• Transactions of the Korean Society of Mechanical Engineers A
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• v.39 no.10
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• pp.1029-1037
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• 2015

Recently, the importance of rotordynamic stability has been increased because of the tendency to employ ultra-high speeds in rotating machinery. In particular, the dynamic characteristics of gas bearings for high-speed rotating machinery need to be identified at various excitation frequencies to predict the rotor's behavior. In this study, we perform dynamic loading tests for gas-foil bearings (GFBs) to determine the bump foil structure and an air-film combined bump-foil structure for varying excitation frequencies. We calculate the dynamic characteristics from the measured force and displacement data. The air film is generated by a pressurized air supply. Based on the results, the stiffness coefficients of the bump structure and the air-film combined bump structure increased, while the damping coefficients decreased at increasing excitation frequencies. Further, the stiffness and damping coefficients of the air-film combined structure show lower values than those of the bump structure. Consequently, we identify the frequency-dependent dynamic characteristics of the bump structure and the effect of gas film on the dynamic characteristics of GFBs. Furthermore, to reveal the effectiveness of the proposed method, we perform experiments and discuss two methods of extracting the dynamic characteristics from the measured data.

• Journal of Korean Tunnelling and Underground Space Association
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• v.10 no.3
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• pp.303-312
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• 2008

Multi-axial compression tests have been frequently adopted for the evaluation of material properties of rock cores and rock fracture model tests. Special care has to be applied on the boundary condition between the specimen and loading platen to draw the precise test results of the multi-axial compression tests. With the use of dry steel platen, the stress rotation will occur, due to the frictional restraint from the boundary between the specimen and loading platen. The restraint will deviate the expected test results under the conditions of the given external pressures. Various methods have been applied to reduce the side restraint along the specimen/loading platen interface. The steel brush type loading platen is one example of the attempts. In this paper, a new type of loading platen is introduced to overcome the limitation caused by the use of the brush type loading platen, which requires some internal space for the installation of the brushes. The new type of loading platen, roller supported steel piston type loading platen. is constituted of shot steel pistons which have sufficient stiffness to deliver the external pressure and the shaft type roller installed at the rear of the pistons. The pistons are designed to follow the local deformation of the specimens. In this paper, structural details of the loading platen are presented and frictional and biaxial compression tests results are shown to verify the required functions of the loading platen. Furthermore, calibration process is followed by a comparison between the test results and numerical analyses.

• Korean Journal of Food Science and Technology
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• v.21 no.6
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• pp.815-819
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• 1989

Rheological properties of chestnut starch suspensions (3 and 4%, db) and gelatinized starch (4%, db) were investigated with a capillary and rotational viscometer, respectively. Starch suspensions had no yield stress and showed dilatant flow behavior in the temperature ranges of $30-65^{\circ}C$. However, starch suspension showed pseudoplastic flow behavior at $70^{\circ}C\; and\;above\; 65^{\circ}C$ for 3 and 4% concentration, respectively Flow activation energy below $50^{\circ}C$ was 0.56 kcal/mole but increased to 51.9-80.8 kcal/mole at $60-70^{\circ}C$. The behavior of gelatinized starch (4%) was pseudoplastic regardless of heating temperature $(65-80^{\circ}C)$ and time (15-60 min). The apparent viscosity of the starch remained constant after heating at $80^{\circ}C$ for 45 min. The swelling power and log apparent viscosity showed similar pattern. The activation energy of the apparent viscosity of the geletinized starch at $70-80^{\circ}C$ was 13.09kcal/mole. The apparent viscosity of thermal-gelatinized $(90^{\circ}C)$ starch was lower than that of 15 psi-gelatinized starch.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.33 no.2
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• pp.95-101
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• 2020

The wind turbine blades should be designed to possess a high stiffness and should be fabricated with a light and high strength material because they serve under extreme combination of lift and drag forces, converting kinetic energy of wind into shaft work. The goal of this study is to understand the basic knowledge required to curtail the process time consumed during the construction of small wind turbine blades using carbon fiber reinforced polymer (CFRP) prepeg composites. The configuration of turbine rotor was determined using the QBlade freeware program. The fluid dynamics module simulated the loads exerted by the wind of a specific speed, and the stress analysis module predicted the distributions of equivalent von Mises stress for representing the blade structures. It was suggested to modify the shape of test specimen from ASTM D638 to decrease the variance in measured tensile strengths. Then, a series of experiments were performed to confirm that the bladder compression molded CFRP prepreg can provide sufficient strength to small wind turbine blades and decrease the cure time simultaneously.

