Thermal Engineering最新文献

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.

Russia is one of the world leaders in the production of sunflower oil, and the utilization of sunflower husks seems to be a very pressing problem. The husk has low humidity (4.4‒12.2%) and a fairly high calorific value (16–19 MJ/kg), but its ash contains a significant amount of potassium, calcium, and magnesium compounds, which cause slagging of the boiler furnace and rapid growth of ash deposits on its convective heating surfaces. Agglomeration and slagging are especially acute when burning crop waste in a fluidized bed of quartz sand, causing defluidization of the layer. This leads to frequent boiler shutdowns to clean the furnaces. Alternative materials to quartz sand are known, but the literature contains little data on their commercial application. The operation of a combustion device with a fluidized bed of quartz sand and olivine as part of a 2-MW heat-generating installation when burning sunflower husks is analyzed. The chemical composition of agglomerates is studied and the mechanism of their formation is described. The experiment on burning husks in a layer of olivine lasted continuously for 600 h. When carrying out periodic measurements of the fractional composition of the olivine layer, the concentrations of carbon oxide, dioxide, and oxygen in the flue gases and the formation of agglomerates was not detected.

Acoustic barriers are used to reduce the noise of power equipment. To increase their efficiency, an cylindrical top edge is installed, which is an add-on on the top edge of the barrier. To study the acoustic properties of the cylindrical top edge, a mathematical model of a 3-m high barrier was built in the COMSOL Multiphysics program. The mathematical model of the barrier without an cylindrical top edge was verified using the Kurze calculation method. The acoustic characteristics of a superstructure in the form of an cylindrical top edge have been studied. It has been determined that the acoustic efficiency of the cylindrical top edge depends both on the position relative to the upper edge of the barrier and on the distance from the noise source to the barrier. The calculation results show that the greatest changes in sound pressure levels when installing an cylindrical top edge are observed at high frequencies, and the minimum at low frequencies. The acoustic efficiency of the cylindrical top edge at geometric mean frequencies corresponding to low frequencies is approximately 1–2 dB and it can reach up to 25 dB at geometric mean frequencies corresponding to high frequencies. The acoustic characteristics of an cylindrical top edge with different installation angles have been studied. It has been shown that the cylindrical top edge with an installation angle of 0° has the highest acoustic efficiency (8–10 dBA) at a distance from the noise source to the barrier of up to 2 m. At distances from 2 to 5 m, the highest acoustic efficiency (4–8 dBA) is observed when using an antidiffraction device with an installation angle of 90°. Using an cylindrical top edge with an installation angle 180° is advisable when the barrier is located next to the design point at a distance from the barrier to it of less than 5 m. When installing an antidiffraction device, a significantly greater acoustic effect is achieved than when increasing the height of the barrier. The results obtained during the research are recommended to be taken into account when implementing noise reduction measures when choosing the location of an acoustic barrier with an cylindrical top edge relative to the noise source and the design point.

The results of practical work on cooling heated targets of research modules of various designs with a dispersed coolant flow are presented. A brief description of the experimental stand and its main systems, nozzle designs and modules are given, allowing for the implementation of different cooling schemes: with parallel and perpendicular arrangement of the nozzle end and the target surface. Temperature fields of heated targets were obtained depending on thermal loads and water and air flow rates. Primary processing of experimental data was carried out, during which the temperature values on the heating and cooling surfaces were determined. Graphs of the dependence of the temperatures of these surfaces on the supplied thermal power for research modules of various designs are shown. The heat-flux density from the cooled surface of the heated target to the dispersed coolant flow and the heat-transfer coefficient were estimated. The dependences of the heat-flux density and heat-transfer coefficient on the temperature difference between the wall and liquid for different designs of cooling systems are shown graphically. An assessment was made of the proportion of heat removed from the heat-loaded elements of the proposed structures through a phase transition. It is shown that the mutual orientation of the nozzle and the heated surface significantly affects the limiting value of the heat-flux density removed from the target in the thermal stabilization mode. It has been established that the cooling efficiency of a dispersed coolant flow with a perpendicular arrangement of the nozzle end and the target surface depends to a large extent on the timely opening of the spray plume, determined mainly by the operating parameters and the distance from the nozzle end to the target.

