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The SiCf/SiC composite tubes were manufactured by three different manufacturing methods: rotation CVI (chemical vapor infiltration), nanorod-assisted CVI, and forced CVI, and the C-ring compression strength of the tubes was evaluated. In the rotation CVI method, the supply route of the source gas greatly influenced the formation of a homogeneous microstructure, and supplying gas from both directions of the tube could improve the microstructure homogeneity. The longer the gas supply time from the inside to the outside, the better was the C-ring strength. In the nanorod-assisted CVI method, the existence of SiC nanorods increased the strength by 25% (338–429 MPa) and the macro-pore fraction by 44% (7.6% to 3.4%) compared to the conventional CVI tube. In tubes with large diameters (100 mm and 150 mm) made of forced CVI, meaningful strength values were measured even at values outside the range of 1

In order to reduce the amount of excess silicon and carbon, reaction-bonded silicon carbide was fabricated by liquid silicon infiltration using a porous carbon preform prepared by chemical vapor deposition. A carbon preform was fabricated at 1100−1400 ℃ at 6.6 kPa for 1 h using acetylene. The deposited carbon preform has a porous structure with nano-sized carbon particles and uniform pore size distribution. Depending on the deposition temperature of the carbon preform, the porosity, the pore size, and the size of the carbon particles were controlled. As the deposition temperature increased, the porosity and average pore size increased, but the average carbon particle size decreased. The residual silicon of the prepared reaction-bonded silicon carbide was 2.7 % when the porosity of the carbon preform deposited at 1100 °C was 51.1 %. The hardness increased up to 21.7 GPa as the amount of the residual silicon reduced.

Improvement of the thermal stability of continuous SiC fiber reinforced SiC ceramic matrix composites (SiCf/SiC CMC) by the pre-treatment of SiC fillers and the suppression of oxidation during polymer impregnation and pyrolysis (PIP) process were investigated. Dense SiCf/SiC CMCs were fabricated using the slurry infiltration and PIP process under a purified argon atmosphere. Structure and mechanical properties of the SiCf/SiC CMC heated at different temperatures were evaluated. The flexural strength of the SiCf/SiC CMC decreased only 15.3%after heating at 1400 ?C, which exhibited a clear improvement compared with the literature data (49.5% loss), where severe thermal deterioration of SiCf/SiC composite occurred at high temperatures by the crystallization and decomposition of the precursor-derived ceramic matrix. The thermal stability of the SiCf/SiC CMC fabricated by PIP process was improved by the pre-treatment of SiC fillers for removing oxides and the strict atmosphere control to prevent oxidation.

Several intermediate steps were applied before the precursor infiltration and pyrolysis process to improve the infiltration of SiC slurry for promoting the infiltration of SiC slurry into fiber voids. These steps include sonication, popping, electrophoretic deposition, vacuum infiltration and cold isostatic pressing (CIP). The intermediate processes, especially popping and CIP, had a beneficial effect on green density enhancement and improving the homogeneous infiltration of the slurry into fiber fabrics. The density of the SiCfiber/SiCfiller green body was 2.20 g/cm3, which corresponded to 68 % of relative density. The SiCf/SiC composite has a high density of 2.65 g/cm3 after seven PIP cycles.

A SiC and ZrC composite coating layer on a tristructural-isotropic (TRISO) coated particle was deposited by fluidized bed chemical vapor deposition method. Methyltrichlorosilane (MTS) and ZrCl4 powder were used as the source materials of the SiC coating and the ZrC coating, respectively. Two kinds of the SiC and ZrC composite coating layers were fabricated with other source gas fraction. The composite coating layer, with a similar ratio of SiC and ZrC, which was observed in the SEM microstructure, was fabricated, and each phase was confirmed by XRD, Raman, and TEM analysis. Only the SiC and ZrC phases were co-deposited; the ZrSi and free carbon phases were not. The composite coating layer with very small ZrC particles embedded in a SiC matrix was fabricated by decreasing the ZrCl4 source gas ratio. Nano-sized ZrC particles in the coating layer were confirmed by high resolution TEM and EDS analysis.

