• 한국인터넷방송통신학회논문지
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• 제17권1호
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• pp.83-88
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• 2017

본 논문은 디지털 부호를 전송할 때 통신 채널에서 발생되는 부호간 간섭을 줄일 수 있는 MMA (Multiple Modulus Algorithm) 적응 등화 알고리즘에 adaptive modulus와 adaptive step size 개념을 적용한 H-MMA (Hybrid-MMA) 알고리즘의 등화 성능평가에 관한 것이다. 기존의 MMA 적응 등화기의 탭 계수 갱신은 등화기 출력를 이용하여 오차 신호를 얻게 되므로 2차원 QAM 신호에서 진폭과 위상을 보상할 수 있지만, 이때 스텝 크기와 modulus는 고정되므로 등화 성능이 저하된다. 이와 같은 문제점을 개선하기 위하여 등화기 출력 신호의 크기에 비례토록 adaptive modulus와 적응 스텝 크기를 변화시킬 수 있는 Hybrid adaptive 개념을 MMA에 적용하므로서 2차원 QAM 신호에서 등화 성능을 개선할 수 있다. 논문에서 제안하는 H-MMA 알고리즘의 성능을 기존의 MMA 알고리즘과 비교하기 위하여 동일한 채널에서 컴퓨터 시뮬레이션을 수행하였다. 시뮬레이션 결과 수렴 속도에서는 제안 H-MMA가 MMA보다 다소 느려지지만, 정상 상태 이후 모든 성능 지수에서 훨씬 우월하며 등화 잡음이 감소됨을 확인하였다.

• 한국방재학회 논문집
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• 제9권5호
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• pp.63-68
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• 2009

본 연구에서 물 흐름에 일정한 농도로 포 소화약제를 정확하게 혼합하는 포 소화설비용 소화약제 혼합장치의 성능향상을 위하여 포소화약제의 정량 혼합특성을 분석하였다. 펌프, 탱크, 압력계, 유량계, 노즐로 구성되는 성능평가 시스템을 사용하여 개발된 프로포셔너의 성능이 실험적으로 평가하였다. 결과적으로 라인프로포셔너의 포소화약제 혼합성능은 벤츄리 효과에 의한 물유량 증가와 오리피스 단면적 증가에 따라 증가하고 오차율 $\pm4%$이내의 범위에서 물 유량과 포소화약제 양의 혼합농도 3%의 성능을 보였다. 프레셔프로포셔너의 경우 물 유량 증가와 입구압력 증가에 따라 혼합성능이 증가하고 오차율 $\pm2%$이내의 범위에서 물 유량과 포소화약제 양의 혼합농도 3%의 성능으로 분석되었다.

• Nuclear Engineering and Technology
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• 제53권4호
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• pp.1236-1249
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• 2021

Although there are still controversial opinions and uncertainty on application of SiC_f/SiC composite cladding as next-generation cladding material for its great oxidation resistance in high temperature steam environment and other outstanding advantages, it cannot deny that SiC_f/SiC cladding is a potential accident tolerant fuel (ATF) cladding with high research priority and still in the engineering design stage for now. However, considering its disadvantages, such as low irradiated thermal conductivity, ductility that barely not exist, further evaluations of its in-pile behaviors are still necessary. Based on the self-developed code we recently updated, relevant thermohydraulic and mechanical models in FROBA-ATF were applied to simulate the cladding behaviors under normal and accident conditions in this paper. Even through steady-state performance analysis revealed that this kind of cladding material could greatly reduce the oxidation thickness, the thermal performance of UO₂-SiC was poor due to its low inpile thermal conductivity and creep rate. Besides, the risk of failure exists when reactor power decreased. With geometry optimization and dopant addition in pellets, the steady-state performance of UO₂-SiC was enhanced and the failure risk was reduced. The thermal and mechanical performance of the improved UO₂-SiC was further evaluated under Loss of coolant accident (LOCA) and Reactivity Initiated Accident (RIA) conditions. Transient results showed that the optimized ATF had better thermal performance, lower cladding hoop stress, and could provide more coping time under accident conditions.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2007년도 춘계학술대회B
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• pp.3353-3358
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• 2007

The present paper is devoted to investigate dynamic effect and steady-state performance of methane autothermal reformer theoretically and numerically. In order to simplify the complicated phenomena in the system, axisymmetric heterogeneous reactor model is developed. As autothermal reaction takes places on catalyst surface between bulk gas and catalyst, volume averaging method is incorporated using porous medium approach. To understand the start-up process which occurs in the reactor is highly important. Therefore, in this paper we get various goverining equations to find out transient and steady solutions and time scale for start-up introducing dimensionless variables. Start-up is a significant issue in reforming reaction for automobile system and fueling of SOFC-based auxiliary power units. This paper deals with characteristics of heat and mass transfer and predicted light-off time in the reformer as oxygen to carbon ratio ($O_2$/C) and amount of feeding gas.

