Thermal Engineering最新文献

Despite the development of experimental and numerical methods for studying the effect of turbulence on the flow structure and gas-dynamic efficiency of turbine cascades, many questions arise when designing and improving the blade rows of high-temperature gas turbines. It is quite difficult to conduct reliable flow measurements or numerical studies for real-life turbomachinery operating conditions, where the range of changes in the intensity and scale of turbulence in the flow is difficult to predict. Therefore, to get closer to understanding how to more adequately take into account the influence of such parameters as the intensity and scale of turbulence when designing turbine rows, a computational study of the gas turbine vane cascade was carried out, which was based on a number of experimental results obtained at the Central Institute of Aviation Motors. To assess the influence of the noted turbulence characteristics on the structure of the flow in the cascade, parametric studies were performed with different intensity values and scales of turbulence specified. In this work, based on experimental data obtained both with and without the use of various turbulators, the influence of the intensity and scale of turbulence on changes in the flow structure and profile losses in the vane cascade is analyzed in the range of values of the reduced (adiabatic) velocity at the exit from the cascade λ_2ad = 0.55–0.95. Computational studies were carried out using the 2D NS software package for the intensity of turbulence at the inlet to the vane cascade Tu = 0.2–10% and at different scales of turbulence.

One of the main characteristics of the maneuverability of power equipment is whether the load may be reduced to the minimum allowable level. The ability of power equipment to operate in a variable regime (with unloading during night time) enables it to participate in the control of frequency and power in the power system. It is important to note that low-load operation of the equipment should not make its performance poorer. In particular, for drum boilers, the required steam conditions and reliable circulation of the working fluid in the evaporation waterwalls should be maintained in the entire operating load range. The numerical-and-experimental studies have substantiated the possibility to reduce the minimum allowable steam load of a typical E-420-13.8-560GM boiler from 210 to 150 t/h. According to the results of thermal design calculations by the Boiler-Designer code, the required steam superheat temperature (560°С) cannot be attained at lower loads. Field tests of an operating boiler have revealed that difficulties with fuel flow control and failures of the furnace’s combustion process arise at loads below 150 t/h. Calculations and experimental measurements performed using flow measuring tubes installed in the furnace waterwalls have demonstrated that the fluid circulation of a fluid in the evaporation water walls remains even on a load decrease to 110 t/h. It is pointed out that more than 100 E-420-13.8-560GM boilers, whose design was modified during long-term operation (for more than 40 years), are operating now. Therefore, the minimum steam load should be finalized only after additional studies for each boiler of this type.

Climate policy is gradually becoming dominant in the world and is beginning to decisively determine the long-term prospects for the development of the global economy and energy. The problem of curbing the rise in global temperature is global; therefore, reducing greenhouse gas emissions as a result of anthropogenic activities must be carried out in the most acceptable way for the global economy and energy sector. The optimal paths for countries around the world to transition to a carbon-neutral economy will vary significantly since they have different economic structures and endowments of energy resources. The article discusses the following technological directions of decarbonization of the economy: intensification of energy conservation, including production, transformation, transportation, and consumption of energy; changing the fuel structure in favor of low-carbon fuels by replacing coal with natural gas; replacing fossil fuels with carbon-neutral biomass; CO₂ capture in energy and industrial installations with its subsequent transportation and disposal; expanding the use of nuclear energy; and transition to the use of carbon-free renewable energy resources. For each of these areas, the potential for their contribution to achieving carbon neutrality in the economy and the existing restrictions on their implementation are identified. The research was carried out in relation to the economy and energy sector of Russia, which is one of the largest consumers and exporters of fossil organic fuels in the world. It is shown that the transition to a carbon-neutral economy must be complex and carried out through a combination of various technological solutions. The implementation of the “electric world” concept in the country, in which all basic energy needs will be met by using electricity produced on a carbon-free basis, until 2060 is hardly possible for technological and economic reasons, so the use of fossil organic fuels during this period will remain inevitable. At the same time, the issue of organizing the capture and disposal of CO₂ must be resolved.

