• 한국연소학회:학술대회논문집
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• 한국연소학회 2001년도 제22회 KOSCI SYMPOSIUM 논문집
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• pp.9-16
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• 2001

When large size nozzle with low jet velocity is used, the buoyancy effect arises from the density difference among propane, air, and burnt gas. Flame characteristics in such buoyant jets have been investigated numerically to elucidate the effect of buoyancy on lifted flames. It has been demonstrated that the cold jet has circular cone shape since upwardly injected propane jet decelerates and forms stagnation region. In contrast to the cold flow, the reacting flow with a lifted flame has no stagnation region by the buoyancy force induced from the burnt gas. To further illustrate the buoyancy effect on lifted flames, the reacting flow with buoyancy is compared with non-buoyant reacting flow. Non-buoyant flame is stabilized at much lower height than the buoyant flame. At a certain range of fuel jet velocities and fuel dilutions. an oscillating flame is demonstrated numerically showing that the height of flame base and tip vary during one cycle of oscillation. Under the same condition. non-buoyant flame exhibits only steady lifted flames. This confirms the buoyancy effect on the mechanism of lifted flame oscillation.

• 대한조선학회논문집
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• 제41권6호
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• pp.8-14
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• 2004

The numerical analysis of a waterjet propulsion system was performed to provide detail understanding of complicated flow phenomena including interactions of intake duct, rotor, stator, and contracted discharge nozzle. The incompressible RANS equations were solved on moving multiblocked grid system. To handle interface boundary between rotor and stator, the sliding multiblock method was applied. The numerical results were compared with experiments and good agreement was obtained. The complicated viscous flow features of the waterjet, such as secondary flow inside the intake duct, the recovery of axial flow by the role of the stator, and tip and hub vortex, etc. were well analyzed by the present simulation. The performance of thrust and torque was also predicted.

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2009년도 제33회 추계학술대회논문집
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• pp.378-381
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• 2009

압축기에 불안정한 특성인 선회실속(Rotating stall)이 발생하면 압력 및 효율이 저하되고, 기계적인 손상도 야기한다. 이러한 불안정성을 개선하고 안정 운전영역을 넓히기 위해 4단 저속 축류압축기에 공기 분사(Air injection) 방법을 적용하여 안정성 개선 실험을 실시하였다. 동익 팁에 축방향으로 공기를 분사할 수 있도록 하기 위해 코안다 효과를 적용한 노즐을 사용하였고, 8개의 인젝터를 1단 동익 상단에 등간격으로 설치하였다. 축류 압축기 80% speed로 운전하면서 선회실속이 발생하기 전에 공기 분사를 실시하였고, 모드(Mode) 발생 유량의 5.4%에 해당하는 공기를 분사하여 약 4%의 안정성 개선 효과를 얻었다.

• 대한기계학회논문집
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• 제12권3호
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• pp.534-540
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• 1988

본 연구에서는 디이젤분무 주위기체의 유동특성중에서 유동속도에 관하여 보 고 하였다. 본모에서는 분무기간중 분사압력의 시간경과특성이 상이한 두 종류의 분 사계를 사용하여 생성된 분무와 주위 기체와의 유동방향의 시간경과 및 유입시기, 정 상유입속도 도달시간등을 분무의 축방향과 반경방향에 대하여 상세한 측정결과를 얻었 기에 보고한다.

• 대한기계학회논문집B
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• 제31권5호
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• pp.405-415
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• 2007

The effects of recirculated exhaust gas on exhaust emissions under four kinds of nozzle tip with the different fuel consumption rates are experimentally investigated by using an once-through boiler with a FGR system. The purpose of this study is to develop the FGR control system for reducing $NO_x$ emissions in boilers. Intake and exhaust oxygen concentrations, and equivalence ratio are considered to figure out the effect of FGR rate on exhaust emissions at various fuel consumption rates. It is found that $NO_x$ emissions are markedly decreased, while soot emissions are increased owing to the drop of intake and exhaust oxygen concentrations, and the rise of equivalence ratio as FGR rates are elevated. One can also conclude that the reduction in $NO_x$ emissions is more considerably influenced by the variation of equivalence ratio due to the FGR rate than the fuel consumption rate.

