• Journal of Hydrogen and New Energy
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• v.17 no.1
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• pp.1-7
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• 2006

The free-piston hydrogen fueled engine is estimated as the next generation power system which can obtain high efficiency and low emission, simultaneously. In order to develop the free-piston hydrogen fueled engine, it is necessary to stable the combustion. The engine combustion, backfire and knock phenomenons were studied by using RICEM for researching combustion characteristics of free-piston engine. As the results, backfire occurrence was not observed in the free-piston engine under limited experimental condition. And knocking occurred in case of higher cylinder wall temperature.

• Journal of Advanced Marine Engineering and Technology
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• v.27 no.5
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• pp.617-623
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• 2003

In the present study, the characteristics of upward bubble flow were experimentally investigated in a liquid bath. The velocity of upward bubble flow was calculated for two different experimental conditions：1) bubble flow without kinetic energy 2) bubble flow with kinetic energy. Bubble flow without kinetic energy starts to undergo the effect of buoyancy l0cm away from the nozzle. Whereas. kinetic energy is dominant before 30 cm away from the nozzle in bubble flow but after this point kinetic energy and inertial force are applied on bubble flow at the same time In addition, as the flow rate increases the maximum velocity point moves to the nozzle. The velocity Profiles near free surface is extremely irregular due to surface flow. Gas volume fraction is high near the nozzle due to gas concentration. but decreases with the increasement of axial position. Gas volume fraction does not vary after the axial position, z=60 in spite of the increasement of flow.

• Proceedings of the Computational Structural Engineering Institute Conference
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• 2006.04a
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• pp.459-466
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• 2006

The objective of this study is to improve the accuracy of damage detection using natural frequency and modal strain energy. The following approaches are used to achieve the goal. First, modal strain energy is introduced and newly GA-based damage detection technique using natural frequency and modal strain energy is proposed. Next, to verify efficiency of the proposed technique, damage scenarios for free-free beams are designed and the vibration modal tests as damage cases are conducted. Finally, feasibility of proposed technique is verified in comparison with a GA-based damage detection technique using natural frequency and mode shape.

• International Journal of Naval Architecture and Ocean Engineering
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• v.5 no.3
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• pp.392-403
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• 2013

For the analysis of radiating noise problems in medium-to-high frequency ranges, the Energy Flow Boundary Element Method (EFBEM) was developed. EFBEM is the analysis technique that applies the Boundary Element Method (BEM) to Energy Flow Analysis (EFA). The fundamental solutions representing spherical wave property for radiating noise problems in open field and considering the free surface effect in underwater are developed. Also the directivity factor is developed to express wave's directivity patterns in medium-to-high frequency ranges. Indirect EFBEM by using fundamental solutions and fictitious source was applied to open field and underwater noise problems successfully. Through numerical applications, the acoustic energy density distributions due to vibration of a simple plate model and a sphere model were compared with those of commercial code, and the comparison showed good agreement in the level and pattern of the energy density distributions.

• The korean journal of orthodontics
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• v.36 no.5
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• pp.360-368
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• 2006

Objective: Surface characteristics of dental materials play an important role in bacterial adhesion. The purpose of this study was to investigate surface characteristics of 5 different light-cured orthodontic adhesives (1 fluoride-releasing composite, 3 non-fluoride-releasing composites, and f resin-modified glass ionomer). Methods: Surface roughness was measured using a confocal laser scanning microscope. Contact angle and surface free energy components were analyzed using the sessile drop method. Results: Surface roughness was significantly different between adhesives despite a relatively small variation (less than $0.05\;{\mu}m$). Lightbond and Monolok2 were rougher than Enlight and Transbond XT. There were also significant differences in contact angles and surface free energy components between adhesives. In particular, considerable differences in contact angles and surface free energy components were found between resin modified glass ionomer and the composites. Resin modified glass ionomer showed significantly smaller contact angles in 3 different probe liquids and had higher total surface free energy and stronger polarity, with notably stronger basic property than the composites. Conclusion: Resin modified glass ionomer may provide a more favourable environment for bacterial adhesion than composite adhesives.

• Journal of Adhesion and Interface
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• v.6 no.3
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• pp.1-9
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• 2005

This study investigated the influence of atmospheric plasma factors such as RF power, treatment time, the gap distance between discharge and sample, and the gas flow rate of Ar on the surface property by using the design of experiment (DOE) method. The plasma treatment time (s), plasma power (W), gap distance (mm) between discharge and sample, and flow rate of Ar gas were in order of important factors for changing the surface free energy of PMMA plates. As a result, the most effective factor for improving the surface free energy of PMMA plates is the distance (mm) from discharge glow to sample plate. Because of the interaction between plasma power (W) and treatment time (s), the power dose (J) factor which multiply plasma power (W) by treatment time (s) should be significantly considered. The optimum condition for maximizing the surface free energy of PMMA plate was found at 1500J of power dose. Through XPS and AFM analysis, we also observed the change of chemical composition, surface morphology and roughness before and after plasma treatment. It is considered that the change of surface free energy of PMMA plate with plasma treatment is influenced by the introduction of polar functional group as well as the increase of surface roughness.