The main objective of this paper is to study and compare the performance characteristics of foil bearings for a typical turboblower during start/stop conditions. The test bearings feature three-segment bump foils and a full smooth top foil with a nominal diameter of 70 mm and an aspect ratio of 1. All foils are made out of Inconel 718 and have a thickness of 0.2 mm (0.008”). Three coatings, applied to the top foil, have been evaluated during the test campaign: PTFE, Molybdenum-Titanium Nitride (MoTiN) and Molybdenum-Aluminum-Titanium Nitride (MoAlTiN). MoTiN and MoAlTiN were applied using physical vapor deposition (PVD) technology. Each bearing was instrumented with sixteen thermocouples located within the bearing sleeve 1 mm away from its inner diameter. Thermocouples allow the measurement of the bearing temperature in the axial and circumferential directions. Bearing displacement was measured using eight proximity probes located at each side of the bearing (four per side). Overall vibrations of the bearing under test were measured via two accelerometers located on top of the bearing housing in the vertical and horizontal directions. In addition, a torque arm mechanism was used to measure the bearing shear force; hence deduce the friction coefficient, friction torque and total power loss. The paper discusses friction and wear results obtained after one hundred start-stop tests for each bearing. A tribological and microstructural analysis is also presented and discussed., ASME Turbo Expo 2020: Turbomachinery Technical Conference and Exposition, September 21 - 25, 2020, Virtual, Online

The hydrothermal corrosion behavior of chemical-vapor-deposited SiC was investigated for application as nuclear fuel cladding in a pressurized water reactor (PWR). This paper presented the effect of grain size, stacking fault, electrical conductivity, and electrochemical properties on the hydrothermal corrosion behavior of the CVD SiC in a simulated PWR water loop at 360 °C and 18.5 MPa. The hydrothermal corrosion of SiC was strongly influenced by electrical conductivity. It was revealed that high electrical conductivity leads to high exchange current density, which allows SiO2/Si(OH)4 to form easily on the SiC surface, allowing it to dissolve rapidly in the hydrothermal environment.

International audience; Silicon carbide fiber-reinforced silicon carbide matrix (SiC/SiC) composites are candidate materials for cladding of light water reactor (LWR) fuels. Loss of fission product gas retention due to the formation of microcrack networks is considered a potential failure mechanism for SiC/SiC-cladded fuels. In this study, a variety of SiC/SiC composite tubes were irradiated with and without an LWR-relevant radial heat flux in the High Flux Isotope Reactor, followed by detailed characterization with X-ray computed tomography (XCT). This first set of XCT data for neutron-irradiated samples confirmed that the internal stresses arising from a combination of temperature gradients and irradiation-induced swelling act as the primary driver for cracking. While the observed cracking patterns varied depending on the tube architectures, the sharp edges of relatively large pores were found to be the common stress concentrator. These findings are useful to help improve the design and manufacturing of SiC/SiC fuel claddings for reduced failure probability.

The integration of foil bearing technology into high-speed oil-free machines has been slow in progress, in part, due to the low load-carrying capacity of the foil thrust bearing. It is crucial this issue is addressed through innovative solutions without overcomplicating the bearing design because simplicity is one of the attractive features of the foil bearing. This work presents novel thrust foil bearing with taper-flat configuration and pocket grooves on the bearing top foil as a secondary pressure boosting mechanism. Parametric study of the pocket dimensions on a rigid bearing reveals that the bearing static performance is the most sensitive to the pocket angular span. Further two-dimensional fluid–structure interaction analyses on foil thrust bearing predict a reduction of power loss by 10% with increased average film thickness. Minimum film thickness also increases when the bearing is lightly loaded but it is reduced 20% at the taper-flat transition area under high loading condition. This issue can be overcome by using stiffer bump foil; however, this is not implemented in this work due to other design constraints. Test results at 90,000 rpm and 140,000 rpm show, by adding the pocket groove pattern on the top foil, the power loss is reduced by 16% compared to the traditional taper-flat configuration.

