• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.21 no.12
• /
• pp.1635-1646
• /
• 1997

The nozzle length to diameter ratio of real diesel nozzles is about 2-8 which is not long enough for a fully developed and stabilized flow. The characteristics of the flow such as turbulence at the nozzle exit which affect the development of the spray can be enhanced by impinging the flow inside nozzle. The flow details inside the impinging nozzles have been investigated both experimentally and numerically. The mean velocities, the fluctuating velocities, and discharge coefficients in the impinging inlet nozzles, round inlet nozzle, and sharp inlet nozzle were obtained at various Reynolds number. The developing feature of the external spray were photographed by still camera and the droplet sizes and velocities were also measured by laser Doppler technique. The spray angle was greater and the droplet sizes near the spray axis were smaller with the impinging flow inside nozzle.

• Proceedings of the Korean Society of Propulsion Engineers Conference
• /
• 1997.11a
• /
• pp.8-9
• /
• 1997

산업의 발달과 환경에 대한 관심이 높아짐에 따라 고효율, 저공해인 가스터빈의 응용범위가 넓어지고 있는 추세이다. 가스터빈 기관의 효율을 높이기 위해서는 터빈 입구온도를 높이는 것이 필수적인데 이는 재질에 의해 제한 받게 되고 이 때문에 효과적인 냉각방법의 필요성이 대두되었다 충돌제트는 국소적으로 높은 열/물질 전달 효과를 얻을 수 있어서 터빈 블레이드 냉각과 연소기 벽면 냉각에 효과적으로 응용 될 수 있다. 이러한 충돌제트의 냉각효과는 제트출구의 초기조건에 매우 민감한데 Kelvin-Helmholts 불안정은 불안정한 자유전단층에서 자연적인 와류생성(roll up)과 개개의 와류고리 형성의 원인이 되고 이 고리의 성장과 병합(pairing)은 제트의 유동특성에 상당히 영향을 미친다. 제트주위에 생성되는 이러한 와류에 의해 제트중심에서 속도와 난류강도가 변하게 된다. 이러한 제트초기의 불안정성은 하류에서의 와류성장에 영향을 끼치기 때문에 와류의 조절에 의한 충돌 면에의 열 전달 효과 상승을 기대할 수 있다. 이 조절방법은 크게 두 가지로 나눌 수 있는데 하나는 제트주의 환형관에 이차유동을 가하여 와류를 직접 제어함으로써 자유전단류(free shear layer flow)의 안정성 원리를 이용하여 열 전달을 촉진하는 것이고 다른 하나는 음향여기(acoustic exitation)를 사용하여 제트주위의 와류형성을 조절하는 것인데, 자연적으로 형성되는 와류의 주파수(와류의 고유주파수)나 부조화 주파수(subharmonic)로 음향여기 시키는 경우 제트 주위 와류는 더욱 증폭되고 그렇지 않은 경우 제트주위 와류의 형성이 억제되어 더 긴 제트코어의 길이 및 제트코어 주위에서 작은 크기의 와류들이 형성된다.

• Journal of the Korean Society of Propulsion Engineers
• /
• v.13 no.2
• /
• pp.42-53
• /
• 2009

Gas turbine engine simulation program has been developed. In compressor and turbine, 2-D NS implicit code is used with k-$\omega$ SST turbulent model. In combustor, 0-D lumped method chemical equilibrium code is adopted under the limitations, the products are only 10 species of molecular and air-fuel is perfectly mixed state with 100% combustion efficiency at constant pressure. Fluid properties are shared on interfaces between engine components. The outlet conditions of compressor have been used as the inlet condition of combustor. The inlet condition of turbine comes from the compressor The back pressure in compressor outlet is transferred by the inlet pressure of turbine. Unsteady phenomena at rotor-stator in compressor and turbine is covered by mixing-plane method. The state of engine can be determined only by given inlet condition of compressor, outlet condition of turbine, equivalence ratio and rotating speed.

