Thermal Engineering最新文献

The cost and efficiency of power installations depend to a great extent on the design characteristics of heat transfer equipment. These characteristics are especially important for gas turbine units with regenerators operating on low boiling working fluids with low temperature differences and high pressure differences. For such highly efficient energy systems with regeneration, the use of plate heat exchangers containing microchannels obtained by chemical etching of their surface holds promise. For improving their thermal-hydraulic efficiency, versions of extended heating surfaces with convergent-divergent channels are proposed in the article. A set of calculated and experimental studies of the newly developed channels for plate heat exchangers was carried out. Convective heat transfer was numerically simulated, the results of which made it possible to determine the influence of key geometrical parameters, namely, the relative segment length L/H and expansion amplitude A/H on the thermal-hydraulic characteristics in a wide range of Reynolds number values (2500 ≤ Re ≤ 15 000). It is shown that with a periodically narrowed and widened channel cross section, flow acceleration and deceleration zones are produced; directed action on the boundary layers takes place, and turbulent mixing is enhanced. This helps eliminate stagnant zones and obtain more uniform distribution of heat removal over the surface. Based on mathematical modeling, correlation dependences for the Nusselt number and the friction coefficient as functions of the Reynolds number have been obtained, which are suitable for engineering assessments of the efficiency of heat exchangers with similar geometrical parameters of the channels. To verify the numerical study results, an experimental setup was developed, and tests of channel models with recording the heat transfer and pressure losses were carried out, which have confirmed the correctness of the theoretical approach and repeatability of the revealed regularities. The obtained results can serve for confirming the rational choice of geometrical parameters and practical applicability of modified plate heat exchangers with chemically etched channels as part of compact regenerators of advanced power facilities.

The article presents a numerical and experimental study of separated flow and heat transfer in limited packages of four inclined oval-trench dimples (OTDs) located on a heated isothermal section of an adiabatic plate in uniform air flow with the Reynolds number Re = 3 × 10⁴. The depth of dimples is equal to 0.25 of their width, and the angle of inclination is 45°. The isothermal section temperature is maintained with saturated steam. The results of numerical calculations carried out by solving the Reynolds averaged Navier–Stokes equations in the VP2/3 software package, which are closed by the differential equations of the shear stress transfer model (RANS with the SST turbulence model) and the equation of energy, were validated by comparing them with the experimental data obtained by means of gradient heatmetry techniques on the thermophysical setup at the St. Petersburg Polytechnic University. Multiblock computation technologies with the use of intersecting meshes of different scales were applied. It has been found that, with shifting the dimples away from the plate leading edge, relative heat transfer coefficient and back flows are enhanced. With a shift from the first to the fourth dimple, the relative Nusselt number increases in the inlet parts at the bottom by a factor of two (from 1.2 to 2.4) and by a factor of 1.7 (from 1.5 to 2.6) in the windward edge area. The stabilization of stagnation pressure on the windward slopes at a level of 0.26 with the minimal pressure equal to −0.15 in the vortex generation zones entails enhancement of back flows. The static pressure differences cause enhancement of back flows in packaged dimples. The study results have revealed fundamental differences in the heat transfer distribution between the first and subsequent dimples, as well as in their end parts.

The article presents the results of experimental studies of the hydraulic and mass transfer characteristics of 1-m high roll polymeric mesh packing having a specific surface area of 240 m²/m³, equivalent diameter of 0.015 m, and free specific volume of 0.95, with a film countercurrent flow regime in the column with a spray density of 5.0‒25.0 m³/(m² h) and air velocity of 0.5‒2.5 m/s. The air pressure difference, volumetric mass transfer coefficient, and air humidification efficiency with water are determined. Generalized empirical formulas for calculating these characteristics are derived. The roll polymeric mesh packing is compared with regular fill packs made of corrugated metal tape and corrugated vertical metal plates with a rough surface in terms of specific pressure difference and mass transfer efficiency. The possibility of using these fill packs in mechanical-draft mini cooling towers is considered. An algorithm for calculating the mini cooling tower efficiency and cooled water temperature after its having been in contact with air is presented. Graphic dependences of the hydraulic and heat-and-mass transfer characteristics on the operating parameters are given. Special attention is paid to an analysis of liquid distribution over the packing surface and liquid film formation, which is a key factor for intense mass transfer. A reasonable ratio between the air velocity and spray density under different operation conditions is determined. The study results can be used in design and modernization of water cooling systems, including those at industrial enterprises, in which it is necessary to maintain highly efficient heat transfer and use the most compact equipment.

