• 한국연소학회지
• /
• 제14권1호
• /
• pp.31-38
• /
• 2009

The effects of acoustic excitation of coaxial air on mixing enhancement and reduction of nitrogen oxides (NOx) emission were investigated. A compression driver was attached to the coaxial air supply tube to impose excitation. Measurements of NOx emission with frequency sweeping were performed to observe the trend of NOx emission according to the fuel and air flow conditions and to inquire about the effective excitation frequency for reducing NOx. Then, Schlieren photographs were taken to visualize the flow field and to study the effect of excitation. In addition, phase-locked particle image velocimetry (PIV) was performed to acquire velocity field for each case and to investigate the effect of vortices more clearly. Direct photographs and OH chemiluminescence photographs were taken to study the variation of flame length and reaction zone. It was found that acoustic forcing frequencies close to the resonance frequencies of coaxial air supply tube could reduce NOx emission. This NOx reduction was influenced by mixing enhancement due to large-scale vortices formed by fluctuation of coaxial air jet velocity.

• 대한기계학회논문집B
• /
• 제22권7호
• /
• pp.1013-1021
• /
• 1998

Various vortical structures are investigated by using three kinds of flow visualization methods in branch pipe flows. There are two typical flow patterns when a jet from the branch pipe with various angles is injected to the main pipe cross flow. The velocity range of cross flow of the main pipe is 0.2 m/s ~ 1.2 m/s and the corresponding Reynolds number, R$_{p}$ is of the range 1.5 * 10$^{3}$ ~ 9.02 * 10$^{3}$. The velocity ratio(R), jet velocity/cross flow velocity, is chosen from 1.3 to 4. The subsequent behavior and development of the ring vortices which are created at the jet boundary mainly depend on the velocity ratio. An empirical relation for the shedding frequency of the ring vortices is derived. It is also found that there are two different vortex shedding mechanism in the mixing of two fluid streams.s.

• 대한기계학회논문집B
• /
• 제27권9호
• /
• pp.1282-1289
• /
• 2003

Transport of a scalar quantity, such as chemical concentration or temperature, is important in many engineering applications and environmental flows. Here we report on results obtained from the large eddy simulations of flow and concentration fields inside the tank performed using a spectral multi-domain technique. The computations were driven by specifying the impeller-induced flow at the blade tip radius (Yoon et al.). This study focused on the concentration development at different molecular diffusivities in a stirred tank operated under turbulent conditions. The main objective of the work presented here is to study the large-scale mixing structure at different molecular diffusivities in a stirred tank by using the large eddy simulation. The time sequence of concentration and flow fields shows the flow dependency of the concentration development. The presence of spatial inhomogenieties is detailed by observing the time variation oflocal concentration at different positions.

• 대한기계학회논문집B
• /
• 제22권11호
• /
• pp.1571-1581
• /
• 1998

A flow visualization study was carried out for the branch pipe mixing flow in which a jet was issued normally to the fully developed pipe flow. An instantaneous laser tomographic method was used for cross flow Reynolds numbers based on the cross flow diameter D ranged $Re_{cf}=5.26{\times}10^3{\sim}1.13{\times}10^4$, diameter ratios d/D = 0.1 ~ 0.2 and velocity ratios R = 0.5 ~ 10. Oil mist with the size of about $10{\mu}m$ diameter was used for the scattering particle. The main purpose of this study was to reveal the physical mechanism and the structure of vortices formation with varying the velocity ratios and diameter ratios in the branch pipe flow. It was found that the physical mechanism and the structures of vortices formation were quite different depending on the velocity ratios. Particularly in the case of R < 1, the typical vortex shows single loop shape and that for the case of R > 1 depicts mushroom-like structure in the cross flow jet.

