• The Korean Journal of Rheology
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• v.8 no.1
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• pp.11-29
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• 1996

스크류 채널 내에 주기적을 배리어를 설치함으로써 단축 스크류 압출 공정에서의 혼합 성능이 높여질수 있음이 S.J. Kim과 T.H. Kwom에 의해 밝혀진 바있다. 그들은 이새 로운 스크류를 통한 혼합이 카오틱하는 점으로부터 이 새로운 스크류를 카오스 스크류라고 명명했다. 우리는 카오스 스크류가 장착된 단축 압출공정에서 역학계 이론과 혼합운동학을 연계하여 연구를 수행하였다. 포인카레 단면을 통한 연구로부터 우리는 배리어의 배열이 islan의 크기에 대단히 밀접하게 관련되어 있음을 발견하였다. 연속적인 쉘 변형은 카오틱 유동에서 유체 요소를 지수 함수 형태로 늘이는 늘임과 접힘으로 이루어진 카오틱 혼합 메 카니즘을보여준다. 유체요소의 국부 늘임은 원리상으로는 계산되어질수 있으나 수치 해석상 의 어려운 점이 있다. 정규 유동에서와 달리 카오틱 유동에서는 입자 추적이 Runge-Kutta 적분중의 시간간격에 대단히 민감하다. 그래서 실제 사용될수 있는 시간 간격에 의해 계산 된 국부 늘임율 및 혼합효율의 정확도가 보장되어지지 않는다. 이러한 점들을 고려하여 우 리는 새로운 혼합 척도로 $\sigma$z를 제안하는데 이값은 비교적 긴 유체선분이 채널방향을 따라 늘어나는 비에 관련된 값이다. 배리어 영역의 길이가 짧을수록 $\sigma$z는 큰값으로 나타나지만 포인카레 단면에 의한 연구에 따르면 배리어의 주기가 너무 짧다면 두 개의 거대한 island 가 존재하는 것으로 밝혀졌다. 그리고 이러한 사실은 유체요소의 늘임비가 크다는 것이 항 상 좋은 혼합성능을 뜻하는 것은 아니라는 점을 보여준다. 이러한 관점에서 볼 때 혼합 스 크류를 설계하는데 있어서는 포인카레 단면을 병행하여 ${\sigma}_z$의 값을 사용하는 것이 바람직할 것이다.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.30 no.6
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• pp.53-60
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• 2002

Based on the analyses of the single transverse injection in supersonic flow fields, the mixing characteristics of dual transverse injection of hydrogen in supersonic air flow are studied with computational methods. Three-dimensional Navier -Stokes and the k-$\omega$ SST turbulence model were used. A parametric study is conducted with the variation of the distance between two injectors. The flow patterns and the mixing characteristics of two injection flows are very different from each other, and the flow patterns and the mixing characteristics of the rear injection flow are strongly influenced by those of the first injection flow. The increase of the distance between two injectors up to a specific distance results in the increase of mixing rate and penetration of fuel. However, the increase of the distance over the specific distance results in the decrease of mixing rate and penetration of fuel. From the results it can be stated that there exists a distance between two injectors for optimum mixing characteristics.

• Proceedings of the Korean Nuclear Society Conference
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• 1997.10a
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• pp.567-572
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• 1997

범용 전산유체해석(Computational Fluid Dynamics) 코드인 CFX-F3D를 이용하여 봉 다발에서의 난류 유동 수치해석을 수행하였다 3$\times$3 봉으로 구성된 부수로 사이의 난류 횡류(Crossflow) 혼합유동과 평행한 4개의 봉으로 이루어진 벽 수로에서의 난류 유동구조를 수치적으로 분석하여 각각의 실험결과와 비교하였다. 부수로 횡류 혼합유동의 경우 예측된 주 유동방향 평균 속도분포는 실험결과와 잘 일치하였으나 벽면과 인접한 부수로에서의 난류강도 분포는 다소 큰 차이가 나타났다. 백수로의 경우 수로 중심선 근처의 주 유동방향의 속도변화는 크게 예측되었고 벽 전단응력은 유로가 협소해지는 영역에서 낮게 예측되었으나 전반적으로 실험결과와 유사한 유동특성을 나타냈다. 이 연구는 봉 다발에서의 난류 유동구조에 대한 이해를 증진시킴과 더불어 CFX-F3D 코드를 평가함으로써 향후 지지격자와 임계열유속 증진장치가 부착된 복잡한 형상의 핵연료 다발에서의 유동장 수치해석의 기반을 마련하였다.

