• 한국해양환경ㆍ에너지학회지
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• 제11권2호
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• pp.92-97
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• 2008

일반적으로 해수가 강으로 침입하거나 담수가 해양으로 유입을 하면 두 유체간의 밀도차에 의한 밀도류의 성질을 가진다. 하지만, 염분과 온도가 동시에 밀도의 차를 결정하는 경우, 특히 이 중 하나가 중력과 불안정한 수직 분포를 갖게되면 이중확산이 혼합을 결정하는 중요한 요인이 된다. 본 연구에서는 고온, 고염의 수괴가 저온, 저염의 수괴로 진입할 때 기계적 혼합과 이중확산에 의한 대류가 혼합에 미치는 상대적 영향을 비교 연구하였다. 실험실에서 차갑고 염도가 없는 주 흐름에 온도가 높고 염분이 높은 밀도류를 조심스럽게 방류시켜 정상상태의 염수침입 형태를 유지하였다. 수평방향으로 따라 밀도율(Turner 1979)이 15정도 되면 온도와 염분의 유출입량비율이 가파르게 변화하였고, 전체적으로 방출구 부분에서 이중확산의 영향이 강하게 나타나게 되며 이 경우에는 총 방출량도 함께 증가하였다. 아울러 밀도율이 낮은 경우 밀도율이 높은 경우에 비해서 약 $6{\times}10$배의 높은 혼합율을 보였다.

• 대한기계학회논문집
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• 제18권12호
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• pp.3297-3304
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• 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.

• 대한기계학회논문집B
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• 제21권11호
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• pp.1422-1436
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• 1997

Numerical investigations were conducted to study the convective phenomena of an initially stably stratified salt water solution with lateral heating in a uniformly rotating annulus. The method of investigation is the finite difference analysis of the basic conservation equation for an axisymmetric, unsteady, double-diffusive convection and calculation is made for R $a_{\eta}$=2*10$^{5}$ and Ta=10$^{7}$ ~ 2.5*10$^{8}$ . Formation of layered flow structure, merging process of layers, the corresponding temperature and concentration distributions, Nu variation with time are examined. Numerical results show that in each layer, the temperature profile looks 'S'-shaped and the concentration profile is uniform due to the convective mixing. At the interface between adjacent layers, the temperature changes smoothly but the concentration changes rapidly. As the effect of the rotation increases, the generation of rolls at hot wall, the formation and merging of layers are delayed. The average Nu shows the trend of conduction heat transferees the effect of the rotation increases.n increases.

• 대한기계학회논문집
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• 제19권7호
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• pp.1731-1740
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• 1995

Experimental investigations have been made to study the double-diffusive nature of convection of an initially stratified salt-water solution due to heating from below in a rotating cylindrical cavity. The objective is to examine the flow phenomena and the heat transfer characteristics according to the changes in temperature gradient, concentration gradient and rotating velocity of cavity. Thermal and solutal boundary conditions at side wall are adiabatic and impermeable, respectively. The top and bottom plate are maintained each at constant temperature and concentration. The cavity is put into a state of solid body rotation. Like the stationary case, the types of initially-formed flow pattern are classified into three regimes depending on the effective Rayleigh number and Taylor number; stagnant flow regime, single mixed-layer flow regime and successively formed multi-mixed layer flow regime. At the same effective Rayleigh number, the number of initially-formed mixed layer and its growth rate decrease as the effect of rotation increases. The temperature and concentration profiles are both uniform in each layer due to convective mixing in the layered-flow regime, but look both liner 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.

• 한국가시화정보학회지
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• 제15권1호
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• pp.41-46
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• 2017

Control of mixing and transport processes are the key areas that can be benefited by understanding the hydrodynamics in gas-liquid two-phase flows. In particular, the enhanced bubble-induced liquid-phase mixing is known to be a function of void fraction distribution, gas phase velocity and so on. To further our insight on the characteristics of the liquid-phase mixing induced by the bubbles, in the present study, we experimentally investigate the mixing performance of a rectangular bubble column while changing the void fraction from 0.006 to 0.075%. A shadowgraphy technique is used to measure the gas-phase properties such as void fraction and size/velocity of bubbles. On the other hand, we use dye visualization with low diffusive buoyant dye to directly measure the level of mixing. Finally, we confirm that the time taken for full mixing scales with the inverse of volume void fraction.

• 대한기계학회논문집
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• 제19권7호
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• pp.1720-1730
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• 1995

Finite-difference analysis was conducted to study the natural convection of a stably stratified salt-water solution in an annulus due to lateral heating. The main purpose of this study is to examine in detail the multi-layered flow structure. Calculation was thus made for R $a_{\eta}$=2*10$^{5}$ and 6.5*10$^{5}$ . Formation of layered flow structure, merging process of layers, the corresponding temperature and concentration distributions, Nusselt number variations with time are examined. Numerical results show that in each layer, the temperature profile looks 'S`-shaped and the concentration profile is uniform due to the convective mixing. The formation of the roll and the layer is governed by natural convection due to the temperature gradient and the merging process of the layer by diffusion of the concentration.ation.

