Thermal Engineering最新文献

Thermal energy is one of the main sources of anthropogenic greenhouse gas emissions. To fulfill Russia’s obligations to reduce greenhouse gas emissions under the Paris Climate Agreement, it is planned to focus in the energy sector on the development and implementation of cleaner technologies for the use of energy fuels, hydrogen and hydrogen-containing mixtures, the decommissioning of obsolete equipment, and the accelerated introduction of new efficient energy plants. As part of the study, an assessment was made of the real possibilities of decarbonization of the Russian heat and power industry through the implementation of priority measures provided for in the Strategy for the Socio-Economic Development of Russia with Low Greenhouse Gas Emissions until 2050. For this purpose, a comparison was made of the carbon intensity of various technologies for generating electrical and thermal energy, taking into account the type of thermal power engineering enterprises and the efficiency of power steam turbine, gas turbine, and combined-cycle gas plants burning various types of fuel. Possibilities for reducing CO₂ emissions were assessed due to improving the quality of solid fuel, the transition from burning coal to burning natural gas, the introduction of combined-cycle gas plants, increasing the efficiency of power plants, decommissioning obsolete equipment, and the use of hydrogen-containing gases and pure hydrogen as fuel.

The need to adapt the world’s industry and economy to constantly tightening climatic standards, as well as a constant growth of energy consumption, facilitate the development of carbon-free electricity generation technologies. Renewable energy and nuclear power plants are referred to energy sources having almost zero carbon dioxide emissions into the atmosphere. However, in view of an insufficient amount of renewable energy resources near large electricity consumers, NPPs play the most important role in the potential transition to the carbon-free economy of Russia. However, they do have certain drawbacks, such as comparatively low energy efficiency, poor maneuverability, and also high specific capital outlays. Combined use of nuclear and fossil fuel may become one of ways for partially removing these drawbacks. The article addresses a thermodynamic analysis of using fossil fuel at an NPP in an external steam superheater with subsequently expanding a part of the steam in a high-temperature turbine. A process circuit solution is proposed whose use makes it possible to obtain an expanded power unit load adjustment range. It has been shown from thermodynamic analysis results that, by subjecting a certain amount of steam from the steam generator to external superheating, it becomes possible to increase the nuclear power unit’s power output and efficiency: the maximal increase in the electric power output can total 338, 382, and 426 MW and that of net electrical efficiency of 0.73, 1.08, and 1.43% at steam superheating temperatures equal to 560, 600, and 640°С, respectively. The hybrid unit employing nuclear and hydrocarbon fuel that operates according to the proposed process cycle circuit includes a smaller amount of main equipment and features wider load adjustment ranges in comparison with standalone NPP and steam turbine thermal power plant: 102.3–132.7, 103.0–136.9, and 103.6–141.2% with respect to the reference process cycle circuit at steam superheating temperatures equal to 560, 600, and 640°С, respectively.

Cycle chemistry monitoring systems are intended for online comprehensive automatic monitoring, analysis, diagnostics, and prediction of the water chemistry in power equipment in all regimes of its operation, including startups and shutdowns, as well as for remote automatic control of one or several processes in the serviced process facility. Basic requirements for cycle chemistry monitoring systems are formulated. Mathematical models, which are based on the material balance, ionic composition of the coolant, and recurrent neural networks, have been developed and studied. They enable us to predict the concentration of impurities along the power unit’s path to prevent failures of the water chemistry. An algorithm has been developed for online quality assessment, based on dimensionless coefficients that provide fair information on the water-chemistry conditions and help to detect failures affecting the water chemistry. A simulation model with a user interface has been developed based on a set of algorithms considering the requirements for cycle chemistry monitoring systems, such as visualization, interactivity, reporting, customization, scalability, continuity, and simplicity. The model facilitates the activities performed by the operational personnel of power plants as to decision-making and prevention of failures of the water chemistry of the power unit, enables us to monitor the process parameters of the power unit in real time, analyze statistical data, predict parameters using algorithms on the basis of the material balance, ionic equilibriums, and neural networks. A user manual has been prepared to help one to understand the program interface. The manual contains a brief description of the system structure, including information and diagnostic functions, basic elements of the mnemonic diagram, and a set of control buttons.

