Thermal Engineering最新文献

The results of experimental studies on the creation of highly efficient designs of vibration-isolating compensators for pipelines with liquid are considered. It is noted that the only way to evaluate the effectiveness of various compensators in reducing vibration at different frequencies currently is to compare their transient vibration stiffness or transient mechanical impedance, which were measured on special stands at a given frequency. The stiffness of the compensator increases significantly with increasing frequency vibrations. Hazardous frequencies may vary between piping systems. For this reason, it is impossible to set an integral criterion for the effectiveness of a vibration-isolating compensator, similar to static stiffness. The results of measurements carried out on a special stand on the transitional vibration stiffness of a new design compensator with thin-layer rubber-metal elements (TRME) are presented. The rigidity decreased by 10 or 100 times or more in the frequency range from 50 to 800 Hz relative to the rigidity of a serial compensator based on rubber cord casing (RCC), including in the presence of water inside it. It has been experimentally shown that the vibration-isolating ability of the same compensator as part of a pipeline system, determined by the value of the dynamic force transmitted by the compensator to the pipeline from the pump, significantly depends on the presence of water in them and its flow, which is not taken into account in known methods. The results of testing compensators with RCC and TRME with a bore diameter of 80 mm as part of a stand with a ring pipeline system, a pump, systems for monitoring the flow of the working fluid, vibrations, pressure pulsations, and dynamic (vibration) forces transmitted by the compensators to the pipeline are presented. In a stand with pipelines, the efficiency of vibration-isolating compensators with TRME is still 10 and 100 times higher than compensators with RCC in the absence of water and decreases by an order of magnitude in the presence of water without its flow when the pump is vibrated by a vibrator. Efficiency decreases even further if water flows through expansion joints and pipelines while the pump is running.

A noniterative implicit method for solving the discrete conservation equations of the KORSAR/GP computer code (hereinafter referred to as KORSAR) two-fluid model is presented. The KORSAR/GP code has been developed jointly by specialists of the Federal State Unitary Enterprise Aleksandrov Research Institute of Technology (NITI) and the special design bureau OKB Gidropress. In 2009, the code was certified at the Federal Service for Environmental, Technological, and Nuclear Supervision (Rostekhnadzor) as applied to numerical safety assessment of VVER-type power reactor plants. The code uses the semi-implicit numerical scheme, which limits the integration time step by the Courant condition with respect to the velocity of a two-phase flow. To cut down the time it takes to calculate prolonged transients in reactor plants, an implicit numerical method, which does not limit the time step by the Courant condition, has been developed on the basis of the SETS (stability-enhancing two-step) method. It is based on the semi-implicit scheme. Prior to its application, discrete phase momentum conservation equations with the convective terms written in implicit form are solved at each time step. After the semi-implicit step, the specific (per unit volume) mass and energy of the phases, which are donor quantities in the convective terms of the transport equations, are calculated at the new time layer. Unlike the SETS method, the implicit method developed for the KORSAR code employs a semi-implicit scheme with linearization of unsteady terms describing the change in the specific mass and energy of a two-phase flow. This approach enables us to solve discrete equations in a noniterative manner. However, the implementation of this procedure requires that the unknown scalar variables, such as the phase specific enthalpies, the vapor volume fraction, and the pressure, be determined in the computational cells. Therefore, the semi-implicit scheme with linearization of unsteady terms with recalculated donor quantities at the end of the time step is reused. The performance and effectiveness of the developed implicit method have been confirmed by solving, using the KORSAR code, a test problem of a two-phase flow in a heated horizontal tube driven by a pressure difference.

The problem of friction and heat transfer in a laminar, transition, or turbulent flow along solid permeable surfaces has been solved using a numerical simulation technique. To derive a compact mathematical description intended for engineering applications in the power industry and other thermal processes, a modern version of the Kolmogorov–Prandtl model with one differential equation (namely, the turbulent kinetic energy conservation equation) was employed. The mathematical model is represented by a system of first-order nonlinear ordinary differential equations for the distributions of flow velocity, friction stress, temperature, turbulent energy, and turbulent energy flux density across the film thickness. The problem of singularity of the mathematical description on a solid wall is discussed. The integral hydrodynamic and thermal characteristics of film flows currently receiving a lot of interest, such as the film Reynolds number and the Stanton number, were obtained. Functional correlations among dimensionless parameters that are relevant for engineering applications, including those for special regimes of film flows with recirculation and mass crossflow on permeable surfaces of structural materials, have been established. The film Reynolds and Stanton numbers are defined as functions of dimensionless parameters at which the relative values of the film thickness, acting forces, and mass crossflow are specified. The obtained correlations can be used in the design and optimization of condensation and steam-generating facilities in the power industry, for elaboration of evaporative coolers for high-stress structural elements in gas turbine and rocket equipment, simulation of hydraulic roughness, and in thin-film materials technologies.

