• 대한기계학회논문집B
• /
• 제40권5호
• /
• pp.329-337
• /
• 2016

연료집합체의 지지격자에 설치된 혼합날개는 난류 강화 기구로서 부수로 내부에서 선회류 또는 연료봉 간극사이에서 횡류를 발생시켜 대류열전달을 증진시키는 역할을 한다. 따라서 혼합날개의 기하학적인 형상 및 배열 형태는 혼합날개의 성능을 결정하는 중요한 인자이다. 본 연구에서는 OECD/NEA의 벤치마크 계산에서 활용된 분할 형태의 혼합날개가 장착된 $5{\times}5$ 연료집합체 내부에서의 유동분포 특성을 파악하기 위해 상용 전산유체역학 소프트웨어인 ANSYS CFX R.14를 사용하여 계산을 수행하였고, 계산결과를 MATiS-H 시험장치의 측정값과 비교하였다. 또한 분할 형태의 혼합날개 형상이 연료집합체 내부유동 형태에 미치는 영향에 대해 설명하였다.

• 대한기계학회논문집B
• /
• 제29권5호
• /
• pp.590-598
• /
• 2005

The present study has been conducted to investigate the effect of discrete ribs and rotation on heat/mass transfer characteristics in a two-pass square duct with $90^{\circ}-rib$ turbulators. The rib turbulator has a square cross section of 1.5 mm. The rib height-to-hydraulic diameter ratio $({e/D_{h})$ is 0.056, and the rib pitch-to-rib height ratio (p/e) is 10. The gap width is the same as the rib height. The rotation number ranges from 0.0 to 0.2 while Reynolds number is fixed to 10,000. In a stationary duct, the heat/mass transfer on the surfaces with discrete ribs is enhanced because the gap flow promotes local turbulence and flow mixing near the ribbed surface. In a rotating duct, the gap flow affects differently the heat/mass transfer on leading and trailing surfaces with discrete ribs. On the leading surface of the first pass, heat/mass transfer is increased due to the gap flow. On the trailing surface of the first pass, however, heat/mass transfer is decreased because the gap flow disturbs reattachment of main flow. The phenomenon, that is, the difference of heat transfer between the leading and the trailing surfaces is distinctly presented by rotation.

• 한국전산구조공학회논문집
• /
• 제15권1호
• /
• pp.59-68
• /
• 2002

원자로 내부구조물의 상부안내구조물집합체는 노심지지배럴과 내부배럴집합체와 함께 원통형의 실린더 구조이며, 유체의 난류하중과 펌프의 맥동하중으로 인한 유체유발하중을 수평방향으로 받는다. 본 논문에서는 이 유체유발하중에 대한 랜덤진동해석과 조화응답해석을 수행한 내용을 기술하였다. 이 해석을 위해 집중질량 보 요소 모델을 사용하였고, 랜덤하중과 펌프맥동하중으로 발생되는 동적응답특성을 평가하였다. 특히 원통형태의 상부안내구조물, 노심지지배럴, 내부배럴집합체 사이에서 형성되는 환형공간의 동수력 연성 효과를 고려하여 모델링 하였고, 상부안내구조물 안쪽에 설치되는 내부배럴집합체의 추가 영향을 검토하였다. 내부배럴집합체의 추가로 인한 하중조건별 최대동적응답은 구조물의 고유진동수에 영향을 받으며, 따라서 구조물의 최대동적응답은 여러 하중 조건별 동적해석 평가를 통해 보수적으로 구하여져야 한다.

• 천문학회보
• /
• 제43권1호
• /
• pp.43.1-43.1
• /
• 2018

Star formation in galaxies predominantly takes place in giant molecular clouds (GMCs). While it is widely believed that UV radiation feedback from young massive stars can destroy natal GMCs by exciting HII regions and driving their expansion, our understanding on how this actually occurs remains incomplete. To quantitatively assess the effect of UV radiation feedback on cloud disruption, we conduct a series of theoretical studies on the dynamics of HII regions and its role in controlling the star formation efficiency (SFE) and lifetime of GMCs in a wide range of star-forming environments. We first develop a semi-analytic model for the expansion of spherical dusty HII regions driven by the combination of gas and radiation pressures, finding that GMCs in normal disk galaxies are destroyed by gas-pressure driven expansion with SFE < 10%, while more dense and massive clouds with higher SFE are disrupted primarily by radiation pressure. Next, we turn to radiation hydrodynamic simulations of GMC dispersal to allow for self-consistent star formation as well as inhomogeneous density and velocity structures arising from supersonic turbulence. For this, we develop an efficient parallel algorithm for ray tracing method, which enables us to probe a range of cloud masses and sizes. Our parameter study shows that the net SFE, lifetime (measured in units of free-fall time), and the importance of radiation pressure (relative to photoionization) increase primarily with the initial surface density of the cloud. Unlike in the idealized spherical model, we find that the dominant mass loss mechanism is photoevaporation rather than dynamical ejection and that a significant fraction of radiation escapes through low optical-depth channels. We will discuss the astronomical.

• 한국해양공학회지
• /
• 제32권4호
• /
• pp.237-243
• /
• 2018

Recently, it was predicted that the size of offshore markets will grow as gas prices edge up. This paper presents experimental results for using strings as a suppression device on a circular cylinder and discusses the various data. A test model was used to investigate the role of strings by varying the thickness of the strings used to suppress a cylinder's lateral force taking into account the effect of turbulence promoted. A substantial amount of experimental data were taken from experiments performed on cylinders at Reynolds number up to a maximum value of $10^5$. The suppression of vortex shedding and a lateral force reduction of up to 70% were observed for the cylinder with strings.

