• 설비공학논문집
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• 제11권6호
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• pp.846-854
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• 1999

Measurements of the local heat transfer coefficients were made on a hemispherically convex surface with a round oblique impinging jet. The liquid crystal transient method was used for these measurements. This method, which is a variation on the transient method, suddenly exposes a preheated wall to an impinging jet while video recording the response of liquid crystal for the surface temperature measurements. The Reynolds number used was 23000 and the nozzle-to-surface distance was L/d=2, 4, 6, 8, and 10 and the jet angle was $\alpha$=$0^{\circ}\; 15^{\circ}\;30^{\circ}C\; and \;40^{\circ}C$. In the experiment, the Nusselt number at the stagnation point decreases as the jet angle increases and has the maximum value for L/d=6. The X-axis Nusselt number distributions exhibit Secondary maxima at $0^{\circ}C\re $\alpha$\re 15^{\circ}C, L/d\le6$ for X/d＜0(upstream) and at $0^{\circ}C\re $\alpha$40^{\circ}C,\;L/d\le4\;and\; at\; 30^{\circ}C\re $\alpha$$\leq$40^{\circ}C,\;L/d\le 6 $for X/d＞0(downstream). The secondary maxima occurs at long distance from the stagnation point as the jet angle increases or the nozzle-to-surface distance decreases. The Y-axis Nusselt number distributions exhibit secondary maxima at Y/d=$\pm$2 for $0^{\circ}C\le a\le30^{\circ}C\; and\; L/d\le4, and \;for\;$\alpha$=40^{\circ}C$and L/d=2. The displacement of the maximum Nusselt number from the stagnation point increases as the jet angle increases or the nozzle-to-surface distance decreases and the maximum distance is about 0.67 times of the nozzle diameter. The ratio of the maximum Nusselt number to the stagnation Nusselt number increases as the jet angle increases.

• 대한기계학회논문집B
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• 제26권3호
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• pp.394-401
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• 2002

The instantaneous flow characteristics of a round jet issuing normally into a crossflow has been studied using a flow visualization technique and particle image velocimetry. The effects of parameters such as jet inflow profile and turbulence intensity of the jet are evaluated for various Reynolds numbers in range between 735 and 3150, which are based on the crossflow velocity and jet-pipe diameter. The jet-to-crossflow velocity ratio is fixed at the value of 3.3. Instantaneous later tomographic images of the symmetry plane of the crossflow jet show that there exist very different natures in the flow structures of the near-field of the jet even though the velocity ratio is same. It is found that when the turbulence intensity of jet is elevated, the shear layer becomes much thicker due to the strong entrainment of the ambient fluid by turbulent interaction between the jet and crossflow. The detailed characteristics of instantaneous velocity and vorticity fields are presented to illustrate the effects of the above parameters on the vertical structures of the crossflow jet.

• 한국자동차공학회논문집
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• 제10권6호
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• pp.51-58
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• 2002

The RPV(Reaction Progress Variable) combustion model has been applied to numerically investigate the effects of Damkohler number on the superequilibrium concentration and flame structure in the nonpremixed turbulent flames. Computations are performed for the two turbulent jet flames of CO/H$_2$/N$_2$(40/30/30 volume percent) having the same jet Reynolds number of 16,700 but different nozzle diameters(4.58mm and 7.72mm). The detailed discussions have been made for the interaction between fluid dynamics and chemistry in the flame field.

• 대한기계학회논문집B
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• 제26권4호
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• pp.531-538
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• 2002

Flow characteristics of a round jet with swirl number of 0.17 have been investigated using a hot -wire anemometry in the initial region within 10D(exit diameter). Swirl effects were observed by comparing centerline flow characteristics, similarities and turbulent budgets of a swirl jet and a free jet, respectively. To obtain similarity of the radial profiles mean velocity and higher moments were measured at the vertical pl anes, located at 2.5, 5.0, 7.5D, 10D, respectively. The centerline velocity characteristics were also measured. It is turned out that similarities of mean and Reynolds stress are established. The jet boundary has wider width than that of a free jet and the shear stress also becomes stronger. In addition the centerline decay becomes faster than that of the free jet, indicating that the swirl induces more entrainment in the initial region of the swirl Jet by transferring the axial mean kinetic energy into the swirl energy and, therefore, has wider boundary, compared with that of free jet.