The aim of the work is to study the effect of diluting gases (nitrogen, carbon dioxide, and steam) and oxygen on the combustion of liquid fuel atomized with superheated steam. Main attention is paid to optimizing the combustion process environmental and power performance characteristics. Experiments were carried out using a laboratory burner with fuel atomized by means of steam into a preliminary gas generation chamber, in which atomized fuel is mixed with blast of various compositions. Operating conditions with addition of diluting agents at room temperature and with heating them to 250°C, as well as cases with the use of oxygen-enriched blasting, were studied. The study results have shown that the temperature of supplied gases plays the key role in the combustion process: in admitting cold dilution agents, the flame temperature decreased by 150–200°C, and the chemical reactions slowed down, whereas when heated gases were admitted, the high flame temperature remained unchanged. With increasing the oxygen fraction, a growth of temperature was observed. It has been determined that the ingress of cold diluting agents in the fuel entailed a more efficient reduction of NO_x and CO emissions in comparison with the operating conditions with admission of a heated mixture. At the same time, the admission of heated gases resulted in lower emissions under the operating conditions with oxygen enrichment. Injection of steam resulted in a lower production of NO_x, but it entailed higher CO emissions and caused the combustion to become unstable. By using the method proposed in the article, it is possible to reduce the NO_x emissions (up to 50%), which is commensurable with this indicator in the case of conventional recirculation of flue gases, but with a lower fraction of them (5–10% against 20–30%). This makes the technology involving steam-assisted atomization and controlled dilution of fuel a promising option for use in the power industry and metallurgy, sectors in which high energy efficiency and environmental safety are of importance.

Electrical conductivity, pH, and oxygen concentration, all continuously measured in circulation water, are the key water chemistry (WC) parameters monitored in a power unit turbine generator water cooling system. With a shift for the use of water cooling in the turbine generator systems at thermal power plants (TPP) and for designing of such systems for turbine generators of the T3V-1200-2AU3 series at NPPs, problems were encountered in meeting the circulation water quality standards and in WC control. The article presents a method for determining by calculation the standardized values of electrical conductivity and pH in metering NaOH in a circulating cooling system (CCS) depending on these indicators in makeup water (water medium with low buffering capacity). The turbine generator water cooling systems serve for intense heat removal from the stator (and rotor) windings and are widely used in the power units at TPPs and NPPs. The cooling (circulation) water quality is standardized with respect to several indicators, the main ones of which are electrical conductivity and pH. In view of quite stringent standards on electrical conductivity, especially for the turbine generator rotor cooling system (below 1.43 μS/cm), there is nothing to do but use deeply demineralized weakly acidic water (рН < 7.0), and it is not permitted to use ammonia. For the acidity to be neutralized with NaOH solution, very precise controlled metering is required: with an insufficient quantity of NaOH, the рН will be below 6.5 (the lower limit), and with an excessive quantity of NaOH, the pH value will exceed 9.0, which is the upper permissible limit for the CCS. A mathematical model of ionic equilibria in the CCS circulation water is constructed and solved. With NaOH solution metered within the range of permissible values, the electrical conductivity and pH value adjustment boundaries are determined depending on the source and makeup water quality. An author’s certificate for a software product has been received. Given the existing makeup water quality and unchanged CCS process circuit arrangement, the performed study makes it possible to recommend installing an automated system for metering NaOH solution into circulation water based on the prescribed electrical conductivity value.

The article analyzes the main achievements in the field of neutralizing effluent water and dangerous halogen containing waste by oxidizing them in water in supercritical state (at temperature t > 374°С and pressure p > 22.1 MPa). Criteria determining the oxidation efficiency are revealed. Problems connected with practical implementation of the discussed approach are formulated, first of all, corrosion of structural materials and deposition of salts. Technical solutions that help improve waste neutralization efficiency, reduce corrosion, and prevent pipelines becoming blocked with solid particles and mineral components are described. The environmental and economic aspects of industrial application of supercritical water oxidation are considered. The conditions for achieving better energy and environmental efficiency of oxidizing waste jointly with conventional fuel kinds are determined. Prospects of applying supercritical water oxidation at thermal power plants for ultrasupercritical steam conditions are noted.

