Results 1 - 10 of 3630
Results 1 - 10 of 3630. Search took: 0.023 seconds
|Sort by: date | relevance|
[en] The results of an extended research performed with the aim of investigating influence air staging application on processes occurred in boiler furnace have been presented in this paper. This subject was developed as a result of the need to obtain valid engineering methods for estimating the intensity of combustion and heat transfer processes under sub-stoichiometric conditions. The used calculation method, presented in previous publications, has been established by linking the differential mathematical model of processes in the furnace and conventional integral calculation procedures of all heating surfaces within the boiler. Such verified calculation method provided the algorithm for qualitative analysis of steam boiler operation regardless of the applied combustion scheme. In this research, by use of such approach, the operation of power boiler within TPP Kostolac has been assessed where combustion system was reconstructed during 2015. Calculation results in case of application of designed combustion system (UNR) and alternative air staging configuration (TC1) have been considered. In addition, the present air distribution scheme with the applied primary measures (R) has been analyzed. Comparison of such gained results listed in the same table ensures the trend of the change occurred by application of the air-staging system which needs to be more closely defined. Results of research showed that air staging throughout the furnace height slows down the combustion with the simultaneous intensification of the heat transfer process. Although this phenomenon led to the reduction in NOx concentration (195/470 mg/Nm3, dry, 6% O2), it decreased the power of considered boiler (725.5/774.0 MW) and increased boiler's efficiency (86.49/85.52%). Furthermore, due to the temperatures of superheated (517.0/540.0 °C) and reheated (524.0/540.0 °C) steam being below the designed level, the safety of the boiler's operation was significantly affected. The study also reveals that the boiler's efficiency rate is, in any considered case with applied air staging system, higher due to the possibility to run the boiler with the lower value of excess air ratio (1.15/1.22). Additionally, results demonstrate that distribution of the amount of air, as well as air introduction location, can significantly influence parameters of superheated and reheated steam as well as the regulation area of the same. © 2018 Elsevier Ltd
[en] Bypass flow in the prismatic gas-cooled very high temperature reactor (VHTR) is not intentionally designed to occur, but is present in the gaps between graphite blocks. Previous studies of the bypass flow in the core indicated that the cooling provided by flow in the bypass gaps had a significant effect on temperature and flow distributions for normal operating conditions. However, the flow and heat transports in the core are changed significantly after a Loss of Flow Accident (LOFA). This study aims to study the effect and role of the bypass flow after a LOFA in terms of the temperature and flow distributions and for the heat transport out of the core by natural convection of the coolant for a 1/12 symmetric section of the active core which is composed of images and mirror images of two sub-region models. The two sub-region models, 9 x 1/12 and 15 x 1/12 symmetric sectors of the active core, are employed as the CFD flow models using computational grid systems of 70.2 million and 117 million nodes, respectively. It is concluded that the effect of bypass flow is significant for the initial conditions and the beginning of LOFA, but the bypass flow has little effect after a long period of time in the transient computation of natural circulation
[en] In the guarded cut-bar technique, a guard surrounding the measured sample and reference (meter) bars is temperature controlled to carefully regulate heat losses from the sample and reference bars. Guarding is typically carried out by matching the temperature profiles between the guard and the test stack of sample and meter bars. Problems arise in matching the profiles, especially when the thermal conductivities of the meter bars and of the sample differ, as is usually the case. In a previous numerical study, the applied guarding condition (guard temperature profile) was found to be an important factor in measurement accuracy. Different from the linear-matched or isothermal schemes recommended in literature, the optimal guarding condition is dependent on the system geometry and thermal conductivity ratio of sample to meter bar. To validate the numerical results, an experimental study was performed to investigate the resulting error under different guarding conditions using stainless steel 304 as both the sample and meter bars. The optimal guarding condition was further verified on a certified reference material, pyroceram 9606, and 99.95% pure iron whose thermal conductivities are much smaller and much larger, respectively, than that of the stainless steel meter bars. Additionally, measurements are performed using three different inert gases to show the effect of the insulation effective thermal conductivity on measurement error, revealing low conductivity, argon gas, gives the lowest error sensitivity when deviating from the optimal condition. The result of this study provides a general guideline for the specific measurement method and for methods requiring optimal guarding or insulation
[en] Highlights: • Optimal thermodynamic parameters of two-phase ejector refrigeration system. • Empirical correlation for primary evaporator temperature. • Calculation of optimal condenser and evaporator dimensions. • Comparison of calculated values with experimental study. - Abstract: Air-conditioning is necessary for the comfort of passengers in commercial buses. However, installing an air-conditioning system can add extra load on the engine and result in extra fuel cost. Therefore, an improvement in the air-conditioning system can lower the fuel consumption of the buses and reduce the size of the evaporator and the condenser. It is known that using two-phase ejector as an expansion valve in the air-conditioning system can improve the system performance. This study offers a model to predict the optimal thermodynamic parameters for a two-phase ejector refrigeration system for buses using R134a under various operating conditions. An empirical correlation is derived to determine the optimal thermodynamic parameters of the system. The effect of evaporation and condensation temperatures on the heat transfer surface area are discussed and graphically illustrated. Moreover, an experimental study to validate the developed model has been carried out in a midibus air-conditioning system. The study findings revealed that the heat transfer surface area can be reduced by about 4% and 55% in the condenser and evaporator, respectively.
