Thermal Engineering最新文献

Different models of stable film boiling of liquids that give heat-transfer characteristics under these conditions are examined. The existing models have been demonstrated to have disadvantages associated with a consideration of certain limiting cases. The model of subcooled liquid film boiling, developed by a research group including the authors of this paper in 2017, takes into account the velocity of natural convection at the liquid/vapor interface. This model demonstrates good agreement with experimental data on cooling of spheres and cylinders, but the expression for the heat-transfer coefficient (HTC) contains an empirical coefficient. A new model of heat transfer during subcooled liquid film boiling based on the Bromley assumptions is proposed. An analysis of the contribution of radiation to heat transfer during film boiling has demonstrated that, according to a rough estimate, the contribution of this factor can be as high as 10% during cooling of high-temperature bodies in water when their surface is superheated to 1000 K. The applicability of the new model of stable film boiling of subcooled liquids and the models examined in this paper was validated by comparison with the authors’ experimental data. The test pieces were spheres and cylinders made of different metals (such as stainless steel, nickel, copper, titanium, FeCrAl alloy, zirconium). They were cooled in saturated or subcooled liquids with different thermophysical properties (such as water, ethanol, water-ethanol mixtures of various concentrations, FC-72, nitrogen) at different system pressures. The experimental data agree best of all with the predictions by the newly developed model. The performed comparisons have demonstrated that this model is more accurate (by 10%) compared to other models of heat transfer during cooling of spheres and cylinders in various liquids (such as water, ethanol, FC-72, isopropanol) in the subcooling range from 10 to 180 K at system pressures from 0.02 to 1.00 MPa.

The work is devoted to determining the characteristics of turbine stages in off-design modes that arise when pressures and temperatures change before or after the stage, a transition to a different rotation speed, or, for example, when the composition of the working fluid changes. As part of the project, a quasi-one-dimensional method for calculating the characteristics of a turboexpander assembly (TEA) stage when changing operating parameters and/or working fluid has been developed, which differs from known methods by using the equations of the state of real gas, adaptation to purely radial stages, and a simplified approach to determining the pressure at the outlet of the guide vane for assessing the degree of reactivity and the ability to switch to another working fluid, including a mixed one. The analytical methodology was verified by comparison with the experimental data of other authors and the results of calculations using CFD methods for radial-axial stages as well as with approaches to the calculation of purely radial turbomachines due to the lack of experimental data for this type of TEA in the public domain. An extended characteristic of a radial stage operating in air (turbo map) was constructed, and the dependences of the isentropic efficiency, degree of reactivity, mass flow and power of the stage on the relative circumferential speed were assessed. An assessment was made of the impact of switching to another working fluid (for example, switching from air to methane was chosen). It is shown that the characteristics do not change qualitatively but they shift from one another along the axis of the relative peripheral velocity. Further development of the technique involves taking into account possible phase transitions (volume condensation) in the flow part.

Development of sorbent compositions from industrial waste is a promising and economically feasible method for solving environmental problems. Power industry enterprises experience an acute need for the development of new environmentally friendly and cheap sorbents for gaseous fuel desulfurization purposes. Owing to removal of sulfur compounds from the fuel, the latter becomes less corrosive in nature, due to which it becomes possible to increase the equipment’s service life and also to decrease the deposits of sulfides on the surfaces of power installations. Based on a review of literature sources, the most important developments for sorbents consisting of industrial waste were determined. The waste of a thermal power plant (TPP) water-treatment facility (WTF) in the form of sludge water is of the greatest interest for removing sulfur compounds from fuel. Sludge water has a complex composition, which depends directly on the source water quality and water-treatment technology. Sludge water is produced at the natural water pretreatment stage, during which suspended matter is removed from source water by adding coagulants, flocculants, and other chemical agents that are specified by the process regulation. The article presents the composition of a sorbent produced from the WTF sludge at one of the Kazan combined heat and power plants (CHPP) for gaseous fuel desulfurization. Laboratory experiments were carried out with this sorbent, as a result of which the sulfur compound absorption efficiency and the strength characteristics of the prepared and formed sorbent were determined. A new method for indicating the extent to which the adsorbent absorption efficiency is decreased by using an indicating sorbent is also proposed. It is very difficult to monitor the level of sulfur compounds in purified gas by means of automatic sensors in view of a high measurement error, due to which an inaccurate result is obtained. An indicating sorbent composition that makes it possible to detect nonadsorbed sulfuric compositions by showing a color change from light to deep yellow is proposed. A method for using this indicating sorbent is described, and experimental data on its ability to absorb sulfur compounds are given.

