Thermal Engineering最新文献

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.

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.