[en] Graphical abstract: Transformation of K components during combustion. - Highlights: • High temperature promotes the formation of gaseous and insoluble K compounds. • Melting decreases K release due to poor mass transfer effect. • The reactions related to K species in bottom ash of co-firing were studied. • The thermodynamics and chemical reaction rate affect the distribution of products. - Abstract: Co-combustion experiments of rice straw with K, Si-riched rice straw and coal were conducted in a horizontal tube furnace at 600–1000 °C. Potassium migration and transformation during co-combustion was investigated via analysis of combustion products by Inductively Coupled Plasma (ICP), X-ray powder diffraction (XRD), Scanning Electron Microscope (SEM) and thermodynamic calculation software respectively. Results indicated that high temperature improved the release of K to the gaseous phase and promoted the generation of insoluble K compounds in the bottom ash. K in the rice straw mainly formed KCl, K2SiO3, while in both coal combustion and co-combustion was in the form of aluminosilicate. At 1000 °C, a serious melting phenomenon was observed in ash of rice straw, bringing a decrease of K release while co-combustion could relieve this condition via increasing the melting temperature of ash effectively. The primary chemistry reaction during the co-combustion was Al migrated from coal ash to rice straw ash, forming relatively steady and high-melting point potassium aluminosilicate rather than potassium sulfate reported before. Furthermore, KCl(g) and K2CO3 were important predecessors to form K2SO4 and K2SiO3 in combustion by theory analysis.
[en] A considerable increase in thermal separation by the use of radiation induced evaporation of binary base fluids using nanofluids has been reported. It was shown for the water-ethanol system, that the vapor phase composition was enhanced about 20 wt% of ethanol above the thermodynamic equilibrium value. Even the azeotropic point seemed to disappear. This paper aims to reproduce the reported results and it tries to analyze possible mechanisms explaining these findings that appear anomalously at first sight.
[en] Highlights: • AHT and RHT have been modelled with the NH3/LiNO3 mixture by the first time. • Single and double stage absorption and resorption heat transformers were modelled. • Double stage cycles achieved higher GTL but lower COP than single cycles. • AHT achieved higher GTL and COP with regard to RHT. • RHT operate at considerably lower pressures than AHT. - Abstract: This paper presents the modeling and thermodynamic analysis of different heat transformers configurations using the alternative mixture NH3-LiNO3. The analyzed configurations are: (i) Absorption Heat Transformer (AHT), (ii) Double Absorption Heat Transformer (DAHT), (iii) Resorption Heat Transformer (RHT) and (iv) Double Resorption Heat Transformer (DRHT). Coefficients of performance and exergetic efficiencies are reported for each one of the systems as function of the main system temperatures. The results showed that the coefficients of performance and exergetic efficiencies are higher for the conventional cycles without the resorption circuit (AHT and DAHT), however, their operating pressures are considerable higher than those reached with the systems with the resorption circuit.
[en] Highlights: • A micro-radial radioisotope thermoelectric generator is manufactured and tested. • The simulated performance of the RTG are compared with the experimental value. • Performance characteristics were determined in different sizes and numbers. • The designed RTG is expected to be a reliable space power supply for MEMS. - Abstract: To satisfy the flexible power demand of the low power dissipation devices in the independent space electric system, a micro-radial milliwatt-power radioisotope thermoelectric generator (RTG) was prepared and optimized in this research. The overall geometrical dimension of the RTG in the experiment was 65 mm (diameter) × 40 mm (height). The RTG, which was built and tested using simulated radioisotope source, eventually obtained an open-circuit voltage of 92.72 mV, an electric power of 149.0 μW, and an energy conversion efficiency of 0.015% at the ambient temperature of 293.15 K and heat source power from 0.1 W to 1 W. On the basis of the structure used in the experiment, the length and cross-sectional area of the thermoelectric leg and the number of thermoelectric modules were effectively optimized through the COMSOL Multiphysics. With the optimized length of 35 mm and cross-sectional area of 1.2 mm2, the RTG with four thermoelectric modules achieved a 15.8 mW output power under 1 W heat source power. The maximum conversion efficiency calculated using COMSOL code increased to 1.58%. According to the optimized electrical output, the micro-radial RTG is expected to be a reliable space power supply for micro components and could satisfy the low power requirements of space missions.
[en] Graphical abstract: The effects of CO2/R41 as an azeotropy refrigerant on system performance are analyzed under various working conditions. The results show that CO2/R41 mixture is a good choice to substitute for pure CO2 in the studied systems because of its low optimal high pressure and high system COP. - Highlights: • The CO2/R41 mixture is an azeotropy refrigerant applied in refrigeration systems. • Comparing with pure CO2, CO2/R41 mixture can decrease the optimal high pressure. • Comparing with pure CO2, CO2/R41 mixture can increase the system COP. - Abstract: In this study, blend of CO2 with R41 is evaluated for application in three different systems including a refrigerated cabinet, an air-source heat pump water heater and a water-source heat pump water heater. The effects of CO2/R41 mixture as an azeotropy refrigerant on system performance are analyzed under various working conditions. The results show that CO2/R41 mixture may be a good choice to substitute for pure CO2 in the studied systems because of its stable chemical properties, low optimal high pressure, high system COP, low compression ratio, low discharge temperature, high refrigerating capacity per unit, and high heating capacity per unit. Furthermore, the exergy efficiencies of the three studied systems are improved to more than 23% with the proposed CO2/R41 (0.5/0.5) mixture refrigerant.