• 정보학연구
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• 제1권2호
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• pp.39-53
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• 1998

CMA나 MCMA 적응 블라인드 등화 알고리즘은 특별한 형태의 신호점을 원하는 신호점로 간주하므로써 완전한 등화에 도달시에도 원래의 신호점과의 불일치로 인한 오차가 불가피하다. 본 논문에서는 축소 신호점 대신에 결정된 원래의 신호점을 이용한 새로운 오차함수를 도입하여 빠른 수렴 속도로 정상상태에 도달하며, 정상상태에서의 오차를 크게 개선한 새로운 적응 블라인드 등화 알고리즘을 제안한다. 컴퓨터 모의실험을 통하여 제안한 알고리즘이 반송파 위상 복원을 가지는 CMA 및 MCMA등과 같은 CMA 계열의 적응 블라인드 등화 알고리즘들에 비해 잔류 심볼간 간섭과 수렴 속도면에서 향상된 성능을 가짐을 확인 하였다.

• 한국항공우주학회지
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• 제39권7호
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• pp.634-642
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• 2011

비행조종작동기 제어용 파일럿 밸브의 내부 유동 및 응답 특성을 분석하기 위해 캐비테이션 모델과 격자 이동 기법을 사용하여 정상/비정상 유동의 수치 해석을 수행하였다. 작동 온도와 시간에 따른 밸브 내부 유동을 분석한 결과, 일정 온도 이상에서 밸브 특성이 안정되는 현상을 발견하였고 캐비테이션이 밸브 성능에 미치는 영향성을 파악하였다. 밸브 내부 압력과 응답 특성을 분석하였으며 비정상 유동이 발달하여 정상 유동 특성과 일치하는 것을 확인하였다.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2003년도 춘계학술대회
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• pp.1479-1484
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• 2003

The heat transfer enhancement by pulsating flow in a plate heat exchanger has been experimentally investigated in this study. The effect of the pulsating flow, such as pulsating frequency and flow rate, on the heat transfer as well as pressure drop in a plate heat exchanger has been studied in detail. Reynolds number in cold side of a plate heat exchanger is varied $100{\sim}530$ while that of hot side is fixed at 620. The pulsating frequency is considered in the range of $5{\sim}30$ Hz. The results of the pulsating flow are also compared with those of steady flow. It is found that the average heat transfer rate as well as pressure drop is increased as flow rate is increased for both steady flow and pulsating flow cases. When pulsating flow is applied to the plate heat exchanger, heat transfer could be substantially increased in particular ranges of pulsating frequency or Strouhal number; $St=0.36{\sim}0.60$ and pressure drop is also increased, compared with those of steady flow.

• 한국전산유체공학회지
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• 제17권3호
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• pp.53-58
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• 2012

Steady flow simulations through a high pressure turbine guide vanes were carried out. The main objective of the present work is to study the performance of turbulence models on the steady flow prediction from aerodynamic and aerothermal points of view. Three turbulence models were compared, namely SST, k-${\omega}$ and ${\omega}$-Reynolds stress models. The laminar results were also compared. The comparison was done with emphasis on the isentropic Mach number and heat transfer coefficient along the blade, and total pressure loss in the wake region. The calculated isentropic Mach number showed reasonable agreement with experimental data along the blade surface for all three turbulent models. For the total pressure loss in the wake region, ${\omega}$-Reynolds stress model showed the best agreement with the experimental data. However, unless using an appropriate transition model, the heat transfer coefficients of all three turbulent models showed poor agreement with experimental data.

• Journal of Power Electronics
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• 제18권3호
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• pp.766-777
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• 2018

In this paper, a sliding mode and proportional plus integral (SM-PI) control combined with self-sustained phase shift modulation (SSPSM) for LLC resonant converters is presented. The proposed control scheme improves the transient response while preserving good steady-state performance. An averaged large signal model of an LLC converter with the ZVS modulation technique is developed for the SM control design. The sliding surface is obtained based on the input-output linearization concept. A system identification method is adopted to obtain the transform function of the LLC resonant converter, which is used to design the PI control. In order to reduce the inherent chattering problem in the steady state, the combined SM-PI control strategy is derived with fuzzy control, where the SM control is responsive during the transient state while the PI control prevails in the steady state. The combination of SSPSM and the SM-PI control provides ZVS operation, robustness and a fast transient response against step load variations. Simulation and experimental results validate the theoretical analysis and the attractive features of the proposed scheme.

• 한국생산제조학회지
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• 제20권2호
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• pp.121-128
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• 2011

As high precision industries such as semiconductor, TFT-LCD manufacturing and MEMS continue to grow, the demand for higher DOF precision stages has been increasing. In general, the stages should accommodate a prescribed range of payloads in order to position various precision manufacturing/inspection instruments. Therefore a nonlinear controller using sliding motion is developed, which bears mass perturbation and makes the upper plate of the stage move in 6 DOF. For the application of the nonlinear control, an observer is also developed based on expected noise covariance. To eliminate the steady state error of step response, integral terms are inserted into the state-space model. The linear term of the controller is designed using optimization scheme in which parameters can be weighted according to their physical significance, whereas the nonlinear term of the controller is designed using trial and error method. A comprehensive simulation study proves that the designed controller is robust against mass perturbation and completely eliminates steady state errors.