Thermal Engineering最新文献

The ammonia-ethanolamine water chemistry used at NPPs with VVER-1200 ensures low rates of corrosion, mass transfer, and growth of corrosion product deposits. The content of corrosion products in the feedwater of the steam generator is less than 1 μg/dm³. This significantly increases the period between flushing the steam generator to remove deposits. However, ethanolamine and ammonia are absorbed by the cation exchange resin in the ion-exchange filters of the secondary circuit purification systems, which leads to the need to regenerate the cation exchange resin and continuously dose reagents to maintain the required pH value in the feedwater. Waste solutions from regeneration containing ethanolamine and large amounts of ammonia must be treated to ensure that the concentrations of these substances do not exceed maximum permissible values when discharged into the environment. To remove ethanolamine and ammonia from regeneration solutions, special installations are created, the operation of which is based on various principles. A pilot plant for cleaning regeneration solutions was manufactured and installed at the Belarusian NPP with VVER-1200. An analysis of the pilot plant’s operation showed that it successfully fulfills its function of protecting the aquatic environment but, at the same time, it is forced to release a significant amount of ammonia into the surrounding air. Removing ammonia is energy-consuming, environmentally unsafe, and requires the additional use of chemical reagents. In this regard, the water-chemical regime of the secondary circuit without ammonia is very promising. Possible options could be either switching from ammonia to dimethylamine or using ethanolamine as the only corrective reagent with the replacement of some of the structural materials of the secondary circuit with steels with a high chromium content, which have higher corrosion resistance compared to those currently used. Both options will simplify the wastewater treatment technology and reduce the environmental impact while maintaining the low corrosion rates achieved by using the ammonia–ethanolamine water chemistry.

The prospects are examined for application of ammonia-fired gas turbine units (GTUs) with thermochemical recuperation of the exhaust gas heat. Examples of operating ammonia-fired gas turbine units are given, and the main operating restrictions for the use of existing gas turbine units are specified. A thermodynamic analysis of a simple gas turbine unit with thermochemical heat recuperation (TCR) was performed in a wide range of operating conditions: the gas temperature at the turbine inlet varied from 700 to 1300°C and the compressor pressure ratio from 5 to 20. It has been established that the thermochemical heat recuperation can increase the GTU efficiency by as much as 9%. The effectiveness of TCR application has been demonstrated to depend on such operating parameters as pressure and temperature. At a temperature above 500°C, the enthalpy of the ammonia decomposition reaction reaches a value close to the maximum of approximately 3.0 MJ/kg NH₃. Thermochemical recuperation leads to the decomposition of ammonia with production of a hydrogen-rich gas (up to 75% (by volume)), which is burned in the combustion chamber, thereby changing the combustion process characteristics. The flame propagation velocity in a gas mixture consisting of hydrogen, nitrogen, and ammonia in different proportions was calculated on the basis of the GRI-Mech 3.0 list of elementary reactions in the Chemkin-Pro module. It has been found that the products of complete thermochemical decomposition of ammonia have a flame propagation velocity that is approximately two times higher than that for methane and more than ten times higher than that for ammonia. Thus, the implementation of the thermochemical heat recuperation in ammonia-fired gas turbine units is expected to increase the energy efficiency and improve the combustion process stability.

Ejectors are devices designed to suck fluid, steam or gas (primary fluid) from a closed space using a powerful jet of steam (secondary fluid), usually operated under specified boundary conditions using specific working fluids. If ejectors are to be used under new boundary conditions, predicting their performance requires either numerical or experimental studies. This paper presents a simple theoretical model capable of accurately predicting the performance of an ejector, given its geometry and boundary conditions, under different operating conditions. The model can predict the entrainment ratio, critical back pressure, and break-up back pressure using a given simple performance curve. The accuracy of the model is validated using computational fluid dynamics (CFD) simulations. Two ejectors with different geometries, dimensions, and boundary conditions are studied using ANSYS Fluent 19.2, and the results are compared with those from two other studies. The model successfully predicts the performance of all four ejectors across a wide range of operating conditions. Finally, the model is extended to any working fluid and temperature and validated numerically using air as the working fluid instead of water vapor. The results show that the model has an entrainment ratio error of less than 2%. It’s worth noting that this model’s applicability is contingent upon simultaneous changes to both the primary and suction streams by the same factor. Under these conditions, the model aligns closely with CFD-simulations.