The nonlinear characteristics of foil bearings often add complexities to analysis and design of foil bearings over wide size ranges. Previously, scaling laws for bearing clearance and areal stiffness of supporting foil structure for differently sized foil bearings were proposed to address these challenges. The scaling laws enable to design large foil bearing by scaling up existing small foil bearing. To verify the proposed scaling law and demonstrate the feasibility of a large foil bearing, a 200mm (in diameter) hybrid air foil bearing with potential use for 2MW generator was designed and manufactured by scaling up an existing 50mm foil bearing following the scaling laws. The 200mm hybrid foil bearing was tested for its static performance. The bearing features a three-pad offset preload with total six hydrostatic injection ports to improve the load carrying capacity at low speed. A great effort was put into designing a test rig to demonstrate the bearing static performance. The test rig is configured with a series of ball bearings-supported test rotor and the hybrid foil bearing is floating on the rotor. A pneumatic actuator is used to apply external load to the foil bearing and a torque rod measures the friction torque of the foil bearing. The test rig was designed with maximum allowable speed of 18,000rpm and the rotordynamics of the test rig was analyzed to ensure its integrity under the testing conditions. Two hybrid modes, i.e., with all injection ports open and only bottom injection ports open, were compared. The hybrid foil bearing can withstand up to 2000N at zero speed under 5 bar absolute supply pressure for both hybrid modes. Twenty start/stop cycles test at 1000rpm over extended period under 2000N in hybrid modes show the bearing temperature changes very little, demonstrating very robust bearing function. The hybrid foil bearing was also tested in hydrodynamic mode at 4400rpm under 2000N, verifying the full functionality of the bearing in hydrodynamic mode. A preview of a 2MW permanent magnet generator featuring the 200mm hybrid foil bearing is also presented as one of the potential applications.

This work presents a novel design of a hydrostatic thrust foil bearing (HSTFB) with an outer diameter of 154mm along with simulation and test results up to specific load capacity of 223kPa (32.3psi). The HSTFB incorporates a high pressure air/gas injection to the thrust foil bearing with a uniform clearance. This bearing has high load capacity, low power loss, and no friction/wear during startup and shutdown. In addition, the HSTFB allows for bidirectional operation. The paper also presents an advanced simulation model which adopts the exact locations of a tangentially arranged bumps to a cylindrical two-dimensional plate model of the top foil. This method predicts top foil deflection with better accuracy than the traditional independent elastic foundation model which distributes the bump locations over the nodal points in the cylindrical coordinates, and with less computational resource than the finite element method applied to the entire bump/top foils. The presented HSTFB, was designed for Organic Rankine Cycle (ORC) generators, but its performance was predicted and measured using air in this paper. The bearing static performance is compared analytically against the rigid counterpart, and presented at different supply pressures, speeds, and minimum film thicknesses. Experimental verification is conducted at 10, 15 and 20krpm. The measured load capacity and frictional loss agree well with the prediction. The measured film thickness also agrees with the prediction after the structural deflection of the thrust runner disc is compensated. Overall, the novel HSTFB demonstrates an excellent static performance and shows good potential for adoption to the intended ORC generators and other large oil-free turbomachines.

Small gas foil bearings (FBs) with shaft diameter below 25 mm can find many applications in air compressors for fuel cells, electrical turbo chargers, small unmanned air vehicles, turbo alternators, etc. These small machines are characterized by very light load to the radial FBs, and thus rotordynamics stability is more challenging than load capacity. However, a main challenge of gas foil thrust bearings (GFTBs) is how to increase the load capacity, and the challenge remains the same regardless of the size. In previous publications on experimental studies on GFTBs, the measured load capacity is well below the prediction due to challenges in testing as well as manufacturing of GFTBs. Difficulty in achieving the design load capacity often leads to increasing the bearing size in actual applications with penalty of higher power loss. This paper presents design feature of a novel GFTB with outer diameter of 38 mm and static performance up to 155 krpm under external load of 75 N using a high-speed test rig. The 38 mm GFTB presented in this paper is a three-layered structure for easy design and manufacturing, and the unique design feature allows easy scale down and scale up to different sizes. Reynolds equations for compressible gas and the two-dimensional thin plate model were adopted for fluid–structure interaction simulation to predict load capacity and power loss of the GFTB. The predicted power loss and load capacity agree well with the measurements.

