• Proceedings of the Korean Vacuum Society Conference
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• 2016.02a
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• pp.227.2-227.2
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• 2016

환형 전자 빔이 진행됨에 따라 표면파의 전단 유동에 의해 발생 및 진행되는 diocotron instability(다이오코트론 불안정성)를 안정시키기 위해, 나선형 자기장이 인가된 상황을 2차원 PIC 시뮬레이션을 통해 구현하고 그 효과에 대한 조사가 수행되었다. 나선형 자기장은 2차원 단면 상에서 회전하는 자기장으로 표현되었고, 이에 대해서 자기장이 회전하는 각도와 회전 주기, 회전 방향을 변수로 하여 그 효과를 나타내었다. 결과적으로, 자기장이 회전 하는 방향과 전자 빔이 회전하는 방향이 반대가 되었을 때, diocotron instability가 진정되는 효과를 얻었으며 이는 불안정성의 근원인 전단 유동을 완화시킴으로써 얻어진 결과임이 확인되었다. 이 때, 동반되는 밀도 분산 현상에 대해서도 고려가 되었고, 결론적으로 인가되는 나선형 자기장에 대하여 가장 적절한 조건에 대한 조사가 수행되었다.

• Korean Chemical Engineering Research
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• v.46 no.1
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• pp.56-62
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• 2008

Characteristics of heat transfer were investigated in a three-phase swirling fluidized bed whose diameter was 0.102 m and 2.5 m in height. Effects of gas and liquid velocities, particle size and liquid swirling ratio ($R_S$) on the immersed heater-to-bed overall heat transfer coefficient were examined. The heat transfer characteristics between the immersed heater and the bed was well analyzed by means of phase space portraits and Kolmogorov entropy(K) of the time series of temperature difference fluctuations. The phase space portraits of temperature difference fluctuations became stable and periodic and the value of Kolmogorov entropy tended to decrease with increasing the value of liquid swirling ratio from 0.1 to 0.4. The value of Kolmogorov entropy exhibited its minimum with increasing liquid swirling ratio. The value of overall heat transfer coefficient (h) showed its maximum with the variation of liquid velocity, bed porosity or liquid swirling ratio, but it increased with increasing gas velocity and particle size. The value of K exhibited its maximum at the liquid velocity at which the h value attained its maximum. The overall heat transfer coefficient and Kolmogorov entropy were well correlated in terms of dimensionless groups and operating variables.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.34 no.4
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• pp.63-69
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• 2006

Experimental tests have been done with swirl injector and helical grain configuration to increase the regression rate of hybrid rocket solid fuel. Two types of injector were designed to evaluate the swirl effect of oxidizer stream on the increase in the regression rate. Results showed Type II injector with swirl number of 3.61 induced the better regression rate than Type I injector. Meanwhile, fuels with two different pitch number of 6 and 100 were used to analyzes the flow characteristics on the enhancement of regression rate. Test with fuels of pitch 6 showed better increase in the regression rate than in the pitch 100 when no swirler was imposed. This is due to the generation of strong turbulences in the oxidizer stream along the pitch 6 configuration. However, the regression rate could be increased further in the fuel with pitch 100 than with pitch 6 when swirl flow was imposed by Type II injector. This result implied that the fuel with pitch 100 could take a role of sustainer of the imposed swirl by swirler II instead of turbulence generator.

• Korean Chemical Engineering Research
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• v.44 no.5
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• pp.505-512
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• 2006