Numerical modeling of the thermal state of the T283-1600 thyristor with various radiators, on the surface of which boiling of the 3M Novec 649 liquid dielectric occurs, was carried out. Calculations were performed in the “in-house” CFD code ANES. Heat-transfer coefficients for nucleate and transition boiling, as well as critical heat flux, were calculated using the formulas of  V.V. Yagov. The change in boiling mode from nucleate to transition was carried out with equal heat flux calculated using the corresponding formulas: approximately 110 kW/m², which is 17% lower than the critical heat flux predicted by Yagov’s formula for technically smooth surfaces. This led to slightly higher calculated temperatures of radiators on surface fragments with a transient boiling regime compared to temperatures during nucleate boiling over the entire cooling surface. The proportion of the surface area covered by the transition boiling regime did not exceed 3.2% of the total radiator area. Various forms of radiators were studied: in the form of fins from several disks and rectangular parallelepipeds with vertical slotted channels. At the same time, the geometric parameters of the fins and channels and their number and dimensions of the radiators were varied. As a result of numerical optimization, a radiator design was determined that meets the required conditions for the maximum temperature of the thyristor on the surface of contact with the radiator. To validate the results of numerical modeling, an experimental setup was created containing an assembly of thyristors with radiators immersed in a 3M Novec 649 dielectric. In normal operation, measuring the temperature of one of the radiators near the contact with the thyristor showed good agreement with the results of numerical simulation.

An axial flow tubular heat exchanger has been experimentally investigated to augment the heat transfer rate with a novel swirl flow of air past the heated tubes. The novel design has been made on the circular baffle plates provided with trapezoidal air deflectors of various inclination angles α, which is the angle made by the deflector surface with baffle plane. The arrangement of tubes, which were supported on baffle plates, was kept the same throughout the experiment analogy with the peripheral longitudinal air flow directed on it. All the tubes were maintained at constant heat flux condition over the entire surface. There were four deflectors developed on each of the baffle plate each deflector with equal inclination angle which generates air swirls inside the circular duct carrying the heated tubes that increase air side turbulence and hence the surface heat transfer rate. For every Re the baffle plates were placed equidistant from each other at different pitch ratios (PR = 0.6, 0.8, 1.0, and 1.2). The Reynolds number Re was kept in the range of 93 500–160 500. The effect of pitch ratios and the inclination angles on the thermo-fluid performance of the heat exchanger has been studied. The investigations reveal an average improvement of 25.1% in the thermo-fluid performance for a heat exchanger provided with the deflector baffle plates (DBP) having an inclination angle of 50° and a pitch ratio of 1.2 compared to that of a heat exchanger with a segmental baffle plate (SBP) tested under similar conditions of operation.

The interaction among the vortices that develop over an axial turbine passage hub leads to pressure losses and, consequently, to a decrease in the stage isentropic efficiency. The turbine performs better if flow separation and secondary flows are reduced. To achieve this, this paper explores by computational fluid dynamics the application of rotor hub contouring to a one-and-a-half-stage axial turbine, the “Aachen Turbine.” The pressure side arm of the rotor horseshoe vortex is guided by a groove in the end-wall rotor hub surface, which is defined parametrically using non-uniform rational B-splines (NURBS). This novel rotor hub groove runs from the leading edge of the rotor blade to the trilling edge of the rotor blade. A three-dimensional steady Reynolds Averaged Navier–Stokes (RANS) k–ω-SST model of the one-and-half-stage turbine with axisymmetric end-walls is validated against reference experimental measurement from the Institute of Jet Propulsion and Turbomachinery at RWTH Aachen in Germany. By contouring the hub of the upstream stator and of the rotor, the overall pressure loss coefficient predicted by openFOAM computational fluid dynamics is reduced by 5.2%, using Kriging optimized groove shape parameters.

The results of the investigation into heat-transfer enhancement at increasing critical heat flux due to modification of a wall’s inner surface are presented. The greater need for new, compact, and energy-efficient heat exchangers on the basis of minichannels for high-tech industries makes this investigation urgent. The potential for application of small diameter channels in systems where various dielectric liquids or freons at moderate and high reduced pressures can be used as a coolant is being actively investigated today. The experiments were performed in a heated vertical minichannel. The wall was modified by the rolling method, which has not yet been used in small diameter channels. The experiments were performed with a forced flow of R125 refrigerant at high reduced pressures of 0.43 and 0.56 in the range of mass flowrates from 200 to 1200 kg/(m² s), which is the most applicable range for minichannel heat exchangers. Heat transfer during forced convection and flow boiling was studied. The experimental setup and the minichannel inner wall modification method are described. Experimental data on forced convection and flow boiling heat-transfer coefficients, critical heat fluxes, and pressure drops are presented. The heat-transfer data were compared with the results obtained previously with the inner surface modified by the action of laser pulses on the outer wall. The convective heat-transfer coefficient in a minichannel with the inner surface modified by rolling was found to be much greater than that in a smooth channel. The obtained convective heat-transfer coefficients are compared with the predictions by empirical formulas derived for large-diameter pipes with the wall surface modified by rolling.