• 한국해안해양공학회지
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• 제18권1호
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• pp.43-52
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• 2006

해수소통을 전제한 경우 새만금호 내측의 염분변화가 만경강의 평수량 및 홍수량 유입에 따라 혼합 확산되는 것을 평가하고, 내부 개발이후 배수수문을 통한 새만금호소수 방류시 관리수위별로 저염수가 외해에 미치는 영향범위를 평가하기 위한 수치모형실험이 실시되었다. 해석에는 2차원 ADCIRC 및 3차원 TIDE3D모형과 3개 연직층을 고려한 3차원 CE-QUAL-ICM모델이 이용되었다. 내부의 확산 모의결과는 시간이 지남에 따라 외해에서 유입되는 염수가 점차 혼합확산되며, 최소 1개월 이상 경과되어야 만경호측에 외해수가 혼합되고, 5개월 지속되어야만 동진호측까지 혼합된다. 호내측의 수위를 관리하기 위해 신시 또는 가력수문을 개방하면 유속이 지수적으로 감소되나 갑문으로부터 전면 10km 이상까지 0.5m/s의 유동장이 형성된다. 이러한 결과는 저염수 방류로 인한 영향이 주기적으로 낙조시에 발생되어 새만금 방조제 전면의 해수순환과 유동 및 생태환경에 적지 않은 영향을 미칠 개연성을 제시하는 것으로 해석된다.

• Advances in aircraft and spacecraft science
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• 제4권1호
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• pp.21-35
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• 2017

Hydrogen peroxide is being considered as a monopropellant in micropropulsion systems for the next generation of miniaturized satellites ('nanosats') due to its high energy density, modest specific impulse and green characteristics. Efforts at the University of Vermont have focused on the development of a MEMS-based microthruster that uses a novel slug flow monopropellant injection scheme to generate thrust and impulse-bits commensurate with the intended micropropulsion application. The present study is a computational effort to investigate the initial decomposition of the monopropellant as it enters the catalytic chamber, and to compare the impact of the monopropellant injection scheme on decomposition performance. Two-dimensional numerical studies of the monopropellant in microchannel geometries have been developed and used to characterize the performance of the monopropellant before vaporization occurs. The results of these studies show that monopropellant in the lamellar flow regime, which lacks a non-diffusive mixing mechanism, does not decompose at a rate that is suitable for the microthruster dimensions. In contrast, monopropellant in the slug flow regime decomposes 57% faster than lamellar flow for a given length, indicating that the monopropellant injection scheme has potential benefits for the performance of the microthruster.

• 한국분무공학회지
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• 제15권3호
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• pp.131-139
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• 2010

Gas motion within the engine cylinder is one of the major factors controlling the fuel-air mixing and combustion processes in diesel engines. In this paper, a special swirl-chamber is designed and applied to a DI (direct injection) diesel engine to generate a strong swirl motion thus enhancing gas motion. Compression, combustion and expansion strokes of this DI diesel engine with the swirl-chamber have been simulated by CFD software. The simulation model was first validated through comparisons with experimental data and then applied to do the simulation of the spray and combustion process. The velocity and temperature field inside the cylinder showed the influences of the strong swirl motion to spray and combustion process in detail. Cylinder pressure, average temperature, heat release rate, total amount of heat release, indicated thermal efficiency, indicated fuel consumption rate and emissions of this DI diesel engine with swirl-chamber have been compared with that of the DI diesel engine with $\omega$-chamber. The conclusions show that the engine with swirlchamber has the characteristics of fast mixture formulation and quick diffusive combustion; its soot emission is 3 times less than that of a $\omega$-chamber engine; its NO emission is 3 times more than that of $\omega$-chamber engine. The results show that the DI diesel engine with the swirl-chamber has the potential to reduce emissions.

• 에너지공학
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• 제15권1호
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• pp.1-7
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• 2006

원자력발전소의 다차원 이상 유동 현상을 적절히 모사하기 위해 일차원 계통해석 코드에 삼차원 유동모델을 적용하였다. 그 중 다차원모델에 새롭게 적용된 이상 난류모델을 검증하기 위해 사각 slab 내부의 단상유동을 계산하여 상용 CFD 코드의 계산결과와 비교하였다. 그 결과 단상유동의 경우 난류 모델의 계산이 적절히 수행됨을 알 수 있었다. 또한 다차원 이상 유동 계산을 검증하기 위해서 RPI에서 수행된 물-공기 다차원 실험의 기포율 분포를 비교하였다. 그 결과 다차원 모델의 이상 유동 계산을 위해서는 일차원 기반의 유동양상 맵 중 수평 분리 유동양상이 제거되어야 함을 알 수 있었다. 이와 같이 유동양상 맵을 수정하여 모사한 계산결과가 실험에서 측정된 기포율의 경향을 잘 따르는 것으로 계산되었다.