The use of condensing heat exchangers (CHEs) in gas-fired boiler units helps cool the flue gases below the dew point. One of the issues that has to be settled in the case of CHEs installed downstream of boilers is to ensure that the flue gas removal stacks will operate without steam condensation on their inner surfaces. To protect smoke stacks against hydrate corrosion, bypassing of part of combustion products not cooled in the CHE is mainly used in practice. The article presents the results obtained from computations of the heat-transfer processes in the combustion products cooled in a CHE as they move in a reinforced concrete smoke stack fitted with clamped lining, which is protected against hydrate corrosion by bypassing. The computations are carried out for three operation modes of the 180-m high smoke stack, through which flue gases are removed from three power-generating boilers of the BKZ-420-140 NGM type installed at the Samara combined heat and power plant (CHPP), a branch of Samara PAO T Plus. The peculiarity and complexity of the computations are connected with the fact that that the flue gas’s thermophysical parameters and motion velocity in the smoke stack vary during the flue gas cooling process. The parameters’ variation pattern depends essentially on the fraction of gases directed in bypass of the CHE. A mathematical model and computer program are developed for computing the heat-transfer processes in flue gases moving in the smoke stack with CHEs installed downstream of the boilers and with the smoke stack protected against hydrate corrosion by the bypassing method. It has been determined that, for a 180-m high three-layer reinforced concrete smoke stack operating at an outdoor air temperature of –30°С and boilers operating at the nominal load, the fraction of bypassed gases makes 30–35%. With the boilers operating at partial loads equal to 75 and 60% of the nominal value, the fraction of bypassed gases makes 35–40 and 40–45%, respectively. The use of condensing heat exchangers in boiler units results in that the levels of temperature difference, free temperature deformation, and thermal stresses in the smoke stack’s structural elements are reduced by a factor of 1.33–2.80 depending on the fraction of gases passed through the CHEs, thereby enhancing the flue gas removing smoke stack performance reliability.

The active development of the Arctic and the Northern Sea Route determines the importance of the rapid development of energy-supply systems for remote regions. A key component of isolated power systems are low-power energy sources. The high cost of fossil fuels in remote regions, coupled with tightening environmental regulations, brings to the fore the challenge of implementing carbon-neutral energy generation technologies. Promising power plants, the performance of which is little dependent on weather conditions, and whose operation is not associated with the generation of greenhouse gas emissions, are low-power nuclear power plants. Currently, some countries are developing and implementing new types of reactor plants whose electrical power does not exceed 300 MW: according to the IAEA, there are more than 70 different projects. Modularity, versatility (in addition to power generation, many projects also provide for the production of thermal energy and hydrogen), increased compactness, and lower capital costs for construction compared to traditional high-power power units make it promising to create low-power reactor plants. This review presents an analysis of the current state of the problems in the design and implementation of such power plants. The technical level of domestic and foreign projects of small modular reactors (SMR) was assessed. Promising areas for the use of thermal energy from small modular installations have been identified, taking into account current trends in energy, including low-carbon and nuclear-hydrogen areas. Possible circuit solutions for the production of electricity based on advanced cycles, including the use of nontraditional working fluids, have been studied. The potential for commercialization of low-power nuclear power plant projects has been considered; the question of successful business implementation of power plants of this type remains open.

District heating system is the main way of heating in cities and towns in China. The development of district heating system still has the problems of low intelligence and low control accuracy, and there is the imbalance of heat supply and demand in heat distribution. Resulting in the energy consumed by the district heating system can account for more than half of the total energy consumption of the building. In order to alleviate this imbalance, this paper studies the control of heat distribution of each heat exchange station in the primary network. The heat model of primary network is established by recurrent neural network (RNN), and the data set used for modeling is the operation data of heat exchange station in reality. Combined with the heat load prediction model, a heat distribution strategy was proposed to optimize the primary flow of the heat exchange station. According to the predicted value, chaotic particle swarm optimization (CPSO) algorithm is used to optimize the primary flow sequence of each heat exchange station, and then the primary flow is adjusted to control the heat distribution of the secondary network. Finally, Simulink simulation model is used to simulate the water supply temperature of the secondary side of the heat exchange station. And analyze the operation status of the secondary side, the results verify the effectiveness of the strategy. The model simulation results show that the heat distribution scheme proposed in this paper can effectively distribute the heat of the heat exchange station according to the heat demand.

The modern structure of energy consumption enhances the nonuniformity of electrical load curves. With the more pronounced nonuniformity of daily and weekly electrical energy consumption, the requirements for the maneuverable characteristics of power units, which include the control range of the power unit load (technological minimum) and the minimum safe load of the power unit (technical minimum), become more demanding. Due to the problem of maintenance and adequate passing of the minimum of electrical loads during nighttime periods and nonworking days, large supercritical pressure (SCP) condensing power units had to be engaged in controlling the loads. This situation is topical for the Russian power industry in the absence of semipeak power units. For SCP power units, it is advisable to perform unloading under sliding pressure conditions throughout the entire steam-water path. The depth of unloading depends mainly on the reliability of the boilers, the hydraulic design of whose heating surfaces had been performed without considering operation at subcritical pressure. The possibility of application of sliding pressure unloading for SCP units was determined by ensuring reliable temperature and hydraulic conditions of the boiler heating surfaces, in which the state of the working fluid changed from subcooled water to slightly superheated steam. Unloading of drum boilers requires maintenance of reliable circulation in the furnace waterwalls and safe temperature conditions of the steam superheating surfaces. The results of the tests of various types of gas-and-oil fired once-through and drum boilers with unloading at sliding or rated subcritical pressures are presented. The reliability indicators of the hydraulic paths of the boilers and the factors limiting deep unloading of power units have been analyzed. The minimum safe loads were determined. Technical solutions for deep unloading were proposed for the hydraulic circuits of the steam-generating part of the flow path of SCP boilers.