• KSTLE International Journal
• /
• 제7권2호
• /
• pp.35-44
• /
• 2006

Turbocharger bearings are under the circumstance of high temperature, moreover rotated at high speed. It is necessary to be designed overcoming the high temperature. So the type of oil inlet port, the inlet oil temperature and the sort of engine oil should be designed, controlled and selected carefully in order to reduce the bearing inside temperature. In this study, the influence of aerated oil on a high-speed journal bearing is also examined by using the classical thermohydrodynamic lubrication theory coupled with analytical models for viscosity and density of air-oil mixture in fluid-film bearing. Convection to the walls and mixing with supply oil and re-circulating oil are considered. The considered parameters for the study of bubbly lubrication are oil inlet port's type, oil aeration level and shaft speed. It is found that the type of oil inlet ports and shaft speed play important roles in determining the temperature and pressure, then the friction and load of journal bearing at high speed operation. Also, the results show that, under extremely high shaft speed, the high shear effects on aerated oil and the high temperature effects are canceled out each other. So, the bearing load and friction show almost no difference between the aerated oil and pure oil.

• Nuclear Engineering and Technology
• /
• 제53권11호
• /
• pp.3625-3634
• /
• 2021

The crossflow is the key phenomenon in turbulent flow inside rod bundles. In order to establish confidence on application of computational fluid dynamics (CFD) to simulate the crossflow in rod bundles, three Reynolds-Averaged Navier Stokes (RANS) models i.e. the realizable k-ε model, the k-ω SST model and the Reynolds stress model (RSM), and the Large Eddy simulations (LES) with the Wall-Adapting Local Eddy-viscosity (WALE) model are validated based on the Particle Image Velocimetry (PIV) flow measurement experiment in a 5 × 5 rod bundle. In order to investigate effects of periodic boundary condition in the gap, the numerical results obtained with four inner subchannels are compared with that obtained with the whole 5 × 5 rod bundle. The results show that periodic boundaries in the gaps produce strong errors far downstream of the spacer grid, and therefore the full 5 × 5 rod bundle should be simulated. Furthermore, it can be concluded, that the realizable k-ε model can only provide reasonable results very close to the spacer grid, while the other investigated models are in good agreement with the experimental data in the whole downstream flow in the rod bundle. The LES approach shows superiority to the RANS models.

• 대한환경공학회지
• /
• 제29권6호
• /
• pp.647-653
• /
• 2007

본 연구의 목적은 기존의 산업용 보일러에서 이산화탄소 배출저감을 위하여 연소가스 재순환에 의한 고온 순산소 연소기술을 개발하는데 있다. 이를 위해 실험실 규모의 LNG 연소기에서 연소 화염특성을 평가하기 위한 조직적인 수치해석 연구가 일차적으로 수행되었다. 특히 본 연구에서 고려한 중요한 변수는 산소부화환경에서 계산된 연소가스의 재순환 정도이다. 배기가스 재순환이 없는 100% 순산소 연소환경에서 화염은 고온의 길고 가는 층류형상의 화염을 보였다. 이는 산화제 중에서 질소성분이 감소함으로써 약화된 난류혼합효과와 $N_2$ 가스에 의한 현열손실의 감소에 기인하는 것으로 판단하였으며 문헌에 발표된 실험과 일치된 결과를 보였다. $O_2/CO_2$ 혼합가스에서 $CO_2$ 가스의 재순환율이 증가될수록 산화제의 유량 증가에 따른 강화된 난류혼합으로 인해 최고 화염온도가 버너 근처로 이동한 반면 전반적인 연소가스 온도는 $N_2$에 비해 $CO_2$의 높은 비열로 인해 낮아지는 현상을 보였다. 결국 80% 이상 $CO_2$ 가스를 재순환한 경우 연소가스의 온도가 급격하게 떨어지는 화염소멸 현상을 보여주었다. 그러나 30% $O_2/70%$ $CO_2$의 혼합 연소조건에서는 기존의 공기연소와 유사한 가스온도를 나타내었다. 이외에도 공기연소와 동일한 유량조건에서 난류강도와 열수지 측면에서 화염특성 변화를 평가하기 위한 면밀한 연구가 수행되었다.