Lubricants are the most important element of mating friction pairs and largely determine their reliability and service life. Components of oil systems of turbine units are susceptible to contamination of the working fluid; therefore, during equipment operation, it is necessary to take oil samples and monitor cleanliness. In many cases, when equipment is stopped for maintenance or is in standby mode, the quality of the oil is not given due attention. Ultimately, this may affect the reliability of the unit. The quality of the oil when starting a turbine is often not the same as when the unit is taken out of service. Increasing filtration efficiency plays a key role in reducing wear rates. Cleaning requirements are most important during turbine commissioning and when equipment is spinning at low speeds. To clean the working fluid during operation, effective full-flow filters are required. The research was carried out on a T-180/210 LMZ turbine unit; Tp-22S turbine oil was used as the working fluid, and the volume of the oil system was 36 m³. After modernizing the filters of the main oil tank (MOT), solid particles in the oil decreased by 5.8 times, the purity corresponds to class six to seven by GOST 17216-2001. After the turbine unit was put into operation after routine repairs, a large amount of contaminants entered the system. The amount of solid particles in the oil increased 27 times. The purity of the oil in the system increased over 14 days of operation of the turbine after routine repairs, and solid contaminants in it during this period decreased by approximately 14 times and corresponds to class eight, and that over 28 days was by approximately 25 times and corresponds to class seven according to GOST 17216-2001. This increase in oil purity is a consequence of filtering out contaminants introduced and formed in the system during routine repairs and the completion of the running-in period of the associated turbine friction pairs. The most sensitive element of the oil system is the control system. As a result of research and compilation of oil-cleanliness data, the recommended level of industrial cleanliness for the hydraulic control system is class eight (GOST 17216-2001). The most common method of reducing the risk to equipment during commissioning operation is the use of additional oil-purification equipment. Oil-purification costs can be offset by reduced maintenance costs and replacement of damaged equipment.

The flame structure, appearance, and emission characteristics of an inverse diffusion porous combustor (IDPC) are investigated experimentally. Unstructured ceramic foam made of silicon carbide (SIC) is used as a porous medium. At stoichiometry conditions, a reactive analysis is performed with methane as a fuel and variations in the pore distribution density (pore density) of ceramic foam SIC. Height of ceramic foam and Reynolds number of air jet (({{operatorname{Re} }_{{air}}})) are varied. Porous medium alters flow momentum in radial and axial directions which affects flame appearance and emissions. Increased radial momentum produces wider and shorter flame in case of IDPC. A bright blue zone is detected at the base of the flame, and a luminous orange or orange-blue zone is observed in the post-combustion zone near the flame tip. As the pore density is enhanced from 10 pores per inch (PPI) to 20 PPI, the flame is detached from the surface of the porous medium at a higher Reynolds number of the air jet. The visible flame height of IDPC is significantly reduced at 10 PPI when compared to a case without a porous medium. The Reynolds number of the air jet and the pore density of the porous medium strongly influence the emission levels of NO_x and CO. The IDPC with porous media height of 28 mm, ({{operatorname{Re} }_{{air}}}) = 8122 and 10 PPI pore density performs optimum in terms of flame shapes and CO and NO_x emissions.

The computational studies carried out previously taking as an example the BKZ-210-140 boiler installed at Tomsk-2 state-owned district power plant (SDPP) have shown that, given the existing scatter in the characteristics of coals fired at the power plant, the temperature of gases at the boiler furnace outlet may vary in a wide range (more than 100°С). Such variability of the operational parameters entails a number of problems, including difficulties with keeping a stable superheated steam temperature, increased risk of heating surfaces becoming slagged, and less efficient fuel combustion. A conclusion has been drawn based on the obtained computation results that the possibility of adjusting the flame’s initial section vector by ±15° will make it possible to solve the above-mentioned problems to a significant extent. A tiltable burner is the key component of the combustion system with adjusting the flame position. Based on an analysis of the current operation conditions of the Tomsk-2 SDPP BKZ-210-140 boiler, technical solutions were developed on the design of a tiltable vortex burner intended for combusting pulverized coal as well as natural gas and fuel oil. The burner’s outlet part is made so that it is possible to tilt it by ±15° in the vertical plane and by ±5° in the horizontal plane, which will make it possible to adjust the combustion mode in an efficient manner. The furnace process is simulated in the ANSYS Fluent software package under different boiler operation conditions. The simulation results show that, in the case of using the new burners, it is possible to improve the furnace process efficiency. By tilting the burner by ±15° in the vertical plane, it becomes possible to obtain the temperature adjustment range at the furnace outlet equal to 120°С. Based on the adopted technical solutions, design documentation for the burner has been developed. An experimental sample of the low-toxic tiltable vortex burner installed in the Tomsk-2 SDPP BKZ-210-140 boiler has been manufactured.

