Thermal Engineering最新文献

A study of film-boiling processes of liquid subcooled to saturation state was conducted using numerical modeling based on the Volume of Fluid (VOF) method and the Lee model for describing heat and mass transfer at the interphase surface. During film boiling, a stable vapor film is formed between the wall and the liquid, eliminating their direct contact, which leads to low heat-transfer intensity. Under conditions of subcooling of the liquid to the saturation state, the intensity of film boiling increases significantly. Despite a thorough understanding of the heat and mass transfer mechanisms involved in this phenomenon, the development of new, more accurate models remains an active area of research. The efforts of researchers are driven by the need to accurately describe the cooling dynamics of high-temperature bodies and predict the onset conditions for high-intensity boiling regimes. Physical models explaining the emergence of these modes are still at the development stage. To verify and clarify them, there is insufficient information about the local characteristics of the vapor film and the features of free convective flow in subcooled liquid. The use of the VOF method allows for detailed tracking of changes in the interphase surface directly during the numerical simulation process. The article presents the results of modeling film boiling of water on the surface of a cylinder with a diameter of 2 mm with superheating of the wall up to 400 K and subcooling of the liquid up to 20 K. The discrepancy between the obtained simulation data and the experimental results published in literary sources does not exceed 10%. Information is provided on the distribution of film thickness, heat flux on the wall, and the interphase surface. According to the simulation, even with slight subcooling, the vapor is not “evacuated” from the vapor cavity, while evaporation is observed on one part of the interphase surface, and condensation on the other. The results of modeling taking into account buoyancy forces, associated with temperature nonuniformity in a subcooled liquid, and without taking them into account practically coincide, which indicates that the natural convection flow is formed mainly due to mass forces caused by difference in phase densities. The obtained data can be useful for creating more accurate empirical models describing the process of stable film boiling in subcooled liquid. All calculations were performed using the ANES CFD code developed at the Department of Engineering Thermal Physics of the National Research University MPEI.

One of the requirements that have to be met is substantiating the safety of newly designed and constructed lead cooled reactors is to evaluate the consequences from possible escape of fission products from nuclear fuel into the coolant during an accident in which part of fuel pins lose their leak tightness. Hence, it is of relevance to develop a model and a computation module based on this model for simulating the interaction of volatile fission products dissolved in the lead melt (iodine and cesium isotopes, and other radionuclides) with the bubbles of gaseous fission products (xenon and krypton isotopes). The consideration of this process is very important in substantiating the safety of lead cooled reactors, because the interaction of volatile fission products dissolved in the lead melt with the bubbles of gaseous fission products has an effect on the content of dissolved radionuclides in the lead melt and on the release of radionuclides into the gas space above the lead melt. The subsequent migration of radionuclides in the reactor gas circuit results in that the activity of radionuclides becomes redistributed in the circuit. It also facilitates the release of activity, as a consequence of loss of leak tightness of the circuit components, into the reactor rooms and the ventilation system. In addition, the bubbles of gaseous fission products in the lead melt interact with the hydrogen isotopes dissolved in the melt (protium and tritium) and facilitate their escape into the reactor gas circuit. The article presents the results from the development of the model and corresponding software unit, and its incorporation into the AEROSOL/LM module, which is part of the EUCLID/V2 integrated code, for calculating the behavior of inert radioactive gas bubbles, including the interaction of bubbles with the radionuclides dissolved in lead melt and the release of bubbles into the reactor gas space. To check how correctly the models are implemented in the code by means of software, the article presents data on the verification of the developed module based on the results of solving test problems.

To date, in the regulatory and technical documents containing requirements for the quality of fire-resistant fluids, there is no unified concept of the rejection indicators of oil, upon reaching the limit values of which the use of the oil is prohibited. In this regard, difficulties arise in making decisions about extending the service life of fire-resistant fluids. Some facilities use oils with an acid number of 3 mg KOH/g and higher, which significantly reduces the reliability of the oil-filled equipment of the power plant, up to the development of emergency situations. The article examines in detail the processes of destruction of fire-resistant fluids based on triaryl phosphates and the conditions for phosphating metals. The results are presented from studies of the surfaces of steel plates after conducting an analysis to determine the corrosion properties of oils in accordance with FR.1.31.2010.08899 Methodology for Measuring the Anticorrosion Characteristics of Samples of Mineral and Fire-Resistant (type OMTI) Turbine Oils (certificate of certification no. MVI 60-09 dated November 17, 2009). It has been shown that, when using oils with a maximum acid number, a film consisting of iron phosphates is formed on the surface of steel coupons, with the simultaneous occurrence of a polycondensation process of oil-decomposition products. Taken together, this leads to an incorrect definition of such an oil quality criterion as corrosion on steel plates. The results of studies of deposits taken from various units of the turbo unit lubrication system are presented, indicating an inevitable decrease in the reliability of the oil system when using oil with a high acid number.

