• 대한전자공학회논문지TC
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• 제44권4호
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• pp.65-71
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• 2007

Doherty 전력증폭기는 일반전력증폭기보다 효율은 뛰어나지만 많은 왜곡성분이 발생한다. 이러한 왜곡성분은 일반적인 진폭왜곡과 위상왜곡, 그리고 메모리 효과에 의한 왜곡성분으로 구분할 수 있다. 본 논문에서는 전기적인 비선형성을 정확히 모델링하고 열 메모리 효과가 Doherty 증폭기에 미치는 영향을 연구함으로써, 전치왜곡기에 적용할 수 있는 열 메모리 효과 보상기를 제안하였다. Doherty 증폭기의 열 메모리 특성을 모델링하기 위하여 순시적으로 소모되는 전력과 순시 접합온도의 정확한 관계식을 정립하여 제안하였다. 제안된 모델의 파라미터는 전치왜곡기를 사용한 Doherty 증폭기의 열 메모리효과를 효율적으로 억제한다. 이러한 열 메모리 보상기를 가진 전치왜곡기는 선형화된 전력증폭기의 출력스펙트럼에서 약 22 dB정도의 ACLR 개선효과를 보인다. 측정결과는 50W급 LDMOS Doherty 전력증폭기로 측정하였으며, 열 메모리 보상기를 가진 전치왜곡기는 ADS로 검증하였다.

• Nuclear Engineering and Technology
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• 제38권2호
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• pp.147-154
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• 2006

Thermal fatigue of the coolant circuits of PWR plants is a major issue for nuclear safety. The problem is especially accute in mixing zones, like T-junctions, where large differences in water temperature between the two inlets and high levels of turbulence can lead to large temperature fluctuations at the wall. Until recently, studies on the matter had been tackled at EDF using steady methods: the fluid flow was solved with a CFD code using an averaged turbulence model, which led to the knowledge of the mean temperature and temperature variance at each point of the wall. But, being based on averaged quantities, this method could not reproduce the unsteady and 3D effects of the problem, like phase lag in temperature oscillations between two points, which can generate important stresses. Benefiting from advances in computer power and turbulence modelling, a new methodology is now applied, that allows to take these effects into account. The CFD tool Code_Saturne, developped at EDF, is used to solve the fluid flow using an unsteady L.E.S. approach. It is coupled with the thermal code Syrthes, which propagates the temperature fluctuations into the wall thickness. The instantaneous temperature field inside the wall can then be extracted and used for structure mechanics computations (mainly with EDF thermomechanics tool Code_Aster). The purpose of this paper is to present the application of this methodology to the simulation of a straight T-junction mock-up, similar to the Residual Heat Remover (RHR) junction found in N4 type PWR nuclear plants, and designed to study thermal striping and cracks propagation. The results are generally in good agreement with the measurements; yet, in certain areas of the flow, progress is still needed in L.E.S. modelling and in the treatment of instantaneous heat transfer at the wall.

• 태양에너지
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• 제18권3호
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• pp.89-94
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• 1998

태양열 응용을 위한 한 방편으로 캡슐화된 PCM을 이용한 유동층 축열조에서 축열 및 방열과정에서 열전달 특성을 살펴보았다. 유동층 축열조는 원통형으로 높이는 40cm, 직경은 5.0cm 이었다. 축열물질은 무기염의 일종인 sodium acetate 였으며, 이것은 파라핀 왁스와 PMMA 로 코팅되었다. 캡슐화된 PCM의 크기는 약 $2{\sim}3mm$였으며, 용융점은 $58^{\circ}C$였다. 축열 및 방열과정시 유동층 축열조의 시간에 따른 온도분포, 순간 열저장 및 방출 속도를 측정하였으며 이로부터 유동층 축열조의 체적 열전달 계수를 도출하였다. 또한 유동층 축열조의 조업변수인 열전달 유체의 유속, 유입온도에 대한 열전달 계수의 영향도 관찰하였다.

• 항공우주시스템공학회지
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• 제1권2호
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• pp.37-42
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• 2007

This paper proposes a numerical modeling approach to simulate the hybrid combustion phenomena. From the physical understandings of hybrid combustion, the computational domain was separated into three regions: the solid fuel, gas phase reactive flow, and the interface between solid and fluid. Moreover, for the accurate calculation, computational grids for these regions was generated at every time step considering the instantaneous moving interface which are governed by the balance equations using thermal pyrolysis. In the domain of reactive flow, by virtue of diffusion flame structure, turbulent combustion modeling was introduced using either mixture fraction approach or mean reaction rate approach.