Application of computation tools resting on contemporary physical and mathematical models for substantiating the design solutions adopted for various heat-transfer equipment components helps save time, manpower, and financial resources of design institutions. The variety of both existing reactors and those being designed, which differ from one another both in design and type of coolants calls for the availability of a versatile thermal hydraulic computer code suited for a wide range of applications. The new-generation HYDRA-IBRAE/LM thermal hydraulic module of the EUCLID integrated code, which has been developed as part of the Proryv (Breakthrough) Project, meets these requirements. The operation of this thermal hydraulic module as part of the integrated code opens the possibility to simulate an essentially wider range of reactor plant operation modes and, as a consequence, those of individual heat-transfer equipment components. The developed thermal hydraulic module, which has been certified at the Scientific and Engineering Center for Nuclear and Radiation Safety (SEC NRS), offers the possibility to analyze the thermal hydraulics of sodium, lead, lead–bismuth, gas, and water coolants in various NPP equipment items. Reactor plant steam generators (SGs) belong to the category of equipment components most complex for modeling since they may contain two types of coolants. The article presents study results demonstrating the code’s abilities to analyze in a correct way the processes in the steam generators of only sodium cooled reactor plants, because these plants exist and are actively operated in Russia and around the world. The data presented in the article allow a conclusion to be drawn that the thermal hydraulic module developed at IBRAE RAS is an efficient tool for numerically analyzing complex heat-transfer processes in reactor plants. By using an extended system of closing correlations implemented in the module, it is possible to perform substantiation of design thermal engineering solutions as applied to individual heat-transfer equipment components.

The relevance of studies into hydrodynamics and heat transfer in minichannels is driven by the increased interest in high-pressure power systems and high-tech devices that employ compact and efficient heat exchangers with a high heat flux. The potential for application of small-diameter channels in various industries, including production of heat exchangers, in which various dielectric liquids or freons can be used as a coolant at moderate and high reduced pressures, is being actively investigated today. High heat fluxes should be removed by boiling as the most efficient heat removal mechanism. Proper designing of heat exchangers employing the boiling process requires reliable methods for calculating heat transfer and pressure drop in two-phase flows. The authors have tested the applicability of the known and most reliable methods for calculating pressure drops and heat-transfer coefficient, which have been developed for conventional channels and minichannels, under conditions of increased reduced pressures as high as p_r = p/p_cr = 0.7. A review of the best-known methods applicable to various diameter (0.16–32 mm) channels is presented, and the predictions by these methods are compared with experimental data. The experiments were performed at a reduced pressure of 0.43, 0.56, and 0.70 in the mass velocity range of G = 200–1000 kg/(m² s). The experimental setup, the test section, and the experimental procedure are described. The studies were done with R125 refrigerant in a 1.1 mm ID vertical round channel with a heated length of 50 mm. The comparison of the experimental data with predictions by the reviewed procedures demonstrated good performance of calculation methods that had been developed for conventional channels and for particular fluids under conditions close to those under which the experiments were carried out. The pressure losses predicted using the homogeneous model at high reduced pressures are in good agreement with the experimental data.

Various literary sources present the results of experiments that were carried out in order to investigate the process of condensation on a horizontal cylinder of a moving steam of freon R-113. These results demonstrate a qualitative disagreement with the trends following from the available theoretical dependencies. The authors of these experimental data indicated some possible reasons for this difference, but a detailed verification of the above assumptions is difficult due to the difficulties in obtaining information about the local characteristics of the condensation process. In this work, the VOF (Volume of Fluid) method is used to simulate the experimental modes of R-113 freon condensation on the surface of a horizontal cylinder from a downward flow moving at a velocity of up to 6 m/s at a pressure close to atmospheric. The Lee model was used to simulate interfacial mass transfer. The selection of its constant was carried out using the algorithm proposed earlier by the authors of this work. Data on changes in the local characteristics of heat transfer during condensation from a moving vapor flow, obtained using the VOF method, are presented. The calculation results are in good agreement with the “unusual” experimental data and confirm the experimentally recorded anomalous (compared to the existing theoretical dependences) increase in the heat-transfer coefficient with an increase in the oncoming flow velocity. It is shown that one of the reasons for the increase in the heat-transfer coefficient is the interaction of the falling condensate film with the vortex structures formed behind the streamlined cylinder. At a certain velocity of the oncoming flow, the falling condensate film is periodically “flooded,” which, in turn, leads to a significant intensification of heat transfer near the lower generatrix of the cylinder. This mechanism is not taken into account in the existing models since, as a rule, it is assumed in them that, after flow separation, the film flows down only due to the action of gravitational forces. A criterion dependence is proposed for determining the boundary of “anomalous” (compared to the theoretical value) heat-transfer intensification.