Due to the potential danger of exposure to aerosol particles on the human body, maximum permissible concentrations of harmful substances are limited by current regulatory documentation. The formation of aerosol particles is possible during beyond design basis accidents at nuclear power plants. The consequences of the radioactive impact of radioactive aerosol particles formed during an accident at a nuclear power plant on the human body are significantly more severe than from the mechanical impact of such particles. An important characteristic of radioactive aerosol particles is their polydispersity (unevenness in size) since particles of different sizes during an accident at a nuclear power plant have different rates of removal from the atmosphere of the nuclear power plant’s containment. Thus, when considering the movement of particles in the containment and the release of aerosol particles into the environment, it is important to correctly model the size distribution of aerosol particles. This paper presents the results of calculating the count and mass distributions of aerosol particles by size in the TOSQAN and Phebus-FP experiments. Methods are given for describing polydisperse systems (using particle size distribution or “average” sizes characterizing the entire distribution) and their influence on processes associated with the transfer of aerosol particles in a containment, and practical recommendations for working with particle size distributions are given. A comparison is made of the use of average size distribution characteristics and the lognormal distribution of aerosol particles to estimate the release during a hypothetical accident at a nuclear power plant with VVER.

A unique substance or material that releases or absorbs enough energy during a phase shift is known as a phase change material (PCM). Usually, one of the first two fundamental states of matter—solid or liquid—will change into the other. Phase change materials for thermal energy storage (TES) have excellent capability for providing thermal comfort in building’s occupant by decreasing heating and cooling energy demands. Because of its latent heat property, a PCM has a high energy density. The building uses PCMs mainly for space heating or cooling, control of building material temperature and increase in building durability, solar water heating, and waste heat recovery from high heat loss locations. Phase change materials for thermal energy storage has been proven to be useful for reducing peak electricity demand or increasing energy efficiency in heating, ventilation, and air-conditioning systems. The primary grid benefit of PCM based thermal energy storage system is load shifting and shedding, which is accomplished by recharging the storage system during off-peak times and substituting heating, ventilation, and air-conditioning system operation during peak times. This study examines PCM based thermal energy storage systems in building applications and benefits, focusing on their substantial limitations, and closes with recommendations for further improvement of design for use.

The results are presented of the search for and systematization of information on typical design solutions for the main heat exchangers of installations with low-boiling working fluids. The organic Rankine cycle (ORC) has been widely accepted as a way for converting waste (exhaust) heat into electrical energy. An increase in the installed capacity of operating commercial ORC power plants and their total capacity is noted in the world every year. At the same time, design options for the main heat exchangers (heater, evaporator-superheater, condenser, regenerative heat exchanger) are not available in open access and presented in catalogues: information about them is not disclosed by the manufacturers and information available in publications is limited and disembodied. An attempt is made in this paper to systematize the available information and, based on an analysis of world and domestic experience in industrial production, formulate an idea of potential engineering solutions for heat and mass transfer installations, which can be offered as prototypes of the considered apparatuses. At the same time, the search for such solutions was focused primarily on apparatuses used in the refrigeration industry, conventional steam turbine power units, and modern ventilation and air-conditioning systems. The advantages and disadvantages of such apparatuses are examined. The results are presented of a comparative analysis of their design, power range, operational features, and the potential effect of these factors on the operation of the overall ORC installation. Approaches to the selection of heat-exchange equipment for ORC installations given in the available publications and proven in practice have been investigated and described.

The article discusses the potential problems that have to be solved in the framework of development and justification of the water chemistry (WC) conditions required to ensure corrosion resistance of the structural materials used in the core and coolant circuit of the power-generating reactor used in the supercritical water cooled VVER-SCW nuclear power plant (NPP). In reactors cooled with water at supercritical temperature and pressure, the integrity of their physical barriers (fuel-rod claddings and reactor coolant circuit boundaries) depends in many respects on the possibility of maintaining the necessary water chemistry conditions that will guarantee the corrosion resistance of equipment and pipeline structural materials for the power unit’s entire service life. The most complex challenge in this regard is to inhibit corrosion and flow-accelerated corrosion processes and to minimize the formation of deposits on the surface of equipment operating in the domain of near-critical and supercritical conditions. The article formulates the limitations that are suggested to be considered in transferring the experience gained from the standardization of water chemistry in supercritical pressure (SCP) power units at thermal and nuclear power plants to the VVER-SCW NPPs. An analysis is carried out that makes it possible to estimate the effect the chemical composition of a supercritical water coolant has on the corrosion state of candidate structural materials for fuel-rod claddings with the aim to get better insight in the main processes occurring in aqueous solutions and for developing (elaborating) a WC conduction technology as applied to ensuring the integrity of the VVER-SCW NPP physical safety barriers.