Surface film condensation processes occur in many technical devices. For designing of industrial condensers, empirical methods are usually used, which, however, are insensitive to some factors affecting the intensity of these processes, which limits the range of their application for design. Using existing methods, it is possible for one to calculate the heat-transfer surface area required to maintain the required heat rate, but it is not possible to specify tube arrangement in the condenser, which determines the flow of the vapor-liquid mixture in the intertube space and, ultimately, the efficiency of the device. To design more efficient condensers, it is necessary to improve the methods, including those based on data obtained using modern methods of numerical modeling of heat and mass transfer processes. One of the promising methods for calculating surface condensation processes in tube bundles is the Volume of Fluid (VOF) method, supplemented by models for taking into account heat and mass transfer at the interphase surface. The VOF method has been implemented in some commercial codes, but the settings of the code parameters and the choice of a suitable mesh and turbulence model for calculating the condensation processes of moving vapor are not at all obvious. Previously, the authors of this work proposed and implemented a modified model of W.H. Lee in the in-house CFD code ANES and the commercial CFD code ANSYS Fluent for calculating heat and mass transfer processes at the interphase surface using the VOF method. The model was verified on typical problems and limited data for tube bundles. In this paper, condensation and turbulent vapor flow models are implemented in the open-source CFD code OpenFOAM. The models were validated on Stefan problems and condensation of moving and stationary vapor of various heat carriers, and cross-verification between OpenFOAM, ANES, and ANSYS Fluent codes was carried out.

In India, solar energy, especially concentrated solar power (CSP), offers a promising path toward the production of clean, renewable energy. The government’s ambitions for increasing the capacity of solar electricity and the abundance of solar resources have brought CSP into further spotlight. Notwithstanding its potential, CSP has encountered noteworthy obstacles impeding its extensive use. These difficulties include the absence of trustworthy data, the limitations of domestic production capacity, and the rivalry posed by photovoltaic (PV) technology. India boasts over 300 clear sky days a year with solar radiation of 1700–1900 kW h per kilowatt peak, making it a country with significant potential for producing electricity from solar power systems per watt. The Indian government has set goals to generate an additional 104 GW of solar electricity by 2025 and 448 GW by 2030. In this review, we try to offer a thorough evaluation of the present status of the CSP in India and discuss the obstacle and future potential of the same. This review aims to provide insightful information for researchers, policymakers, industry stakeholders, and practitioners in the renewable energy field.

Climate change with Egypt’s increasingly hot weather and its plans towards energy transition, addressing an approach for clean heating, ventilation, and air condition solutions is becoming requisite. This paper examines the potential of utilizing solar absorption cooling systems in institutional buildings by presenting a case study of a proposed solar absorption cooling system for a library building with an area of 4402 m², located at the British University in Egypt. The proposed solution is to replace 30% of the existing conventional air conditioning units with a hot-water driven single-effect absorption chiller powered by solar thermal vacuum tube solar collectors, coupled with a stratified hot water storage tank. The potential area of the solar collectors was calculated to be 856 m². A detailed analysis was done using TRNSYS (Transient Simulation System Software) as a simulation tool, to find that the optimum stratified hot water storage tank size is to be 10 m³; with a specific volume per solar collectors’ area of 0.01 m³/m². The proposed system covers the cooling demand of the library building for 8 to 9 months of the year without an auxiliary heater and saves almost 95% of the electrical energy consumed by the replaced conventional air conditioning system.

The issues are examined associated with application of high-temperature mechanical filtration of the coolant for rendering proper water chemistry (WC) in the primary circuit of a supercritical water-cooled power reactor (VVER-SCW reactor). A critical analysis of the use of high-temperature filters (HTFs) in a dual-circuit VVER-SCW nuclear power plant is presented. The main challenges associated with the introduction of the high-temperature filtration technology under the primary circuit operating conditions, which was developed and implemented in the design of the unified VVER-1000 unified reactor plant, are outlined. The features of HTF operation under supercritical conditions are examined. The causes responsible for the change in the behavior of corrosion products on transition to operation in the region of near-critical and supercritical parameters are analyzed. The calculated and experimental values of the solubility of corrosion products in the water coolant under near-critical and supercritical conditions are assessed. It has been demonstrated that the transition to near-critical and supercritical operating conditions increases the degree of oversaturation of the coolant with iron and nickel, thereby facilitating further dominance of the fraction of suspended particles in it compared to compounds in a truly dissolved state. The effect the transition to the near-critical and supercritical operating conditions has on the characteristics of suspended particles of corrosion products in the coolant and the HTF efficiency was analyzed. A proposal has been made that the effects of radiolysis in supercritical water enhances the importance of colloidal systems formed from the corrosion products. It has been demonstrated that, if high-temperature mechanical filtration systems of the coolant are used at VVER-SCW power units, one can expect that the problems encountered during their (HTFs) operation at series power units with a VVER-1000 reactor will worsen. Potential ways are proposed for improving the technology of high-temperature filtration in the primary circuit of VVER-SCW nuclear power units.

