• 대한기계학회:학술대회논문집
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• 대한기계학회 2004년도 춘계학술대회
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• pp.2123-2128
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• 2004

In this study, analysis is made for the effects of groove shape on the thermal performance of a axial groove heat pipe. The mathematical models of two-phase flow in grooved heat pipe are presented for the capillary limitation in steady state. Generally, the heat pipe performance depends on the capillary pressure and liquid flow. The friction force of liquid flow through the groove increases with the groove width decreased, and then the capillary pressure is improved in the gas-liquid interface of groove. Therefore, the optimal groove width shaper exists for the maximum thermal performance of heat pipe. In this paper, the optimal groove shape and scale are presented by considering both capillary pressure and liquid flow.

• 한국유체기계학회 논문집
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• 제18권1호
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• pp.44-50
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• 2015

Labyrinth seals are commonly used in various kinds of turbomachinery to reduce leakage flow. In the present 3D CFD analysis of see-through-type labyrinth air seal, the methodology of determining leakage and rotordynamic coefficients is suggested with the relative coordinate system for steady-state simulation. The leakage flow and rotordynamic forces predicted by using different solvers and turbulent models of FLUENT are compared with the results of the existing bulk-flow analysis code LABYSEAL.FOR and experiment. The present CFD result of direct stiffness(K) shows only improvement in prediction. The results of leakage and rotordynamic coefficients as well as computing time are sensitive against the used solver and turbulent model.

• 대한전기학회논문지:전력기술부문A
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• 제48권11호
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• pp.1410-1416
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• 1999

This paper aims at investigating the operating-mode characteristics of two-module Thyristor Controlled Series Compensator (TCSC) as an equivalent of the multi-module TCSC in a simple three-phase power transmission system. The load flow program is developed to analyze the steady-state characteristics of two-module TCSC system and to find the thyristor firing angles for the required real power flow. The stability calculation program is developed with Poincare mapping theory. Simulation studies of the TCSC power transmission system using EMTP are performed to evaluate the transient characteristics of two-module TCSC as a real power flow controller and to rpove the results of the load flow calculation and the stability analysis. In the process of the study, the operating-mode characteristics of two-module TCSC are evaluated and compared to those of single-module TCSC.

• Nuclear Engineering and Technology
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• 제55권9호
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• pp.3320-3325
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• 2023

The scaled water-cooled Reactor Cavity Cooling System (RCCS) experimental facility reproduces a passive safety feature to be implemented in Generation IV nuclear reactors. It keeps the reactor cavity and other internal structures in operational conditions by removing heat leakage from the reactor pressure vessel. The present work uses Flownex one-dimensional thermal-fluid code to model the facility and predict the experimental thermal-hydraulic behavior. Two representative steady-state cases defined by the bulk volumetric flow rate are simulated (Re = 2,409 and Re = 11,524). Results of the cavity outlet temperature, risers' temperature profile, and volumetric flow split in the cooling panel are also compared with the experimental data and RELAP system code simulations. The comparisons are in reasonable agreement with the previous studies, demonstrating the ability of Flownex to simulate the RCCS behavior. It is found that the low Re case of 2,409, temperature and flow split are evenly distributed across the risers. On the contrary, there's an asymmetry trend in both temperature and flow split distributions for the high Re case of 11,524.

• 천문학회보
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• 제46권2호
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• pp.64.1-64.1
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• 2021

Accretion flows around black holes and neutron stars emit high energy radiation with varying spectral and timing properties. Observed timing variations, both short and long-term, point to the existence of a mechanism, dictated by the flow dynamics, and not by the stellar surface or magnetic fields, that is common in both. Spectral energy distributions of multiple sources indicate that the Comptonization process, the dominant mechanism for changing states in X-ray, takes place inside the flow that has similar physical properties in both the objects. In a series of observational and numerical studies, we enquire about the following: 1. Is there a steady state configuration for accreting matter around black holes that can explain spectral and timing properties? 2. Could a similar formalism explain spectral and timing properties of accretion around neutron stars? 3. Could there be a generalized flow configuration for accreting matter around such compact objects? Furthermore, we show that a unified spectral model can be constructed based on the generalized flow configuration, common to black holes and neutron stars.

• 한국추진공학회지
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• 제2권2호
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• pp.47-55
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• 1998

중형항공기용 터보팬엔진의 설계점, 탈설계점에 대한 정상상태 및 천이상태 성능해석을 수행하였다. 정상상태 성능해석은 설계점으로 선정한 지상최대추력조건과 탈설계점으로 선정한 최대상승조건과 순항조건에서 수행하였으며 부분부하 성능해석 결과 저압압축기 회전속도 90％RPM에서 가장 연료소모율이 적어 경제적임을 확인하였다. 천이상태 성능해석은 각각의 비행조건에서 연료의 Step증가, Ramp증가, Ramp감소, Step 증가 후 Ramp 감소의 경우에 대하여 수행하였다. 천이상태의 성능해석을 위해서는 압축기와 터빈의 일에 대한 조합이 필요한데, 일의 조합방정식에 포함된 잉여 토오크의 적분에는 Modified Euler Method를 적용하였다. 천이상태 성능해석 결과, 모든 비행조건에서 Step 및 Ramp 증가의 경우 고압압축기의 터빈입구온도가 제한온도를 초과하며, 최대상승조건에서 연료를 Step 또는 5.5초 이내의 Ramp 증가시킬 경우 고압압축기에서 실속이 발생함을 확인하였다.