Different models of stable film boiling of liquids that give heat-transfer characteristics under these conditions are examined. The existing models have been demonstrated to have disadvantages associated with a consideration of certain limiting cases. The model of subcooled liquid film boiling, developed by a research group including the authors of this paper in 2017, takes into account the velocity of natural convection at the liquid/vapor interface. This model demonstrates good agreement with experimental data on cooling of spheres and cylinders, but the expression for the heat-transfer coefficient (HTC) contains an empirical coefficient. A new model of heat transfer during subcooled liquid film boiling based on the Bromley assumptions is proposed. An analysis of the contribution of radiation to heat transfer during film boiling has demonstrated that, according to a rough estimate, the contribution of this factor can be as high as 10% during cooling of high-temperature bodies in water when their surface is superheated to 1000 K. The applicability of the new model of stable film boiling of subcooled liquids and the models examined in this paper was validated by comparison with the authors’ experimental data. The test pieces were spheres and cylinders made of different metals (such as stainless steel, nickel, copper, titanium, FeCrAl alloy, zirconium). They were cooled in saturated or subcooled liquids with different thermophysical properties (such as water, ethanol, water-ethanol mixtures of various concentrations, FC-72, nitrogen) at different system pressures. The experimental data agree best of all with the predictions by the newly developed model. The performed comparisons have demonstrated that this model is more accurate (by 10%) compared to other models of heat transfer during cooling of spheres and cylinders in various liquids (such as water, ethanol, FC-72, isopropanol) in the subcooling range from 10 to 180 K at system pressures from 0.02 to 1.00 MPa.

The work is devoted to determining the characteristics of turbine stages in off-design modes that arise when pressures and temperatures change before or after the stage, a transition to a different rotation speed, or, for example, when the composition of the working fluid changes. As part of the project, a quasi-one-dimensional method for calculating the characteristics of a turboexpander assembly (TEA) stage when changing operating parameters and/or working fluid has been developed, which differs from known methods by using the equations of the state of real gas, adaptation to purely radial stages, and a simplified approach to determining the pressure at the outlet of the guide vane for assessing the degree of reactivity and the ability to switch to another working fluid, including a mixed one. The analytical methodology was verified by comparison with the experimental data of other authors and the results of calculations using CFD methods for radial-axial stages as well as with approaches to the calculation of purely radial turbomachines due to the lack of experimental data for this type of TEA in the public domain. An extended characteristic of a radial stage operating in air (turbo map) was constructed, and the dependences of the isentropic efficiency, degree of reactivity, mass flow and power of the stage on the relative circumferential speed were assessed. An assessment was made of the impact of switching to another working fluid (for example, switching from air to methane was chosen). It is shown that the characteristics do not change qualitatively but they shift from one another along the axis of the relative peripheral velocity. Further development of the technique involves taking into account possible phase transitions (volume condensation) in the flow part.

Development of sorbent compositions from industrial waste is a promising and economically feasible method for solving environmental problems. Power industry enterprises experience an acute need for the development of new environmentally friendly and cheap sorbents for gaseous fuel desulfurization purposes. Owing to removal of sulfur compounds from the fuel, the latter becomes less corrosive in nature, due to which it becomes possible to increase the equipment’s service life and also to decrease the deposits of sulfides on the surfaces of power installations. Based on a review of literature sources, the most important developments for sorbents consisting of industrial waste were determined. The waste of a thermal power plant (TPP) water-treatment facility (WTF) in the form of sludge water is of the greatest interest for removing sulfur compounds from fuel. Sludge water has a complex composition, which depends directly on the source water quality and water-treatment technology. Sludge water is produced at the natural water pretreatment stage, during which suspended matter is removed from source water by adding coagulants, flocculants, and other chemical agents that are specified by the process regulation. The article presents the composition of a sorbent produced from the WTF sludge at one of the Kazan combined heat and power plants (CHPP) for gaseous fuel desulfurization. Laboratory experiments were carried out with this sorbent, as a result of which the sulfur compound absorption efficiency and the strength characteristics of the prepared and formed sorbent were determined. A new method for indicating the extent to which the adsorbent absorption efficiency is decreased by using an indicating sorbent is also proposed. It is very difficult to monitor the level of sulfur compounds in purified gas by means of automatic sensors in view of a high measurement error, due to which an inaccurate result is obtained. An indicating sorbent composition that makes it possible to detect nonadsorbed sulfuric compositions by showing a color change from light to deep yellow is proposed. A method for using this indicating sorbent is described, and experimental data on its ability to absorb sulfur compounds are given.

