Thermal Engineering最新文献

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.

In the recent years, photovoltaic thermal (PVT) devices have immensely grown in popularity. PVT devices harness clean energy and are considered the best alternative to renewable energy when considering the simultaneous generation of both useful heat and electricity, thus providing a higher overall energy yield and improved performance as compared to single-function systems. These devices use either air, water or nanofluid as coolant. Although much work has been done in the field of PVT systems using water/nanofluid as coolant, there is a gap in literature on the use of PVT air collector (PVTAC) for the same. The aim of this review is to study the development of these devices through a study of the various parameters such as mass flow rate, different absorber configurations, use of coatings and glazing on which the performance of such systems depends. This study is done with exergy efficiency as performance evaluator. This review also discusses the various applications of the PVT air collectors. It is to be noted that this study is focused primarily on the recent development in these devices. The conclusion offers merits and demerits of the system as well as recommendations for future scope of study.

The design options for horizontal condensers for freon, petroleum products, and other chemicals are presented. The layout of such devices, in which air or water is used to cool steam, should preferably be carried out with minimal vertical dimensions. The process of condensation in horizontal and vertical pipes has been studied in a number of works; however, it is recommended in each specific case to have specific experimental data for engineering calculations. The article describes an experimental setup for studying the complete condensation of a promising freon, R245fa, in horizontal and inclined pipes. A copper pipe with grooves for installing thermocouples is built into the working section, which is cooled by water using the “pipe in pipe” scheme. Thermocouples are placed in the gap through which the cooling water flows to measure its temperature as it heats up, which allows determining the local value of the heat-transfer coefficients of the freon. As a result of experiments on the condensation of R245fa freon, the values of the average heat-transfer coefficient along the length of the pipe were obtained at different angles of inclination of the pipe and the mass velocity of the liquid, and it was established that, with an increase in inclination to 5°–10° to the horizon, the heat-transfer coefficient during condensation increases to the greatest extent. The distribution of the heat-transfer coefficient along the length of the pipe for complete condensation of R245fa was also obtained. At the initial stage, the value of the heat-transfer coefficient decreases rapidly and then stabilizes. The experimental results are useful for calculating heat-exchange devices, such as horizontal and slightly inclined air condensers for steam of various substances.

An active thermal protection envelope (ATPE) is a new kind of systems for maintaining temperature conditions in buildings and structures, which emerged after the “warm floor” and “warm walls” systems. The article substantiates the relevance of studying the thermal characteristics and practical application of an ATPE comprising tubular heat transfer devices (THTDs). A 1D analytical solution and a 2D numerical solution of the heat transfer problem in an ATPE are developed. For the 2D solution, a numerical scheme that takes into account conjugate heat transfer between the heat distribution layer (HDL) and thermal insulating layer (TIL), as well as the modeling procedure, are presented. For verifying the results, numerical and analytical calculations were carried out, and the temperature distributions in the heat distribution layer for one of the ATPE versions were compared. The 1D analytical solution is in good agreement with the 2D numerical calculation results. The temperature differences arising in the HDL and at its surface, as well as the THTD temperature overheating are determined. A tubular heat transfer device overheating calculation method for carrying out practical computations is proposed. The Biot number value at which the standardized temperature distribution parameters at the thermal protection structure inner surface are achieved is estimated. A conclusion is drawn that, owing the use of an ATPE equipped with tubular heat transfer devices, the heat carrier temperature can be approached closest to the indoor temperature. This means that the heat supply systems of buildings and structures can be made more efficient in exergetic and energy respects at the expense of insignificantly larger heat losses, especially in the case of using low-grade heat sources, and also during heat transformation and storage. Formulas for calculating the THTD placement pitch, minimal HDL temperature, and THTD specific power are presented.