• 한국전산유체공학회지
• /
• 제20권4호
• /
• pp.14-20
• /
• 2015

Without electirc power to cool down the hot reactor core, passive systems utilizing natural circulation are becoming a big specialty of recent neculear systems after the severe accident in Fukusima. When we consider the natural circulation in a pool, thermal mixing phenomena may start from single phase circulation and can continue to two phase condition. Since the CUPID code, which has been developed for two-phase flow analysis, can deal with the phase transition phenomena, the CUPID would be pertinent to natural convection problems in single- and two-phase conditions. Thus, the CUPID should be validated against single- and two-phase natural circulation phenomena. For the first step of the validation process, this study is focused on the validation of single-phase natural circulation. Moreover, the CUPID code solves the fluid properties by the relationship to pressure and temperature from the steam table considering non-condensable gas effects, so that the effects from variable properties are included. Simple square thermal cavity problems are tested for laminar and turbulent conditions against numerical and experimental data. Throughout the investigation, it is found that the variable properties can affect the flow field in laminar condition, but the effect becomes weak in turbulence condition, and the CUPID code implementing steam table is capable of analyzing single phase natural circualtion phenomena.

• 한국전산유체공학회:학술대회논문집
• /
• 한국전산유체공학회 2011년 춘계학술대회논문집
• /
• pp.302-310
• /
• 2011

A fluid in an enclosure can be heated by electric heating, chemical reaction, or fission heat. In order to remove the volumetric heat of the fluid, the walls surrounding the enclosure must be cooled. In this case, a natural convection occurs in the pool of the fluid, and it has a dominant role in heat transfer to the surrounding walls. It can augment the heat transfer rates tens to hundreds times larger than conductive heat transfer. The heat transfer by a natural convection in a regular shape such as a square cavity or semi-circular pool has been studied experimentally and numerically for many years. A pool of an inverted triangular shape with 10 degree inclined bottom walls has a good cooling performance because of enhanced boiling critical heat flux (CHF) compared to horizontal downward surface. The coolability of the pool is determined by comparing the thermal load from the pool and the maximum heat flux removable by cooling mechanism such as radiative or boiling heat transfer on the pool boundaries. In order to evaluate the pool coolability, it is important to correctly expect the thermal load by a natural convection heat transfer of the pool. In this study, turbulence models with modifications for buoyancy effect were validated for unsteady natural convections by volumetric heating. And natural convection in the triangular pool was evaluated by using the models.

• Journal of Power Electronics
• /
• 제16권2호
• /
• pp.685-694
• /
• 2016

This paper presents a reliability assessment model for the power semiconductors used in wind turbine power converters. In this study, the thermal loadings at different timescales of wind speed are considered. First, in order to address the influence of long-term thermal cycling caused by variations in wind speed, the power converter operation state is partitioned into different phases in terms of average wind speed and wind turbulence. Therefore, the contributions can be considered separately. Then, in regards to the reliability assessment caused by short-term thermal cycling, the wind profile is converted to a wind speed distribution, and the contribution of different wind speeds to the final failure rate is accumulated. Finally, the reliability of an actual power converter semiconductor for a 2.5 MW wind turbine is assessed, and the failure rates induced by different timescale thermal behavior patterns are compared. The effects of various parameters such as cut-in, rated, cut-out wind speed on the failure rate of power devices are also analyzed based on the proposed model.

• 한국해안해양공학회지
• /
• 제19권5호
• /
• pp.476-483
• /
• 2007

이어도에 건설된 다목적 해양과학기지에서는 주위 대류 및 해류의 데이터 수집을 통해 기상, 어장예보 등의 기초 자료를 제공하는 것을 한 가지 목적으로 한다. 그런데 이어도 기지 구조물에 의한 해수 유동의 변화는 과학기지의 관측 데이터에 영향을 주어 정확한 자료 수집을 어렵게 하므로, 이어도 기지 구조물이 주위 유동에 미치는 영향을 분석하고 유동 정보 관측을 평가/보정하는 방법에 대한 연구가 필요하다. 본 연구에서는 이를 위한 기초연구로, 해류와 구조물 사이의 상호 작용을 모사 할 수 있는 알고리즘을 연구, 적용방법을 논의한다. 그 결과, 3차원 전산유체역학을 이용한 수치해석을 통해 이어도 기지 구조물 및 수중암초가 주위 유동에 미치는 영향을 연구하고 정확한 데이터 측정방법을 제안한다.

• 한국항공우주학회지
• /
• 제34권4호
• /
• pp.63-69
• /
• 2006

하이브리드 로켓 연료의 연소율 향상을 위해서 스월 유동을 이용한 방법과 나선형 그레인 방법의 두 가지 방법을 사용하여 연소율 증가에 대한 실험 연구를 수행하였다. 스월 유동을 적용하기 위해서 두 가지의 인젝터를 사용하였으며 스월 수 3.61인 인젝터를(Type II) 사용할 때 더 높은 연소율 증가를 나타내었다. 그러나 두 가지 인젝터 모두에서 공통으로 연료 앞부분에서 연소가 집중적으로 발생하는 현상이 발생하였다. 나선형 그레인에 의한 연소율 증가를 실험하기 위하여 피치 6과 피치 100인 두 종류 연료를 사용하였다. 스월 유동이 없는 인젝터를 사용할 경우 피치 6 그레인에서는 강한 난류의 발생에 따른 연소율 향상이 더 크게 일어나는 것을 확인할 수 있었다. 그러나 스월 인젝터와 나선형 그레인을 동시에 적용하여 실험한 결과에 의하면, Type II 인젝터와 피치 100 그레인를 적용할 경우에 가장 높은 연소율 증가를 이루었다. 이것은 인젝터에서 발생한 스월 유동이 나선형 그레인을 통해 연료의 출구까지 유지되었기 때문인 것으로 판단된다.