• Proceedings of the Korean Institute of Surface Engineering Conference
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• 2015.05a
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• pp.112-112
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• 2015

The purposed of this work was to determine nanotube shape variation on the Ti-xNb alloys with alloying elements and applied potentials. Samples were prepared by arc melting, followed by followed by homogenization for 12 hr at $1000^{\circ}C$ in argon atmosphere. This study was evaluated the phase and microstructure of Ti-xNb alloys using an X-ray diffraction (XRD) and optical microscopy (OM). The morphology of the samples was investigated with a field-emission scanning electron microscope (FE-SEM) and energy dispersive X-ray spectroscopy (EDS). The nanotube on the alloy surface was formed in 1 M $H_3PO_4$ with small additions of NaF 0.8 wt.%. All anodization treatments were carried out using a scanning potentiostat (Model 362, EG&G, USA) at constant voltage 30 V for 120 min, respectively. The morphology of the samples was investigated with a field-emission scanning electron microscope (FE-SEM) and energy dispersive X-ray spectroscopy (EDS). Surface characteristics of nanotbue formed on Ti-xNb alloys was investigated by potentiodynamic test and potentiostatic in 0.9% NaCl solution at $36.5{\pm}1^{\circ}C$. It was observed that the changed ${\alpha}$ phase to ${\beta}$ phase with Nb content.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2004.03a
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• pp.186-191
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• 2004

The reactive flowfield of the transverse injecting combustor has been studied using Euler-Lagrange method in order to develop an efficient solution procedure for the understanding of liquid spray combustion in the transverse injecting combustor which has been widely used in ramjets and turbojet afterburners. The unsteady two-dimensional gas-phase equations have been represented in Eulerian coordinates and the liquid-phase equations have been formulated in Lagrangian coordinates. The gas-phase equations based on the conservation of mass, momentum, and energy have been supplemented by combustion. The vaporization model takes into account the transient effects associated with the droplet heating and the liquid-phase internal circulation. The droplet trajectories have been determined by the integration of the Lagrangian equation in the flow field obtained from the separate calculation without considering the iterative effect between liquid and gas phases. The reported droplet trajectories had been found to deviate from the initial conical path toward the flow direction in the very end of its lifetime when the droplet size had become small due to evaporation. The integration scheme has been based on the TEACH algorithm for gas-phase equation, the second order Runge-Kutta method for liquid-phase equations and the linear interpolation between the two coordinate systems. The calculation results has shown that the characteristics of the droplet penetration and recirculation have been strongly influenced by the interaction between gas and liquid phases in such a way that most of the vaporization process has been confined to the wake region of the injector, thereby improving the flame stabilization properties of the flowfield.

• Journal of Electrical Engineering and Technology
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• v.7 no.6
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• pp.905-910
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• 2012

This paper reports an investigation of pulse width modulation (PWM) techniques for twophase brushless DC (BLDC) motors fed by a two-phase eight-switch inverter in a fan application. The three-phase BLDC motor is widely applied in industry; however, a lower-cost two-phase BLDC motor and drive circuit has been greatly in demand in recent years. In this paper, we introduce a mathematical model of the two-phase BLDC motor with sinusoidal back electromotive forces (EMFs) based on traditional three-phase BLDC motors. To simplify the drive algorithm and speed up its application, we analyze the principle of block commutation for a two-phase BLDC motor drive in the 180-electricaldegree conduction mode, and we further propose five PWM schemes to improve the commutation performance of the two-phase BLDC drive. The effectiveness of the proposed PWM methods is verified through experiments.

• Journal of Electrical Engineering and Technology
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• v.13 no.3
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• pp.1317-1325
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• 2018

A generator-grid coupling calculation model is established to study the leading-phase operational capability of a 770 MW jumbo hydro-generator in a Chinese ultra-mega hydropower station. The static and dynamic stability of the generator are analyzed and calculated to obtain stability limits under leading-phase operating conditions. Three-dimensional (3D) time-varying nonlinear moving electromagnetic and temperature field models of the generator end-region are also established and used to determine the magnetic field, loss, and temperature of the end-region under the leading-phase operating condition. The simulation results agree with data measured from the actual 770 MW hydro-generator. This paper provides reliable reference data for the leading-phase operation of a jumbo hydro-generator, which will help to improve in the design and manufacture of future hydro-generators.

• Journal of the Korea Institute of Military Science and Technology
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• v.9 no.2 s.25
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• pp.26-33
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• 2006

A sonar dome is basically designed and installed to protect sonar array from shocks, sea wave slaps and floating matters. The acoustic wave passing through sonar dome, however, can be distorted in magnitude and phase. This paper presents a numerical method for predicting the steady-state sound pressure on the surface of transducer array in the sonar dome and typical results of sonar beam pattern affected by sonar dome. A beam tracing model with phase information and a multi-layered elastic boundary model are involved. A full three-dimensional sonar dome is modeled as a GRP acoustic window, a rubber coated steel baffle and a rubber coated steel hull. A transducer array is modeled as thick steel cylinder. There are some assumptions such as incidence of plane wave, specular reflection on boundary and directionality of transducer element.