Cladding thermal conductivity is an important physical property in assessing the performance of silicon carbide (SiC)-cladded fuels for nuclear reactors. However, there is a significant lack of reliable data, particularly for irradiated materials, because the geometry complicates the measurement. This study investigates the thermal diffusivity of coupons with a curvature, machined from SiC fiber–reinforced SiC matrix composite tubes, with and without neutron irradiation under light water reactor–relevant temperature and dose conditions. The tested materials included full composite and duplex SiC composite tubes. The measurements were conducted using a modern flash diffusivity apparatus. The analyzed area on the specimen during diffusivity testing was reduced for improved measurement accuracy due to sample curvature. Post-irradiation measurements showed that the effects of neutron irradiation on thermal conductivity (e.g. thermal defect resistivity) are different between SiC composite plates versus tubes. The difference was explained by higher matrix density of the tube than the plate. This study provides reliable thermal properties of prototypic SiC composite tubes useful for fuel performance modeling of SiC-based cladding.

In order to improve the hydrothermal corrosion resistance of nuclear fuel cladding, TiN and TiCrN were deposited on the surface of a Zr-based alloy tube as an environmental barrier coating using arc ion plating and DC sputtering under an Ar+N2 atmosphere. A hydrothermal corrosion test was carried out at 360 °C and 20 MPa for 120 days in simulated PWR primary water chemistry. To similar the water chemistry, 1200 ppm H3BO3, 2.2 ppm LiOH, and 25 cc·H2/kg·H2O were added to pressurized water. The pH and conductivity were controlled to be 23 μS/cm and 6.5, respectively. After the hydrothermal corrosion test, a microscale of FeTiO3 covered the entire surface of TiN due to its rapid corrosion rate. On the other hand, the thickness of the oxide layer on TiCrN was reduced to a range of tens of nanometers. The corrosion product on the surface was identified as FeCr2O4. This spinel-structured FeCr2O4 effectively suppressed the corrosion of the TiCrN and improved the corrosion resistance.

To improve the oxidation resistance of SiC composites at high temperature, the feasibility of using Ti3SiC2 coated via electrophoretic deposition (EPD) as a SiC fiber reinforced SiC composite interphase material was studied. Through fiber pullout, Ti3SiC2, due to its lamellar structure, has the possibility of improving the fracture toughness of SiCf/SiC composites. In this study, Ti3SiC2 coating was produced by EPD on SiC fiber; using Ti3SiC2-coated SiC fabric, SiCf/SiC composite was fabricated by hot pressing. Platelet Ti3SiC2 powder pulverized into nanoparticles through high-energy wet ball milling was uniformly coated on the SiC fiber in a direction in which the basal plane of the particles was parallel to the fiber. In a 3-point bending test of the SiCf/SiC composite using Ti3SiC2-coated SiC fabric, the SiCf/SiC composite exhibited brittle fracture behavior, but an abrupt slope change in the strength-displacement curve was observed during loading due to the Ti3SiC2 interphase. On the fracture surface, delamination between each layer of SiC fabric was observed.

Multi-layered SiC composites consisting of monolithic SiC and a SiC f /SiC composite are one of the accident tolerant fuel cladding concepts in pressurized light water reactors. To evaluate the integrity of the SiC fuel cladding under normal operating conditions of a pressurized light water reactor, the hydrothermal corrosion behavior of multi-layered SiC composite tubes was investigated in the simulated primary water environment of a pressurized water reactor without neutron fluence. The results showed that SiC phases with good crystallinity such as Tyranno SA3 SiC fiber and monolithic SiC deposited at 1200 °C had good corrosion resistance. However, the SiC phase deposited at 1000 °C had less crystallinity and severely dissolved in water, particularly the amorphous SiC phase formed along grain boundaries. Dissolved hydrogen did not play a significant role in improving the hydrothermal corrosion resistance of the CVI-processed SiC phases containing amorphous SiC, resulting in a significant weight loss and reduction of hoop strength of the multi-layered SiC composite tubes after corrosion.

Multi-layered SiC composites have been considered as a nuclear fuel cladding material of light water reactors, LWRs, because of their excellent high temperature strength and corrosion resistance under accident conditions. During a design basis accident of a LWR such as a loss-of-coolant accident, the peak temperature of the fuel clad rapidly increases as the production of decay heat continues. The emergency core cooling systems then automatically supply the reactor core with emergency cooling water. The fuel clad consequently suffers from thermal shock. In this study, the structural integrity of multi-layered SiC composite tubes after thermal shock was investigated. Several kinds of multi-layered SiC composite tubes consisting of CVD SiC and CVI SiCf/SiC were water-quenched from 1200°C to room temperature. The triplex SiC composite tube retained its tubular geometry during quenching. The strength degradation after thermal shock was less than 13% for the specimens with a PyC interphase. The residual stress distribution within the tubes during thermal shock was evaluated by a finite element method. This article is protected by copyright. All rights reserved.

