Thermal Engineering最新文献

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.

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.

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.

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.

For substantiating liquid metal cooled reactor plants, the EUCLID/V2 integrated code is being developed, and its verification and validation are carried out for certifying it at the Scientific and Technical Center for Nuclear and Radiation Safety (NTC NRB). One of the integrated code’s main parts is the severe accident block, which includes the SAFR module for calculating the destruction of fuel pins, fuel assemblies (FAs) and the entire core, as well as the HEFEST-FR module for calculating the melt retention and cooling down in the sodium-cooled reactor core catcher. The HEFEST-FR module implements the possibility to perform 2D simulation of the structural elements and fuel melt behavior in liquid metal cooled reactors. In accordance with the NTC NRB requirements, for the HEFEST-FR module to be used as part of the EUCLID/V2 code for analyzing the safety of fast reactors, it must be validated with the use of available experimental data; the validation shall be accompanied with an uncertainty and sensitivity analysis and assessment of the calculation result error. The article presents the results obtained from verification of the EUCLID/V2 integrated code HEFEST-FR module through solving the analytical problem of settling a stationary temperature of a homogeneous bounded cylinder uniformly heated from below with boundary conditions of the third kind and through solving the Stefan single-phase problem, as well as the results of validating the HEFEST-FR module based on the SCARABEE BF1 experiment. It is shown that the average absolute value by which the numerical calculation deviates from the analytical solution of the problem of settling a stationary temperature of a homogeneous bounded cylinder uniformly heated from below with boundary conditions of the third kind makes approximately 1.1 K. The maximum relative deviation of the results of calculations carried out using the computer program from the results of analytical solution of the Stefan problem (determination of the melt front) makes 0.46%. An assessment of the errors of modeling using the melt retention module as part of the EUCLID/V2 code (HEFEST-FR) based on the BF1 test of the SCARABEE experiment has shown that the temperature calculation error lies in the interval [‒82.3; 182.5] K, and the error of calculating the radial heat flux lies in the interval [‒55.2; 31.2] kW/m².

Various design versions of the rotor of hydro-steam turbines (HSTs) and their application fields are reviewed. It is shown that the design with nozzles arranged over the periphery has certain shortcomings resulting in a decreased energy efficiency, including a thermodynamically unjustified increase of pressure at the nozzle inlet, which results in excessively high velocities in the nozzle “throat,” a short period of time for which the evaporating medium resides in the nozzle divergent part, and poor aerodynamic characteristics of the peripheral area, which cause increased friction losses during the impeller rotation in a two-phase medium. A hydro-steam turbine impeller design with helical nozzle-channels is proposed. Such design has features that create prerequisites for increasing the turbine efficiency, including a longer time for which the medium resides in the nozzle, a possibility to obtain aerodynamically smooth lateral and peripheral surfaces of the impeller, and better conditions for moisture separation from the medium surrounding the rotating impeller. The conditions under which superheated water enters the impeller are considered, and statements on shaping the impeller profile part are formulated. A procedure for determining the nozzle-channel divergent part’s camber line shape is proposed proceeding from the minimal force interaction between the liquid phase fragments and channel walls. An algorithm for determining the areas of the channel divergent part’s cross sections when the velocity increase and pressure decrease patterns become monotonic in nature as the flow moves from the inlet to the outlet is developed. A solid-state 3D model of the HST four-nozzle impeller obtained in designing the turbine is presented.

A description is given of the model of formation of iodine oxide nanoparticles in the volume of the containment of a nuclear reactor during radiation-chemical reactions and clustering with subsequent formation of nanoparticles. The results of test calculations are presented. Nanoparticles of radioactive iodine oxides pose a potential hazard as a source of long-term radiation exposure to NPP personnel. They also penetrate outside the power plant through the air-ventilation system even if the enclosure remains sealed. In addition, calculations have shown that the formation of radioactive iodine oxide nanoparticles is closely related to the formation of volatile iodine compounds in the gas phase. Moreover, the intensity of formation of the latter depends significantly on air humidity. The main feature of the proposed model is the consideration of the kinetics of phenomena for processes in both the gas and aerosol phases when exposed to ionizing radiation on oxygen and water vapor in the air. The main result of the calculations is confirmation of the fundamental possibility of the formation of aerosol nanoparticles consisting of iodine oxides in a humid atmosphere even with a relatively low dose rate of radiation energy absorbed by the air. At the same time, radiolysis of water vapor has a weak effect on the size and concentration of iodine oxide nanoparticles. However, taking into account the chemical interaction of radiolysis products, in particular hydrogen radicals, induced by water vapor radiolysis, significantly affects the formation of volatile iodine compounds with hydrogen: HI and HOI. The obtained results, despite their preliminary nature, are important since they indicate the inevitability of the formation of suspended nanoparticles of iodine oxides and hydrogen iodide in the atmosphere of the reactor premises in the event of emergency situations with the release of radionuclides beyond the first circuit of the reactor. Therefore, this mechanism must be taken into account when developing models of the formation and behavior of fission product aerosols at NPPs.