture at the exits from static mixers were obtained and pressure losses in the structures were determined. The optimal design of the mixer is proposed, which consists of four elements in the form of a 180° plate, each element of which has a length (half-spin period) equal to two diameters of the pipe. Adjacent elements are twisted in opposite directions and adjoin each other at an angle of 90°. It is shown that it is possible in a fragment of the fuel pipeline, including a static mixer and a straight section of the pipe with a length of not more than five diameters, to achieve the required uniformity of the composition and temperature of the fuel mixture in the outlet section of the said fragment.

The effect of heat conduction through the wall along the flow of heat carriers on the effectiveness of heat exchangers (HEs) when the wall ends are not thermally insulated from the environment has been investigated. An analytical solution of the problem for cocurrent flow of heat carriers was obtained for the same ratios of thermal equivalents of the heat carriers β and heat-transfer coefficients α on both sides of the wall separating hot and cold heat carriers and for a counter flow at β = α = 1. The solution to the problem depends on the number of heat-transfer units Ntu, parameter ({{C}_{A}}) describing the axial wall heat conduction, the Biot number Bi, which determines relative heat transfer from the wall ends to the environment, and temperatures of the fluid in contact with the wall ends. The effect of axial wall heat conduction becomes more pronounced with decreasing parameter ({{C}_{A}}). Two cases are examined: case I with surrounding fluid temperatures assumed as equal to the inlet and outlet temperatures of the hot heat carrier and case II with surrounding fluid temperatures assumed as equal to the inlet and outlet temperatures of the cold heat carrier. The results obtained demonstrate that, at low Biot numbers (Bi < 10^–3), the effectiveness of a heat exchanger ε for any values of ({{C}_{A}}) in a cocurrent HE hardly differs at all from the effectiveness of the HE in the absence of the axial wall heat conduction effect, ε₀, and for a counter flow at low ({{C}_{A}}) the HE effectiveness is noticeably less than ε₀ and decreases by two times at (Ntu) ( gg ) 1. At high Biot numbers (Bi > 1), the effect of axial wall conduction can increase the effectiveness of either heat carrier, while the temperature of the other heat carrier will change slightly during its flow through the heat exchanger. The predictions indicate that the best way for increasing the HE effectiveness is to employ the same thermal equivalents of two heat carriers and heat-transfer coefficients on both sides of the wall separating the hot and the heat carriers (β = α = 1).