Characteristics of particle holdup and heat transfer were investigated in a liquid-particle swirling fluidized bed whose diameter was 0.102 m and 2.5 m in height. Effects of liquid velocity, particle size and swirling liquid ratio($R_s$) on the particle holdup and immersed heater-to-bed overall heat transfer coefficient were examined. The particle holdup increased with increasing particle size and swirling liquid ratio but decreased with increasing liquid velocity.The local particle holdup was relatively high in the region near the heater when the $R_s$ value was 0.1~0.3, but the radial particle holdup was almost uniform when the $R_s$ value was 0.5, whereas, when the $R_s$ value was 0.7, the local particle holdup was relatively low in the region near the heater. The heat transfer characteristics between the immersed heater and the bed was well analyzed by means of phase space portraits and Kolmogorov entropy(K) of the time series of temperature difference fluctuations. The phase space portraits of temperature difference fluctuations became stable and periodic and the value of Kolmogorov entropy tended to decrease with increasing the value of $R_s$ from 0.1 to 0.5. The Kolmogorov entropy exhibited its maximum value with increasing liquid velocity. The value of overall heat transfer coefficient(h) showed its maximum value with the variation of liquid velocity, bed porosity or swirling liquid ratio, but it increased with increasing particle size. The value of K exhibited its maximum at the liquid velocity at which the h value attained its maximum. The particle holdup and overall heat transfer coefficient were well correlated in terms of dimensionless groups of operating variables.

• Transactions of the Korean Society of Mechanical Engineers
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• v.10 no.5
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• pp.670-679
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• 1986

본 논문은 내관 외벽에 나선형 인공조도가 있는 동심 이중관 사이의 유동특성 을 관찰하기 위해 일정 나선각에 대하여 피치비를 변화시키면서 선회속도, 축방향속도 압력을 측정하였으며, Han등의 연구에서 발표된 조도함수를 도입하여 유한차분법으로 수치계산을 수행하였다.

• Proceedings of the KSEEG Conference
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• 2002.04a
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• pp.377-379
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• 2002

• Proceedings of the KSEEG Conference
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• 2002.04a
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• pp.406-408
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• 2002

• The Korean Journal of Rheology
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• v.7 no.2
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• pp.101-109
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• 1995

아주작은 Reynolds수 조건하의 나선형 미세입자가 갖는 저항 및 이동계수를 계산하 는 일반적인 방법을 논하였다. 이러한 문제의 해를 구하기 위해서는 입자의 중심선을 따라 분포되어 있는 stokeslet으로 수식화된 유동장을 필요로 한다. 입자에 작용하는 수동력학적 힘과 토오크는 이러한 stokeslet의 적분에 의하여 구할 수 있는데 유동특성에 따라 2계 이상 의 텐서에 비례한다. Johnson에 의하여 유도된 적분방정식은 LU-분해법을 사용하여 해를 구하였다. 해석 결과의 정확성은 침상체를 모델로 하여 Brenner의 엄밀해와 비교하였으며, 일치된 결과를 얻었다.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.34 no.12
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• pp.59-66
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• 2006

To understand the role of helical geometry on the regression rate enhancement, two competing underlying mechanisms such as turbulence enhancement and swirling motion production were studied by numerical calculations. Experimental results showed that the enhancement of heat transfer rate has the very close relation to the increase in regression rate even though the percentage of increase in heat transfer rate is different from that in regression rate. This discrepancy is presumably due to the change of turbulent flow feature caused by so-called "blowing mass flux" from the fuel surface. In this regard, the results of RANS calculation show that the blowing velocity is responsible for the reduction of the swirl generation and the increase in the turbulent kinetic energy. And the dominancy of one of the mechanisms causes the increase in the regression rate. Meanwhile, the increase in turbulent kinetic energy due to the mixing of blowing flow and free stream flow does not contribute for the enhancement of the heat transfer rate to the surface because the blowing flow pushes boundary layer away from the solid surface.

• Journal of Advanced Marine Engineering and Technology
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• v.37 no.8
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• pp.905-910
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• 2013

The flow analysis has been made by applying the turbulent models in the helically coiled tubes of heat transfer. The k-${\varepsilon}$ and Spalart-Allmaras turbulent models are used in which the structured grid is applied for the simulation. The velocity vector, the pressure contour, the change of residuals along the iteration number and the friction factors are simulated by solving the Navier-Stokes equations to make clear the Reynolds number effect. The helical tube increases the centrifugal forces by which the wall shear stress become larger on the outer side of the tube. The centrifugal force makes the heat transfer rate locally larger due to the increase of the flow energy, which finds out the close relationship between the pressure drop and friction factor in the internal flow. The present numerical results are compared with others, for example, in the value of friction factor for validation.