• 대한전기학회:학술대회논문집
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• 대한전기학회 2006년도 춘계학술대회 논문집 전기기기 및 에너지변환시스템부문
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• pp.99-102
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• 2006

This paper proposes a DITC(Direct Instantaneous Torque Control of SRM (Switched Reluctance Motor) using a novel 4-level converter for smooth torque control and high efficiency. DITC of SRM is very useful for smooth torque control, but the driving efficiency is low due to advanced current that does not produce torque. For the high efficiency control of SRM, fast excitation and demagnetization of phase current are required. A novel 4-level converter and a new control scheme are present to improve the driving efficiency. The proposed DITC of SRM using 4-level converter is verified by computer simulation.

• 대한전기학회:학술대회논문집
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• 대한전기학회 1996년도 하계학술대회 논문집 A
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• pp.492-494
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• 1996

This paper describes a study on the series resonant inverter using the double full-bridge, and the output voltage of the proposed inverter is controlled by phase shift angle of the outputs of two inverters. These control schemes can be reduce the switching loss and EMI, etc, which the inverter is drived by auto following control of output frequency, because it is impossible for switching devices to be always turned on and off at zero voltage or zero current. Theoretical characteristics of the proposed double inverter circuit are compared with Pspice simulation and experimental results.

• 전력전자학회:학술대회논문집
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• 전력전자학회 2003년도 춘계전력전자학술대회 논문집(1)
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• pp.251-255
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• 2003

This paper proposes a novel UPQC(unified power quality conditioner) based on H-bridge modules, isolated through single-phase multi-winding transformers. The dynamic performance of proposed system was analyzed by simulation with EMTDC/PSCAD, assuming that the UPQC is connected with the 22.9kV distribution line. The proposed system can be directly connected to the transmission line without series injection transformers. It has flexibility in expanding the operation voltage by increasing the number of H-bridge modules and can compensate reactive power, harmonics, voltage sag and swell, voltage unbalance. The control strategy for the proposing UPQC was derived using the instantaneous power method. The proposing UPQC has the ultimate capability of improving power quality at the point of installation on power distribution systems and can be utilized for the future distribution system.

• 한국광학회지
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• 제13권1호
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• pp.70-72
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• 2002

좁은 선폭을 갖는 연속 레이저 빔이 제이조화된 Nd:YAG레이저빔과 Bethune셀을 사용하여 좁은 선폭을 갖는 펄스 레이저빔이 생성되었다. 증폭된 펄스빔의 주파수 chirping정도는 헤테로다인 방법을 이용하여 측정되었으며, 헤테로다인 맥놀이신호로부터 순간적인 위상 변화량을 추출함으로서 분석되었다. 10 ns 펄스에 의해 MHz 이하의 선폭을 가지는 연속 레이저빔으로부터 증폭된 주파수 chirping은 80MHz정도로 매우 좁은 선폭의 증폭된 펄스 레이저빔이 얻어졌다.

• 대한조선학회논문집
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• 제40권5호
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• pp.17-25
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• 2003

Velocity field behind a container ship model with a rotating propeller has been investigated using PIV (particle image velocimetry) system. Four hundred instantaneous velocity fields were measured at 4 different blade phases and ensemble-averaged to investigate the spatial evolution of vortical structure of near wake within one propeller diameter downstream. The phase-averaged mean velocity fields show the potential wake and the viscous wake formed due to the boundary layers developed on the blade surfaces. The interaction between bilge vortex developed along the hull surface and the tangential velocity component of incoming flow causes to have asymmetric flow structure in the transverse plane.

• 소음진동
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• 제4권3호
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• pp.365-375
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• 1994

Periodic longitudinal vibrations of a video and audio tape and the like used for transducing pictures and sounds may be induced by self-excited vibrations which are caused by frictions against the heads and guides, and their eccentricity, noncircularity or irregularity. It is important to analyze the longitudinal vibration of a tape because it causes distortions of a reproduced signal. It is difficult to measure directly the longitudinal vibration. In this paper a method estimating longitudinal vibration using impulse responses in the transverse direction is presented. And, the parameters boundary domain where the transverse and longitudinal vibration can be decoupled is introduced. In the domain where the both vibrations are decoupled, analytic methods using frequency characteristics and transient responses of the transverse vibration, respectively, are presented. The time domain method predicted more exactly the instantaneous phase of the longitudinal vibration than frequency domain method did.