Safe operation of nuclear reactors depends in many respects on the insight into and prediction accuracy of the processes through which the fission product aerosols are generated, transferred, and deposited in the course of possible accidents. Aerosols are finely dispersed particles produced as a result of destruction of nuclear fuel and structural materials in the reactor core. They may be transferred with a gas and vapor flow; they can deposit on various surfaces and cause radioactive contamination of equipment and rooms. The article presents the results from verification and validation of the aerosol particle deposition model in the AERCONT aerosol module in the composition of the TITAN-2/V1.0 integrated computer code developed for calculating the behavior of gases, vapors, and fission product aerosols in the gas–vapor coolant in the primary circuit and rooms of the containment of an NPP with a reactor based on the VVER technology. The deposition of fission product aerosols determines the occurrence and accumulation of radioactive deposits on the walls of the process circuit and rooms. Therefore, validation of models for calculating this process is necessary and is of practical interest from the viewpoint of radiation safety. The article presents a brief description of the model simulating the behavior of multicomponent and polydispersed fission product aerosols, methods for numerically solving the system of differential equations, and a comparison of the model with similar approaches. The results of validating the particle deposition model against experimental data are given. Particle sizes, temperature gradients, and flow regime have an effect on the deposition rate, which are of crucial importance for assessing the contamination sources and elaborating efficient strategies for mitigating the consequence during the evolvement of accidents at NPPs.

The paper presents the results of studies of pyrolysis liquid (PL) obtained during high-temperature destruction of rubberware wastes, such as large-size tires of quarry dump trucks. It has been established that the pyrolysis gas consists mainly of methane and hydrogen. After pyrolysis and subsequent carbon dioxide activation, the specific surface area of the solid carbon-containing residue was 110 m²/g, and its adsorption activity for methylene blue was 83.8 mg/g. The pyrolysis liquid contains fractions similar to gasoline, diesel fuel, and atmospheric residue, enabling it to be used as fuel oil or fuel additive. In this work, the composition of nine fractions of PL, including the fractionator bottoms, formed during vacuum distillation at the corresponding boiling points was established. It has also been found that the pyrolysis liquid with a density of 882 kg/m³ at t = 40°С has a kinematic viscosity of (nu ) = 2.60 mm²/s, while the density at t = 20°С is 896 kg/m³ and the kinematic viscosity is 4.2 mm²/s. The flash point attains 38°С, the heating value of the PL fractions ranges from 42 to 48 MJ/kg. Chromatographic analysis of nine samples obtained during vacuum distillation of the pyrolysis liquid was performed. The fractions contained such components as ethanol and compounds of cyclohexanone, pentanone, and methanol, which can be recommended for use in the chemical industry. The fraction ignition time delay does not exceed 1 s at 700°С. It has been demonstrated that the vacuum distillation yields fractions whose boiling point is lower than that at atmospheric pressure with the difference in boiling points between the fractions being as great as 45°C.

The results of experimental investigation into the coolant hydrodynamics within the entrance section of a fuel assembly in the core of the RITM reactor are presented. The investigation was aimed at capturing the effect of orifices and absorber grids of various designs on the development of the coolant flow in a fuel-rod bundle of a fuel assembly. To attain this goal, the experiments were performed in a scale model of the entrance section of the fuel assembly with all the structural elements of the standard assembly—from the throttle orifice to the second spacer grid. The spacing between model elements was increased by a scale factor equal to 5.8 relative to the standard spacing of their arrangement. The experiments were performed using two methods for investigating the coolant hydrodynamics: the pneumometric method and the tracer injection method. The studies were carried out at several cross-sections along the length of the model, and the studied region covered the entire cross-section of the model. The measurement sections were located considering the design features of the model. The hydraulic resistance coefficients (HRCs) of throttle orifices in fully open and maximally closed positions were experimentally determined. The features of the coolant flow at the inlet of the fuel assembly are visualized using maps of axial velocity distribution in the measurement sections as well as maps of injected tracer concentration distribution. A comparative analysis of the efficiency of application of two types of absorber grids was carried out. The experimental results were used to substantiate design solutions in modifying individual elements of the fuel assembly, as well as to confirm the reliability of new cores. In addition, the obtained experimental data can be used to validate the LOGOS CFD code developed in Russia.