The thermal conductivity of SiC ceramics and FCM fuel composites, consisting of a SiC matrix and TRISO coated particles, was measured and analyzed. SiC ceramics and FCM pellets were fabricated by hot press sintering with Al 2 O 3 and Y 2 O 3 sintering additives. Several factors that influence thermal conductivity, specifically the content of sintering additives for SiC ceramics and the volume fraction of TRISO particles and the matrix thermal conductivity of FCM pellets, were investigated. The thermal conductivity values of samples were analyzed on the basis of their microstructure and the arrangement of TRISO particles. The thermal conductivity of the FCM pellets was compared to that predicted by the Maxwell-Eucken equation and the thermal conductivity of TRISO coated particles was calculated. The thermal conductivity of FCM pellets in various sintering conditions was in close agreement to that predicted by the Maxwell-Eucken equation with the fitted thermal conductivity value of TRISO particles.

Tantalum carbide, which is one of the ultra-high temperature ceramics, was deposited on graphite by low pressure chemical vapor deposition from a TaCl 5 -C₃H 6 - Ar-H₂ mixture. To maintain a constant TaCl 5 /C₃H 6 ratio during the deposition process, TaCl 5 powders were continuously fed into the sublimation chamber using a screw-driven feeder. Sublimation behavior of TaCl5 powder was measured by thermogravimetric analysis. TaC coatings have various phases such as Ta+α-Ta₂C, α-Ta₂C+TaC 1-x , and TaC 1-x depending on the powder feeding methods, the C₃H 6 /TaCl 5 ratio, and the deposition temperatures. Near-stoichiometric TaC was obtained by optimizing the deposition parameters. Phase compositions were analyzed by XRD, XPS, and Raman analysis.

Crack healing phenomena were observed in mullite and mullite + Yb2SiO5 environmental barrier coating (EBC) materials during thermal shock cycles. Air plasma spray coating was used to deposit the EBC materials onto a Si bondcoat on a SiCf/SiC composite substrate. This study reveals that unidirectional vertical cracks (mud cracks) formed after several thermal shock cycles, however, the cracks were stable for 5000 thermal shock cycles at a maximum temperature of 1350 &deg, C. Moreover, the crack densities decreased with an increasing number of thermal shock cycles. After 3000 thermal shock cycles, cracks were healed via melting of a phase containing SiO2 phase, which partially filled the gaps of the cracks and resulted in the precipitation of crystalline Al2O3 in the mullite. Post-indentation tests after thermal shock cycling indicated that the mullite-based EBC maintained its initial mechanical behavior compared to Y2SiO5. The indentation load&ndash, displacement tests revealed that, among the materials investigated in the present study, the mullite + Yb2SiO5 EBC demonstrated the best durability during repetitive thermal shocks.

The creep behavior of a nickel-base superalloy, Alloy 617, which is considered as a candidate material for the very high temperature gas cooled reactor, was studied. Creep rupture tests were carried out at 800°C, 900°C and 1000°C in static and flowing helium environments as well as in air. Creep rupture life in static helium was longer than that in air, while it was shorter in flowing helium environments. Microstructure observation of the creep tested specimens showed that the shorter creep rupture life in flowing helium was associated with the thicker oxide layer, greater decarburization depth, and deeper internal oxidation happened during the creep tests. The degree of such oxidation damage was quantified for the creep tested specimens and correlated with the creep rupture life in different environments.

The influence of high-temperature annealing on the microstructure of zirconium carbide (ZrC) was investigated in relation to its application as a coating layer of a nuclear fuel in a very high temperature gas cooled reactor. ZrC was deposited as a constituent coating layer of TRISO coated particles by a fluidized bed chemical vapor deposition method using a ZrCl 4 -CH 4 -Ar-H 2 system. The grain growth of ZrC during high-temperature annealing was strongly influenced by the co-deposition of free carbon. Sub-stoichiometric ZrC coatings have experienced a significant grain growth during high-temperature annealing at 1800 °C and 1900 °C for 1 h. On the other hand, a dual phase of stoichiometric ZrC and free carbon experienced little grain growth. It was revealed that the free carbon of the as-deposited ZrC was primarily distributed within the ZrC grains but was redistributed to the grain boundaries after annealing. Consequently, carbon at the grain boundary retarded the grain growth of ZrC. Electron backscatter diffraction (EBSD) results showed that as-deposited ZrC had (001) a preferred orientation that kept its favored direction after significant grain growth during annealing. The hardness slightly decreased as the grain growth progressed.