• Journal of the Korean Society of Manufacturing Technology Engineers
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• v.7 no.6
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• pp.117-125
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• 1998

The paper attempts to estimate the incubation time of a cavity in the interface between a power law creep particle and an elastic matrix subjected to a uniaxial stress. Since the power law creep particle is time dependent, the stresses in the interface relax. The volume free energy associated with Helmholtz free energy includes strain energies caused by applied stress and dislocations piled up in interface(DPI). The energy due to DPI is found by modifying the result of Dundurs and Mura[4]. The volume free energies caused by both applied stress and DPI are a function of the cavity size(r) and elapsed time(t) and arise from stress relaxation in the interface. Critical radius $r^*$ and incubation time $t^*$ to maximise Helmholtz free energy is found in present analysis. Also, kinetics of cavity formation are investigated using the results obtained by Riede [7]. The incubation time is defined in the analysis as the time required to satisfy both the thermodynamic and kinetic conditions. Through the analysis it is found that 1) strain energy caused by the applied stress does not contribute significantly to the thermodynamic and kinetic conditions of a cavity formation, 2) in order to satisfy both thermodynamic and kinetic conditions, critical radius $r^*$ decreases or holds constant with increase of the time until the kinetic condition(eq. 2.3) is satisfied. there for the cavity may not grow right after it is formed, as postulated by Harris [15], and Ishida and Mclean [16], 3) the effects of strain rate exponent (m), material constant $\sigma$0, volume fraction of the particle to matrix(f)and particle size on the incubation time are estimated using material constants of the copper as matrix.

• Journal of Hydrogen and New Energy
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• v.26 no.6
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• pp.638-644
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• 2015

This paper describes the continuing effort to analysis and design on dynamic and electrical behavior of gamma-type free piston Stirling engine/generator with dual-opposed linear generator for domestic micro-CHP (Combined Heat and Power) system. The double acting Stirling engine/generator has one displacer and two power piston which are supported by flexure springs. Two power pistons oscillate with symmetric sinusoidal displacement and are connected with moving magnet type linear generators for power generation. To operate Stirling engine/generator, combustion heat of natural gas is supplied to hot-end and heat is rejected from cold-end by cooling water. The temperature difference across the displacer induces the oscillating motion, and it can be explained with mass-spring vibration system. The purpose of this paper is to describe the design process of linear generator for the double acting free-piston Stirling engine.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.33 no.4A
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• pp.450-457
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• 2008

One of very essential techniques for enlarging lifetime of energy-constrained wireless personal area network (WPAN) devices is energy-efficient transmission technique. If the WPAN is operated based on a TDMA protocol, the satisfaction of QoS requirements at each allocated time slot is another important factor to be considered. We therefore propose an energy-efficient transmission scheme for WPANs operating with a contention-free medium access protocol such as TDMA, as well as satisfying QoS requirement. The proposed algorithm determines the optimum combination of transmit power, physical data rate and fragment size required to simultaneously minimize the energy consumption and satisfy the required QoS in each assigned time duration, considering all the possible energy-minimization related parameters. The proposed algorithm demonstrated the improved performance results in terms of throughput and energy consumption via computer simulation.

• Journal of Hydrogen and New Energy
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• v.27 no.1
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• pp.114-120
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• 2016

This paper describes generating efficiency characteristics of the double acting Stirling engine/generator for domestic small-scale CHP (Combined Heat and Power) system. In small distributed generation applications, Stirling engine has competition from fuel cell, microturbine and etc. In order to be economical in the applications, a long life with minimum maintenance is generally required. Free piston Stirling engine (FPSE) has no crank and rotating parts to generate lateral forces and require lubrication. Double acting Stirling engine/generator has one displacer and two power piston which are supported by flexure springs. Two power pistons oscillate with symmetric displacement and are connected with moving magnet type linear generators for power generation from PV work. In experiments, 1 kW class double acting free piston Stirling engine/generator is fabricated and tested. Heat is supplied to hot end of engine by the combustion of natural gas and converted to electric power by linear generators which are assembled with power pistons. The electric parameters such as voltage, current and phase are measured with for variable flow rate of fuel gas. Especially, generating efficiency of FPSE is measured with three different measurement methods. Generating efficiency of the double acting Stirling engine/alternator is about 24%.