• 한국추진공학회지
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• 제22권3호
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• pp.53-64
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• 2018

본 논문에서는 초음속 유동장 내 연료 수직 분사 조건에서 분사구의 형상에 따른 연료/공기 혼합 특성을 분석하고자 하였다. 이를 위해 동일한 분사구 출구 면적과 유량 조건에 대해 수소와 공기에 대한 비반응 유동장 전산 해석을 수행하였다. 해석 결과의 검증을 위하여 자유류 마하수 3.38, 제트-자유류 운동량 플럭스비 1.4 인 평판 분사 시험을 모의하였다. 5개의 서로 다른 형상을 갖는 분사구를 이용하여 형태에 따른 박리 구간, 분사 제트의 구조의 차이를 살펴보고 분사구 후류에서 수소의 침투 높이와 수소-공기의 혼합에 따른 가연 면적에 변화를 확인함으로써 분사구 형상에 따른 연료/공기 혼합 특성을 정량적으로 비교하였다.

• 대한토목학회논문집
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• 제8권3호
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• pp.53-60
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• 1988

가로흐름으로 방류(放流)되는 연직상향평면부력(鉛直上向平面浮力)?의 거동(擧動)이 실험과 기본방정식(基本方程式)의 적분기법(積分技法)에 의하여 해석된다. 적분기법(積分技法)을 상사법칙(相似法則)과 특성(特性)길이를 도입한 연직상향 및 수평방향흐름영역에 대한 점근해(漸近解)이며, 실험은 상이한 속도비와 방류밀도 Froude 수를 변화시키면서 수행되었다. 실험결과(實驗結果)와 해석적해(解析的解)는 잘 일치하여, ?중심선(中心線)의 경로(經路)와 온도(溫度)는 멱법칙(冪法則)으로 표현될 수 있는 것으로 나타났다.

• 한국자동차공학회논문집
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• 제13권4호
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• pp.49-57
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• 2005

In this study, in-cylinder flow of the swirl chamber type diesel engine numerically simulated by VECTIS code. The flow fields during the intake and compression process were also investigated in detail. Numerical results revealed that the generation and distortion of the swirling, tumbling vortices and those influences on turbulence kinetic energy by shape of the jet passage, angle and area. It was also found that flow characteristics were affected by inflow velocity that depends on change of the jet passage shape. Swirl ratio was increased according to decrease of jet passage area, and was affected by piston motion according to increase of jet passage angle. Tumbling vortices had the similar in various cases, but tumble ratio was increased with the inflow velocity. The generation of turbulence kinetic energy was considerably influenced by complex effects of swirling and tumbling vortices.

• 한국산업보건학회지
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• 제16권3호
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• pp.290-301
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• 2006

A push pull hood system is frequently applied to control contaminants evaporated from an open surface tank. Efficiency of push pull hood system is affected by various parameters, such as, cross draft, vessel shapes, tank surface area, liquid temperature. A previous work assisted by flow visualization technique qualitatively showed that a strong cross draft blown from the pull hood to push slot could destroy a stable wall-jet on the surface of tank, resulting in the abrupt escape of smoke from the surface. In this study, the tracer gas method was applied to determine the effect of cross-draft on the capture efficiency qualitatively. A new concept of capture efficiency was introduced, that is, linear efficiency. This can be determined by measuring the mass of tracer gas in the duct of pull hood while the linear tracer source is in between push slot and pull hood. By traversing the linear tracer source from the push slot to the pull hood, it can be found where the contaminant is escaped from the tank. Total capture efficiency can be determined by averaging the linear efficiencies. Under the condition of cross-draft velocities of 0, 0.4, 0.75, 1.05 and 1.47m/s, total capture efficiencies were measured as 97.6, 95.4, 94.6, 92.7 and 70.5% respectively. The abrupt reduction of efficiency with cross-draft velocity of 1.47m/s was due to the destruction of tank surface wall-jet by the counter-current cross-draft. The same phenomenon was observed in the previous flow visualization study. As an alternative to overcome this abrupt efficiency drop, the 20% increase of hood flow rates was tested, resulting in 20% efficiency increase.

• 대한기계학회논문집B
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• 제28권8호
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• pp.887-894
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• 2004

A technique of mixing enhancement by using an elliptic jet screech reflector has been examined experimentally in an underexpanded sonic round jet where jet screech tone is generated. Since jet screech is known to enhance jet spreading, a reflector was designed to focus jet screech waves near the nozzle lip at an underexpanded jet. The reflector has an elliptic cross section of which one focus is located near the nozzle lip and the other in the jet screech source region in a plane including the jet axis. In the jet with the elliptic reflector, the mass flow rate showed a significant increase in the jet entrainment when compared to that for the small disk reflector. This was attributed to the increased screech amplitude near the nozzle lip as well as the mode change of the jet. The jet mixing was also increased by the amplified jet screech at two other underexpanded jets, but the jet oscillation mode did not change.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2000년도 춘계학술대회논문집
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• pp.71-77
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• 2000

