A health grade system against mechanical faults of power transformers has been little investigated compared to those for chemical and electrical faults. This paper thus presents a statistical health ...grade system against mechanical faults in power transformers used in nuclear powerplant sites where the mechanical joints and/or parts are the ones used for constraining transformer cores. Two health metrics-RMS and root mean square deviation of spectral responses at harmonic frequencies-are first defined using vibration signals acquired via insite sensors on 54 power transformers in several nuclear powerplants in 16 months. We then investigate a novel multivariate statistical model, namely copula, to statistically model the populated data of the health metrics. The preliminary study shows that the proposed health metrics and statistical health grade system are feasible to monitor and predict the health condition of the mechanical faults in the power transformers.
Aerodynamic loads for a horizontal axis wind turbine of the National Renewable Energy Laboratory (NREL) Phase VI rotor in yawed condition were predicted by using the blade element momentum theorem. ...The classical blade element momentum theorem was complemented by several aerodynamic corrections and models including the Pitt and Peters’ yaw correction, Buhl’s wake correction, Prandtl’s tip loss model, Du and Selig’s three-dimensional (3-D) stall delay model, etc. Changes of the aerodynamic loads according to the azimuth angle acting on the span-wise location of the NREL Phase VI blade were compared with the experimental data with various yaw angles and inflow speeds. The computational flow chart for the classical blade element momentum theorem was adequately modified to accurately calculate the combined functions of additional corrections and models stated above. A successive under-relaxation technique was developed and applied to prevent possible failure during the iteration process. Changes of the angle of attack according to the azimuth angle at the specified radial location of the blade were also obtained. The proposed numerical procedure was verified, and the predicted data of aerodynamic loads for the NREL Phase VI rotor bears an extremely close resemblance to those of the experimental data. KCI Citation Count: 7
This paper describes an optimal design process of a permanent-magnet thrust bearing. The bearing consists of two sets of permanent magnet rings. One set is located inside the other. An axial ...displacement between the two sets creates an axial force, thereby achieving a thrust bearing function. In order to obtain an optimal design of the bearing where the required load capacity of bearing is achieved with the least magnet volume, we derived analytical design equations based on the equivalent current sheet (ECS) method. We considered two types of magnet arrays: axial arrays and Halbach arrays. Optimization is carried out for these two types. The results show that the Halbach array can achieve the load capacity requirement with smaller magnet volume than the axial array. We also found that there exists an optimal axial separation of rings for the axial array. The efficacy of the ECS method is verified against the results from three-dimensional finite element analyses and experiments. It is found that the Halbach array is more sensitive to the underlying assumptions of ECS method than the axial array.
Understanding water hammer is very important to the prevention of excessive pressure build-up in pipelines. Many researchers have studied this phenomenon, drawing effective solutions through the ...time- and frequency-domain approaches. For the purposes of enhancing the advantages of the frequency-domain approach and, thereby, rendering investigations of the dynamic characteristics of pipelines more effective, we propose partial fraction expansion of the transfer function between the unsteady flow source and a given section. We simulate the proposed approach using a vibration element inserted into a simple pipeline, deducing much useful physical information pertaining to pipeline design. We conclude that locating the resonance of the vibration element between the first and second resonances of the pipeline can mitigate the excessive pressure build-up attendant on the occurrence of water hammer. Our method of partial fraction expansion is expected to be useful and effective in analyses of unsteady flows in pipelines.
A 35-kWh superconductor flywheel energy storage (SFES) system using hybrid bearing sets, which is composed of a high-temperature superconductor bearing and an active magnet damper, has been developed ...in the Korea Electric Power Corporation Research Institute (KEPRI). To keep the flywheel system safe, backup bearings were adapted, and a ball bearing was selected through simulations. The 35-kWh SFES was tested up to 6500 r/min successfully. During rotation tests, there were few accidents. An accident occurred while decreasing speed, at around 4000 r/min. Although the backup bearing was damaged to keep the flywheel system safe, the shaft was also damaged. Contact due to the elongation of the flywheel shaft was considered as the reason for the accident. Some results of the accident are described and analyzed.
The stability of flywheels in an energy storage system supported by active magnetic bearings (AMBs) is studied in this paper. We designed and built two flywheel energy storage systems (FESS) that can ...store up to 5 kWh of usable energy at a maximum speed of 18,000 rpm. One is optimized to store as much energy as possible, resulting in a flywheel with a strong gyroscopic coupling. The other has a much smaller gyroscopic coupling for ease of control. To analyze the stability of the system accurately, we derived the dynamic models of the rotor using finite-element method and integrated them with the models of the bearings, amplifiers, and sensors to obtain the simulation model of the system. We validated the model through experiments and compared the stability of these two systems.
미분탄 입자의 고속가열 열분해거동 해석 장지훈(JIHOON JANG); 한가람(KARAM HAN); 유근실(GEUN SIL YU) ...
한국수소 및 신에너지학회 논문집,
2019, Letnik:
30, Številka:
3
Journal Article
Odprti dostop
The pyrolysis characteristics of pulverized coal particle was numerically analyzed with the drop tube furnace. Based on the simulated gas flow field in the drop tube furnace, the particle velocity, ...temperature and volatile evolution were calculated with the fourth order Runge-Kutta method. The effects of changes in reactor wall temperature and particle diameter on the pyrolysis behavior of coal particle were investigated. The particle heating rate was very sensitive to the reactor wall temperature and particle size, that is, the higher wall temperature and the smaller particle size resulted in the higher heating rate and the consequent quicker volatile evolution.
Aerodynamic loads for a horizontal axis wind turbine of the National Renewable Energy Laboratory (NREL) Phase VI rotor in yawed condition were predicted by using the blade element momentum theorem. ...The classical blade element momentum theorem was complemented by several aerodynamic corrections and models including the Pitt and Peters' yaw correction, Buhl's wake correction, Prandtl's tip loss model, Du and Selig's three-dimensional (3-D) stall delay model, etc. Changes of the aerodynamic loads according to the azimuth angle acting on the span-wise location of the NREL Phase VI blade were compared with the experimental data with various yaw angles and inflow speeds. The computational flow chart for the classical blade element momentum theorem was adequately modified to accurately calculate the combined functions of additional corrections and models stated above. A successive under-relaxation technique was developed and applied to prevent possible failure during the iteration process. Changes of the angle of attack according to the azimuth angle at the specified radial location of the blade were also obtained. The proposed numerical procedure was verified, and the predicted data of aerodynamic loads for the NREL Phase VI rotor bears an extremely close resemblance to those of the experimental data.
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