• 한국연소학회:학술대회논문집
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• 한국연소학회 2014년도 제49회 KOSCO SYMPOSIUM 초록집
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• pp.59-62
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• 2014

Predicting coal combustion by computational fluid dynamics (CFD) requires a combination of complicated flow and reaction models for turbulence, radiation, particle flows, heterogeneous combustion, and gaseous reactions. There are various levels of models available for each of the phenomena, but the use of advanced models are significantly restricted in a large-scale boiler due to the computational costs and the balance of accuracy between adopted models. In this study, the influence of coal devolatilization model and turbulent mixing rate was assessed in CFD for a commercial boiler at 500 MWe capacity. For coal devolatilization, two models were compared: i) a simple model assuming single volatile compound based on proximate analysis and ii) advanced model of FLASHCHAIN with multiple volatile species. It was found out that the influence of the model was observed near the flames but the overall gas temperature and heat transfer rate to the boiler were very similar. The devolatilization rate was found not significant since the difference in near-flame temperature became noticeable when it was multiplied by 10 or 0.1. In contrast, the influence of turbulent mixing rate (constant A in the Magnussen model) was found very large. Considering the heat transfer rate and flame temperature, a value of 1.0 was recommended for the rate constant.

• 물과 미래
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• 제29권4호
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• pp.187-198
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• 1996

난류전단 흐름에서의 비정상 수평 선오염원의 확산에 관한 수치모형을 개발하였다. 계산기법으로는 비정상 이송확산 방정식을 종방향 이송 및 연직방향 확산으로 분리하고, 이들 방정식을 방시간 간격에 대하여 번갈아 계산하는 단계분리 유한차분기법을 사용하였다. 종방향 이송방정식에 대해서는 Holly-Preissmann 기법을, 연직방향 확산방정식에 대해서는 Crank-Nicholson 기법을 각각 적용하였다. 개발된 모형을 난류전단 흐름에서의 정상 수평 선오염원의 확산 문제에 적용하여 계산결과를 반해석해와 비교함으로써 모형을 검증하였다. 또한 난류전단 흐름내로 순간적으로 방류된 면오염원의 확산문제에 계산모형을 적용하였다. 마찰계수에 대한 민감도 분석 결과, 동일한 무차원 시각에서의 혼합 정도는 마찰계수에 관계없이 거의 일정한 것으로 나타났다. 또한 동일한 정도의 혼합상태에 도달하는 데 소요되는 유하거리는 마찰계수의 제곱근에 반비례함을 알 수 있었다.

• 오토저널
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• 제11권3호
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• pp.37-48
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• 1989

The effects of burnt gas mixing, which can be caused by turbulent eddy mixing, on the interaction and extinction characteristics of premixed flames are analyzed using large activation energy asymptotics adopting counterflow as a model problem. The results show that the burnt gas mixing, preferential diffusion and heat loss affect the fuel consumption rate, flame temperature and the oxidizer concentration at the flame which influence the flame behavior and the extinction characteristics.

• 대한기계학회논문집B
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• 제22권3호
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• pp.280-293
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• 1998

Numerical study on turbulent and mean structures of a turbulent boundary layer with longitudinal and spanwise pressure gradient is carried out by using Reynolds-stress-model (RSM). The existence of pressure gradient in a turbulent boundary layer causes the skewing or divergence of rates of strain, which contributes to production of turbulent kinetic energy. Also, this augmentation of production due to extra rates of strain can increase the turbulent mixing and cause the anisotropy of turbulent intensities in the outer layer. This paper uses the Reynolds Stress Model to capture anisotropy of turbulent structures effectively and is devoted to compare the results computed by using RSM and the standard k-.epsilon. model with experimental data. It is concluded that the RSM can produce the more accurate predictions for capturing the anisotropy of turbulent structure than the standard k-.epsilon. model.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2003년도 추계학술대회
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• pp.986-991
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• 2003

A turbulent wake generated by a cylinder in nozzle contraction affects to the jet flow characteristics. In this study, a computational work to investigate the effect of the turbulent wake on two-dimensional subsonic jet was carried out with three different kinds of nozzle. Computations are applied to the two-dimensional unsteady, Navier-Stokes equations. Several kinds of turbulent models and wall functions are employed to validate the computational predictions. It was known that the wake flow enhanced the spread of the jet flow, compared with no wake flow condition. It was also found that the jet core is shortened by the wake flow developed from a control cylinder.