The results of experimental studies of the applicability of the float-discrete method for measuring the level of a heavy liquid-metal coolant (HLMC) using sealed magnetically controlled contacts as a sensitive element are presented. These contacts register the coolant level in the field of a permanent magnet located on the surface of a heavy liquid-metal coolant. The performance of such a level sensor was studied using a control tank with a lead-bismuth coolant under conditions close to natural ones. This method is simple, but its main problem is maintaining the integrity of sealed magnetically controlled contacts when exposed to high temperatures. The experiments were carried out using a float-discrete level sensor prototype on a high-temperature stand with a lead-bismuth coolant. The data collected during the processing of the results confirm with reliable accuracy the applicability of the float-discrete method for monitoring the level of a heavy liquid-metal coolant. An HLMC level measuring device operating according to this method makes it possible to monitor the level in tanks while maintaining the tightness of the circuit. Due to this, it is possible to abandon the currently common methods for determining the level of HLMC using electric contact level sensors in which the sealing of the circuit is impossible. This device can be used on various experimental stands with liquid-metal coolants as well as in reactor plants and accelerator-controlled systems in the temperature range of 210–230°C, for example MYRRHA. To ensure the operability of the level transmitter at higher temperatures, it is necessary to upgrade the reed switch cooling system.

A technique for modeling turbulent flow in a channel with impermeable and permeable walls in the presence of heat supply to the wall is proposed. To close the equations of the boundary layer, a three-parameter differential model of shear turbulence is used, which is supplemented by a transfer equation for a turbulent heat flux. Calculations are carried out for a developed turbulent flow in a round pipe with impermeable and permeable walls for air and binary gas mixtures with a low molecular Prandtl number with parameters corresponding to those in earlier experiments. The results of studies on the effect of the Prandtl number on heat transfer in a pipe with impermeable walls for a coolant with constant physical properties are consistent with the experimental data and empirical dependences of W.M. Kays and B.S. Petukhov for the Nusselt number in the range of Prandtl numbers of 0.2–0.7. It is shown that a positive pressure gradient arising in a pipe under strong gas suction leads to a violation of the similarity of the velocity and temperature profiles and, as a consequence, to a violation of the Reynolds analogy. The use of the transport equation for a turbulent heat flux makes it possible to take into account the complex dependence of the turbulent Prandtl number on the molecular Prandtl number in the viscous sublayer and in the logarithmic boundary layer. The influence of the variability of thermophysical properties and the turbulent Prandtl number on the characteristics of heat transfer in a pipe is estimated. Thus, the difference between the Nu number determined under the assumption of a constant turbulent Prandtl number and the results obtained in calculations using the equation for turbulent heat flux increases with a decrease in the molecular Prandtl number and an increase in the intensity of gas suction.

On the example of a micro–gas-turbine plant (MGTU) of the C30 Capstone type, an analysis of various options for the use of modern electric energy storage devices as part of a buffer battery was carried out and compared. Gas microturbines with a unit capacity of several tens to hundreds of kilowatts appeared on the market in the 1970s and have become increasingly widely used in autonomous and distributed generation systems. Their competitiveness in comparison with diesel and gas reciprocating power plants is ensured primarily by achieving comparable efficiency values with competitors as a result of the use of a regenerative thermodynamic cycle with highly efficient recuperative heat exchangers and high-speed turbogenerator equipment with air bearings instead of oil bearings. This significantly reduces the operational requirements for the frequency of maintenance of power plants, and also expands the possibilities of using various types of liquid and gaseous fuels (polyfuel) available in the operation area. An important feature of micro–gas-turbine power plants is the DC link and the buffer storage of electrical energy in the power output circuit, which allow one to effectively control the current parameters (regulate them) without changing the engine speed. In traditional versions of such power plants, as a rule, lead-acid batteries are used as a buffer energy storage. The authors considered options for replacing them with supercapacitors and batteries of various types, taking into account such operational factors as the predominance of low ambient temperatures during most of the year (arctic conditions), difficulties in logistics, maintenance conditions for power plants of these batteries, and their considerable cost. The weight and size characteristics of drives are estimated based on different types of elements with an emphasis on products of Russian manufacturers. It is concluded that when operating an MGTU in harsh climatic conditions, it is advisable to use supercapacitor batteries in their buffer storage, despite their low specific energy consumption and high cost.