• 대한기계학회논문집B
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• 제25권8호
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• pp.1087-1096
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• 2001

The dynamic structures of unsteady CH$_4$/Air jet diffusion flame with a flame-vortex interaction were numerically investigated. A timed-dependent, axisymmetric computational model and a low mach number approximation were employed in the present calculation. A two-step global reaction mechanism which considers 6 species, was used to calculate the reaction rates. The predicted results including the gravitational effect show that the large outer vortices and the small inner vortices can be well simulated without any additional disturbances near nozzle tip. It was found that the temperature and species concentrations have deviated values even for the same mixture fraction in the flame-vortex interaction region. It was also shown that the flame surface is not deformed by the inner vortex in upstream region, while in downstream region, the flame surface is compressed or stretched by the outer vortex roll-up. The present unsteady jet flame configuration accompanying a flame-vortex interaction is expected to give good implications for the unsteady structures of turbulent flames.

• 한국연소학회:학술대회논문집
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• 대한연소학회 2003년도 제27회 KOSCO SYMPOSIUM 논문집
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• pp.251-256
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• 2003

The capability of pilot injection with small fuel quantity at all engine operating conditions is one of the main feature of the common rail system. The purpose of the pilot injection is to lower the engine noise and to reduce the NOx emissions. This study describes the pilot spray structure characteristics of the common-rail diesel injectors, solenoid-driven and piezo-driven type, with different electric driving characteristics So, three common-rail injectors with different electric current wave were used in this study. The pilot spray characteristics such as spray speed, spray tip penetration, and spray angle were obtained by spray images, which is measured by the back diffusion light illumination method with optical system for high-speed temporal photography. Also the CFD analysis was carried out for fuel behavior under high pressure in between needle and nozzle of solenoid-driven injector to know the condition of initial injection at experiment test. It was found that pilot injection of common-rail system was effected by rate of injection and temperature of injected fuel and electrical characteristic of the driven injector.

• 한국자동차공학회논문집
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• 제1권3호
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• pp.34-45
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• 1993

This study aims to analyze the mixing characteristics of hydrogen considered as a new fuel for internal combustion engines. As the physical property of helium gas is similar to that of hydrogen, helium gas was used in this study. To analyze the steady and unsteady behavior of jet, helium gas was injected into the stationary atmosphere at the normal temperature and pressure. Concentration of helium gas in the center of jet flow is in inverse proportion with axial distance from the nozzle tip. This agrees with the free jet theory of Schlichting. The relative equation for dimensionless concentration to radial/axial distance the axial distance of potential core region, the cone angle a of the jet flow and the relative equation for arriving distance of the front of jet flow to the lapse of time are obtained. But free jet theory of Schlichting in the dimensionless concentration is not in agreement with the present experimental results of the distance of the radial direction. It needs more study. When the arrival frequency of jet flow is used as a parameter, the transition area changing from unsteady flow area into steady flow area becomes gradually wider downstream, but its ratio for the whole unsteady flow area gradually decreases.

• 동력기계공학회지
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• 제14권3호
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• pp.5-12
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• 2010

The purpose of this study is to analyze heterogeneous distribution of branch-like structure at downstream region of inner spray. The previous many studies about diesel spray structure have yet stayed in the analysis of 2-D structure, and there are very few of informations which are concerned with 3-D analysis of the structure. The heterogeneous distribution of droplets in inner spray affects the mixture formation of diesel spray, and also the combustion characteristics of the diesel engines. Therefore, in order to investigate 3-D structure of evaporative spray the laser beam of 2-D plane was used in this study. Liquid fuel was injected from a single-hole nozzle (l/d=5) into a constant-volume vessel under high pressure and temperature in order to visualize the spray phenomena. The incident laser beam was offset on the central axis. From the images analysis taken by offset of laser beam, we examine formation mechanism of heterogeneous distribution by vortex flow at the downstream of the diesel spray. As the experimental results, the branch-like structure formed heterogeneous distribution of the droplets consists of high concentration of vapor phase in the periphery of droplets and spray tip of branch-like structure. Also the 3-D spatial structure of the evaporative diesel spray can be verified by images obtained from 2-D measurement methods.

• 한국자동차공학회논문집
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• 제2권5호
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• pp.11-22
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• 1994

The pupose of this study is to measure droplet size and velocity simultaneously for a transient diesel fuel spray in a quiescent chamber at atmospheric temperature and pressure. Generally, diesel combustion phenomena is mainly governed by characteristics of injection system and fuel spray. Therefore we need to clarify these characteristics for developing more economical diesel systems. In this study, correlation between droplet size and velocity was measured at downstream distance from nozzle. Governing parameters are pump speed and fuel quantity for the detailed nature in this transient diesel fuel spray. It is observed effect to the droplet size and velocity distribution. Velocity(peak, mean, rms), number density and droplet size were investigated simulaneously using PDA in the spray. Various results are presented to illustrate the effects of operation factors and correlation between the droplet diameter and velocity.