Thermal Engineering最新文献

Steam boilers of various arrangements (or profiles) exist in the world. The most widely used ones include, for example, semitower boilers (or L-paso boilers). In Russia, there is not sufficient experience in designing such boilers, especially in their practical application. The paper analyzes advantages and disadvantages of semitower boilers. As with tower boilers, their main advantage is a small footprint. However, compared to tower boilers, the L-paso boilers have a lower height. According to the performed analysis, the semitower configuration is best suited to boilers fired with gaseous or liquid fuel. In Russia, due to its climatic conditions, semitower boilers can be used for replacements of obsolete (type E) natural circulation gas-and-fuel oil-fired boilers with a steam output of 210–420 t/h. As an example, the paper presents a brief description of the L‑paso (type Ep) natural circulation reheat boiler manufactured by the well-known Babcock–Wilcox Co. This boiler, which is a part of a 158-MW power unit, has been successfully fired with sulfur fuel oil for many years. A customized numerical model of the boiler was developed in the Boiler Designer software package. Multivariant calculations of the boiler were performed on the basis of this model. An analysis of the predictions has confirmed that the boiler can operate in a wide range of loads while maintaining the design steam conditions. The furnace heat release rate q_F, the furnace cross-section heat release rate q_V, and the flue gas temperature at the furnace outlet (vartheta _{{text{f}}}^{{''}}) have been demonstrated to considerably exceed the values allowed by the applicable Russian regulations for similar boilers. This fact is explained. The gas velocities in the boiler gas ducts are noticeably higher, and the gas and air velocities in the air heater are approximately the same as in the Russian-made boilers. The steam enthalpy increments Δh and the mass velocity ρw in the superheater stages basically correspond to the concepts of Russian specialists.

Due to the advanced capabilities of modern computational fluid dynamics (CFD) codes and developed models and algorithms, numerical simulation has become an efficient tool for studying two-phase flows, analyzing the entire totality of the processes occurring in them, and obtaining the data on flow local characteristics, which are difficult to measure directly. Active efforts taken for incorporating new models into various CFD codes should be accompanied by their cross-verification, the results of which can serve as a basis for selecting the most accurate, efficient, and universal models and algorithms. In this article, the results obtained from the solution of the problem about the condensation of R-142b refrigerant saturated vapor in a horizontal tube in the wall conjugate statement using two CFD codes, ANES and ANSYS Fluent, are analyzed. The copper tube’s inner diameter is 28 mm, its length is 2.75 m, wall thickness is 2 mm, and the total mass flux is 47 kg/(m² s). The studies are of relevance for heat recovery installations based on the organic Rankine cycle. The calculations were carried out using the modified Lee model that we suggested previously, and which has been implemented in the ANES CFD code developed at the Department of Engineering Thermophysics, NRU MPEI. The cross verification of the VOF algorithms implemented in the ANES and ANSYS Fluent codes has shown that the results of modeling the saturated vapor condensation in a horizontal tube obtained using the above-mentioned codes are in good agreement with each other and are close to the empirical dependences recommended in the literature sources (M. Shah) for calculating the condensation in a horizontal channel. Data on the distribution of local heat-transfer characteristics over the tube’s inner wall are presented, which demonstrate that the heat-transfer coefficient features an essential nonuniformity over both the tube length and perimeter.

This paper presents the results of computational-theoretical and experimental studies of a model of discharge collecting channels of a typical centrifugal fan in an air-cooled turbogenerator. An experimental test bench was created and a measurement system was developed to determine losses with different configurations of cooling air discharge channels. It was found that the original design of the turbogenerator fan’s discharge collecting chamber has low aerodynamic efficiency due to high internal losses, which reduce the technical and economic performance of the turbogenerator. One cost-effective way to increase fan performance by reducing losses is through aerodynamic optimization of the collecting chamber contours. Analysis of computational-theoretical and experimental research results of the typical fan collecting chamber design showed that the system of guide ribs has the main influence on loss levels and aerodynamic efficiency, since these ribs simultaneously provide structural rigidity and reliability while forming the flow path geometry. An optimized flow path for the collecting chamber was developed and tested without requiring changes to the overall fan housing dimensions. The improvement in aerodynamic characteristics is associated with modifying the guide rib system design through flow channel reprofiling. The optimization of the fan collecting chamber design increased useful power output by reducing aerodynamic losses in the turbogenerator’s air-cooling system. The design optimization, which ensures smooth increase in flow area with reduced positive pressure gradients in diffuser sections of the flow path, led to a relative efficiency increase of 24% while simultaneously reducing the metal consumption of the air-cooled turbogenerator centrifugal fan collecting chamber structure.