Lubricants are the most important element of mating friction pairs and largely determine their reliability and service life. Components of oil systems of turbine units are susceptible to contamination of the working fluid; therefore, during equipment operation, it is necessary to take oil samples and monitor cleanliness. In many cases, when equipment is stopped for maintenance or is in standby mode, the quality of the oil is not given due attention. Ultimately, this may affect the reliability of the unit. The quality of the oil when starting a turbine is often not the same as when the unit is taken out of service. Increasing filtration efficiency plays a key role in reducing wear rates. Cleaning requirements are most important during turbine commissioning and when equipment is spinning at low speeds. To clean the working fluid during operation, effective full-flow filters are required. The research was carried out on a T-180/210 LMZ turbine unit; Tp-22S turbine oil was used as the working fluid, and the volume of the oil system was 36 m³. After modernizing the filters of the main oil tank (MOT), solid particles in the oil decreased by 5.8 times, the purity corresponds to class six to seven by GOST 17216-2001. After the turbine unit was put into operation after routine repairs, a large amount of contaminants entered the system. The amount of solid particles in the oil increased 27 times. The purity of the oil in the system increased over 14 days of operation of the turbine after routine repairs, and solid contaminants in it during this period decreased by approximately 14 times and corresponds to class eight, and that over 28 days was by approximately 25 times and corresponds to class seven according to GOST 17216-2001. This increase in oil purity is a consequence of filtering out contaminants introduced and formed in the system during routine repairs and the completion of the running-in period of the associated turbine friction pairs. The most sensitive element of the oil system is the control system. As a result of research and compilation of oil-cleanliness data, the recommended level of industrial cleanliness for the hydraulic control system is class eight (GOST 17216-2001). The most common method of reducing the risk to equipment during commissioning operation is the use of additional oil-purification equipment. Oil-purification costs can be offset by reduced maintenance costs and replacement of damaged equipment.

The flame structure, appearance, and emission characteristics of an inverse diffusion porous combustor (IDPC) are investigated experimentally. Unstructured ceramic foam made of silicon carbide (SIC) is used as a porous medium. At stoichiometry conditions, a reactive analysis is performed with methane as a fuel and variations in the pore distribution density (pore density) of ceramic foam SIC. Height of ceramic foam and Reynolds number of air jet (({{operatorname{Re} }_{{air}}})) are varied. Porous medium alters flow momentum in radial and axial directions which affects flame appearance and emissions. Increased radial momentum produces wider and shorter flame in case of IDPC. A bright blue zone is detected at the base of the flame, and a luminous orange or orange-blue zone is observed in the post-combustion zone near the flame tip. As the pore density is enhanced from 10 pores per inch (PPI) to 20 PPI, the flame is detached from the surface of the porous medium at a higher Reynolds number of the air jet. The visible flame height of IDPC is significantly reduced at 10 PPI when compared to a case without a porous medium. The Reynolds number of the air jet and the pore density of the porous medium strongly influence the emission levels of NO_x and CO. The IDPC with porous media height of 28 mm, ({{operatorname{Re} }_{{air}}}) = 8122 and 10 PPI pore density performs optimum in terms of flame shapes and CO and NO_x emissions.