• Transactions of the Korean Society of Mechanical Engineers
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• v.12 no.3
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• pp.551-560
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• 1988

Gas-particle two phase flow and heat transfer in a cavity receiving thermal radiation through selectively transparent walls have been analyzed by a finite difference method. Particles injected from the upper hole of the cavity are accelerated downward by gravity and exit through the lower hole while they absorb, emit and scatter the incident thermal radiation. Gas phase is heated through convection heat transfer from particles, and consequently buoyancy induced flow field is formed. Two-equation model with two-way coupling is adopted and interaction terms are treated as sources by PSI-Cell method. For the particulate phase, Lagrangian method is employed to describe velocities and temperatures of particles. As thermal radiation is incident upon horizontally, radiative heat transfer in the vertical direction is assumed negligible and two-flux model is used for the solution of radiative heat flus. Gas phase velocity and temperature distributions, and particle trajectories, velocities and temperatures are presented. The effects of particle inlet condition, particle size, injection velocity and particle mass rate are mainly investigated.

• Nuclear Engineering and Technology
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• v.37 no.6
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• pp.595-604
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• 2005

A new instrument, an average BDFT (Birectional Flow Tube), was proposed to measure the flow rate in single and two phase flows. Its working principle is similar to that of the Pilot tube, wherein the dynamic pressure is measured. In an average BDFT, the pressure measured at the front of the flow tube is equal to the total pressure, while that measured at the rear tube is slightly less than the static pressure of the flow field due to the suction effect downstream. The proposed instrument was tested in air/water vertical and horizontal test sections with an inner diameter of 0.08m. The tests were performed primarily in single phase water and air flow conditions to obtain the amplification factor(k) of the flow tube in the vertical and horizontal test sections. Tests were also performed in air/water vertical two phase flow conditions in which the flow regimes were bubbly, slug, and churn turbulent flows. In order to calculate the phasic mass flow rates from the measured differential pressure, the Chexal drift-flux correlation and a momentum exchange factor between the two phases were introduced. The test results show that the proposed instrument with a combination of the measured void fraction, Chexal drift-flux correlation, and Bosio & Malnes' momentum exchange model could predict the phasic mass flow rates within a $15\%$ error. A new momentum exchange model was also proposed from the present data and its implementation provides a $5\%$ improvement to the measured mass flow rate when compared to that with the Bosio & Malnes' model.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.14 no.2
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• pp.128-135
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• 2002

An existing numerical model fo r determining the wave field and pore pressures inside rubble mound breakwater was reformulated here especially to enhance the predictability of interior pore pressures. The pore pressures strongly depend on the nonlinear wave field occurring along frontal slope which is very difficult to be numerically reproduced. In the present study, hence, the amplitude and phase informations of wave pressures along the frontal slope are obtained directly through a hydraulic model test and are incorporated into the numerical model. The interior wave field is analyzed by a boundary element method, and thereby the pore pressures are determined. It was found that the calculated pore pressures agreed quite well with experimental values.

• Journal of Biomedical Engineering Research
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• v.29 no.4
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• pp.267-271
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• 2008

The RF shimming technique has been used to improve the transmit RF field homogeneity in highfield MRI. In the RF shimming technique, the amplitude and phase of the driving currents in each coil element are optimized to get homogenous flip angle or uniform image intensity. The inductive and capacitive coupling between the coil elements may degrade the RF field homogeneity if not taken into account in the optimization procedure. In this paper, we have analyzed the coupling effects on the RF shimming using a sixteen-element TEM RF coil model operating at 300 MHz. We have found that the coupling effects on the RF shimming can be reduced by putting high dielectric material between the active rung and the shield.

• 한국전산유체공학회:학술대회논문집
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• 1996.05a
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• pp.125-130
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• 1996

A numerical analysis is conducted on heat transfer and fluid flow of a plasma spraying process under the DC-RE hybrid electromagnetic field. Plasma flow is analyzed by using Eulerian approach and the equation of particle motion is simultaneously solved using a trajectory analysis with a lumped-heat-capacity model. Axisymmetric two dimensional electromagnetic fields governed by Maxwell's equations are solved based on a vector potential concept. The effects of the RF electromagnetic field on the temperature and velocity fields of the turbulent plasma flow are clarified. Control characteristics of phase changes and dispersed features of particles by applying the RF electromagnetic field are also clarified in an attempt to improve the plasma spraying process