• Korean Chemical Engineering Research
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• v.59 no.4
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• pp.521-531
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• 2021

Vacuum Pressure Impregnation (VPI) is a process that enhances physical properties by coating some types of epoxy resins on windings of stator used in large rotators such as generators and motors. During vacuum and pressurization of the VPI process, resin gas is generated by vaporization of epoxy resin. When the tank is opened for curing after finishing impregnation, resin gas is leaked out of the tank. If the leaked resin gas spreads throughout the workplace, there are safety and environmental problems such as fire, explosion and respiratory problems. So, exhaust system for resin gas is required during the process. In this study, a case study of exhaust efficiency by location of vent was conducted using Computational Fluid Dynamics (CFD) in order to design a system for exhausting resin gas generated by the VPI process. The optimal exhaust system of this study allowed more than 90% of resin gas to be exhausted within 1,800 seconds and reduced the fraction of resin gas below the Low Explosive Limit (LEL).

• Journal of the Korean Society of Propulsion Engineers
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• v.13 no.3
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• pp.50-57
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• 2009

A gas mechanical pulsator is developed for the study of combustion instabilities in various combustors such as LRE combustor. First, it shows that the mass flow rates and the perturbation frequencies can be successively controlled by the inlet pressure and the rotating speed of a rotating disk with many holes. Second, the device is used as an acoustic amplification source as a substitute for the speaker in the previous acoustic tests and its results show almost the same resonant frequency and damping characteristics compared with the previous results. In conclusion, the result shows that it can be used as a substitute for a speaker in the studies of LRE combustion instabilities, which has a flow and no limitation of amplification, and a device for making a perturbation source in gas flow.

• Korean Journal of Food Science and Technology
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• v.33 no.1
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• pp.55-59
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• 2001

Pressure-puffing system or extruder has been used to puff rice kernel or rice flour. Most of the study on rice puffing were the effect of process conditions such as moisture content and heating temperature on physical and chemical characteristics of popped rice. The study on mechanism and development of instant puffed rice like popcorn has been limited. Extruded waxy rice pellets were puffed in a microwave oven after drying and conditioning. Extruded pellets were formed with extrusion conditions of $20{\sim}27%$ moisture content, 2.76 MPa $CO_2$ gas injection pressure and 200 rpm screw speed. Under these conditions, puffed waxy rice pellets in microwave oven had low density and soft texture. Density and texture of puffed waxy rice pellet could be optimized by control of moisture content, $CO_2$ gas injection pressure and screw speed that affect significantly when puffing extruded waxy rice pellet.

• Journal of the Korean Society of Propulsion Engineers
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• v.20 no.1
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• pp.76-81
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• 2016

At negative-g condition, a float-type valve can open the passage of fuel in order to it flows continuously through a pressurized fuel tank system. Since specialized test conditions and high-cost supports are required, it is truly difficult to test the valve in a real high-speed test. Therefore, this paper contains performance analysis of a float-type fuel supply valve through flight tests conducting roll and negative-g maneuvers.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2008.06a
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• pp.324-324
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• 2008

Piezoelectric ceramics is an active element that makes stator to vibrate to generate rotational force in ultrasonic motors. In drive of ultrasonic motors, many factors that affect to the resonance characteristics of piezoelectric ceramics exist. For example, those factors are bonding condition with elastic body, the magnitude of electric field, the normal force for frictional drive and the emission of heat due to vibration and friction and so on. Therefore, it is important to research the inclination of property variation of piezoelectric ceramics in circumstance that has complex elements. In this paper, we focused and analyzed the resonance characteristics of ultrasonic motor due to the magnitude of the driving voltage and normal force.

• Proceedings of the KIEE Conference
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• 2000.07b
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• pp.741-743
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• 2000

In this paper the relationship between the pressing force applied to rotors and the characteristics of ultrasonic motor are discussed. The characteristics of ultrasonic motor using a piezoelectric vibrator were systematically studied. And these were applied to the construction of a card forwarding device. The principle of ultrasonic motor is to use an elliptical motion generated on the side of the vibrator, and the elliptical motion of the ultrasonic motor was obtained by complex oscillation of $L_1-B_4$ mode. As the experimental results. the forwarding speed of the card increased linearly as the pressing force applied to rotors increased. The forwarding speed of the card was 16.0 cm/s when the pressing force applied to rotors was 1 N. The forwarding force of the card increased linearly as the pressing force applied to rotors increased. The forwarding force of the card was 398 mN when the pressing force applied to rotors was 1 N. Therefore, this ultrasonic motor can be expected to be used for card-forwarding device and so on.