Nuclear fuel cladding used in a nuclear power plant must possess superior oxidation resistance in the coolant atmosphere of high temperature/high pressure. However, as was the case for the critical LOCA (loss-of-coolant accident) accident that took place in the Fukushima disaster, there is a risk of hydrogen explosion when the nuclear fuel cladding and steam reacts dramatically to cause a rapid high-temperature oxidation accompanied by generation of a huge amount of hydrogen. Hence, an active search is ongoing for an alternative material to be used for manufacturing of nuclear fuel cladding. Studies are currently aimed at improving the safety of this cladding. In particular, ceramic-based nuclear fuel cladding, such as SiC, is receiving much attention due to the excellent radiation resistance, high strength, chemical durability against oxidation and corrosion, and excellent thermal conduction of ceramics. In the present study, cladding with protective films was fabricated using a process that forms a matrix phase by polymer impregnation of polycarbosilane (PCS) after filament-winding the SiC fiber onto an existing Zry-4 cladding tube. It is analyzed the oxidation and microstructure of the metal cladding with composite protective films using a drop tube furnace for thermal shock test.

Due to its stability at high temperature and its layered structure, Ti3SiC2 MAX phase was considered to the interphase of SiC f /SiC composite. In this study, Ti₃SiC₂ MAX phase powder was deposited on SiC fiber via the electrophoretic deposition (EPD) method. The Zeta potential of the Ti₃SiC₂ suspension with and without polyethyleneimine as a dispersant was measured to determine the conditions of the EPD experiments. Using a suspension with 0.03 wt.% ball milled Ti₃SiC₂ powder and 0.3 wt.% PEI, Ti₃SiC₂ MAX phase was successfully coated on SiC fiber with an EPD voltage of 10 V for 2 h. Most of the coated Ti₃SiC₂ powders are composed of spherical particles. Part of the Ti₃SiC₂ powders that are platelet shaped are oriented parallel to the SiC fiber surface. From these results we expect that Ti₃SiC₂ can be applied to the interphase of SiC f /SiC composites.

We report on the mass production of plasma activated water (PAW) by a non-thermal atmospheric pressure plasma. As a non-thermal plasma source, we used planar dielectric barrier discharge (DBD) and two types cylindrical DBDs. The manufacturing characteristics of PAW were investigated for each DBD type. The results show that the radical transfer characteristics from plasma to water have a dominant effect on the PAW properties. Of the two cylindrical DBD reactors, the "direct" type in which the mesh electrode was replaced with the conductor (Al) showed superior characteristics over "indirect" type. DBD reactor with an Al electrode with the length of 1 m long achieved a PAW production capacity of 520 liters per hour and production efficiency of 65 l kWh−1. Such a large-capacity PAW manufacturing device can be applied to various industries including agriculture, sterilization, and water treatment.

Air foil bearings (AFBs) are introduced as promising bearings for oil-free turbomachinery applications. AFBs provide reliable operation at high speed and high temperature with negligible power loss. Hybrid Air Foil Bearing (HAFB) technology utilizes the radial injection of externally pressurized air into the traditional hydrodynamic AFB’s film thickness through orifices attached to the top foil. Previous studies have reported enhancement in the rotordynamic stability of HAFBs compared to traditional hydrodynamic AFBs. HAFBs have several orifices distributed in the circumferential direction. In this study, the effect of the circumferential location of radial injection on the rotordynamic performance of the rotor-HAFB is studied. Analytical and experimental evaluations of the rotordynamic performance of a rotor supported by two single-pad HAFBs are presented. Parametric studies are conducted using three sets of single-pad HAFBs. The circumferential locations of orifices are different for each set. The presented simulation analyses consist of time-domain orbit simulation and frequency-domain modal analysis. Imbalance responses of rotor-HAFB were measured with various orifice locations and the results agree well with predictions. Comparison of the rotordynamic performance of HAFBs with different orifice configurations demonstrate substantial improvement in rotordynamic stability as well as enhancement in the stiffness and damping coefficients of HAFBs by choosing the best circumferential location for radial injection to control rotor eccentricity and attitude angle.Copyright © 2018 by ASME