• 대한기계학회논문집
• /
• 제16권11호
• /
• pp.2150-2158
• /
• 1992

본 연구에서는 복잡한 유동형태를 지닌 충돌분사류에 대한 유동특성을 연구하 기 위하여 단순화된 실험모델로써 형상이 동일한 두 원형분류의 충돌에 의한 충돌분류 의 혼합현상 및 유동구조 등을 질량유량비의 변화에 따라 유체역학적으로 구명하고자 하였으며, 본 연구 결과는 연소기관에서의 연소효율 증대 및 구조개선등의 공학적 응 용을 위한 기본자료로 활용하고, 이론적 연구에 의한 난류의 유동구조 및 유동특성 에 대한 타당성 입증과 이론적 모델의 보완을 위한 실험자료로 이용하고자 한다. 충돌유동에 영향을 미치는 주요인자는 노즐직경, 충돌각, 충돌질량유량비, 온도, 밀도 등이며, 이 인자들 중에서 충돌질량유량비와 출돌각이 충돌후 형성되는 난류혼합유동 에 지배적인 영향을 미치므로, 본 연구에서는 두 원형분류의 충돌질량유량비를 가변할 수 있는 장치를 고안하였으며, 두 분류의 충돌각을 45˚로 고정하고, 고속측과 저속측 노즐의 질량유량비를 1.0, 0.8, 0.6, 0.4로 설정하여 질량유량비에 따른 혼합 유동구 조의 구명을 위한 실험적인 연구를 수행하였다. 충돌후의 혼합유동의 특성을 연구하 기 위하여 유동중심궤적, 유동반폭, 유동단면, 2차원 및 3차원 유동장, 평균속도분포 등을 온라인 컴퓨터시스템을 이용하여 측정분석하였다.

• Journal of Advanced Marine Engineering and Technology
• /
• 제34권3호
• /
• pp.360-368
• /
• 2010

A SCR catalytic filter system is used for reducing $NO_x$ and soot emissions simultaneously from diesel combustors. The amount of ammonia (as a reducing agent) must be controlled with the amount of $NO_x$ to obtain an optimal $NO_x$ conversion. Hence, gas mixing between ammonia and exhaust gases is vital to ensure that the SCR catalyst is optimally used. If ammonia mass distribution is not uniform, slip potential will occur in rich concentration areas. At lean areas, on the other hand, the catalyst is not fully active. The better mixing is indicated by the higher uniformity of ammonia mass distribution which is necessary to be considered in SCR catalytic filter system. The ammonia mass distributions are depended on the flow field of fluids. In this study, the velocity field of gaseous flow is investigated to characterize the transport of ammonia in SCR catalytic filter system. The influence of different injection placements on the ammonia mass distribution is also discussed. The results show that the ammonia mass distribution is more uniform for the injector directed radially perpendicular to the main flow of inlet at the gravitational direction than that at the side wall for both laminar (Re = 640) and turbulent flows (Re = 4255). It is also found that the mixing index decreases as increasing the heating temperature in the case of ammonia injected at the side wall.

• 한국항공우주학회지
• /
• 제35권12호
• /
• pp.1108-1114
• /
• 2007

end-burning 연소실의 주요 설계인자들을 구축하기 위해 기존에 수행되었던 인젝터 배열 및 포트 직경 변화, O/F비 변화 외에 산화제 분사각 변화에 따른 연소 특성을 해석하였다. 연료면과 평행한 분사각(Case 1), 연료면을 향해 기울여진 분사각(Case 2)과 노즐을 향해 기울여진 분사각(Case 3)을 설정하여 모델을 구성하였다. 연료면을 향한 분사각의 경우 상류에서 가장 효율적인 혼합특성을 보였으나 상당량의 미연가스가 노즐 밖으로 배출됨을 알 수 있었다. 반면 Case 1과 Case 3은 낮은 혼합특성을 보였으나 연소효율은 연료면을 향한 경우보다 월등한 것으로 판명되었다. Case 1, Case 3 모두 유사한 경향을 나타내었으나 노즐을 향한 Case 3은 짧은 체류시간으로 인해 연료면과 평행한 Case 1에 비해 낮은 연소성능을 갖는 것으로 평가되었다.