• Journal of Aerospace System Engineering
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• v.12 no.4
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• pp.81-89
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• 2018

In the present study, a numerical investigation was conducted to analyze the effect of the distance between the adjacent injectors on the characteristics of flow structure, fuel penetration, and air/fuel mixing. Numerical results were validated with experimental data using a single injection. Subsequently, the same injector geometry and properties were applied on a non-reacting flow simulation with multiple injectors. Total pressure loss, penetration height, and mixing efficiency were compared with the distance between the injectors. The results showed that each injected gas merged into a single stream, resulting in the 2D-like flow fields under the condition of short distance and lower mixing efficiency along with higher total pressure loss. When the distance between the injectors increased, total pressure loss reduced and mixing efficiency increased due to the weakening of interactions between the injected gases.

• Journal of the Korean Society of Propulsion Engineers
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• v.17 no.1
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• pp.52-60
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• 2013

Ejector-diffuser system makes use of high-pressure primary stream to entrain the low-pressure secondary stream through pure shear action between two streams. In general, the flow field in the ejector-diffuser system is highly complicated due to turbulent mixing, compressibility effects. A fatal drawback of the ejector system is in its low efficiency. Many works have been done to improve the performance of the ejector system, but not yet satisfactory. In the present study, a mixing guide vane was installed at the inlet of the secondary stream for the purpose of the performance improvement of the ejector system. A CFD method has been applied to simulate the supersonic flows inside the ejector-diffuser system. The present results obtained were validated with existing experimental data. The mixing guide vane effects are discussed in terms of the entrainment ratio, total pressure loss as well as pressure recovery.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.40 no.3
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• pp.157-164
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• 2016

The performance of convective heat transfer in rod bundle flow was experimentally evaluated using a twist-vane spacer grid. A $4{\times}4$ square-arrayed rod bundle was prepared as the test section, with a pitch-to-diameter ratio(P/D) of ~1.35. To check the convective heat transfer performance, the circumferential and longitudinal variations in rod-wall temperatures were measured downstream of the twist-vane spacer grid. In the circumferential measurements, the rod-wall temperature toward the twist-vane tip showed the lowest value, which might be due to the deflected water flow caused by the twist-vane. On the other hand, the wall temperature of the longitudinal measurements near the twist-vane spacer grid decreased dramatically, which implies that the convective heat transfer performance was enhanced. A heat transfer enhancement of ~35 % was achieved near downstream of the twist-vane spacer grid, as compared with the upstream value. Based on the present experimental data, a correlation for predicting the heat transfer performance of a twist-vane spacer grid was proposed.

• Aerospace Engineering and Technology
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• v.7 no.1
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• pp.40-48
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• 2008

Air mixing system which is composed of air pressure control system, hot pipe system and air mixer, is the facility for mixing hot air($1000^{\circ}C$, 10kg/s) from storage air heater (SAH) and decompressed air($20^{\circ}C$, 15kg/s) from high pressure air supply system. Air pressure control system reduce the pressure of the air, from 32MPa to 3.5 MPa and supply the decompressed air to air mixer. The hot pipe system supply hot air from SAH to air mixer which mix hot with the decompressed air from air pressure control system. Fully mixed air flow rate is 25kg/s and mixed temperature is up to $400^{\circ}C$. So, we can expand the operating envelop of the supersonic ground test facility to low Mach number and low altitude region.

• Transactions of the Korean Society of Mechanical Engineers
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• v.15 no.5
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• pp.1774-1780
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• 1991

본 연구에서는 유한 길이를 갖는 수직 덕트내 공기 유동의 속도 분포를 LDV를 사용하여 측정하며 유동 특성에 미치는 가열정도, 덕트간격 및 입구소도등의 영향을 보고하고자 한다. 또한 속도분포, 온도분포 및 열전달에 미치는 부력의 영향을 수치 적으로 예측하여 그 타당성을 검증하고자 한다.

• Journal of the Korean Society of Propulsion Engineers
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• v.18 no.4
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• pp.81-89
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• 2014

Tandem-ejector has been devised for engine-bay cooling. In this study, 1-D model has been developed to analyze Tandem-ejector. In the model, the primary, the secondary and the tertiary flow conditions have been analyzed with isentropic process. The mixing process has been analyzed with conservation laws based on the control volume analysis. The total pressure loss of the primary flow has been analyzed under the matching condition between the static pressure of Tandem-ejector discharge flow and atmospheric pressure. Consequently, 1-D model can predict Tandem-ejector performance accurately and provide the performance map.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2008.05a
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• pp.281-284
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• 2008

The flow of combustor in scramjet engine is supersonic speed. So residence time and mixing ratio are very important factors for efficient combustion. This study used open cavity on fuel/air mixing model and laser schlieren was carried out to investigate flow characteristics around a jet orifice and a cavity. A source of illumination has 10 ns endurance time so it can observe unsteady flow characteristics efficiently. Pressure was measured by varying momentum flux ratio. And the change of critical ignition point was observed to change of momentum flux ratio.