Experimental data on the thermal resistance of thermosiphons designed with different structural junctions (“pipe in pipe,” “flat shelve”) are obtained, and the possibility is shown to decrease it as a result of heat transfer enhancement in the evaporator by using a combined coating in the form of longitudinal grooves 0.1 mm in radius, filled with aluminum oxide nanoparticles 100–200 nm in size. The effect the coating has on the thermosiphon thermal resistance was studied in an experimental section in the form of a 100 mm long pipe 38 × 3 mm in diameter made of grade AISI304 stainless steel. As a result of applying the coating, the thermosiphon thermal resistance is decreased by 2.4–3.0 times at heat fluxes in the range 200–1700 W/m² and evaporator slope angles 0°–30°. Experiments with slope angles equal to 0°, 10°, and 20° were carried out, and it is pointed out that the maximal effect is reached at the evaporator horizontal and slightly inclined positions. For calculating the heat transfer in a thermosiphon, a procedure based on the Yu.A. Kuzma-Kichta formula is proposed. The predicted thermosiphon thermal resistance values agree with the experimental data within 20%, a circumstance that extends the model applicability area.

Loop heat pipes (LHPs) are two-phase heat transfer devices with capillary pumping of the working fluid inside a sealed, closed loop, including an evaporator and a condenser connected by smooth-walled pipelines for separate movement of vapor and liquid, the diameter of which usually varies from 2 to 8 mm. This design feature enables easy integration of LHPs into densely packed systems. The LHP evaporator is equipped with a fine-pored capillary structure (wick) (pore radius from 1 to 20 μm), which creates high capillary pressure. Loop heat pipes are used in energy-efficient systems for utilizing low-potential heat, heating or cooling remote objects, and for uniform heat distribution over a large surface area of ​​heat sinks. Currently, the most popular area of ​​application for LHPs is electronic cooling systems operating under a wide variety of conditions. Various liquids can be used as working fluids in LHPs, depending on their operating temperature range, chemical compatibility with structural elements, and the required operating characteristics of the LHPs. This paper presents the theoretical foundations and operational conditions for LHPs. It also discusses the issue of selecting a working fluid to enhance the heat transfer capacity of LHPs. Quality criteria are proposed for assessing working fluids based on their efficiency in these devices. The results of a comparison of six different working fluids using these criteria are presented, showing that ammonia is the most effective working fluid in the operating temperature range relevant for electronics, from 40 to 70°C. Based on the p, t diagram of the working cycle of the working fluid, the thermal resistance of the “heat source–LHP–heat sink” system is visualized, as well as the thermal resistance of the main structural elements.

The article summarizes the key scientific and technological achievements in the field of applying gas hydrates in the power industry. The modern ideas about the specific features relating to the synthesis, transportation, storage, regasification, and use of hydrates for thermal conversion of low-grade fuels and waste, separation of gaseous and liquid media, as well as sequestration of anthropogenic emissions are analyzed. It is shown that the experimental data and the results of mathematical modeling and various-scale tests that have been obtained for the last years in the world’s scientific society form a basis for the development of technologies for applying natural and man-made hydrates. Concepts for integrated use of hydrates in the power systems at mines and in supply of energy resources to settlements are outlined. The technical and economic constraints connected with the development of gas hydrate technologies, their target indicators, and potential solutions of the formulated problems are determined. For the gas hydrate technologies to be efficiently introduced in the energy sector, the following so-called target business metrics (threshold indicators) should be achieved: gas content is no less than 180 vol/vol.;¹ purification ratio of gas obtained after separation of smoke, natural, casing-head, and synthesized gases is no less than 90%; maximal loss of gas in storing and transporting hydrates is not more than 0.1%; hydrate storage term is up to six months; stable combustion temperature during joint low-emission combustion of hydrates with low-grade fuels (sludges, heavy coal-tar products, cakes, etc.) is not lower than 1100°С. For solving these problems, it is necessary to carry out fundamental and applied studies, as well as various-scale tests for integrating gas hydrates into modern power systems with ensuring environmental cleanliness and economic efficiency. The conditions under which essential advantages are achieved in comparison with alternative technologies in each hydrate application area have been determined.