• Archives of Plastic Surgery
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• 제41권6호
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• pp.668-672
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• 2014

Background Negative-pressure wound therapy (NPWT) is believed to accelerate wound healing by altering wound microvascular blood flow. Although many studies using laser Doppler have found that NPWT increases perfusion, recent work using other modalities has demonstrated that perfusion is reduced. The purpose of this study was to investigate the influence of NPWT on tissue oxygenation of the foot, which is the most sensitive region of the body to ischemia. Methods Transcutaneous partial pressure of oxygen ($TcpO_2$) was used to determine perfusion beneath NPWT dressings of 10 healthy feet. The sensor was placed on the tarso-metatarsal area of the foot and the NPWT dressing was placed above the sensor. $TcpO_2$ was measured until it reached a steady plateau state. The readings obtained at the suction-on period were compared with the initial baseline (pre-suction) readings. Results $TcpO_2$ decreased significantly immediately after applying NPWT, but gradually increased over time until reaching a steady plateau state. The decrease in $TcpO_2$ from baseline to the steady state was 2.9 to 13.9 mm Hg (mean, $9.3{\pm}3.6$ mm Hg; $13.5{\pm}5.8%$; P<0.01). All feet reached a plateau within 20 to 65 minutes after suction was applied. Conclusions NPWT significantly decrease tissue oxygenation of the foot by 2.9 to 13.9 mm Hg. NPWT should be used with caution on feet that do not have adequate tissue oxygenation for wound healing.

• 대한전기학회:학술대회논문집
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• 대한전기학회 2005년도 추계학술대회 논문집 전력기술부문
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• pp.46-48
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• 2005

This paper discusses a probabilistic method for power system security assessment. The security analysis relates to the ability of the electric power systems to survive sudden disturbances such as electric short circuits or unanticipated loss of system elements. It consists of both steady state and dynamic security analyses, which are not two separate issues but should be considered together. In steady state security analysis including voltage security analysis, the analysis checks that the system is operated within security limits by OPF (optimal power flow) after the transition to a new operating point. Until now, many utilities have difficulty in including dynamic aspects due to computational capabilities. On the other hand. dynamic security analysis is required to ensure that the transition may lead to an acceptable operating condition. Transient stability, which is the ability of power systems to maintain synchronism when subjected to a large disturbance. is a principal component in dynamic security analysis. Usually any loss of synchronism may cause additional outages and make the present steady state analysis of the post-contingency condition inadequate for unstable cases. This is the reason for the need of dynamic studies in power systems. Probabilistic criterion can be used to recognize the probabilistic nature of system components while considering system security. In this approach. we do not have to assign any predetermined margin of safety. A comprehensive conceptual framework for probabilistic static and dynamic assessment is presented in this paper. The simulation results of the Western System Coordinating Council (WSCC) system compare an analytical method with Monte-Carlo simulation (MCS).

• 동력기계공학회지
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• 제22권1호
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• pp.34-40
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• 2018

In this paper, Performance of 1MW OTEC system using R32 with varying seawater temperature range is studied. Steady state cycle is designed and its output and generation efficiency were 1,014kW and 2.72%, respectively. Compared to dynamic cycle, system performance and change during long term operation is studied. The simulation is performed by decreasing surface seawater temperature from $29^{\circ}C$ to $25^{\circ}C$ with 20 minute of reaction time. Dynamic cycle with same condition applied to steady state cycle and it showed output and efficiency of 1,020kW and 2.75% respectively. Seawater temperature decreased from $29^{\circ}C$ and the vapor fraction of refrigerant decreased below 1 at $28^{\circ}C$. While the vapor fraction was above 1, the turbine output decreased by 0.017kW per second. After the seawater temperature reached $26.2^{\circ}C$, the turbine output decreased by 1.03kW per second. However, Driving the turbine below the saturation temperature caused the occurrence of surging and the influx of liquid refrigerant. When the liquid separator having a capacity of 1.0 m3 was used, the flow into the turbine was confirmed after 5 minutes from the first liquid refrigerant coming into the separator.

• 한국지반공학회논문집
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• 제26권4호
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• pp.47-58
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• 2010

불포화 흐름을 고려하지 않는 도해법 (Casagrande, 1961; Cedergren, 1997)과 근사식(Sakamoto, 1998)에 의한 중심코어형 사력댐 코어죤의 침투량은 실제 침투거동과는 차이가 있고, 저수위 변화에 대한 침투량 변화경향의 파악도 곤란하다. 본 연구에서는 소양강댐에 대한 2차원 정상상태 및 비정상상태 침투류해석으로 저수위, 포화투수계수, 불포화수리특성 변화에 대한 정상상태 침투량과 정상상태 도달시간에 미치는 영향요인을 분석하였다. 분석결과 침투량은 정규화된 선형 관계식으로 예측이 가능하고, 불포화 예측변수 n이 정상상태 침투량과 정상상태 도달시간에 가장 큰 영향을 주었다. 본 연구에서 제시한 예측기법은 복잡한 3차원 해석 및 여러 단면에 대한 2차원 해석을 수행하지 않고도 댐 설계자 및 안전관리 실무자가 중심코어형 사력댐의 침투량을 산정하여 실측치와의 정량적 분석에 손쉽게 이용할 수 있을 것으로 생각된다.