Experimental data are presented on nucleate boiling heat transfer of dielectric liquid HFE-7100 in horizontal layers. The liquid layer height was varied in a wide range while changing the pressure in the working chamber. The experiments were performed on a flat surface of a 120-mm outer diameter stainless-steel plate in an experimental heat-transfer vacuum setup whose working chamber was a thermal syphon. The experimental data were compared with the Yagov and Gogonin correlations which had been obtained for pool boiling. The Gogonin correlation has been demonstrated to properly generalize the experimental data in all ranges of reduced pressures and liquid layer heights. This correlation explicitly describes the influence of such parameters as the heating surface roughness and the ratio of the thermophysical properties of the liquid and the heat-transfer wall, which have a pronounced effect on the heat-transfer coefficient during boiling of dielectric liquids. Besides, this correlation is convenient since it can be used with parameters that can be monitored during the experiment. A generalization is presented of experimental data by the Pioro empirical correlation recommended for generalizing data on nucleate boiling in thin liquid layers. It has been demonstrated that the Pioro correlation with carefully selected coefficients and power exponents can generalize with an acceptable accuracy the experimental data obtained under given conditions on a heat-transfer wall exposed to a working fluid. To quantitatively assess the agreement between the computational correlations and experimental data on nucleate boiling of dielectric liquid HFE-7100 in horizontal layers, the mean error and rms deviation were used.

As conventional exergy analyses do not reveal the exergy destruction rates in a thermal system component caused by inefficiencies of interconnected components, actual potentials for improving the component performances cannot be provided by such analyses. This study analyses a combined-cycle gas turbine power plant using advanced exergy analysis methodologies, which address the shortcomings of conventional exergy analysis by evaluating the exergy destruction rates that are endogenous and exogenous, avoidable and unavoidable. Avoidable exergy destruction rates in the entire plant were found to be 31% of the total exergy destruction rates, indicating a significant potential for improving the plant. Exergy destruction rates for most of the plant components were largely endogenous (95.2%), signifying that contributions of cross-component interactions were limited. Avoidable endogenous exergy destruction rates account for 28.4% of the overall exergy destruction rates in the plant, while avoidable exogenous exergy destruction rates account for 2.1%. A component-level ranking of the plant components ranked the pumps in the plant as first for improvement whereas the highest priority was allocated to the combustion chambers (CC) by a plant-level ranking. A parametric study of the influence of CC operating conditions on the plant’s performance showed that CC operating temperatures more significantly affected plant exergy destruction rates than the CC operating pressures.

The article addresses matters concerned with decreasing the prime cost of thermal energy in the remote and isolated regions of Russia’s Arctic zone (AZ) that have a high wind energy potential by using wind turbines (WTs) jointly with boiler houses operating on expensive imported fossil fuel for heat-supply purposes. The use of wind turbines will make it possible to decrease the participation of boiler houses in the supply of heat to consumers, save fossil fuel, and, thereby, help decrease the prime cost of thermal energy. A procedure for calculating the levelized cost of thermal energy is developed and described in detail. The procedure is adapted to analyzing the efficiency of alternative options of using WTs jointly with boiler houses for heat-supply purposes, among which the alternative ensuring the minimal levelized cost of thermal energy is regarded as the most efficient one. By using the obtained technique, the economic efficiency of applying WTs jointly with boiler houses in the heat-supply systems of remote and isolated regions is evaluated taking the westernmost part of Russia’s Arctic zone as an example. It has been determined that, in such regions, in which the final annual average cost of fossil fuel is more than 1.5 times higher than the fuel cost in the cities and industrial centers of the AZ westernmost part as a consequence of a high transport component, the WTs are most efficient when used jointly with boiler houses operating on diesel fuel. For boiler houses operating on fuel oil and coal, the effect from using WTs is not so high. It is also shown that the cheaper the fuel, the less efficient or even inefficient at all the use of WTs becomes in comparison with the heat-supply option from a boiler house without connecting a WT. For the regions considered, the joint production of thermal energy by WTs and boiler houses operating on diesel fuel, fuel oil, and coal makes it possible to decrease its levelized cost by 7‒55, 5‒20, and 2‒7%, respectively.

Energy efficient and environment friendly power generation is the primary goal for any power generating industries. This paper proposes a thermodynamic approach based on E³ (energy, exergy and environment) analysis for performance improvement of power plant during low main steam and high reheater (RH) temperature conditions through a suitable operation technique. Thermodynamic modeling of a 500 MW Subcritical (SubC) coal based thermal power plant is carried in “Cycle-Tempo” at different conditions. Partial withdrawl of final feed water heater (high pressure heater—HPH-6) from service without RH spray condition during low main steam (MS) temperature and high RH steam temperature condition will help to increase the MS temperature by about 0.85–1.00°C and thereby, the net plant energy and exergy efficiency will be improved by about 0.09 and 0.08% point, respectively. Partial withdrawl of HPH-6 with RH spray condition will deteriorate the plant energetic and exergetic plant performance and this will guide the operation engineer for which extend withdrawl of HPH-6 can be done for getting higher plant performance. The net energy efficiency of turbogenerator (TG) cycle decreases with partial withdrawl of HPH-6 due to decrease in the feed water temperature by about 7°C and more relative energy rejection of the cycle. The net exergy efficiency of TG cycle increases due to less relative exergy destruction rate causing from improvement in steam quality. However, the use of RH spray increases the irreversiblities in the plant and the spray does not expand in high pressure turbine (HPT) which in turn decrease the exergy efficiency. The boiler energy efficiency increases due to decrease in fluegas exit loss as the fluegas exit temperature drops from about 140 to 133°C due to partial withdrawl of HPH-6. The exergy efficiency of boiler also decreases due to increase in exergy destruction in final super heater (FSH), reheater and economizer. For a 500 MW SubC coal power plant, hourly about 930 kg of coal and about 1183 kg of CO₂ emission can be saved and reduced through this operation technique namely, partial withdrawl of HPH-6 without RH spray condition for controlling low MS temperature. Hence, the proposed analysis will help to take proper operational technique for mitigating coal crisis and safeguarding the environment as well.