Automating the operation of equipment at modern thermal power plants to the maximal possible extent is becoming an increasingly more urgent problem. For coal-fired boilers, the development of furnace operation mode control systems is of special importance. A significant scatter in the characteristics of the coal delivered for combustion have a strong influence on the boiler’s operation mode and its technical and economic indicators. Essential changes in the combustion mode frequently give rise to problems connected with gas temperature fluctuations at the furnace outlet, with maintaining a stable superheated steam temperature, slagging of heating surfaces, degraded combustion efficiency, etc. For estimating the influence of coal properties on the operation mode of the E-210-13.8KT boiler (the factory designation is BKZ-210-140) at the Tomsk GRES-2 thermal power plant, computational studies of gas temperature at the furnace outlet were carried out using the Boiler Designer software package. With an essential variation in the coal characteristics, the calculated values of temperature varied from 1103 to 1150°С at 100% load and from 910 to 948°С at 50% load. The adjustment of fireball direction at the burner outlet by ±15° made it possible to change the gas temperature at the furnace outlet by approximately 90°С. In the case of introducing a fireball direction adjustment system, it would be possible to solve, to a significant extent, the boiler-operation problems mentioned above. An algorithm for automatically adjusting the combustion mode has been developed, which, in case of having been implemented, would make it possible to achieve more reliable operation of boiler unit components, decrease the risk of the heating surfaces becoming intensely fouled with slag, and maintain a stable superheated steam temperature in different boiler-operation modes. A swirl movable burner able to vary the fireball direction at the burner outlet by ±15° should become the combustion system’s key component.

The industrial production of carbon sorbents from coal is a promising and relevant direction. The starting material is mainly brown coal, which is characterized by a high yield of volatile substances and low ash content. Of particular interest to the coal industry is the development of technology for producing sorbents from low-grade coals with a large specific surface area, high adsorption activity, and low cost. Existing methods for producing sorbents from coals that meet such criteria should be based on various thermophysical principles of influence on the source material. The work investigated one-stage and two-stage methods for producing sorbents from coal grades D and DG mined in Kuzbass. The one-stage technique consisted of steam gasification of the starting material in a fluidized bed. The two-stage technique was based on preliminary decarbonization in a muffle furnace followed by activation with superheated water vapor in a fluidized bed. As a result of experimental studies, samples of carbon sorbents were obtained from coals of low metamorphism. Analysis of textural characteristics showed that the specific surface area of the sorbents is up to 250 m²/g and adsorption activity up to 100 mg/g. It has been established that the composition of the mineral mass of the original coals significantly affects the adsorption activity of the resulting sorbents. Estimates show that the higher the ash basicity index, the higher the adsorption activity of the resulting carbon sorbent. With a one-stage method for producing sorbents from coal grades D and DG in a fluidized bed, a fairly high specific surface area is achieved with a relatively low adsorption activity in comparison with a two-stage method.

The development of compact, safe, and efficient methods for storing hydrogen is one of the key problems of hydrogen energy. Currently used technologies for storing hydrogen in the form of compressed gas or cryogenic liquid require significant capital investments and maintenance costs for compressor and cryogenic equipment, are characterized by high energy costs, and their implementation requires special safety measures as well as the use of hydrogen-neutral structural materials. A promising way to solve these problems for medium-scale storage systems is the use of metal hydrides, which provide the simplest, most compact, and safe hydrogen storage compared to traditional methods. However, the high cost of hydride-forming materials hinders the implementation of this approach. The use of alloys based on the TiFe intermetallic compound would reduce the costs of metal hydride hydrogen storage by more than five times. This circumstance is the reason for the growing interest of specialists in the field of hydrogen energy technologies in hydrogen-storage materials based on titanium-iron alloys. Although hydrogen systems with the TiFe intermetallic compound and its derivatives have been studied for more than 50 years, the search for ways to increase the resistance of their hydrogen sorption characteristics to poisoning by oxygen-containing impurities in the gas and solid phases has become particularly relevant in recent years. This article provides an overview of research and development aimed at obtaining, studying the properties, and using titanium-iron alloys with improved hydrogen sorption characteristics. An analysis of the data presented in the scientific literature is presented, and approaches to the development of highly efficient hydride-forming materials based on the TiFe intermetallic compound and hydrogen-storage systems based on them are formulated.