The purpose of the review is to find the best currently available correlations for calculating heat transfer and pressure drop in the main heat-transfer equipment items in organic Rankine cycle (ORC) units. The search is limited to the designs of apparatuses, which are the best ones in the opinion of the authors of this paper, for a conventional two-circuit ORC-unit, where thermal oil cools a heat source in the first circuit and transfers heat to refrigerant in the vapor generator (hereinafter referred to as the evaporator). Besides the evaporator, the second circuit of the unit includes a “refrigerant–water” or “refrigerant–air” condenser and a regenerative heat exchanger which heats up liquid refrigerant upstream of the evaporator with the exhaust vapor of the turbine (or expander). The criteria are presented for selecting working fluids for such units depending on the heat source temperature. The working fluids that have found the widest application at each temperature level (such as cyclopentane, benzene, toluene, MM, MDM, R1233zd, R245fa, R601, R601a, RC318, R134a) are listed, and their characteristics and thermodynamic properties are presented at specified condensation (25°C) and boiling (200, 120, and 70°C) points. The analysis of these data, including information on the proposed working fluids, has yielded nominal parameters of ORC-units. Thousands of fundamental and engineering works are devoted to the study of boiling and condensation processes, the interest in which has been growing over the past 10–15 years. The development of new energy conversion technologies and the appearance of new working fluids, materials, and methods of surface treatment has given a second wind. This paper reviews correlations for heat-transfer coefficients and hydraulic resistance factors in apparatuses with refrigerant boiling in round tubes, condensation in tubes and channels and in the shell side (on tube bundles), and heating and cooling of single-phase refrigerant in tubes and channels. The correlations for engineering calculation of the main heat-transfer equipment of ORC-units, which are the most convenient ones in the authors’ opinion, are presented.

A mathematical model of a high-temperature cylindrical reactor for heterogeneous pyrolysis of methane during its filtration through a moving layer formed by granules of carbonized wood is presented. The carbon matrix was modeled by spheres of the same diameter with a simple cubic packing. The carbon matrix was heated through the reactor wall. Preheated methane was fed into the lower part of the reactor. The process of pyrocarbon formation as a result of heterogeneous pyrolysis of methane was described by one gross reaction taking into account hydrogen inhibition and changes in the reaction surface. It was assumed that the rate of pyrocarbon deposition is directly proportional to the partial pressure of methane. The system of two-dimensional, nonstationary differential equations describing the operation of the reactor in a cyclic mode with periodic unloading of a portion of carbon–carbon composite and synchronous loading of carbonized wood granules was solved numerically using the DIFSUB algorithm. The reactor radius and operating parameters (specific mass flow rate of methane, carbon composite unloading frequency) were varied in calculations. Based on the obtained results, the dependences of the quality of the carbon–carbon composite (average density and maximum density spread), the composition of the hydrogen-containing gas mixture at the reactor outlet, the degree of methane conversion, the reactor productivity for carbon composite and hydrogen on the operating parameters, and the reactor radius were analyzed. Data are provided on energy consumption for heating methane and carbonized granules loaded into the reactor as well as for compensation of the endothermic effect accompanying methane pyrolysis.

The prospects for achieving carbon neutrality in economically developed countries that are members of the Organization for Economic Cooperation and Development (OECD) and other countries are examined. An analysis of the energy and land use structure in these countries was carried out. Scenario assessments of the dynamics of changes in carbon indicators of the study economies have been developed, and a comparison has been made with forecasts from leading global energy agencies. It has been shown that, at the current rate of decarbonization and development of the carbon capture and storage (CCS) industry, it is impossible for countries in both groups to fulfill their commitments to achieve climate neutrality in 2050–2070; this goal cannot be achieved before the end of this century. The central challenge in achieving climate neutrality is the rapid and large-scale implementation of CCS technologies in all their possible manifestations. Using a set of global climate system models, calculations of the global average temperature (GAT) were performed for the proposed scenarios, and their results were compared with other works. Despite the fact that climate change occupies almost a leading place on the global agenda, the actual results of efforts in this area are far from those declared, and it is now impossible to cap warming to within 1.5°C. The key task is to minimize the time the global climate system remains in the dangerous extreme zone (above 1.5°C), which will require the emergence of a global economy with negative greenhouse gas (GHG) emissions.

Heat exchange during condensation of freons has been studied quite well; however, various flow regimes of the steam-condensate mixture may arise during condensation inside heat-exchange pipes. There is a large amount of experimental data on the condensation of freons inside pipes with different internal diameters. However, the results obtained by different authors are contradictory, and experimental dependencies can give a high error in the event of a discrepancy between the calculated and actual flow regimes of the steam-condensate mixture. Due to the difficulty of identifying these modes for each such case, reliable recommendations for the calculation and design of heat exchangers must be based on experimental data. In order to obtain such materials, an experimental stand was developed and manufactured, allowing the study of condensation processes of various working fluids in a horizontal cooled tube. The working section of the stand was a copper pipe with an external diameter of 32 mm and a wall thickness of 2 mm, built into an external steel pipe with a diameter of 45 × 3 mm with an annular gap of 3.5 mm. Five chromel-copel thermocouples were installed in the gap to measure the water temperature; they were led to the measuring instruments through the wall of the outer pipe. Thermocouples were also installed in the copper pipe wall. The stand’s thermocouples were precalibrated, and the freon and cooling water consumption was determined by the differences on the flow diaphragms with an error not exceeding 1.5%. The temperatures of cooling water and condensing freon R142b along the length of the heat-exchange pipe were obtained for some flow regimes with different parameters of the working fluid at the pipe inlet. A sharp decrease in the local heat-transfer coefficient along the length of the heat-exchange pipe during complete condensation is shown and is especially significant at its inlet section. The obtained data will be used in the design of heat exchangers with condensation of R142b freon in horizontal pipes.

