• Proceedings of the KSME Conference
• /
• 2005.11a
• /
• pp.13.2-13.2
• /
• 2005

• Journal of computational fluids engineering
• /
• v.16 no.3
• /
• pp.66-74
• /
• 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.

• 한국전산유체공학회:학술대회논문집
• /
• 2005.10a
• /
• pp.61-66
• /
• 2005

Natural convection flows in a cubical, air-filled cavity that has one pair of opposing faces isothermal at different temperatures, Th and Tc, the remaining faces having a linear variation from Tc to Th are numerically simulated by a new solution code(PowerCFD) using unstructured cell-centered method. Solutions are obtained for configurations with a Rayleigh number as high as 105 and three inclination angles ${\theta}$ of the isothermal faces from horizontal: namely ${\theta}=0$, 45 and $90^{\circ}$. Interesting features are presented in detail and comparisons are made with benchmark solutions and experimental results found in the literature. It is found that the code is capable of producing accurately the nature of the laminar convection in a cubical, air-filled cavity with differentially heated walls.

• Journal of Biosystems Engineering
• /
• v.10 no.1
• /
• pp.24-38
• /
• 1985

경사진 밀폐 공간에서 마주 보는 두 벽면의 온도 차로 인하여 발생되는 자연 대류 현상은 여러 공학 분야에서 볼 수 있는 중요한 열전달 현상으로서, 최근 들어 평판형 태양열 집열기를 설계하려는 사람들에게 많은 관심의 대상이 되고 있다. 평판형 태양열 집열기의 경우 덮개판으로 부터의 대류 열손실을 감소시킴으로서 집열 효율을 높일 수 있을 뿐만 아니라 사용목적에 따라 소형 집열기를 제작할 수 있어 경제적으로 유리하게 될 것이다. 밀폐된 공간에서 최초에 정지 상태에 있는 얇은 유체층을 하부에서 가열시켜 주면 열팽창 현상이 일어나고, 이것에 의한 부력이 점도나 열전도도 등의 안정화 요인을 극복할 수 있을 정도로 커지면 System이 불안정하게 되어 자연 대류 현상이 수반되며 이 때문에 열전달율이 급격히 증가하게 된다. 이러한 현상의 지배 방정식은 연립 비선형 편미분 방정식으로 특수한 경계 조건외에는 일반적으로 해석적 해를 구하기가 어렵기 때문에 실험적 연구가 많이 실시되어 왔고 이들 결과의 대부분은 전반적인 열전달 특성치만을 구하는데 집중되어 왔다. 본 연구에서는 수치 해석법의 하나인 유한 차분법을 도입하여 이차원으로 가정한 경사진 평판형 밀폐 공간에서의 자연 대류 현상의 지배 방정식을 유한 차분화시켜, $$2.74{\times}10^3\leq_-Gr\leq_-2.0{\times}10^6$$, Pr=0.73, $$15^{\circ}\leq_-a\leq_-150^{\circ}$$, 종횡비는 1, 2, 3, 5, 9에 대하여 정상 상태에서의 해를 구하면서 수치적으로 실험하였다. 본 연구에서 얻어진 결론을 요약하면 다음과 같다. (1) 해석적으로 구하기 어려운 경사진 밀폐 공간에서 자연대류현상의 지배 방정식을 유한 차분법으로 해결할 수 있으며, 대류항과 확산항을 따로 고려한 유한차분법이 효과적임을 확인하였다. (2) 저온과 고온 벽면에서의 온도를 각각 균일하게 놓고 단변으로 이루어진 벽면은 완전히 절연되어 있는 경우에 대하여 수치해를 구한결과, 이전의 해석적 및 실험적 결과와 일치하였으며, 시간의 경과에 따른 온도 및 유선의 변화를 현상학적으로 관찰할 수 있었다. (3) 평균 열전달 계수에 미치는 경사각의 효과를 살펴본 결과 종횡비가 1 인 경우 경사각이 $45^{\circ}$에서, 종횡비가 2, 3, 5, 9인 경우 경사각이 $60^{\circ}$에서 각각 평균 열전달 계수 최대치가 나타났다. (4) Ra수(Rayleigh number) 가 증가될수록, 경사각에 상관없이 평균 열전달 계수도 증가되었다. 한편 Ra수 및 경사각의 변화에 따라 종횡비가 증가될수록 평균 열전달 계수는 경사각이 $90^{\circ}$인 경우를 제외하고는 감소됨을 볼 수 있었다. 경사각이 $90^{\circ}$인 경우, 평균 열전달 계수는 종횡비가 2 인 곳에서 최대치를 얻을 수 있었으며, 종횡비가 계속 증가될수록 평균 열전달 계수는 점차 감소되어짐을 볼 수 있었다.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
• /
• v.18 no.9
• /
• pp.722-728
• /
• 2006

Natural convection flows in a cubical air-filled slanted cavity that has one pair of opposing faces isothermal at different temperatures, $T_h\;and\;T_c$, respectively, the remaining four faces having a linear variation from $T_c\;toT_h$ are numerically simulated by a solution code (PowerCFD) using unstructured cell-centered method. Special attention is paid to three-dimensional flow and thermal characteristics according to a new orientation (diamond type) for the cubical-cavity benchmark problem in natural convection. Comparisons of the average Nusselt number at the cold face are made with experimental benchmark solutions found in the literature. It is found that the code is capable of producing accurately the nature of the laminar convection in a cubical air-filled slanted cavity with differentially heated walls.

• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.33 no.6
• /
• pp.435-442
• /
• 2009

Three-dimensional heat transfer characteristics for natural convection flows are numerically investigated in the doubly-inclined cubical-cavity according to the variation of a newly defined orientation angle �� of the hot wall surface from horizontal plane at moderate Rayleigh numbers. Numerical simulations of laminar flows are conducted in the range of Rayleigh numbers($10^4{\leq}Ra{\leq}10^5$) and $0^{\circ}{\leq}{\alpha}90^{circ}$ with a solution code(PowerCFD) employing unstructured cell-centered method. Comparisons of the average Nusselt number at the cold face are made with benchmark solutions and experimental results found in the literature. It is found that the average Nusselt number at the cold wall has a maximum value around the specified orientation ${\alpha}$ at each Rayleigh number. Special attention is also paid to three-dimensional thermal characteristics in natural convection according to new orientation angles at Ra��= $1{\times}10^5$, in order to investigate a new additional heat transfer characteristic found in the range of above Ra = $6{\times}10^4$.

• Transactions of the Korean Society of Mechanical Engineers
• /
• v.18 no.12
• /
• pp.3297-3304
• /
• 1994

Experimental investigation have been made to study the double-diffusive nature of convection of an initially stratified salt-water solution heated from below in a cylindrical cavity. The objective is to examine the process of mixed-layer formation, the flow phenomena, the heat transfer characteristics, and temperature and concentration distribution according to the changes in the effective Rayleigh number based on the reference height which represents the relation of temperature and concentration gradient. The types of initially formed flow pattern are categorized in three regimes depending on the effective Rayleigh number ; stagnant flow regime, single mixed-layer flow regime and successively formed multiple mixed-layer flow regime. The temperature and concentration profiles are both uniform in each layer due to convective mixing in the layered flow regime, but both linear in stagnant flow regime and single mixed-layer flow regime. At the interface between adjacent layers, the temperature changes smoothly but the concentration changes rapidly. The layers expand by diffusion of concentration through the interface along with its random fluctuation.

• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.30 no.4 s.247
• /
• pp.337-342
• /
• 2006

Natural convection flows in a cubical air-filled cavity that has one pair of opposing faces isothermal at different temperatures, $T_h\;and\;T_c$, respectively, the remaining four faces having a linear variation from $T_c\;to\;T_h$ are numerically simulated by a solution code(PowerCFD) using unstructured cell-centered method. Special attention is paid to three-dimensional flow and thermal characteristics according to the variation of inclination angle $\theta$ of the isothermal faces from horizontal: namely $\theta=0^{\circ},\;15^{\circ},\;30^{\circ},\;45^{\circ},\;50^{\circ},\;60^{\circ},\;75^{\circ}\;and\;90^{\circ}$. Comparisons of the average Nusselt number at the cold face are made with experimental benchmark results found in the literature. It is demonstrated that the average Nusselt number at the cold face has a maximum value around the inclination angle of $50^{\circ}$. It is also found that the code is capable of producing accurately the nature of the laminar convection in a cubical air-filled cavity with differentially heated walls.

• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.31 no.12
• /
• pp.1002-1008
• /
• 2007

Natural convection flows in a doubly-inclined cubical air-filled cavity are numerically simulated by a solution code(PowerCFD) using unstructured cell-centered method. For a physical realizability, the cavity has one pair of opposing isothermal faces at different temperatures, $T_h\;and\;T_c$, respectively, the remaining four faces having a linear variation from $T_c\;to\;T_h$. The paper redefines a new doubly-inclined orientation for the cubical-cavity benchmark problem. Special attention is paid to three-dimensional thermal characteristics in natural convection according to the new orientation at $Ra=4\times10^4$. Comparisons of the average Nusselt number at the cold face are made with benchmark solutions and experimental results found in the literature. It is found that the average Nusselt number at the cold face has a maximum value at the doubly-inclined angle ranging from $40^{\circ}\;to\; 45^{\circ}$ We also report the effect of new orientation on the type of temperature structure in a doubly-inclined cubical-cavity.

• 한국전산유체공학회:학술대회논문집
• /
• 2011.05a
• /
• 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.