This paper details the design and performance of a high-speed (up to 190,000rpm) gas foil thrust bearing (GFTB) test rig to measure bearing load capacity. Several GFTB test rigs were reported in the literature for operating speed up to 90krpm. A few recently presented works show successful runs at 135krpm for testing gas thrust bearing with viscoelastic support and 130krpm tilting pad thrust bearing with compliant structure. However, a GFTB test rig for speed range over 100krpm has not been reported. At high speed operation, the gas film thickness of the GFTB is around a few microns which makes it difficult to achieve in testing. In many cases, the measured thrust load from experiments is well below the predicted data due to difficulty in testing and instrumentation. Difficulty in validating the actual load capacity of the bearings leads to increasing the thrust bearing size to ensure sufficient load capacity in actual applications, which results in higher power consumption. This work presents detail feature of a novel GFTB test rig and test results of 38mm GFTB. The developed test rig runs up to 190krpm and measures bearing load capacity, frictional torque and temperature across bearing ID and OD. The test rig is suitable for testing GFTB with OD from 30 mm to 40 mm. The test facility successfully tests a 38 mm GFTB to its predicted load capacity of 75N (110kPa).Copyright © 2018 by ASME

This paper investigates the effect of gas foil thrust bearing (GFTB) on the rotordynamic performance of the rotor-gas foil bearing (GFB) system. A rigid rotor supported on two gas foil journal bearings (GFJB) and a pair of GFTBs is studied using a five degree of freedom (5-DOF) model. The studies were performed in both frequency domain using excitation frequency-dependent bearing coefficients (modal analyses) and non-linear analyses (time domain orbit simulations). Modal analyses were performed for both symmetrically and asymmetrically supported rotor systems. For the symmetric rotor, the modal stiffness for the conical mode increases with the axial force, while cylindrical mode is not affected. The axial force has little effects on the modal damping for both the cylindrical mode and conical mode. Thus, the natural frequency and threshold speed (stability limit) for the conical mode increases as the axial force increases, while these values for the cylindrical mode remain almost constant. For the asymmetric rotor, the modal stiffness for both the cylindrical mode and conical mode increases with the axial force, and thus both natural frequency and threshold speed increase with the axial force. Rotor lateral vibrations were also predicted using synchronous bearing coefficients (of both GFJB and GFTB) for both conical and cylindrical modes. The predicted rotor lateral responses show the critical speed increases with axial force for both cylindrical mode and conical mode. The nonlinear analysis using time-domain orbit simulation was also performed including the effect of axial force on the GFTB. The effect of axial force on the stability of the rotor system were discussed. The predicted results showed that the stability of rotor system improved as the axial force increases for Case 1 when the out of phase imbalances were added on the rotor. However, the stability of the rotor system for Case 2 not only influenced by the axial force but also influenced by how asymmetry the rotor is. For the in phase imbalances, the onset speed of subsynchronous motion decreases as axial force increases for the large asymmetric rotor bearing system and the decrement of the onset speed of subsynchronous decreases as the asymmetry of the rotor bearing system decreases. For the out of phase imbalances, the onset speed of subsynchronous motion also decreases as axial force increases for the large asymmetric rotor, but an opposite trend was shown as the asymmetry of the rotor decreases.

The corrosion behavior of chemically vapor deposited SiC was investigated in relation to its application as fuel cladding in a pressurized water reactor (PWR). Corrosion tests were conducted in deoxygenated, pressurized water at 360 °C with control of dissolved hydrogen. The dissolved hydrogen dramatically reduced the corrosion rate of SiC compared with that in water without the dissolved hydrogen. It was revealed that the dissolution of the surface oxide of SiC primarily contributed to the weight loss at the initial stage of corrosion. Further weight loss was minimal for 210 d because dissolved hydrogen effectively retarded the formation of SiO2.