Currently, power facilities that operate turbine lubrication and control systems using fire-resistant oil only use fire-resistant fluids from foreign manufacturers (Reolube-OMTI, Reolube 46RS, and Fyrquel-L). The impossibility of domestic production of fire-resistant oil is connected with the loss of a special industrial raw material base in Russia in the 1990s. Restoring the entire technological cycle is not a matter for the immediate future. To maintain the operational readiness of oils used in process equipment, extend their service life, and reduce the volume of replacement, it is necessary to organize a high-quality cleaning process. For this purpose, the All-Russia Thermal Engineering Research Institute developed technology for the comprehensive cleaning of fire-resistant liquids and created equipment for its implementation at energy facilities. The results are presented of the analysis of complex cleaning and restoration of oils with their draining from the oil system and “on the go.” The quality indicators of the oils in both variants have been significantly improved—the acid number, deaeration and demulsification time, moisture content, and corrosive aggressiveness of the oil have been reduced, the industrial purity class has been lowered, etc.—and values for individual indicators have been achieved that meet the requirements for fresh oils. It has been shown that it is advisable to carry out complex oil cleaning “on the go,” which helps to clean the oil system from accumulated deposits due to the simultaneously occurring process of sludge dissolution and also allows to significantly reduce the rate of degradation of the restored oil under operating conditions.

Reducing greenhouse gas emissions remains a topical issue in fundamental and applied scientific research, including in terms of analyzing developed and applied CO₂ capture technologies. The main focus is on methods of carbon dioxide burial in stable geological formations, absorption, filtration, etc. The absorption of carbon dioxide during photosynthesis is usually associated with terrestrial biota, although aquatic organisms have a higher productivity of photosynthesis. The use of microalgae as photosynthetic agents is determined mainly by their value for obtaining high-quality food and feed additives, pharmaceutical products, and biofuels, but it is important to consider their effectiveness in the associated absorption of CO₂. When producing products with a long carbon sequestration period, this method can be included in the list of effective carbon capture technologies. To estimate the specific energy costs for CO₂ absorption, proven cultivation methods were considered: open-plane cultivators (microalgae Arthrospira platensis, growth rate from 20 to 40 g/m² per day on dry matter) and cylindrical closed photobioreactors (microalgae Chlorella vulgaris, growth rate 0.7 g/dm³ per day in dry matter). Based on experimental results of microalgae cultivation under conditions of elevated CO₂ concentrations, it is shown that specific energy consumption is in the range from 27 to 768 GJ/t when cultivating A. platensis microalgae and from 59 to 373 GJ/t in microalgae cultivation of C. vulgaris. The greatest energy costs are required for heating and lighting microalgae plantations as well as for separating biomass from the culture liquid for microalgae with small cell sizes. Specific energy consumption can be reduced by maximizing the use of natural light and waste heat from industrial facilities and optimizing biomass collection systems.

The effect of dimensionless operating parameters (Reynolds (Re) and Prandtl (Pr) numbers) on the dimensionless heat-transfer coefficient (Nusselt (Nu) number) is examined in a liquid metal flow in a round tube. The Nu number dependences at Pr ( ll ) 1 (liquid metals) are often presented as Nu = f (Pe), where Pe = Re Pr is the Peclet number. The simplified dependence for Nu relies very much on the fact that determination of the dependence Nu = f (Re, Pr) from the experiments with liquid metal coolants is a challenging matter since such experiments involve great difficulties. Moreover, the measurement error in in such experiments is 10–20% or higher, which is comparable with the deviation of the Nusselt number under the effect of the Prandtl number. In addition, when making experiments under earthly environment conditions, the effect of natural convection on the experimental results cannot be eliminated. In this work, to study the dependence of the Nusselt number on the Prandtl number, a series of calculations of a liquid metal flow in a round tube at a constant Peclet number was performed using the direct numerical simulation (DNS) technique. The predictions demonstrate an increase in the Nusselt number by approximately 10% as the Prandtl number drops from Pr = 0.025 (mercury) to Pr = 0.005 (liquid sodium) at Pe = 125. The influence of the Pr number on the Nu number decreases (in percentage terms) as the Pe number increases.