The development of technologies in many industries has imposed strict requirements on the control of thermal performance control of power modules. For example, the maximum operating temperature of modern bipolar power transistors of fourth generation exceeds 175°С at a heat flux (HF) above 1 MW/m². Removal of such heat fluxes requires boiling-based cooling systems. The heat release of a power module cannot be controlled without direct measurement of its heat flux. In this work, heterogeneous gradient heat-flux sensors (HGHFS) are employed, which can directly measure local heat fluxes. These sensors are a reliable tool for investigating phase transition processes. Since surface finning considerably increases the heat-transfer surface area, finned models with one, three, and five longitudinal fins are examined. The first critical heat flux during saturated water boiling on a horizontal surface was determined experimentally. The HGHFS signal was compared with a thermocouple signal. It has been established that the onset of a boiling crisis cannot be determined using temperature measurements since the heat flux already exceeds the first critical heat flux by the time the temperature begins to rise. The delay of the thermocouple signal relative to the HGHFS signal is 0.5 s. The local heat flux during boiling on finned surfaces is compared with the heat flux during boiling on a flat surface. Heat-transfer enhancement was obtained for all studied surfaces. The temperature of the simulated power module could be reduced by 11.7–20.5% relative to the temperature of a horizontal plate. With a finning ratio of 7.4, the temperature drop decreased by 20.5%.

The results of activities on studying the cooling of high-temperature surfaces and phase change heat transfer enhancement are briefly analyzed. A set of works aimed at modernizing the experimental setup intended to model thermally stressed components of power installations is carried out. The heat transfer process that takes place in the cases of applying hydraulic and pneumatic atomizers has been studied on the setup. A technique for modifying a surface using the electronic erosion method is proposed and described. Two new heat transfer surfaces of a test section were fabricated using the new method, and their macrophotographs and roughness profiles have been obtained by means of a microscope and contact profilometer. The efficiency with which the modified and nonmodified surfaces are thermally stabilized by a dispersed flow at coolant flowrates equal to 2.1 × 10^–3 and 4.3 × 10^–3 kg/s using hydraulic and pneumatic atomizers was experimentally studied. The dependences of heat flux on the cooled surface temperature were analyzed. It is shown that the heat flux removed from the modified surface cooled with liquid sprayed by the hydraulic atomizer is by 20–50% higher (its value increases with increasing the coolant flowrate), than it is for the nonmodified surface in the range of surface temperatures from 120 to 140°C. The heat removal efficiency is better for the surface having a higher roughness. The removed heat flux convective component and phase change component in the case of surface cooling with dispersed flow are calculated. A conclusion has been drawn that the phase change makes a key contribution in this process. The quantity of dispersed coolant required to implement the above-mentioned cooling modes is estimated, and the dependence of its flowrate on the heat flux is obtained.

A brief description of drum steel 15NiCuMoNb5 (WB36) is presented. The relevance of the analysis of static crack resistance of this steel and its welded joints is substantiated. The metal (WB36 steel) of welded blanks simulating natural drum elements in three modifications (batches) was studied: base, welded, and fusion zones. The tests were carried out at room temperature. The testing methodology complied with the requirements of GOST 25.506-85. For all samples, clearly expressed type IV failure diagrams were obtained (according to GOST 25.506-85). Taking into account their (destruction) viscous nature, the J-integral criterion was used as a characteristic of the static crack resistance of the material. By means of special processing of diagrams with the allocation of the plastic component of the destruction energy, the J-integral values were determined for each sample. The final analysis of the test results is performed in graphical format in accordance with the requirements of the standard. Based on the obtained data, J-integral dependencies on the length of the grown static crack were found for each batch of metal studied. Taking into account the significant spread of experimental points for each modification of the metal, it is proposed to generalize the obtained dependencies and consider their generalized version as an estimated characteristic of the critical J-integral of the metal (steel WB36) of welded products, which corresponds in quantitative terms to a range of values of approximately 0.4–0.6 MJ/m². The characteristics of the critical stress intensity factors (SIF) calculated from these values were approximately 300 to 370 MPa m^0.5. A calculation of the load-bearing capacity of a drum with a surface crack in the longitudinal weld zone carried out using the research results showed that the maximum permissible depth for extended cracks should not exceed approximately one third of the shell wall thickness.