Abnormally high-priced transactions in urban land speculation bring detrimental effects on economy, environment, and society. Governmental agencies around the world are striving hard to monitor and control land speculation by introducing various policy objectives and tools for an efficient urban development planning. One of the major challenges in controlling land speculation is to quickly identify the spatiotemporal locations of concern (hot spots) by monitoring the spatial clustering pattern changes over time and to alert the appropriate decision-making agencies for timely policy intervention. In this paper, we introduce a framework to rapidly detect the spatiotemporal hot spots of speculative land transactions in near-real-time data by exploiting the prospective monitoring procedures. We applied this method in the city of Hwasung, Republic of Korea, as an empirical illustration and found that the locations Jeongnam, Bongdam, Mado, and Dongtan were identified as hot spots with high, concentrated transac...

Objectives: The aim of this study was to investigate the exposure and risk assessment of residents near asbestos mines in Korea. Methods: To assess asbestos types and airborne concentrations, air monitoring was performed in the neighborhoods of Kwangcheon (KC) and Sinsuk (SS) mines, which were leading South Korean mines in the past. In addition, activity-based-sampling (ABS) of residents' particular activities were conducted in order to estimate the Excess Lifetime Cancer Risks (ELCRs) for the residents. Conclusions: The average concentration of airborne asbestos in KC was 0.0014 f/cc and 0.0015 f/cc by PCM and TEM, respectively. In SS it was equal at 0.0012 f/cc by PCM and TEM. No statistically significant difference was found in the average concentration of airborne asbestos between the two mines. The average asbestos concentration of ABS was 0.0048 f/cc (PCM) and 0.0042 f/cc (TEM) in KC, while it was 0.0137 f/cc (PCM) and 0.0125 f/cc (TEM) in SS. It was found that the average asbestos concentration of ABS in SS was statistically significantly higher than that of KC (p

The SiC ceramics are under investigation for the fuel cladding in the light water nuclear reactors because of its excellent high temperature strength and corrosion resistance against hot steam under the severe accident conditions. In this study, the SiC triplex tubes consisting of a SiC inner layer, a SiC/PyC/SiC intermediate layer, and a SiC outer layer were fabricated by the chemical vapor processes. The hoop strength and fracture behaviors of the SiC triplex tube were investigated. The SiC triplex tubes fabricated at the high ratio of H2/MTS had a quite high average strength with a relatively small standard deviation. The hoop strength of the composite tubes tends to increase with the volume fraction of the reinforced fibers. The highest fiber volume fraction was obtained using Tyranno SA3-0.8k with the dense winding patterns such as bamboo-like mosaic pattern, which resulted in the high hoop strength compared to other fibers of Tyranno SA3-1.6k and Hi-Nicalon Type S. Hoop strength also increased slightly as the winding angle increased from 45° to 65°. Fracture behaviors of the SiC triplex tube were investigated via the observation of microstructure of the failed samples.

Air foil bearing (AFB) technology has made substantial advancement during the past decades and found its applications in various small turbomachinery. However, rotordynamic instability, friction and drag during the start/stop, and thermal management are still challenges for further application of the technology. Hybrid air foil bearing (HAFB), utilizing hydrostatic injection of externally pressurized air into the bearing clearance, is one of the technology advancements to the conventional AFB. Previous studies on HAFBs demonstrate the enhancement in the load capacity at low speeds, reduction or elimination of the friction and wear during starts/stops, and enhanced heat dissipation capability. In this paper, the benefit of the HAFB is further explored to enhance the rotordynamic stability by employing a controlled hydrostatic injection. This paper presents the analytical and experimental evaluation of the rotordynamic performance of a rotor supported by two three-pads HAFBs with the controlled hydrostatic injection, which utilizes the injections at particular locations to control eccentricity and attitude angle. The simulations in both time domain orbit simulations and frequency-domain modal analyses indicate a substantial improvement of the rotor-bearing performance. The simulation results were verified in a highspeed test rig (maximum speed of 70,000 rpm). Experimental results agree with simulations in suppressing the subsynchronous vibrations but with a large discrepancy in the magnitude of the subsynchronous vibrations, which is a result of the limitation of the current modelling approach. However, both simulations and experiments clearly demonstrate the effectiveness of the controlled hydrostatic injection on improving the rotordynamic performance of AFB.Copyright © 2017 by ASME