The ammonia-ethanolamine water chemistry used at NPPs with VVER-1200 ensures low rates of corrosion, mass transfer, and growth of corrosion product deposits. The content of corrosion products in the feedwater of the steam generator is less than 1 μg/dm³. This significantly increases the period between flushing the steam generator to remove deposits. However, ethanolamine and ammonia are absorbed by the cation exchange resin in the ion-exchange filters of the secondary circuit purification systems, which leads to the need to regenerate the cation exchange resin and continuously dose reagents to maintain the required pH value in the feedwater. Waste solutions from regeneration containing ethanolamine and large amounts of ammonia must be treated to ensure that the concentrations of these substances do not exceed maximum permissible values when discharged into the environment. To remove ethanolamine and ammonia from regeneration solutions, special installations are created, the operation of which is based on various principles. A pilot plant for cleaning regeneration solutions was manufactured and installed at the Belarusian NPP with VVER-1200. An analysis of the pilot plant’s operation showed that it successfully fulfills its function of protecting the aquatic environment but, at the same time, it is forced to release a significant amount of ammonia into the surrounding air. Removing ammonia is energy-consuming, environmentally unsafe, and requires the additional use of chemical reagents. In this regard, the water-chemical regime of the secondary circuit without ammonia is very promising. Possible options could be either switching from ammonia to dimethylamine or using ethanolamine as the only corrective reagent with the replacement of some of the structural materials of the secondary circuit with steels with a high chromium content, which have higher corrosion resistance compared to those currently used. Both options will simplify the wastewater treatment technology and reduce the environmental impact while maintaining the low corrosion rates achieved by using the ammonia–ethanolamine water chemistry.

The prospects are examined for application of ammonia-fired gas turbine units (GTUs) with thermochemical recuperation of the exhaust gas heat. Examples of operating ammonia-fired gas turbine units are given, and the main operating restrictions for the use of existing gas turbine units are specified. A thermodynamic analysis of a simple gas turbine unit with thermochemical heat recuperation (TCR) was performed in a wide range of operating conditions: the gas temperature at the turbine inlet varied from 700 to 1300°C and the compressor pressure ratio from 5 to 20. It has been established that the thermochemical heat recuperation can increase the GTU efficiency by as much as 9%. The effectiveness of TCR application has been demonstrated to depend on such operating parameters as pressure and temperature. At a temperature above 500°C, the enthalpy of the ammonia decomposition reaction reaches a value close to the maximum of approximately 3.0 MJ/kg NH₃. Thermochemical recuperation leads to the decomposition of ammonia with production of a hydrogen-rich gas (up to 75% (by volume)), which is burned in the combustion chamber, thereby changing the combustion process characteristics. The flame propagation velocity in a gas mixture consisting of hydrogen, nitrogen, and ammonia in different proportions was calculated on the basis of the GRI-Mech 3.0 list of elementary reactions in the Chemkin-Pro module. It has been found that the products of complete thermochemical decomposition of ammonia have a flame propagation velocity that is approximately two times higher than that for methane and more than ten times higher than that for ammonia. Thus, the implementation of the thermochemical heat recuperation in ammonia-fired gas turbine units is expected to increase the energy efficiency and improve the combustion process stability.

Ejectors are devices designed to suck fluid, steam or gas (primary fluid) from a closed space using a powerful jet of steam (secondary fluid), usually operated under specified boundary conditions using specific working fluids. If ejectors are to be used under new boundary conditions, predicting their performance requires either numerical or experimental studies. This paper presents a simple theoretical model capable of accurately predicting the performance of an ejector, given its geometry and boundary conditions, under different operating conditions. The model can predict the entrainment ratio, critical back pressure, and break-up back pressure using a given simple performance curve. The accuracy of the model is validated using computational fluid dynamics (CFD) simulations. Two ejectors with different geometries, dimensions, and boundary conditions are studied using ANSYS Fluent 19.2, and the results are compared with those from two other studies. The model successfully predicts the performance of all four ejectors across a wide range of operating conditions. Finally, the model is extended to any working fluid and temperature and validated numerically using air as the working fluid instead of water vapor. The results show that the model has an entrainment ratio error of less than 2%. It’s worth noting that this model’s applicability is contingent upon simultaneous changes to both the primary and suction streams by the same factor. Under these conditions, the model aligns closely with CFD-simulations.