• 한국전산유체공학회지
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• 제13권4호
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• pp.39-44
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• 2008

Numerical simulations of laminar flow over a sphere are conducted to investigate the effect of the Reynolds number on the characteristics of vortex shedding. The Reynolds numbers considered are between 300 and 475, covering unsteady planar-symmetric and asymmetric flows. Results show that the unsteady planar-symmetric flow can be categorized into two different regimes: single-frequency regime and multiple-frequency regime. The former has a single frequency component due to periodic shedding of the vortices with the same strength in every shedding cycle, while the latter has multiple frequency components due to cycle-to-cycle variation in the strength of shed vortices with the shedding angle fixed. The multiple-frequency planar-symmetric flow, which is newly found in the present study, occurs at Re=330${\sim}$360 between the single-frequency planar-symmetric flow and the asymmetric flow. On the other hand, the asymmetric flow occurs at Re${\geq}$365, where the vortices shed from the sphere show variation both in strength and shedding angle unlike the planar-symmetric flow. Also, it is shown that the breaking of planar symmetry is closely related to the imbalance of vortical strength between a pair of streamwise vortices.

• 연구논문집
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• 통권28호
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• pp.49-58
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• 1998

포핏밸브형 2행정 기관의 소기 시스템 형상 변화에 따른 소기유동 및 소기효율의 개선 효과를 해석하기 위해, 각 흡기포트 형상별 소기 유동 가시화와 소기효율 정량화 연구를 수행하고자 한다. 소기시스템의 형상은 기계연구원에서 독자 개발한 RSSV(Rotatable Shrouded Scavenging Valve)의 슈라우드 각도를 변화시켰으며, 소기 효율 정량화를 위해 정상상태하에서 등유와 착색된 물을 사용한 single-cycle method. 동적모사장치(dynamic simulatior)에서 $CO_2$와 $O_2$가스를 사용한 가스추출법(gas sampling method)을 적용하였으며 실제 기관에서의 소기 효율을 측정하기 위해 압력 선도를 해석하였다. 위 실험들의 결과로부터 $90^circ$슈라우드 밸브가 가장 높은 효율을 가지는 것으로 판명되었으며, 특히 이들 실험방법들은 2행정 엔진 개발에 있어 시간과 비용을 절감하는데 아주 유용한 기법임을 알 수 있었다.

• 한국신재생에너지학회:학술대회논문집
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• 한국신재생에너지학회 2006년도 춘계학술대회
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• pp.96-98
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• 2006

Proper water management is vital to achieve high performance and durability of PEFC (Polymer Electrolyte Fuel Cell). The effects of the residual water from PEFC after purge in shut-down processes on GDL/MEAs were investigated with freeze/thaw cycles Freeze/thaw cycle tests were conducted with single cells which were designed from transparent acryl plates. Single cells which contain several amount of residual water were cycles from $80^{\circ}C$ to $-28^{\circ}C$. The resistance changes of the single cells which have various amount of residual water were evaluated by ac-impedance analysis with 24 times of freeze/thaw cycles. Also, after the freeze/thaw cycles, the property changes were characterized by visual methods such as SEM, EPMA. Though it was difficult to observe noticeable property changes in the visual characterizations, the resistance of cells dramatically increased with the amount of remained water.

• Communications for Statistical Applications and Methods
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• 제4권2호
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• pp.515-522
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• 1997

In this paper, we find the second derivative of mean busy cycle with respect to a parameter of inter-arrival time distribution. We show that this derivative can be estimated from single sample path. We do the similar thing for the mean number of arrivals during busy cycle.

• 동력기계공학회지
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• 제20권1호
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• pp.11-17
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• 2016

In this study, the simulation for performance comparison between basic single stage organic rankine cycle, multi stage reheater cycle and multi stage reheater & recuperator cycle was carried out. The multi stage reheater cycle and multi stage reheater & recuperator cycle was designed to improve the efficiency for organic rankine cycle using heat source from industrial waste heat and heat sink from deep ocean water. R245fa was selected as a refrigerant for the cycle and system efficiencies were simulated by the variation of the heat sink temperature and the cycle classification. Performance characteristics were simulated by using the Aspen HYSYS. It was confirmed that the system efficiency was decreased by the increase of heat sink temperature. These results can be considered to be applied as geo-ocean thermal energy conversion in where plenty of geothermal or ocean thermal resource exist.

