高純度Fe-(4,5,6)wt%Si合金における磁気・機械特性及び磁区構造とその挙動

九州工業大学博士学位論文（要旨）学位記番号：生工博甲第257号 学位授与年月日平成28年3月25

Rare earth technology: magnetic cooling and magnetic separation

This dissertation deals with two prospectives of rare earth technology. It’s application in magnetic cooling as well as its harvesting and recycling phase. The emphasis is on mapping and manipulating the transport processes of energy and mass, during magnetic cooling and rare earth magnetic separation, under the influence of magnetic field. Distinguished by the driving force of flow field, they belong to the context of magnetohydrodynamics and ferrohydrodynamics, respectively. Multiple aspects are investigated with respect to magnetic cooling. First, the transient dynamics of heat transfer from two periodically magnetized gadolinium (Gd) plates into a heat transfer fluid (n-decane) is studied. It demonstrates that the propagation of the thermal fronts emanating from the Gd plates after magnetization or demagnetization obeys a √t-dependence. A finite time required for magnetization and demagnetization causes a spatially delayed propagation of the thermal fronts. The diffusive heat flux, derived from the temperature profiles, experiences a drop down by about 80% after first 3 seconds while the percentage of thermal energy transferred into n-decane experiences a maximum there. With a stagnant fluid, this work provides reasons for lower bounds of geometry and operation frequency of a simplified parallel-plate structure in the diffusive limit. Furthermore, the potential of magnetohydrodynamic (MHD) convection to increase heat transfer during magnetic cooling is tested. To do this, a section of an active magnetic regenerator is considered, namely a flat gadolinium plate, immersed in an initially stagnant heat transfer fluid (NaOH) which is placed in a cuboid glass cell. To create the MHD flow, a small electric current is injected by means of two electrodes and interacts with the already present magnetic field. As a result, a Lorentz force is generated, which drives a swirling flow in the present model configuration. By means of particle image velocimetry and Mach-Zehnder interferometry, the flow field and its impact on the heat transfer at the gadolinium plate is analyzed. For the magnetization stage, a heat transfer enhancement by about 40 % can be achieved even with low currents of 3 mA is found. In parallel to enhance the heat transfer by an actively stirring of the heat transfer fluid by means of MHD, alternative fluid candidate is also investigated. A room temperature eutectic liquid metal GaInSn, with superior Pr≈ 0.03, and comparable viscosity like that of water is tested in a segment of parallel plate AMR. Due to the high electric conductivity, velocity field of GaInSn contrasting to that of aqueous based ones is strongly influenced by magnetic field due to Lorentz force. Therefore, preliminary velocity measurements by means of ultrasound doppler velocimetry with a quasi homogeneous static magnetic field (220 mT) in a duct channel at the non-conducting Shercliff walls are conducted. The Hartmann walls are constituted of two parallel Gd plates. The second part of this dissertation, rare earth harvesting and recycling, aims to answer the question of why an enrichment of paramagnetic ions can be observed in a magnetic field gradient despite the presence of a counteracting Brownian motion. For that purpose, a rare-earth chloride (DyCl3) solution is studied in which weak evaporation is adjusted by means of small differences in the vapor pressure. The temporal evolution of the refractive index field of this solution, as a result of heat and mass transfer, is measured by means of a Mach–Zehnder interferometer. A numerical algorithm is developed that splits the refractive index field into two parts, one space-dependent and conservative and the other time-dependent and transient. By using this algorithm in conjunction with a numerical simulation of the temperature and concentration field, it is able to show that 90% of the refractive index in the evaporation-driven boundary layer is caused by an increase in the concentration of Dy(III) ions. A simplified analysis of the gravitational and magnetic forces, entering the Rayleigh number, leads to a diagram of the system’s instability. Accordingly, the enrichment layer of elevated Dy(III) concentration is placed in a spatial zone dominated by a field gradient force. This leads to the unconditional stability of this layer in the present configuration. The underlying mechanism is the levitation and reshaping of the evaporation-driven boundary layer by the magnetic field gradient

Fault Diagnosis for Power Electronics Converters based on Deep Feedforward Network and Wavelet Compression

A fault diagnosis method for power electronics converters based on deep feedforward network and wavelet compression is proposed in this paper. The transient historical data after wavelet compression are used to realize the training of fault diagnosis classifier. Firstly, the correlation analysis of the voltage or current data running in various fault states is performed to remove the redundant features and the sampling point. Secondly, the wavelet transform is used to remove the redundant data of the features, and then the training sample data is greatly compressed. The deep feedforward network is trained by the low frequency component of the features, while the training speed is greatly accelerated. The average accuracy of fault diagnosis classifier can reach over 97%. Finally, the fault diagnosis classifier is tested, and final diagnosis result is determined by multiple-groups transient data, by which the reliability of diagnosis results is improved. The experimental result proves that the classifier has strong generalization ability and can accurately locate the open-circuit faults in IGBTs.Comment: Electric Power Systems Researc

Unbalance Compensation of a Full Scale Test Rig Designed for HTR-10GT: A Frequency-Domain Approach Based on Iterative Learning Control

Unbalance vibrations are crucial problems in heavy rotational machinery, especially for the systems with high operation speed, like turbine machinery. For the program of 10 MW High Temperature gas-cooled Reactor with direct Gas-Turbine cycle (HTR-10GT), the rated operation speed of the turbine system is 15000 RPM which is beyond the second bending frequency. In that case, even a small residual mass will lead to large unbalance vibrations. Thus, it is of great significance to study balancing methods for the system. As the turbine rotor is designed to be suspended by active magnetic bearings (AMBs), unbalance compensation could be achieved by adequate control strategies. In the paper, unbalance compensation for the Multi-Input and Multi-Output (MIMO) active magnetic bearing (AMB) system using frequency-domain iterative learning control (ILC) is analyzed. Based on the analysis, an ILC controller for unbalance compensation of the full scale test rig, which is designed for the rotor and AMBs in HTR-10GT, is designed. Simulation results are reported which show the efficiency of the ILC controller for attenuating the unbalance vibration of the full scale test rig. This research can offer valuable design criterion for unbalance compensation of the turbine machinery in HTR-10GT

Thermophysical Properties of Lignocellulose: A Cell-scale Study down to 41K

Thermal energy transport is of great importance in lignocellulose pyrolysis for bio-fuels. The thermophysical properties of lignocellulose significantly affect the overall properties of bio-composites and the related thermal transport. In this work, cell-scale lignocellulose (mono-layer plant cells) is prepared to characterize their thermal properties from room temperature down to 41 K. The thermal conductivities of cell-scale lignocellulose along different directions show a little anisotropy due to the cell structure anisotropy. It is found that with temperature going down, the volumetric specific heat of the lignocellulose shows a slower decreasing trend against temperature than that of microcrystalline cellulose, and its value is always higher than that of microcrystalline cellulose. The thermal conductivity of lignocellulose decreases with temperature from 243 K to 317 K due to increasing phonon-phonon scatterings. From 41 K to 243 K, the thermal conductivity rises with temperature and its change mainly depends on the heat capacity's change