• 설비공학논문집
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• 제2권4호
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• pp.279-287
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• 1990

An experimental study of the heat transfer characteristics between circular water jet and cylindrical inner surface is presented. The ratios of the semi-cylinder's inner diameter and the nozzle outlet diameter were varied parametrically, as were the Reynolds number and the supplementary water heights. The measurements showed that cirucmferential distribution of the heat transfer coefficient peaked at the stagnation point and, there occurred a kind of a secondary maximum of heat transfer that moved toward to stagnation point as the ratio d/D increased. The local heat transfer coefficient increases as the Reynolds number becomes larger, and the rate of increase is subjected to the influence of d/D & position of angle. Also, optimum heights of supplementary water which brings about the augmentation of heat transfer are S/D=1 for the stagnation point, the position of $15^{\circ}$ & $30^{\circ}$ angle, but for the positions of $45^{\circ}$ angle (d/D=10~11.67), $60^{\circ}$ & $75^{\circ}$ angle, the heat transfer coefficients in the case of using supplementary water are smaller than simple jet (S/D=0).

• 대한기계학회논문집B
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• 제28권10호
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• pp.1139-1146
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• 2004

The effect of triangular tabs attached at the perimeter of jet nozzle on heat transfer enhancement was investigated experimentally. The modified flow structure was visualized using a smoke-wire method. Four different types of jet nozzle having 0, 4, 6 and 8 tabs were tested at jet Reynolds number Re=15,000 to investigate the effect of tabs on the variation of heat transfer rate. The local and average Nusselt numbers are increased with increasing the number of tabs. At nozzle-to-plate distance of L/D=4, the average Nusselt number was increased about 9.9% at Re=15,000 in the impingement region for the case of 8 tabs attachment. As the nozzle-to-plate distance increases, however, the heat transfer enhancement effect of triangular tabs is reduced. For the case of 4 tabs, the heat transfer enhancement is not so distinctive at L/D=8. As the protrusion depth of tabs into the jet flow increases, the heat transfer rate is also enhanced when the nozzle-to-plate distance is smaller than L/D=6.

• 대한기계학회논문집B
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• 제29권5호
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• pp.537-544
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• 2005

The present study numerically investigates two-dimensional fluid flow in the confined jet flow in the presence of applied magnetic field. Numerical simulations to calculate the fluid flow and heat transfer in the confined jet are performed for different Reynolds numbers in the absence and presence of magnetic fields in the range of $0{\le}N{\le}0.05$, where N is the Stuart number (interaction parameter) which is the ratio of electromagnetic force to inertia force. The present study reports the detailed information of flow in the channel at different Stuart numbers. As the intensity of applied magnetic fields increases, the vortex shedding formed in the channel becomes weaker and the oscillating amplitude of impinging jet decreases. The flow fields become the steady state if the Stuart number is greater than a critical value. Thus the pressure coefficients at the stagnation point also vary as a function of Stuart number.

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2006년도 제27회 추계학술대회논문집
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• pp.353-356
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• 2006

본 논문에서는 초음속 유동장내에 분사된 이차제트 주변에서의 열전달 현상을 고찰 하였다 초음속 유동장내에 등열유속조건이 적용된 표면을 설치하고, jet to freestream momentum ratio의 변화에 따른 2차분사를 하여, 2차분사 노즐 주변의 표면온도변화를 적외선카메라를 통하여 측정하였으며, 이를 바탕으로 대류열전달계수를 계산하였다.

• 한국항공우주학회지
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• 제35권2호
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• pp.94-100
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• 2007

초음속 제트 마하수 1.07부터 1.2 범위에서 축대칭 제트 스크리치 톤을 해석하였다. 축대칭 모드는 낮은 마하수 축대칭 제트의 지배적인 스크리치 톤 모드이다. 난류 해석을 위해 수정된 Spalart-Allmaras 모델을 RANS (Reynolds-averaged Navier-Stokes) 방정식에 사용하였다. 스크리치 톤 해석에서 중요한 음파의 전파, shock-cell 구조 및 거대한 불안정 파를 정확히 계산하기 위해 비반사 특성 경계조건과 연계한 고차정확도의 ENO 기법을 사용하였다. 수치 해석결과는 다른 연구자들의 실험 및 계산 결과와 잘 일치하였으며, 따라서 본 연구에 사용된 수치 기법들이 초음속 제트 유동 및 소음연구에 타당함을 확인하였다.

• 에너지공학
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• 제18권2호
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• pp.108-113
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• 2009

균일한 열유속을 갖는 가열된 평판에 부착된 발포 알루미늄에 대한 충돌제트의 열유동 특성에 대한 실험적 연구가 수행되었다. 열전달 특성에 대한 기공도, 다공성 물질의 두께, Reynolds 수의 영향이 고찰되었다. 실험결과 가열평판에 부착된 발포 알루미늄에 의해 열전달의 증가를 얻을 수 있었고 다공성 물질의 삽입에 의한 열전달 증가는 열전달 표면적의 증가와 압력 손실에 의한 운동량 감소에 의해 영향을 받는 것을 확인하였다.