An axisymmetric supersonic jet is simulated at a Mach number of 1.5 and a Reynolds number of $10^5$ to identify the mechanism of sound radiation from the jet. The present simulation is performed based on the high-order accuracy and high-resolution ENO(Essentially Non-Oscillatory) schemes to capture the time-dependent flow structure representing the sound source. In this simulation, optimum expansion jet is selected as a target, where the pressure at nozzle exit is equal to that of the ambient pressure, to see pure shear layer growth without effect of change in jet cross section due to expansion or shock wave generated at nozzle exit. Shock waves are generated near vortex rings, and discernible pressure waves called Mach wave are radiated in the downstream direction with an angle from the jet axis, which is characteristic of high speed jet noise. Furthermore, vortex roll-up phenomena are observed through the visualization of vorticity contours.

• 대한기계학회논문집
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• 제12권4호
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• pp.876-891
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• 1988

본 연구에서는 실린더내의 혼합유동을 고찰하기 위하여 두 분류를 충동 유동 시켜 충돌 후 혼합 유동 상태를 온라인 측정 시스템에 의하여 계측하였다.실린더내 의 난류에 의한 혼합현상을 구명하기 위하여 혼합영역을 중심으로 3차원유동 성분들의 평균속도, 난류응력 등을 측정하여 일반 자유분류의 반실험식과 비교 검토하였으며, 본 실험 결과에 잘 일치하는 반실험식을 가정식으로부터 컴퓨터에 의한 점근적 방법으 로 구하였다.

• 한국분무공학회지
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• 제16권4호
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• pp.176-185
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• 2011

The empirical correlations for the prediction of penetration height of liquid jet in crossflow are reviewed and classified in this study. Around thirty different correlations had been proposed by many investigators. It has generally known that the penetration height of a liquid jet in a cross-flow is a function of the liquid to air momentum flux ratio and the normalized downstream distance from the injector. However, several researchers incorporated the Weber number, liquid-to-water or air viscosity ratio, pressure ratio or Reynolds number, temperature ratio in the empirical correlations. The existing correlations can be grouped as correlations in a power-law, logarithmic, and exponential forms, respectively. Correlations in a power-law form can be further classified as three groups such as basic form, Weber number form and other parameters form. It should be pointed out that correlations in a logarithmic form in terms of Weber number or any other parameters could not be found. Universal correlation has still not been established due to the significant discrepancies between various correlations suggested to date. Several of the studies reported the significant discrepancies of predicted values by the existing correlations. The possible reasons for discrepancies will be summarized as measurement technique, assumptions made in defining terms in the liquid to air momentum flux ratio, difficulties in defining the boundaries of the liquid jets, and nozzle/injector geometry. Evaluation of validity for the correlations proposed recently by several investigators is essentially required. Those include eight power-law forms, two logarithmic forms, and one exponential form.

• 한국태양에너지학회 논문집
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• 제23권4호
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• pp.55-61
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• 2003

Heat transfer characteristics for an air jet vertically impinging on a flat plate with a set of hybrid rods was investigated numerically using the RNG k-$\varepsilon$turbulent model. A commercial finite-volume code FLUENT is used. The rods had cross sections of half circular and rectangular shapes. The heating surface was heated with a constant heat flux value of $1020W/m^2$. Parameters investigated were the jet Reynolds number, nozzle -to-plate spacing, the rod pitch and rod-to-plate clearance. The local and average Nusselt number were found to be dependent on the rod pitch and the clearance because installing rods disturbed the flow. Higher convective heat transfer rate occurred in the whole plate as well as in the wall jet region.

• Journal of Mechanical Science and Technology
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• 제14권12호
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• pp.1386-1395
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• 2000

The mear field structure of round turbulent jets with initially asymmetric velocity distributions is investigated experimentally. Experiments are carried out using a constant temperature hot-wire anemometry system to measure streamwise velocity in the jets. The measurements are undertaken across the jet at various streamwise stations in a range starting from the jet exit plane and up to a downstream location of twelve diameters. The experimental results include the distributions of mean and instantaneous velocities, vorticity field, turbulence intensity, and the Reynolds shear stresses. The asymmetry of the jet exit plane was obtained by using circular cross-section pipes with a bend upstream of the exit. There pipes used here include a straight pipe, and 90 and 160 degree-bend pipes. Therefore, at the upstream of the upstream of the pipe exit, secondary flow through the bend mean streamwise velocity distribution could be controlled by changing the curvature of pipes. The jets into the atmosphere have two levels of initial velocity skewness in addition to an axisymmetric jet from a straight pipe. In case of the curved pipe, a six diameter-long straight pipe section follows the bend upstream of the exit. The Reynolds number based on the exit bulk velocity is 13,400. The results indicate that the near field structure is considerably modified by the skewness of an initial mean velocity distribution. As the skewness increases, the decay rate of mean velocity at the centerline also increases.