The article presents a kinetic model describing the lead vapor oxidation in the vapor bubble volume to produce lead oxide and hydrogen with their subsequent dissolution in the lead melt volume. The model is implemented in the approximation of homogeneous distribution of reagents and oxidation reaction products in the bubble volume. An analytical solution for stationary oxidation conditions is obtained. It is shown that vapor bubbles in the lead melt volume are a sort of chemical “microreactors” producing lead oxide and hydrogen, which subsequently dissolve in the melt volume. However, such hydrogen generation mechanism does not pose any threat for the primary coolant circuit of fast lead cooled reactors in terms of hydrogen accumulation and explosion hazard in view of essentially low intensity of the hydrogen generation source. The article presents the results of water to hydrogen conversion assessments carried out with the use of a homogeneous kinetic model for interaction of water vapor with lead vapor in the vapor bubble volume. The model incorporates mechanisms governing lead evaporation into the bubble volume, oxidation of lead vapor as it interacts with water vapor in the bubble volume, and dissolution of reaction products in the lead melt surrounding the bubble. One more important result of equilibrium thermodynamic computations is connected with a possible change in the composition of iron oxides in the melt after the injection of water from the steam generator leak into the melt. The ingress of water into the lead melt may cause a change in the composition of iron oxides, thereby increasing the fraction of hematite and decreasing the fraction of magnetite. This may entail a change in the composition of the protective oxide film on the structural steel surface to make it more brittle.

Organic fuel combustion products are one of the main sources of air pollution. When burning fossil fuels, pollutants harmful to human health, such as nitrogen and sulfur oxides, fly ash with particles of unburned fuel, carbon oxides, polycyclic aromatic hydrocarbons, and metal oxides, are released into the atmosphere. The largest emissions of pollutants into the atmosphere are accompanied by the combustion of solid fuels. In 2014, significant changes took place in the environmental legislation of the Russian Federation [1], which oblige energy enterprises to comply not only with sanitary and hygienic standards but also with technical and technological standards. To maintain technological indicators for specific NO_x emissions on boiler equipment at the regulatory level, it will be necessary to fully apply primary methods for suppressing the formation of nitrogen oxides, use low-toxic burners and fuel combustion schemes, and, in some cases, expensive gas purification from NO_x will be required. Based on statistical data, the BKZ-420-140 (E-420-140) boiler was selected as a prototype for research. When BKZ-420-140 boilers in the factory version operate using vortex burners when burning brown coal, the concentration of nitrogen oxides in the flue gases is more than 800 mg/m³, which significantly exceeds current standards. In order to reduce emissions of nitrogen oxides while ensuring high operating efficiency of boiler equipment, a solid-fuel combustion scheme using direct-flow burners and nozzles has been proposed for this boiler. It is shown that it will be possible to increase the economic and environmental efficiency of BKZ-420-140 boilers’ operation by implementing the developed solid-fuel combustion scheme using direct-flow burners and nozzles on them.

The calculation method developed by the authors is supplemented by the heat balance of a circulating fluidized bed (CFB) reactor for burning coke residue in a CFB reactor‒gas-generator system. The following options are examined: with the supply of additional dried fuel to the reactor as an alternative method for keeping the required gasification temperature of particles at the CFB reactor outlet and application as a fuel of a mixture of wood biomass and Kuznetsk coal benefication products. The thermal cycle of the plant has been modified as applicable, and the calculation results are presented. It has been demonstrated that gasification of a mixture of biomass and coal benefication enables increasing the overall system capacity (production of hydrogen + electricity) without a considerable growth in the biomass consumption. In this case, the hydrogen production decreases, and the hydrogen production efficiency drops but the efficiency of electricity generation rises. The hydrodynamic calculation of CFB reactors was performed to attain the specified flowrates of circulating material required to maintain proper temperatures in the reactors. The flowrate of circulating particles can be increased by increasing the pressure difference (loading level or weight of material in the reactors). The overall dimensions of the reactors have been determined, and their layout is presented. A procedure for calculating capital and operating expenditures is outlined, and these expenditure components are estimated. The cost of hydrogen production using biomass without CO₂ emission over the life cycle of the plant was estimated (USD 1.45/kg). Approximately 2/3 of the formed CO₂ is already ready for storage. Therefore, we have only to remove CO₂ from the flue gas flow from the CFB reactor of the gas generator. This level corresponds to available foreign data on similar plants operating on natural gas and is lower than that provided by the widely used technology of steam reforming of natural gas with CO₂ capture.