• Journal of the Korea Academia-Industrial cooperation Society
• /
• v.17 no.10
• /
• pp.618-627
• /
• 2016

This study numerically analyzes the characteristics of the velocity distribution for each location of a square-sectional $180^{\circ}$ bent duct using a Reynolds Stress Turbulent model. The flow parameters were varied, including the working fluids, inlet velocity, surface roughness, radius of curvature, and hydraulic diameter. The boundary conditions for computational fluid dynamics analysis were inlet temperatures of air and water of 288 K and 293 K, inlet air velocity of 3-15 m/s, inner surface roughness of 0-0.001 mm, radius of curvature of 2.5-4.5 D, and hydraulic diameter of 70-100 mm. The working fluid characteristics were highly affected by changes in the viscous force. The maximum velocity profiles in the bent duct were indicated when the $90^{\circ}$ section was in the region of X/D=0.8 and the $180^{\circ}$ section was in the region of Y/D=0.8. Lower surface roughness and higher radius of curvature resulted in a higher rate of velocity change. Also, an efficient measuring location downstream of the bent duct is suggested since the flow deviations were the most stable when the straight duct length was in the region of L/D=30. The minimum deviations at the same velocity conditions according to the hydraulic diameter were mostly indicated in the range of L/D=15-30 based on the standard deviation characteristics.

• Proceedings of the Korea Water Resources Association Conference
• /
• 2023.05a
• /
• pp.8-8
• /
• 2023

하천에 설치된 횡단구조물은 어류의 이동을 방해하거나 불가능하게 만들지만, 어도를 통해 어류의 이동 및 서식생태에 미치는 영향을 최소화할 수 있다. 하천에 서식하는 대부분의 어종은 연중 회유하는 특성을 갖기 때문에 어류의 자유로운 이동을 위해서 어도는 필수적으로 설치해야 하는 시설이다. 어도는 어류의 유영능력을 기반으로 수리시설물의 낙차나 하천의 유량 등을 고려하여 설계한다. 이처럼 어도 내의 흐름은 어류의 소상능력에 영향을 미치는 주요 인자로 평가되고 있으며 어도 내 흐름에 따라 어도의 이동효율에 차이가 발생한다. 따라서 본 연구에서는 수치모의를 통해 계단식 어도 내의 어류 친화적 흐름의 특성을 연구하였다. 어류 친화적 흐름은 어류가 쉽게 유영할 수 있는 흐름을 의미하며, 유속, 난류운동에너지, 에너지소산을 기준으로 판단할 수 있다. 이를 위해 3차원 수치모형 FLOW-3D를 이용하였으며, 개수로 실험을 통해 모델의 검증을 수행하였다. 연구에서 사용한 어도의 형식은 국내에서 전체 어도의 20%를 차지하고 있는 계단식 어도로 선정하였다. 수위와 유속에 따라 흐름조건을 설정하였으며, 실험결과를 통해 어류의 유영능력에 영향을 미치는 3가지 항목의 값을 계산하여 어도 내의 어류 친화적 흐름의 적합성을 판단하였으며, 이를 바탕으로 최적의 흐름조건을 제시하였다. 향후, 본 연구결과는 계단식 어도를 설계하는 것에 기초자료가 될 수 있을 것이라 기대된다. 그러나 어도의 설계는 어도의 설치위치나 어도의 입구 위치 및 형태 등과 같은 설치계획에 따라 어도의 이동효율이 좌우될 수 있기 때문에 하천환경을 고려하는 추가적인 연구가 필요할 것으로 판단된다.

• Journal of the Korean Society of Propulsion Engineers
• /
• v.18 no.1
• /
• pp.73-84
• /
• 2014

The present paper includes recent research works on the estimation of physical properties like equivalence ratio and heat release rate of flame through chemiluminescence measurement. Modern combustion devices require a precise control to increase combustion stability as well as to suppress pollutant emissions. The determination of combustion characteristics from chemiluminescence provides practical advantages over other techniques. However, the technique is dependent on equivalence ratio, combustion pressure, inlet temperature, turbulent intensity and fuel type. The intensity ratio of $OH^*$ and $CH^*$ has a strong relation with an equivalence ratio for methane/air premixed flames. The global measurement of chemiluminescence is accepted as a good indicator for a global heat release rate.

• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.37 no.4
• /
• pp.367-375
• /
• 2013

In this study, the characteristics of the heat transfer coefficient and pressure drop were numerically analyzed according to the axis ratio (AR), pitch, location of vortex generator, and bump phase of the tube surface about an elliptical fin-tube heat exchanger. The boundary condition for CFD analysis was decided as a tube surface temperature of 348 K and inlet air velocity of 1-5 m/s. RSM 7th turbulent model was chosen as the numerical analysis for the sensitivity level. The analysis results indicated that the AR and transverse pitch decreased whereas the heat transfer coefficient increased. On the other hand, there was little difference in the longitudinal pitch. Furthermore, the heat transfer rate was more favorable when the vortex generator was located in front of the tube. Also, the bump phase of the tube surface indicated that the pressure drop and heat transfer were more favorable with the circle type than with the serrated type.

• Transactions of the Korean Society of Mechanical Engineers
• /
• v.14 no.6
• /
• pp.1652-1660
• /
• 1990

Combined radiative-convective heat transfer in a hot gas tube flow has been investigated numerically and experimentally. In the numerical analysis, a standard k-.epsilon. model is used for the evaluation of turbulent shear stresses and spherical harmonics method with the Weighted Sum of Gray Gases Model for the solution of radiative transfer equation. In the experimental study measured are the velocity and temperature of the hot gas flow generated by the propane gas combustion, and tude wall heat flux distribution. Numerical results are compared with experimental ones and it is confirmed that P-3 provides quite reliable results in the analysis of the combined radiation-convection system.

• Transactions of the Korean Society of Mechanical Engineers
• /
• v.14 no.3
• /
• pp.702-715
• /
• 1990

This paper describes behaviors of two-phase open channel flow inside the flash chamber of a horizontal Multi-Stage-Flash evaporator numerically along with the experimental observations. Bubble trajectories and the velocity and temperature distributions of the liquid phase were predicted by using the particle-source-in-cell(PSI-Cell) method with the appropriate bubble motion/growth equations. Size and number of bubble nuclei embedded in the incoming liquid(brine) were taken into account as important parameters in addition to the conventional ones such as the velocity, degree of inlet superheat, inlet opening height, and the liquid level. Bubble motions, which are unsteady, appeared to be mostly determined by the buoyancy and the drag forces. The calculations, though a number of simplifying assumptions were made, reasonably simulated the hydrodynamic behaviors of the two-phase horizontal stream observed in the experiments. The simulated temperature distributions also agreed fairly well with the other's measurements. Non-equilibrium allownaces, evaluated from the simulated temperature distributions, were within the range of those obtained from the existing correlations, and reduced with the increases of the number and size of incoming bubble nuclei due to vigorous flashing.

• Journal of the Korean Society of Propulsion Engineers
• /
• v.22 no.1
• /
• pp.16-27
• /
• 2018

A basic numerical analysis was performed to confirm the possibility of combining a dual bell nozzle and an Expansion-Deflection(E-D) nozzle. The dual bell nozzle was designed based on the first-stage nozzle of the Korean Space Launch Vehicle that is being developed, and the E-D nozzle concept was applied to the dual bell nozzle. The inlet condition was analyzed by applying eight types of frozen flow analysis, and k-${\omega}$ SST was selected as the turbulence model. The number of optimal grids was obtained as 240,000 through the grid sensitivity analysis. As a result, it was confirmed that the transition altitude increased owing to over-expansion when the E-D nozzle concept was applied to the dual bell nozzle, and the specific impulse gain was obtained at high altitudes compared with the KSLV-II first-stage engine.