Thermal Engineering最新文献

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.

The Government of the Russian Federation has approved a Comprehensive Plan to Increase the Volume of Recycling of Ash and Slag Waste of Hazard Class V, aimed at achieving the target indicator: increasing the share of useful utilized ash and slag from thermal power plants and boiler houses to reach 50% of the annual volume of their formation by 2035. Global achievements are considered using examples of advanced experience from China, India, and EU member countries that have achieved significant results in the field of ash and slag disposal from thermal power plants. The main areas of application of dry ash in Russia are presented, and information on the products of processing of ash and slag from the energy sector is also provided. An analysis of current information related to the use of ash and slag from thermal power plants and the activities of various organizations in these countries aimed at achieving a 100% level of ash and slag utilization was carried out. Increased emphasis is being placed on the requirements of the regulations issued by the Ministry of Environment, Forests, and Climate Change to achieve 100% utilization of fly ash in India. The important role of coal generation is confirmed by data on the distribution of energy sources in the structure of world consumption and the volumes of electricity generation at coal-fired power plants in China, India, EU member countries, and Russia. Statistics on the volumes of formation and use of ash and slag from thermal power plants in the above-mentioned countries are provided. The experience of EU countries in legislative regulation in the field of handling ash and slag from the energy sector is considered based on the European Union Regulation on the Registration, Evaluation, Authorization and Restriction of Chemicals (REACH), which came into force in 2007. Particular attention is paid to the necessity and expediency of increasing utilization level of coal combustion products at thermal power plants in Russia.

The article presents the results obtained from a study of the processes occurring in the primary circuit coolant of a KLT-40S propulsion light water reactor in conducting the ammonia water chemistry (WC) in it. At the fuel residence cycle initial stage, a high concentration of acetate ions (200‒220 ppb) was noted in the coolant, and their content turned to be prevailing among other anion impurities. In the course of reactor operation, their concentration in the coolant decreased as a consequence of radiolysis and thermal oxidation processes. Other anions, including those that cause local corrosion of structural materials (chloride and sulfate ions) were present in the coolant of both the operating and shutdown reactor in low quantities: not more than 15 ppb. The results obtained from the study of the behavior of anion impurities in the primary circuit coolant point to the possibility of long-term operation of the nuclear power facility (NPF) without the need to connect ion exchange purification filters, the operation of which in normal operation modes can cause contamination of process media with the products of their own destruction. Proceeding from the experience gained from operation of propulsion light water reactors, ways for improving the primary coolant circuit water chemistry (WC) for small capacity nuclear power plants (SNPPs) are proposed; in particular, it is recommended to maintain the standardized concentrations of reference anions in the coolant of the shutdown reactor and also to maintain the coolant parameters at the diagnostic level in the course of cooling down the reactor. It is proposed to adopt, as the standardized indicators of the SNPP primary circuit coolant, the concentrations of chloride and sulfate ions for the shutdown reactor equal to 50 ppb and adopt, as diagnostic indicators in cooling down the reactor, the concentrations of chloride and sulfate ions equal to 100 and 200 ppb, respectively.

Advances in computer technology have significantly expanded the possibilities for studying heat and mass transfer processes using Computational Fluid Dynamics (CFD) methods and, in particular, vapor condensation in pipes. One of the promising methods of numerical research is Volume of Fluid (VOF), which allows direct modeling of the behavior of the interphase surface in complex unsteady flows with mass transfer. Currently, the main efforts of researchers are aimed at the active development and testing of effective VOF models and algorithms and the selection of optimal characteristics of the grids used that are necessary for modeling a moving interphase surface and modes in which the vapor flow can be turbulent and the flow in the condensate film can consistently change from laminar (laminar-wave) to turbulent. An important issue remains the influence of taking into account real three-dimensionality in problems traditionally considered as two-dimensional: condensation of vapor on the surface of a horizontal cylinder, bundles of horizontal tubes, or in a vertical cooled tube. For this purpose, the authors previously performed methodological calculations, including verification of models and VOF algorithms as applied to condensation processes in pipes. Based on the results obtained in a two-dimensional (2D) formulation when modeling condensation in a vertical pipe of turbulent vapor flow, the optimal sizes of grid cells in the liquid film and vapor in the radial and longitudinal directions were selected, various turbulence models were tested, and the method for determining the constant in the Lee model was verified. When comparing the calculated values and data obtained experimentally at the Department of Engineering Thermal Physics of the National Research University MPEI, their good agreement was observed (arithmetic mean deviation 14.4%). This paper examines the results of modeling the specified problem in a three-dimensional (3D) formulation. Based on the performed calculations, the operability of the proposed algorithms, methods, and grid parameters was confirmed when transferring them from a two-dimensional to a three-dimensional problem statement. The values obtained from 3D modeling are in better agreement with the experimental data (average arithmetic deviation 10.2%); the accuracy of calculations relating to the laminar-wave mode of condensate film movement is significantly increased.

Implementation of modern methods for the design and upgrading of low-emission combustion chambers for gas turbine engines requires performance of a wide variety of computational experiments with appropriate fuel surrogates, which are hydrocarbon compositions capable of simulating the essential physical and chemical characteristics of the fuel. Complex multicomponent surrogates of commercial aviation kerosene fuels have been developed in this work. Surrogates consist of hydrocarbons from the main structural classes of compounds specific for aviation kerosene fuels and reproduce the key physical and chemical characteristics of fuels, such as the H/C ratio, molecular weight, density, lower heating value, and heat of evaporation. The surrogates were tested against temperature-independent and temperature-dependent characteristics of Jet A, Jet A-1, and TS-1 fuels, including their distillation curves. Surrogates have been identified, which offer the best agreement with the published data on temperature-dependent and temperature-independent characteristics of Jet A, Jet A-1, and TS-1 fuels.