• 한국태양에너지학회 논문집
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• 제27권4호
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• pp.19-26
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• 2007

This paper concerns the study of absorption refrigeration and heat pump cycle to use sewage. Simulation analysis on the double-effect absorption refrigeration cycle with parallel and two-stage heat pump cycle has been performed. The working fluid is Lithium Bromide and water solution. The absorption refrigeration cycle use sewage as a cooling water for the absorber and condenser, and absorption refrigeration cycle does that as a chilled water for the evaporator of the first stage cycle. And the two-stage cycle consists of coupling double-effect with parallel and single effect cycle so that the first stage absorber and condenser produces heating water to evaporate refrigerant in the evaporator of the second stage. The effects of operating variables such as a absorber temperature on the coefficient of performance have been studied for absorption refrigeration and heat pump cycle.

• 설비공학논문집
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• 제25권5호
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• pp.225-232
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• 2013

Heat pump systems have been widely adopted in buildings for cooling and heating, due to their higher energy efficiency. Recently, the demand for hot water supply from the heat pump system has been increasing. To increase the water supply temperature with higher system efficiency and reliability, a heat pump water heater adopting cascade cycle was investigated in this study. The cascade heat pump water heater consisted of a low-stage cycle using R410A, and a high-stage cycle using R134a. A simulation program for the cascade heat pump water heater was developed, and verified by comparison with experimental data. The performance of the cascade heat pump water heater was optimized, by varying the compressor rotating speeds of the low- and high-stage cycles. At low ambient temperatures, the performance of the cascade cycle was compared with that of the single-stage cycle. The system efficiency of the cascade cycle was higher than that of the single-stage cycle, showing a lower compression ratio and compressor discharge temperature.

• 한국소프트웨어감정평가학회 논문지
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• 제15권1호
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• pp.103-107
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• 2019

최근 저전력 고속 디지털 데이터 통신을 구현 하기위해 많은 기술들이 개발되고 있는 추세이며 듀티사이클 보정에 관련된 기술도 그중 하나이다. 본 논문에서는 전압제어 링 발전기용 저-면적 듀티사이클 보정 회로를 제안하였다. 듀티사이클 보정 회로는 전압제어 링 발진기의 180도 위상차이를 이용하여 듀티사이클을 보정하는 회로이며, 제안된 저-면적 듀티사이클 회로는 기존의 플립플롭을 TSPC(True Single Phase Clocking) 플립플롭으로 변경하여 회로를 구성하였고 이로 인하여 저-면적 고성능 회로를 구현하였다. 일반적인 플립플롭을 대신하여 TSPC플립플롭을 사용하여 기존 회로 대비 저-면적으로 회로 구현이 가능하며 고속 동작에 용이하여 저-전력용 고성능 회로에 활용될 것으로 기대된다.

• Journal of Communications and Networks
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• 제14권1호
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• pp.40-50
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• 2012

The Berkeley-medium access control (B-MAC) is a lightweight, configurable and asynchronous duty cycle medium access control (MAC) protocol in wireless sensor networks. This article presents an analytic modelling of single-hop B-MAC protocol under a Poisson traffic assumption.Our model considers important B-MAC parameters such as the sleep cycle, the two stage backoff mechanism, and the extended preamble. The service delay of an arriving packet and the energy consumption are calculated by an iterative method. The simulation results verify that the proposed analytic model can accurately estimate the performance of single-hop B-MAC with different operating environments.

• 한국연소학회:학술대회논문집
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• 한국연소학회 2012년도 제45회 KOSCO SYMPOSIUM 초록집
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• pp.225-228
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• 2012

Thermodynamic and economic analysis on various type of gas turbine combined cycle power plants was presented to build up the criteria for optimization of power plants. The efficiency considered about energy level difference between electricity and heat was introduced. The efficiency on power and heat generation of power plants whose have different purpose was estimated and power generation costs on various type of combined heat and power plants : fired/unfired, condensing/non-condensing mode, single/double pressure HRSG.