• 한국연소학회지
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• 제14권2호
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• pp.32-41
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• 2009

The transported probability density function model combined with the consistent finite volume (FV) method has been applied to simulate the turbulent bluff-body reacting flows. To realistically account for the non-isotropic turbulence effects on the turbulent bluff-body reacting flows, the present PDF transport approach is based on the joint velocity- turbulent frequency-composition PDF formulation. The evolution of the fluctuating velocity of a particle is modeled by a simplified Langevin equation and the particle turbulence frequency is represented by the modified Jayesh - Pope model. Effects of molecular diffusion are represented by the interaction by exchange with the mean (IEM) mixing model. To validate this hybrid FV/PDF transport model, the numerical results are compared with experimental data for the turbulent bluff-body reacting flows.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2004년도 춘계학술대회
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• pp.1819-1824
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• 2004

The common method to improve heat transfer in Nuclear fuel rod bundle is install a mixing vane in space grid. The previous split mixing vane is guides cooling water to swirl flow in sub-channel of fuel assembly. But, this swirl flow decade rapidly after mixing vane and the effect of enhancing the heat transfer vanish behind this short region. The large scale secondary vortex flow was generated by rearranging the inclined angle direction of mixing vanes to the coordinated directions. This LSVF mixing vanes generate the most strong secondary flow vortices which maintain about 35 $D_H$ after the spacer grid and the streamwise vorticity in subchannel with LSVF mixing vane sustain two times more than that in subchannel with split mixing vane. The turbulent kinetic energy and the Reynolds stresses generated by the mixing vanes have nearly same scales but maintain twice more than previous type.

• Nuclear Engineering and Technology
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• 제24권3호
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• pp.263-273
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• 1992

핵연료 집합체의 속도분포, 압력강하는 열수력 설계와 안전해석에 중요하다. 본 실험적 연구의 목적은 봉다발 지지 격자 하류에서의 수력학적 혼합을 고찰하는데 있다. 이 연구에서 가압경수로형 5X5 봉다발 부수로의 상세한 수력학적 특성들을 1차익 He-Ne LDV를 이용하여 측정하였다. 축방향 유속, 난류강도와 압력강하를 주로 측정하였고 LDV의 정렬을 조정하여 측방향의 유속, 난류강도, Reynolds 전단응력 등도 역시 측정하였다. 봉다발의 마찰계수와 지지 격자의 손실계수는 측정된 압력 강하로부터 평가하였다. 서로 다른 종류의 지지 격자의 수력학적 혼합성능을 이웃하는 부수로 간에서의 난류 횡류 혼합률을 예측함으로써 고찰할 수 있었다.

• Journal of the Korean Wood Science and Technology
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• 제25권3호
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• pp.58-65
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• 1997

This research was carried out to evaluate the effect of process variables on mechanical properties of the wood fiber-thermoplastic fiber composites by turbulent air mixing method. The turbulent air mixer used in this experiment was specially designed in order to mix wood fiber and thermoplastic polypropylene or nylon 6 fiber, and was highly efficient in the mixing of relatively short plastic fiber and wood fiber in a short time without any trouble. The adequate hot - pressing temperature and time in our experimental condition were $190^{\circ}C$ and 9 minutes in 90% wood fiber - 10% polypropylene fiber composite and $220^{\circ}C$ and 9 minutes in 90% wood fiber 10% nylon 6 fiber composite. Both in the wood fiber - polypropylene fiber composite and wood fiber- nylon 6 fiber composite, the mechanical properties improved with the increase of density. Statistically, the density of composite appeared to function as the most significant factor in mechanical properties. Within the 5~15% composition ratios of polypropylene or nylon 6 fiber to wood fiber, the composition ratio showed no significant effect on the mechanical properties. Bending and tensile strength of composite, however, slightly increased with the increase of synthetic fiber content. The increase of mat moisture content showed no significant improvement of mechanical properties both in wood fiber - polypropylene fiber composite and wood fiber nylon 6 fiber composite. Wood fiber - nylon 6 fiber composite was superior in th mechanical strength to wood fiber-polypropylene fiber composite, which may be related to higher melt flow index of nylon 6 fiber(22g/10min) than of polypropylene fiber(4.3g/10min).

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